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The Gas Flare Stack Process

Bachelor Thesis 2017 65 Pages

Engineering - Chemical Engineering

Excerpt

Table of Contents

ABSTRACT

ACKNOWLEDGMENT

1. INTRODUCTION
1.1 General Introduction
1.2 Gas Flaring Composition

2. FLARE CLASSIFICATION
2.1 Height of the Flare
2.1.1 Elevated Flare
2.1.2 Ground Flare
2.2 Method to enhance mixing at the flare tip
2.2.1 Steam-Assisted Flares
2.2.2 Air-Assisted Flares
2.2.3 Non-Assisted Flares
2.2.4 Pressure-Assisted Flares

3. PROCESS DESCRIPTION
3.1 Gas Transport Piping
3.2 Knock-out Drum
3.3 Liquid Seal
3.4 Flare Stack
3.5 Purge Gas Reduction (Gas Seal)
3.6 Burner Tip
3.7 Pilot Burners
3.8 Steam Jets
3.9 Controls
3.9.1 Flame Front Generator (FFG):
3.9.2 Electronic spark ignition
3.9.3 Ballistic Pellet Ignition

4. DESIGN PROCEDURE
4.1 Flame Properties
4.1.1 Burning Velocity
4.1.2 Flame Stability
4.1.3 Flame Length
4.2 Radiation
4.3 Size of Flare
4.3.1 Diameter
4.3.2 Height
4.3.3 Flare Tip
4.3.4 Jets
4.4 Sample of Calculation
4.4.1 Calculation of Flare Diameter
4.4.2 Calculation of Flame Length
4.4.3 Calculation of Flame Distortion caused by Wind Velocity
4.4.4 Calculation of Flare Stack Height

5. COST ESTIMATION
5.1 Estimating Total Capital Investment
5.1.1 Equipment Costs
5.1.2 Installation Costs
5.2 Estimating Total Annual Costs
5.2.1 Direct Annual Costs
5.2.2 Indirect Annual Costs

6. INSPECTION (MAINTENANCE), SAFETY AND RECOVERY
6.1 Inspection
6.1.1 Maintenance
6.1.2 Spare Parts
6.1.3 Malfunctions
6.1.4 Operator Training
6.2 Safety
6.2.1 Emergency planning
6.2.2 Maintenance
6.2.3 Ventilation
6.2.4 Noise
6.3 Recovery

7. FLARE GAS IMLICATIONS
7.1 Environmental Implications
7.1.1 Climate Change
7.1.2 Acid Rain
7.1.3 Agriculture
7.1.4 Thermal Emissions and Luminosity
7.1.5 Noise Pollution
7.2 Health Implications
7.2.1 Radiation Effect on Humans
7.2.2 Adverse Effects
7.2.3 Hematological Effects
7.3 Other Implications
7.3.1 Economic Loss
7.3.2 Pollution
7.3.3 Social

CONCLUSION

REFERENCES

ABSTRACT

Gas flaring is a combustion device to burn associated, unwanted or excess gases and liquids released during normal or unplanned over-pressuring operation in many industrial processes, such as oil-gas extraction, refineries, chemical plants, coal industry and landfills.

This paper provides an overview of the gas flaring in industry, concentrated on its composition, relevant environmental and health impacts, process conditions, preferable ignition systems, cost factors of flares (such as height, diameter, flame length, horizontal distance to nearest tower, wind velocity, gas temperature, civil and erection costs).

Also, the effect of wind velocity, Gas temperature, flare height, Gas Mach number, on the process and equipment design specifications are studied with the connection of inspection and maintenance to ensure worker and process safety.

ACKNOWLEDGMENT

The author acknowledges with thanks and appreciation, the contribution, assistance and a life time of continuous motivation from the family, friends and the supervisor in charge of the project with her constant support, facilitating the preparation and presentation of this paper and everyone else who has been a source of help.

List of Figures

Figure 1: Self-Supported Stack Schematic.

Figure 2: Guy Wired Supported Stack Schematic.

Figure 3: Derrick Supported Stack.

Figure 4: Demountable Derrick Schematic.

Figure 5: Open Ground Flare.

Figure 6: Enclosed Ground Flare.

Figure 7: Diagram of an overall Flare Stack System in a Petroleum Refinery

Figure 8: Vertical Two Phase Knock-out Drum Schematic

Figure 9: Horizontal Two Phase Knock-out Drum Schematic.

Figure 10: Vertical Liquid Seal Schematic.

Figure 11: Molecular Seal Schematic

Figure 12: Velocity Seal Schematic.

Figure 13: Burner Tip Close-up Image.

Figure 14: Pilot Burner Close-up Image.

Figure 15: Steam Jets Schematic.

Figure 16: FFG at Site.

Figure 17: Launching Ignition Pellets

Figure 18: Typical Flare Installation P&ID.

Figure 19: Approximate Flame Distortion due to Lateral Winds on Jet Velocity from a Flare Stack (Courtesy of API, 1220L Street, NW, Washington D.C)

Figure 20: Dimensional References for Sizing a Flare Stack (Courtesy of API, 1220 L Street, NW, Washington D.C)

Figure 21: Flame Length vs Heat Release.

Figure 22: Cost vs Flare Height.

Figure 23: Dimensional References of Flame Distortion.

Figure 24: Kurdistan Region Gas Flaring Practices (May 2014) (Provided and Licensed by SkyTruth Global Flaring Map)

List of Tables

Table 1: Radiation from Gaseous Diffusion of Flames

Table 2: Design Data

Table 3: Capital Cost Factors for Flare Systems

Table 4: Capital Costs for Flare System

Table 5: Malfunction Mechanisms, Symptoms and their Corrections.

Table 6: Allowable Waste Gas Composition.

Table 7: Thermal Radiation & Noise Level as a Function of Distance.

Table 8: Exposure Times Necessary to Reach Pain Threshold

Table 9: Pollutants of Flare and Their Health Effect

1. INTRODUCTION

1.1 General Introduction

The flame at the top of an oil production site is an iconic image for the oil and gas industry. Yet few people know why the flare is there and its purpose. The extraction of highly flammable liquids and gases from the earth is obviously a precise and potentially dangerous operation. It involves precise technology combined with experience and expertise developed over decades. Despite well publicized incidents that occasionally take place, the industry is safe and consistently getting safer.

Burning excess gas by flare is a critical part of that safety regime. Flare stacks are often used for burning off flammable gas released by pressure relief valves during unplanned over-pressuring of plant equipment. This often takes place during start-ups and shutdowns in production when the volume of gas being extracted can be uncertain. In this respect flare stacks provide a critical means by which to ensure safety – the alternative to allowing the gas to escape would be a significant build-up of pressure and the risk of explosion. It is not always the case that gas is flared for safety reasons. When crude oil is extracted and produced from onshore or offshore oil wells, raw natural gas also comes to the surface. In areas of the world lacking pipelines and other gas transportation infrastructure, this gas is commonly flared.

The flare is a last line of defense in the safe emergency release system in a refinery or chemical plant. It uses to dispose of purged and wasted products from refineries, unrecoverable gases emerging with oil from oil wells, vented gases from blast furnaces, unused gases from coke ovens, and gaseous water from chemical industries. It provides a means of safe disposal of the vapor streams from its facilities, by burning them under controlled conditions such that adjacent equipment or personnel are not exposed to hazards, and at the same time obeying the environmental regulation of pollution control and public relations requirements. (Ling, July 2007)

Flaring is a volatile organic compound combustion control process in which the VOCs are piped to a remote, usually elevated, location and burned in an open flame in the open air using a specially designed burner tip, auxiliary fuel, and steam or air to promote mixing for nearly complete (> 98%) VOC destruction. Completeness of combustion in a flare is governed by flame temperature, residence time in the combustion zone, turbulent mixing of the components to complete the oxidation reaction, and available oxygen for free radical formation. Combustion is complete if all VOCs are converted to carbon dioxide and water. Incomplete combustion results in some of the VOC being unaltered or converted to other organic compounds such as aldehydes or acids.

The flaring process can produce some undesirable by-products including noise, smoke, heat radiation, light, SO , NO , CO, and an additional source of ignition where not desired. However, by proper design these can be minimized. (Leslie B. Evans, William M. Vatavuk, Diana K. Stone, Susan K. Lynch, Richard F. Pandullo, September 2000)

Downstream processing of hydrocarbons is a tricky business. It must be safe, not only for personnel but also the longevity of the equipment. Regulations of all types shape how systems can be designed, and those regulations can change, requiring modifications to equipment during operation. The process of flaring falls firmly into this category. And as flaring restrictions become more stringent, not only new builds but also existing structures must be kept up-to-date, requiring studies of flare processes and solutions.

Conducting flare designs requires significant engineering knowledge:

a. How does the deployment of a multiphase flow meter in a system affect flaring requirements?
b. How do you mechanically overcome pressure differences associated with re-injecting separated well fluids and gases back into a production line?
c. What are the recognized flaring categories and how do they shape flaring process and its design contributions?

The differences in government and environment approach may at times strongly drive the options available to designers of downstream constructions and modifications. Client budget and goals also affect options: does the company want to simply meet regulatory requirements under a fixed budget, or does it want to spend a little extra to reduce flaring to as low a level as possible to reduce environmental impact beyond what’s regulated?

Flare studies also evaluate a variety of components to determine if they are appropriate for a build or upgrade. Configurations of relief valves, hydraulic flare header systems, and real-time monitoring equipment are analyzed and simulated using standard and software-based tools to determine the best configurations for efficient relief loads and flare capacities. In addition to efficiency, flare studies also take into account safety analyses, ensuring, when appropriate, flame arrestors and control valves are in place to prevent over pressurization and equipment damage.

1.2 Gas Flaring Composition

Generally, the gas flaring will consist of a mixture of different gases. The composition will depend upon the source of the gas going to the flare system. Associated gases released during oil-gas production mainly contain natural gas. Natural gas is more than 90 % methane (CH4) with ethane and a small amount of other hydrocarbons; inert gases such as N2 and CO2 may also be present. Gas flaring from refineries and other process operations will commonly contain a mixture of hydrocarbons and in some cases H2. However, landfill gas, biogas or digester gas is a mixture of CH4 and CO2 along with small amounts of other inert gases. There is in fact no standard composition and it is therefore necessary to define some group of gas flaring according to the actual parameters of the gas. Changing gas composition will affect the heat transfer capabilities of the gas and affect the performance of the measurement by flow meter.

2. FLARE CLASSIFICATION

Flares are generally categorized in two ways: (1) by the height of the flare tip (i.e., ground or elevated), and (2) by the method to enhance mixing at the flare tip (i.e., steam-assisted, air assisted, pressure-assisted, or non-assisted). Elevating the flare can prevent potentially dangerous conditions at ground level where the open flame (i.e., an ignition source) is located near a process unit. Further, the products of combustion can be dispersed above working areas to reduce the effects of noise, heat, smoke, and objectionable odors.

2.1 Height of the Flare

In industrial, the most common utilized flare systems are elevated flares and ground flares.

Selection of the type of flare is influenced by several factors, such as:

- Availability of space
- The characteristics of the flare gas (composition, quantity and pressure)
- Economics
- Investment and operating costs
- Public, government and environmental regulations.

2.1.1 Elevated Flare

Elevated flare is the most commonly used type in refineries and chemical plants. Have larger capacities than ground flares. The waste gas stream is fed through a stack from 50 to 250 meters tall and is combusted at the tip of the stack head, so that the heat generated will not cause safety problems and for the reduction of noise.

The elevated flare, can be (steam assisted, air assisted or non- assisted). Elevated can utilize steam injection / air injection to make smokeless burning and with low luminosity up to about 20% of maximum flaring load. If adequately elevated, this type of flare has the best dispersion characteristics for malodorous and toxic combustion products. Capital costs are relatively high, and an appreciable plant area may be rendered unavailable for plant equipment, because of radiant heat considerations.

All elevated flares contain similar types of secondary equipment to process the flue gas prior to being combusted. Mist eliminators and gravity settlers remove liquid from the gas stream. In the flare tip, a seal prevents dangerous air intrusion, an ignition system keeps the pilot lit, and equipment measures flow rates and temperatures. In addition many flare systems contain equipment to eliminate smoke produced during combustion.

2.1.1.1 Self-Supported Stacks

This is the simplest and most economical design for applications requiring short-stack heights because they require relatively less installation space and are simple to erect, reducing capital costs. Self-supporting flare systems have no wires or framework to support them. Instead, the material thickness is set to provide the desired strength, limiting the height to under (50 meters overall height); however, as the flare height and/or wind loading increases, the diameter and wall thickness required become very large and expensive.

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Figure 1: Self-Supported Stack Schematic

2.1.1.2 Guy Wired Supported Stacks

This is the most economical design in the 50 – 150 meters height range. The design can be a single-diameter riser or a cantilevered design. Normally, sets of 3 wires are anchored 120 degrees apart at various elevations (1 to 6). Investment for guy wires flare systems is generally lower than the other types of structural support because their installation require large amounts of land to accommodate the wires.

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Figure 2: Guy Wired Supported Stack Schematic

2.1.1.3 Derrick Supported Stacks

This is the most feasible design for stack heights above 200 meters. Derrick supported flares can be the optimum solution for flare systems installed inside plants, when higher elevation is required to limit ground radiation, thermal expansion and available area is limited as a result of other present equipment. They use a single-diameter riser supported by a bolted framework of supports which offer greater support than guyed stacks, but cost 50 to 100% more to build. Derrick supports can be fabricated from pipe (most common), angle iron, solid rods, or a combination of these materials.

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Figure 3: Derrick Supported Stack

2.1.1.4 Demountable Derrick

This is a variation on a derrick supported system, this type of elevated flare tip is mounted on top of a riser supported by a steel trussed (derrick) structure which is designed so that multiple flares can be installed in a single derrick structure. Similar to a conventional derrick supported flare system, demountable derricks require three or four additional foundations for the derrick legs. Due to the economic impact of shutting a flare down and the requirement for large cranes, users welcome a means to be able to lower the flare tip to grade level for maintenance, inspection, or repair activities that allows one flare to be worked on or replaced while the other flare(s) remain in service.

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Figure 4: Demountable Derrick Schematic (Figure removed due to copyright reasons)

A demountable derrick allows a user the ability to lower the flare tip and pilots to grade through the use of a winch system (typically three different winches), guide rails, tilting tables, and flanged riser sections. Demountable derricks eliminate concerns associated with process temperature designs.

2.1.2 Ground Flare

A ground flare is where the combustion takes place at ground level. It varies in complexity and may consist either of conventional flare burners discharging horizontally with no enclosure or of multiple burners in refractory-lined steel enclosures. The type, which has been used almost exclusively, is the multi jet flare (enclosed type).

Compared to elevated flare, ground flare can achieve smokeless operation as well, but with essentially no noise or luminosity problems, provided that the design gas rate to the flare is not exceeded. However, it has poor dispersion of combustion product because it stack is near to ground, this may result in severe air pollution or hazard if the combustion products are toxic or in the event of flame-out. Capital, operating and maintenance requirements cost are high.

Because of poor dispersion, multi jet flare is suitable for "clean burning" gases when noise and visual pollution factors are critical. Generally, it is not practical to install multi jet flares large enough to burn the maximum release load, because the usual arrangement of multi jet flare system is a combination with an elevated over-capacity flare. (Ling, July 2007)

2.1.2.1 Open Ground Flare

An open ground flare can come in several different configurations. Most systems have a series of headers spaced across open ground with multiple burner tips to distribute the flame with a radiation fence typically surrounding the area. Open ground flares are able to burn larger quantities at a time. These units are almost completely smokeless and have a very high gaseous conversion rate. Open Ground Flares are necessary for handling large gas flow rate with smokeless operations nevertheless they require large clearances.

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Figure 5: Open Ground Flare. (Figure removed due to copyright reasons)

The open ground flare units contain a limited amount of space for the burners. This means that the units must be kept at a distance for safety reasons. These systems also require a gas pressure of 3-5 psi, which is much higher than any other type of flare system. While open ground flare units are effective, their bright light, noise and heat emissions require that the unit be used in a more secluded area.

2.1.2.2 Enclosed Ground Flare

An enclosed ground flare is much more common than an open ground flare. This combustion system utilizes a refractory shell to enclose the incineration process. An enclosed flare's burner heads are inside a shell that is internally insulated. This shell reduces noise, luminosity, and heat radiation and provides wind protection. A high nozzle pressure drop is usually adequate to provide the mixing necessary for smokeless operation and air or steam assist is not required. They are equipped with a vertical combustion chamber and the height must be adequate for creating enough draft to supply sufficient air for smokeless combustion and for dispersion of the thermal plume. The internal processes of the enclosed ground flare are the same as that of the elevated flare. Enclosed flares generally have less capacity than open flares and are suitable for managing low and medium gas flow rates (~100,000 kg/hr or less) due to the cost of providing an enclosure around the flame and are used to combust continuous, constant flow vent streams, although reliable and a very high combustion efficiency under any atmospheric conditions operation can be attained over a wide range of design capacity. Stable combustion can be obtained with lower Btu content vent gases than it is possible with open flare designs probably due to their isolation from wind effects.

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Figure 6: Enclosed Ground Flare

2.2 Method to enhance mixing at the flare tip

2.2.1 Steam-Assisted Flares

Steam-assisted flares are single burner tips, elevated above ground level for safety reasons that burn the vented gas in essentially a diffusion flame. They reportedly account for the majority of the flares installed and are the predominant flare type found in refineries and chemical plants.

Steam assisted flares are designed to dispose of heavier waste gases which have a greater tendency to smoke and are typically found in downstream applications where high efficiency combustion of heavy hydrocarbons is required. In order to prevent incomplete combustion and ensure an adequate air supply and good mixing, steam is injected into the waste stream (using peripheral steam rings, center steam sparkers, and/or inner induction tubes) to promote turbulence for mixing and to induce air into the flame. Steam flares are used in applications where high-pressure steam is available on site. (Leslie B. Evans, William M. Vatavuk, Diana K. Stone, Susan K. Lynch, Richard F. Pandullo, September 2000)

The injection of steam has two principal effects:

- High-pressure steam flow causes turbulence in the waste stream which improves mixing and therefore improves combustion efficiency.
- Additional air is induced into the waste gas providing the oxygen necessary for augmented smokeless capacity.

2.2.2 Air-Assisted Flares

Air-assisted flares are comprised of two risers (waste gas and air) and a blower system that provides supplemental combustion air, this type of flares use forced air to provide the combustion air and the mixing required for smokeless operation. Upon mixing, the high-pressure air flow causes turbulence in the waste gas stream, improving mixing, and ultimately combustion efficiency. These flares are built with a spider-shaped burner (with many small gas orifices) located inside but near the top of a steel cylinder (two feet or more in diameter). Combustion air is provided by a fan in the bottom of the cylinder. The amount of combustion air can be varied by varying the fan speed.

The principal advantage of the air-assisted flares is that they can be used where steam is not available. Although air assist is not usually used on large flares (because it is generally not economical when the gas volume is large) and the number of large air assisted flares being built is increasing. These flares generally dispose of heavier waste gases which have a greater tendency to smoke.

2.2.3 Non-Assisted Flares

The non-assisted flare is just a flare tip without any auxiliary provision for enhancing the mixing of air into its flame. Its use is limited essentially to gas streams that have a low heat content and a low carbon/hydrogen ratio that burn readily without producing smoke and for installations where smokeless combustion of heavy hydrocarbons is not required. These streams require less air for complete combustion, have lower combustion temperatures that minimize cracking reactions, and are more resistant to cracking. Utility flare tips are one of the lower capital cost options for safe disposal of waste gases.

2.2.4 Pressure-Assisted Flares

Pressure-assisted flares use the vent stream pressure to promote mixing at the burner tip. If sufficient vent stream pressure is available, these flares can be applied to streams previously requiring steam or air assist for smokeless operation. High Pressure Flares use the actual pressure of the waste gas to create turbulence, entraining combustion air into the base of the flame to obtain smokeless combustion.

Pressure-assisted flares generally (but not necessarily) have the burner arrangement at ground level, and consequently, must be located in a remote area of the plant where there is plenty of space available. They have multiple burner heads that are staged to operate based on the quantity of gas being released. The size, design, number, and group arrangement of the burner heads depend on the vent gas characteristics.

3. PROCESS DESCRIPTION

For flares, VOC’s are led via a pipe to a remote place, normally high, and are then combusted in open air via an open flame or sent to an enclosed ground flare. In order to realize effective combustion, a well-designed burner outlet, a pilot flame, steam or air injection for good turbulence and mixing are needed, along with extra fuel.

Most flames work via a diffusion flame. With a diffusion flame, air is mixed with the outer edge of the fuel gas/flue gas so that the fuel gas is surrounded by a combustible gas mixture. A stable flame is obtained when this mixture is ignited. The heat transfer takes place via heat diffusion between the boundary layer and the fuel gas.

Soot particles will be formed due to the cracking of VOC’s. The glow created by these soot particles lends a yellow color and clarity to the flame. In large diffusion flames, a burning section can be sealed off from the open air due to gas burbles and turbulence. This causes soot to be formed and there is local instability which makes the flame flicker. The elements of an elevated steam-assisted flare generally consist of gas vent collection piping, utilities (fuel, steam, and air), piping from the base up, knock-out drum, liquid seal, flare stack, gas seal, burner tip, pilot burners, steam jets, ignition system, and controls. The Figure diagram of a steam-assisted elevated smokeless flare system showing the usual components that are included.

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Figure 7: Diagram of an overall Flare Stack System in a Petroleum Refinery . (Figure removed due to copyright reasons)

3.1 Gas Transport Piping

Process vent streams are sent from the facility release point to the flare location through the gas collection header. The piping (generally schedule 40 carbon steel) is designed to minimize pressure drop. Ducting is not used as it is more prone to air leaks. Valving should be kept to an absolute minimum and should be “car-sealed” (sealed) open. Pipe layout is designed to avoid any potential dead legs and liquid traps. The piping is equipped for purging so that explosive mixtures do not occur in the flare system either on start-up or during operation.

3.2 Knock-out Drum

Liquids that may be in the vent stream gas or that may condense out in the collection header and transfer lines are removed by a knock-out drum. Gravity causes the liquid to settle to the bottom of the vessel, where it is withdrawn. The vapor travels upward at a designed velocity which minimizes the entrainment of any liquid droplets in the vapor as it exits the top of the vessel. The knock out or dis-entrainment drum is typically either a horizontal or vertical vessel located at or close to the base of the flare, or a vertical vessel located inside the base of the flare stack. Liquid in the vent stream can extinguish the flame or cause irregular combustion and smoking. In addition, flaring liquids can generate a spray of burning chemicals that could reach group level and create a safety hazard.

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Figure 8: Vertical Two Phase Knock-out Drum Schematic . (Figure removed due to copyright reasons)

For a flare system designed to handle emergency process, this drum must be sized for worst case conditions (e.g., loss of cooling water or total unit de-pressuring) and is usually quite large. For a flare system devoted only to vent stream VOC control, the sizing of the drum is based primarily on vent gas flow rate with consideration given to liquid entrainment. (Cheremisinoff, PRESSURE SAFETY DESIGN PRACTICES FOR REFINERY AND CHEMICALOPERATIONS, 1998)

Both the horizontal & vertical design is a common consideration for the Knock out drum, which is determined based on the operating parameters as well as other plant conditions. If a large liquid storage capacity is desired and the vapor flow is high,

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Figure 9: Horizontal Two Phase Knock-out Drum Schematic

A horizontal drum is often more economical. Also, the pressure drop across horizontal drums is generally the lowest of all the designs. Vertical knockout drums are typically used if the liquid load is low or limited plot space is available.

A vapor-liquid separator may also be referred to as a flare KO drum, flash drum, knock-out drum, knock-out pot, compressor suction drum or compressor inlet drum.

The separator is designed into 3 zones with different functions.

- Zone 1: Inlet distribution zone

The inlet distribution zone provides 2 functions. First the installed internals – such as inlet gas distributors or an open half pipe – knock out the majority of the incoming liquid. Second the internals distribute the incoming gas flow as uniformly as possible. The function of the inlet distribution zone is often underestimated in its complexity and requires special care in its design.

- Zone 2: Fine separation zone

In the fine separation zone all remaining liquid is removed from the gas flow. Technologies used for the separation are inertial impaction and direct interception - covering a particle range starting from 2-3µm. The fine separators can be equipped with perforated plates – for flow straightening purposes – ensuring the optimum function of the separator.

- Zone 3: Liquid collection and drain zone

In this zone all pre-separated liquid (from zone 1) and from the fine separator (zone 2) is collected and drained off. The bottom section of the vertical KO drum holds the collected and separated liquid coming from zone 1 and 2. The down comers (drain pipe typically coming from zone 2) are mounted as close as possible to the vessel shell wall and will not be positioned in close proximity to the inlet gas distributors. The height of zone 3 depends on process parameters such as hold-up time, surge time and the amount of liquid passing through the vessel.

3.3 Liquid Seal

Purge systems are the perfect solution for preventing air infiltration in the flare stack and header system; however, it is possible to lose purge gas supply. When purge gas is lost or interrupted, the flare system and plant can once again face the possibility of flashback and/or catastrophic explosion. Process vent streams are usually passed through a liquid seal before going to the flare stack. The liquid seal can be downstream of the knockout drum or incorporated into the same vessel.

Figure 10: Vertical Liquid Seal Schematic

The liquid seal drum is a specially designed vessel containing a predetermined level of water in the base of the drum. As the waste/process gas enters the drum through the flare system header, it is diverted down into the water and forced to bubble through the liquid seal. The gas then travels up through the flare stack and tip for combustion.

The main purpose of the liquid seal drum: Firstly , is to stop flame propagation and prevent possible flame flashbacks caused when air is inadvertently introduced into the flare system and the flame front pulls down into the stack by quenching the flame with a barrier of water.

Second, the liquid seal acts as a large check valve so that gas cannot travel upstream for any reason.

A third function, is to dis-entrain liquid droplets. Thus, the liquid seal drum provides flame arresting capabilities, acts as a check valve, and knocks liquids out of the waste gas. Liquid seals can be designed as a separate vessel or as an integral part of the flare riser structure.

The liquid seal also serves to maintain a positive pressure on the upstream system and acts as a mechanical damper on any explosive shock wave in the flare stack. Other devices, such as flame arresters and check valves, may sometimes replace a liquid seal or be used in conjunction with it. Purge gas also helps to prevent flashback in the flare stack caused by low vent gas flow. (API, JANUARY 2000)

3.4 Flare Stack

For safety reasons a stack is used most of the time to elevate the flare. The flare must be located so that it does not present a hazard to surrounding personnel and facilities. As described in details in Chapter 2.

3.5 Purge Gas Reduction (Gas Seal)

Keeping oxygen out of the flare system is a critical safety concern. At low waste gas flow rates, air may tend to flow into flare tip through the top and travel down the inside wall of the tip due to wind or thermal contraction of stack gases and create an explosion potential. To prevent this, purge or sweep gas is used. Purge gas can be any gas that will not condense. Natural gas and nitrogen are the two most commonly used. Purge reduction devices are used to reduce the amount of purge gas required to keep the oxygen level below 8% and minimize utility costs. There are two types of purge reduction devices commonly in use:

v Molecular Seal: (also referred to as a buoyancy seal, flare seal, stack seal, labyrinth seal, or gas barrier), are installed just below the flare tip flange. The seal internals force the waste and purge gas travelling up the stack to make a 180 degree turn down and then it must turn again to go up to the flare tip. The light gases will collect at the top of the seal creating a barrier to any oxygen that may have come down the flare stack. This design provides a greater reduction in required purge gas but are considerably larger and more expensive than velocity seals. Molecular Seals can also collect liquids which can freeze in cold environments.

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Figure 11: Molecular Seal Schematic . (Figure removed due to copyright reasons)

v Velocity Seal: These are known by the names (internal-gas seal, fluidic-seal, and arrestor seal). A velocity seal is simply a cone or chevron that is located inside the flare tip just above the flare tip flange. The cone shaped design breaks the flow of air into the system by disrupting the flow passage of the air to the wall and creates a velocity differential barrier in the purge gas. A velocity seal doesn’t collect liquids or require draining, making it suitable for cold weather environments. They are less expensive but do not reduce the amount of purge gas as much as a molecular seal, although they are usually proprietary in design and their presence reduces the operating purge gas requirements. (Charles E. Baukal, JR., 2014)

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Figure 12: Velocity Seal Schematic

3.6 Burner Tip

The burner tip or flare tip, is designed to give environmentally acceptable combustion of the vent gas over the flare system's capacity range and play the role to keep an optimum burn and control over all flow rates. The burner tips are normally designed to make sure that the tip does not come into contact with the flame making the tips reliable and long lasting. Consideration is given to flame stability, ignition reliability, and noise suppression. The maximum and minimum capacity of a flare to burn a flared gas with a stable flame is a function of tip design. Flame stability can be enhanced by flame holder retention devices incorporated in the flare tip inner circumference.

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Figure 13: Burner Tip Close-up Image

Burner tips with modern flame holder designs can have a stable flame over a flare gas exit velocity. The actual maximum capacity of a flare tip is usually limited by the vent stream pressure available to overcome the system pressure drop. (Cheremisinoff, INDUSTRIAL GAS FLARING PRACTICES, Apr 1, 2013)

3.7 Pilot Burners

Presence of a flame and pilot gas supply should be stable and continuous (also required by EPA regulations). Reliable ignition is obtained by continuous pilot burners designed for stability and positioned around the outer perimeter of the flare tip. The number and duty of the pilot burners should be determined by the size of the flare and its application. For most flare systems the pilot burner cannot be accessed for service. Maintenance or replacement is not possible while the flare is in operation.

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Figure 14: Pilot Burner Close-up Image

The pilot burner is essentially a robust premixed burner unit designed to provide a stable flame for igniting the waste gas exiting the flare tip and features three main components (venturi or mixer, gas jet or orifice and the burner nozzle or pilot tip). The flow of the pilot gas through the orifice and into the mixer draws in air through the inspirator assembly. This air then mixes with the fuel gas and flows up to the burner nozzle (pilot tip) where it is ignited and the resultant pilot flame is stabilized. Reliable operation of the pilot burner is governed by achieving the right air fuel ratio for the pilot gas being used. Too much air and the pilot will be unstable, too little air and the pilot will struggle to light unless enough secondary air is available around the pilot nozzle.

Pilot burners have to be designed to withstand extreme weather conditions (rain, wind, humidity and high temperatures) along with direct flame impingement from the flare tip as the pilot burner nozzle is typically subjected to thermal cycling by being frequently engulfed in flame from the flare itself.

The pilot burners are ignited by an ignition source system, which can be designed for either manual or automatic actuation. Automatic systems are generally activated by a flame detection device using either a thermocouple, an infra-red sensor or, more rarely, (for ground flare applications) an ultra-violet sensor.

3.8 Steam Jets

A diffusion flame receives its combustion oxygen by diffusion of air into the flame from the surrounding atmosphere. The high volume of fuel flow in a flare may require more combustion air at a faster rate than simple gas diffusion can supply. High velocity steam injection nozzles, positioned 1-15 around the outer perimeter of the flare tip,

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Figure 15: Steam Jets Schematic

Increased gas turbulence in the flame boundary zones, drawing in more combustion air and improving combustion efficiency. For the larger flares, steam can also be injected concentrically into the flare tip.

The injection of steam into a flare flame can produce other results in addition to air entrainment and turbulence. Three mechanisms in which steam reduces smoke formation have been presented.

Briefly, one theory suggests that steam separates the hydrocarbon molecule, thereby minimizing polymerization, and forms oxygen compounds that burn at a reduced rate and temperature not conducive to cracking and polymerization.

Another theory claims that water vapor reacts with the carbon particles to form CO, CO2, and H2, thereby removing the carbon before it cools and forms smoke.

An additional effect of the steam is to reduce the temperature in the core of the flame and suppress thermal cracking.

The physical limitation on the quantity of steam that can be delivered and injected into the flare flame determines the smokeless capacity of the flare. Smokeless capacity refers to the volume of gas that can be combusted in a flare without smoke generation. The smokeless capacity is usually less than the stable flame capacity of the burner tip.

Significant disadvantages of steam usage are the increased noise and cost. Steam aggravates the flare noise problem by producing high-frequency jet noise. The jet noise can be reduced by the use of small multiple steam jets and, if necessary, by acoustical shrouding. Steam injection is usually controlled manually with the operator observing the flare (either directly or on a television monitor) and adding steam as required to maintain smokeless operation. To optimize steam usage infrared sensors are available that sense flare flame characteristics and adjust the steam flow rate automatically to maintain smokeless operation. Automatic control, based on flare gas flow and flame radiation, gives a faster response to the need for steam and a better adjustment of the quantity required. If a manual system is used, steam metering should be installed to significantly increase operator awareness and reduce steam consumption. (Ludwig, Ernest E., 1999)

3.9 Controls

Flare system control can be completely automated or completely manual. Components of a flare system which can be controlled automatically include the auxiliary gas, steam injection, and the ignition system. Fuel gas consumption can be minimized by continuously measuring the vent gas flow rate and heat content (Btu/scf) and automatically adjusting the amount of auxiliary fuel to maintain the required minimum heating value for steam-assisted flares. Steam consumption can likewise be minimized by controlling flow based on vent gas flow rate. Steam flow can also be controlled using visual smoke monitors. Automatic ignition panels sense the presence of a flame with either visual or thermal sensors and reignite the pilots when flameouts occur.

In order to light the pilot burner and in turn the flare tip, an ignition system is required. Ignition of the premixed gas at the pilot burner nozzle is the most reliable arrangement and can be achieved by flame or spark. A number of different ignition systems are available, the most common being:

3.9.1 Flame Front Generator (FFG):

Sometimes referred to as the ’fire ball’ ignition type. These systems are also very common and among the oldest technologies. The FFG mixes plant or instrument air with pilot ignition gas (typically natural gas or propane) at grade and fills a line running up to the pilot burner.

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Figure 16: FFG at Site

At grade an electrically generated spark ignites the mixture and the resulting fireball travels up to the pilot and ignites the main pilot fuel gas. These systems are also available in Manual or automated version.

The disadvantage is that FFGs are temperamental. Moisture or solids can block the fireball from making it up to the pilot. It is also not uncommon for the gas and air mixture to get too lean or too rich and the spark doesn’t ignite. Sometimes over anxious operators don’t allow the line running up to the pilot to completely fill and the fireball “stops short.” All of these disadvantages can be suppressed with maintenance and training.

The advantage of these systems is that all items (flow control and the sparking device) needing maintenance or repair are at grade and can be serviced while the flare is in operation. It is not uncommon for an automatic HEI system to be the primary method of ignition and a manual FFG to be the backup system.

3.9.2 Electronic spark ignition

This type of a flare pilot burner is simple and easy to automate and more frequently becoming the preferred flare ignition method. There are two basic forms of these systems; high energy (HE) and high tension (HT).

High Energy Ignition (HEI) is probably the most common and primary method of pilot ignition for newer systems. HEI systems use an electric probe inserted near the pilot burner to create a spark and ignite the pilot fuel gas/air mixture. A capacitor is used to discharge the spark across a low tension spark plug in a short time and with a high current. Pilot flame monitoring is achieved through the use of thermocouples.

Over the last several years these systems have proven very reliable. These systems can easily be automated so that an HEI system will attempt to relight the pilot without the need for operator action.

3.9.3 Ballistic Pellet Ignition

This system, originally developed in Norway for offshore use. Instead of a fireball made of natural gas and air, it comprises a launching cabinet containing the ignition pellets, a guide tube and a pellet collector.

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Figure 17: Launching Ignition Pellets. (Figure removed due to copyright reasons)

The launcher uses compressed air to drive a pellet through the guide tube up to the flare tip/deck. Once the pellet exits the guide tube, the fuse is released and the pellet explodes producing a shower of sparks over the flare tip and any associated pilot burners thereby lighting the gas. The empty pellet is retained in the collector. Flame monitoring can be carried out optically or via thermocouples attached to the pilot burners (if fitted). These systems are often found in production facilities but not very often in process plants.

Figure 18: Typical Flare Installation P&ID. (Figure removed due to copyright reasons)

4. DESIGN PROCEDURE

The sizing of flares requires determination of the required stack diameter and the required stack height.

Since the flare tip is open to the atmosphere, high gas velocities are expected at this point. Very high tip velocities cause a phenomenon known as blow-off where the flame front is lifted and could eventually turn into a blow-out. Very low velocities could damage the flare tip due to high heat intensities and smoking. In this case ingress of air in the system and creation of a flammable mixture is possible. Therefore, determination of the right flare diameter is important as far as operation of the system is concerned.

The location and height of flare stacks should be based upon the heat release potential of a flare, the possibility of personnel exposure during flaring, and the exposure of surrounding plant equipment. There are exposure limitations set forth which must be taken into consideration. This in effect fixes the distance between the flame and the object. Now if there are limitations on the location (distance), then the stack height can be calculated, otherwise an optimum tradeoff between height and distance should be applied.

Wind velocity, by tilting the flame in effect changes the flame distance and heat intensity. Therefore, its effect should be considered in determining the stack height.

If the flare is blown-out (extinguished), or if there are environmental hazards associated with the flare output, the possibility of a hazardous situation downwind should be analyzed.

4.1 Flame Properties

4.1.1 Burning Velocity

A flame is a rapid self-sustaining chemical reaction occurring in a distinct reaction zone. Two basic types of flames are: (1) the diffusion flame, found in conventional flares, which occurs on ignition of a fuel jet issuing into air: and (2) the aerated flame, which occurs when fuel and air are premised before ignition. The burning velocity, or flame velocity, is the speed at which a flame front travels to its surface and into the unburned combustible mixture.

4.1.2 Flame Stability

In the case of a flare, the flame front is normally at the top at the stack. However, at low gas velocities, back mixing of air may occur in the top of the stack. Experiments show that if a sufficient flow of combustible gas maintained to produce a flame visible from ground level, there usually will not be significant back mixing of air into the stack. At lower gas flows there is the possibility of combustion at a flame front part way down the stack with a resultant high stack temperature, or of flame extinguishment with subsequent formation of an explosive mixture in the stack and ignition from the pilot light.

In an aerated flame from a premising device a phenomenon known 'flashback' may occur. This results from the linear velocity of the combustible mixture becoming less than the flame velocity, causing the flame to travel back to the point of mixture.

In the case of either aerated or diffusion flames, if the fuel flow rate is increased until it exceeds the flame velocity at every point, the flame will be lifted above the burner until a new stable position in the gas stream above the port is reached as a result of turbulent mixing and dilution with air. This phenomenon is called 'blow off'. (Extinguishment of the flame is referred to as 'blowout'). By means of a sufficiently large pilot flame, it is possible to anchor the flame of the main stream in the boundary regions where velocity gradient would otherwise far exceed the critical value for the blow off. However, there is evidence that flame stability can be maintained at Mach numbers of about 0.4, depending on discharge properties and type of tip used. Both blow off and flashback velocities are greater for fuels with high burning velocities. Small amounts of hydrogen in a hydrocarbon fuel widen the stability range, because blow off velocity increases much faster than flashback velocity.

4.1.3 Flame Length

The flare tip exit velocity is calculated as follows:

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The flame vertical length, is estimated by using the following equation:

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Equation 4.3 (above) is valid for 1.2 0.022. Similarly, the flame horizontal length, is estimated by using the following equation:

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Equation 4.4 is valid for 1.22 0.005. Both and can also be calculated using Figure 19.

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Figure 19: Approximate Flame Distortion due to Lateral Winds on Jet Velocity from a Flare Stack (Courtesy of API, 1220L Street, NW, Washington D.C)

The center of the flame from the top of the flare stack can be calculated as follows:

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Where:

= vertical distance of the flame center from the top of the flare stack in m.

= horizontal distance of the flame center from the top of the flare stack in m.

From the geometry of the flare stack (Figure 20), the flare stack height can be easily calculated. (Datta, 2014)

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Figure 20: Dimensional References for Sizing a Flare Stack (Courtesy of API, 1220 L Street, NW, Washington D.C)

4.2 Radiation

Equation 4.7 may be used to determine the distance required between a location of atmospheric venting and point of exposure where terminal radiation must be lifted:

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Where:

= Minimum distance from midpoint of flame to object being considered, in m.

= Fraction of heat radiated.

= Heat release, in kcal/h.

= Allowable radiation, in kcal/m2/h.

The F factor allows for the fact that not all the heat released in a flame can be released as radiation. Measurements of radiation from flames indicate that the fraction of heat radiated (radiant energy per total heat of combustion) increases toward a limit somewhat as the burning rate does with increasing diameter of flame. (Crawford, 1988)

Data from the US Bureau of Mines for radiation from gaseous diffusion of flames are given in Table 1. These data apply only to the radiation from a gas. If liquid droplets of hydrocarbon larger than 150 pm in size are present in the flame, these values should be increased somewhat.

Table 1: Radiation from Gaseous Diffusion of Flames

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Figure 21: Flame Length vs Heat Release.

The calculated distance is based on a vertical flame burning in still air. Flame under the influence of wind will tilt in the direction the wind is blowing. Until more data become available, the lateral wind effect on flames from flare stacks may be obtained from Figure 19 which relates horizontal reach and vertical lift of flames to the lateral wind velocity relative to stack velocity. (Hossein Shokouhmand, Shahab Hosseini, October 24, 2007)

4.3 Size of Flare

The sizing of flares requires determination of the required stack diameter and the required stack height. Factors also governing sizing are wind effect, dispersion, and ground flares. An example covering the full design of a flare stack is given in Section 4.4.

4.3.1 Diameter

Flare stack diameter is generally sized on a velocity basis. Although pressure drop should be checked. Depending on: (1) the volume ratio of maximum conceivable flare flow to anticipated average flare flow; (2) the probable timing, frequency. and duration of those flows; and (3) the design criteria established for the project to stabilize flare burning, it may be desirable with low pressure flare tips and to permit a velocity of up to 0.5 Mach for a peak, short-term, infrequent flow (emergency release) and 0.2 Mach for normal conditions, (where Mach equals the ratio of vapor velocity to sonic velocity in that vapor at the same temperature and pressure and is dimensionless). These API 521 recommendations are conservative. Some suppliers are designing "utility-type" tips for rates up to 0.8 Mach for emergency releases. For high-pressure flare tips, most manufacturers offer "sonic" flares that are very stable and clean burning; however, they do introduce a higher backpressure into the flare system. Smokeless flares should be sized for the conditions under which they are to operate smokelessly. (McDaniel, July 1983)

The formula relating velocity (as Mach number) to flare tip diameter can be expressed as follows:

(4.8)

Where:

= Ratio of vapor velocity to sonic velocity in that vapor.

= Vapor relief rate in kg/h.

= Pressure of vapor just inside flare tip in kgf/cm2.

= Flare stack tip diameter in m.

= Temperature of vapor just inside flare tip in degrees R.

= Ratio of specific heats, for vapor being relieved.

= Molecular weight of vapor in kg/mol

Pressure drops as high as 0.14 kgf/cm2 have been used satisfactorily at the flare tip. Too low a tip velocity can cause heat and corrosion damage. The burning of the gases becomes quite slow, and the flame is greatly influenced by the wind. The low-pressure area on the downwind side of the stack may cause the burning gases to be drawn down along the stack for 3 m or more. Under these conditions, corrosive materials in the stack gases may attack the stack metal at an accelerated rate, even though the top 2.5 - 3m of the flare is usually made of corrosion-resistant material.

(4.9)

Where:

acceleration due to gravity, 32.3 ft/s2.

= gas velocity, ft/s.

= pressure drop at the tip, inches of water.

= density of gas, lbm/ft3.

4.3.2 Height

The flare stack height is generally based on the radiant heat intensity generated by the flame. If toxic or corrosive pollutants are present in the stream, a check should be made on the maximum concentration level and its location to determine what might occur if the flare pilot flame should be lost.

To utilize the formula for radiant heat given in Section 4.2, two factors must be established: the fraction of heat radiated, and the allowable radiation intensity, it appears that an value of 0.2 for methane, 0.3 for higher molecular weight hydrocarbons, and 0.15 for hydrogen would be the maximum thermal radiation values expected with near-ideal combustion conditions. Since efficient combustion would seldom be expected at peak flaring rates, use of values of approximately two-thirds of those cited herein are suggested as representing a more practical approach.

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Figure 22 shows that there is a linear relation between flare height and cost. For example, a 10 m increase in flare height causes 40 thousand USD more cost. Therefore, a lower flare with bigger R is preferable. There is a constraint in lower limit of flare height. In order to diffuse the pollution, the flare needs to be risen up to a height so that the wind speed is enough to diffuse the pollution. (Hossein Shokouhmand, Shahab Hosseini, October 24, 2007)

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Figure 22: Cost vs Flare Height.

4.3.3 Flare Tip

The flare tip exit velocity is calculated as follows:

= (4.13)

Where:

= Flare tip exit velocity in m/s.

= Actual volumetric flowrate in m3/s.

= diameter of the pipe in m.

4.3.4 Jets

The number of jets is based on gas velocity. For 25 mm standard pipe, the recommended maximum velocity permits a flow rate of 72.2 m3/h (actual or standard) of gas per jet. The following empirical equation can be used:

(4.14)

Where:

= Number of jets ( should be rounded off upward to a whole number)

= Flare design capacity, m3/h (for a flare system, actual and standard cubic meters are virtually equivalent).

The jets are laid out on an approximately square pitch, with spacing in both directions varying between roughly 450 mm and 600 mm.

(4.15)

Where:

= Center-to-center spacing of adjacent burner lines, mm

= Center-to-center spacing of jets along a burner line, mm

= Inside diameter of stack, m.

This equation is based on a negligible wall effect. The design values of and must be determined from a scale drawing, which is made to allow the required number of jets to be installed in the available area. This area is restricted by the limitation that no jet should be placed closer than 300 mm from the inside wall of the stack. The spacing is also affected by air flow considerations, which may require the layout to be modified. (Standards, April 2012)

4.4 Sample of Calculation

Basis: Design for velocity of Mach number = 0.2

Table 2: Design Data

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4.4.1 Calculation of Flare Diameter

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4.4.2 Calculation of Flame Length

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= Heat liberated in kcal/h = (heat of combustion)

From Figure 21,

4.4.3 Calculation of Flame Distortion caused by Wind Velocity

A method is given to determine flame distortion.

From Figure 19,

Figure 23: Dimensional References of Flame Distortion.

4.4.4 Calculation of Flare Stack Height

Design basis:

- Fraction of heat radiated, . Use two-thirds of F in Hajek and Ludwig's equation.
- Heat liberated, ) (
- Maximum allowable radiation at 45.72m from flare stack,

Figure 24: Kurdistan Region Gas Flaring Practices (May 2014) (Provided and Licensed by SkyTruth Global Flaring Map)

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5. COST ESTIMATION

5.1 Estimating Total Capital Investment

The capital costs of a flare system are presented in this section and are based on the design/sizing procedures discussed in Chapter 4.

Total capital investment TCI, includes the equipment costs EC for the flare itself, the cost of auxiliary equipment, the cost of taxes, freight, and instrumentation, and all direct and indirect installation costs.

The capital cost of flares depends on the degree of sophistication desired (i.e., manual vs automatic control) and the number of appurtenances selected, such as knock-out drums, seals, controls, ladders, and platforms. The basic support structure of the flare, the size and height, and the auxiliary equipment are the controlling factors in the cost of the flare. The capital investment will also depend on the availability of utilities such as steam, natural gas, and instrument air.

The total capital investment is a battery limit cost estimate and does not include the provisions for bringing utilities, services, or roads to the site, the backup facilities, the land, the research and development required, or the process piping and instrumentation interconnections that may be required in the process generating the waste gas. These costs are based on a new plant installation; no retrofit cost considerations such as demolition, crowded construction working conditions, scheduling construction with production activities, and long interconnecting piping are included. These factors are so site-specific that no attempt has been made to provide their costs.

5.1.1 Equipment Costs

The expected accuracy of these costs is ± 30% (i.e., "study" estimates).

The standard construction material is carbon steel except when it is standard practice to use other materials, as is the case with burner tips.

The flare costs, presented in Equations 5.1, 5.2, 5.3 are calculated and are based on support type as follows:

Self-Support Group: (5.1)

Guy Support Group: (5.2)

Derrick Support Group: (5.3)

Where:

= Tip diameter (in)

= Stack height (ft) (30 ft or 9 m minimum)

The equations are least-squares regression of cost data. It must be kept in mind that even for a given flare technology (i.e., elevated, steam assisted), design and manufacturing procedures vary from vendor to vendor, so that costs may vary. Once a study estimate is completed, it is recommended that several vendors be solicited for more detailed cost estimates.

Each of these costs includes the flare tower (stack) and support, burner tip, pilots, utility (steam, natural gas) piping from base, utility metering and control, liquid seal, gas seal, and galvanized caged ladders and platforms as required. Costs are based on carbon steel construction, except for the upper four feet and burner tip, which are based on 310 stainless steel.

The gas collection header and transfer line requirements are very site specific and depend on the process facility where the emission is generated and on where the flare is located. For the purposes of estimating capital cost, it is assumed that the transfer line will be the same diameter as the flare tip and will be 100 feet (30 m) long. Most installations will require much more extensive piping, so 100 (30 m) feet is considered a minimum. (Leslie B. Evans, William M. Vatavuk, Diana K. Stone, Susan K. Lynch, Richard F. Pandullo, September 2000)

The costs for vent stream piping , are presented separately in Equation 5.4 or 5.5 and are a function of pipe or flare diameter .

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Where:

= Pipe or flare diameter (in)

The costs , include straight, Schedule 40, carbon steel pipe only, are based on 100 feet (30 m) of piping, and are directly proportional to the distance required.

The costs for a knock-out drum, , are presented separately in Equation 5.6 and are a function of drum diameter, (in), and height, (in).

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Where:

= Vessel thickness (in) determined based on the diameter.

= Height (in)

= Drum diameter (in)

Flare system equipment cost EC presented in Equation 5.7, is the total of the calculated flare, knock-out drum, and piping costs.

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Purchased equipment costs PEC (Equation 5.8), is equal to equipment cost EC plus factors for ancillary instrumentation (i.e., control room instruments) (0.10), sales taxes (0.03), and freight (0.05) or,

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5.1.2 Installation Costs

The total capital investment TCI (Equation 5.9), is obtained by multiplying the purchased equipment cost PEC by an installation factor of 1.92.

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These costs were determined based on the factors in Table 3.

Direct installation costs cover foundations and supports, equipment handling and erection, piping, insulation, painting, and electrical.

Indirect installation costs cover engineering, construction and field expenses, contractor fees, start-up, performance testing, and contingencies. Depending on the site conditions, the installation costs for a given flare could deviate significantly from costs generated by these average factors.

5.2 Estimating Total Annual Costs

The total annual cost, TAC is the sum of the direct and indirect annual costs.

5.2.1 Direct Annual Costs

Direct annual costs include labor (operating and supervisory), maintenance (labor and materials), natural gas, steam, and electricity. Unless the flare is to be dedicated to one vent stream and specific on-line operating factors are known, costs should be calculated based on a continuous operation of 8,760 h/yr and expressed on an annual basis. Flares serving multiple process units typically run continuously for several years between maintenance shutdowns.

Operating labor is estimated at 630 hours annually. A completely manual system could easily require 1,000 hours. A standard supervision ratio of 0.15 should be assumed.

- Maintenance labor is estimated at 0.5 hours per 8-hour shift. Maintenance materials costs are assumed to equal maintenance labor costs. Flare utility costs include natural gas, steam, and electricity.

The total natural gas cost , to operate a flare system includes pilot , auxiliary fuel , and purge costs :

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Where:

= Annual volume of pilot gas.

= Cost per

and are similarly calculated.

Steam cost to eliminate smoking is equal:

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Where:

= Annual steam consumption.

= Cost per

The use of steam as a smoke suppressant can represent as much as 90% or more of the total direct annual costs.

5.2.2 Indirect Annual Costs

The indirect (fixed) annual costs include overhead, capital recovery, administrative (G & A) charges, property taxes, and insurance.

Overhead is calculated as 60% of the total labor (operating, maintenance, and supervisory) and maintenance material costs. The system capital recovery cost CRC is based on an estimated 15-year equipment life. For a 15-year life and an interest rate of 7%, the capital recovery factor is 0.1098. The system capital recovery cost is the product of the system capital recovery factor CRF and the total capital investment TCI or:

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Table 3 : Capital Cost Factors for Flare Systems

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Once the required flare tip diameter and stack height have been calculated (in previous chapter) then equipment costs can be calculated. Since the height is 21 m, the flare will be self-supporting. The costs are determined from Equation 5.1.

In which = 0.46 m = 17.71 in

= 21 m = 68.89 ft

Knock-out drum costs are determined using Equation 5.6, where height and thickness is depended on the drum diameter.

In which

Transport piping costs are determined using Equation 5.4.

The total auxiliary equipment cost

The total capital investment is calculated using the factors given in Table 3. The calculations are shown in Table 4.

Table 4: Capital Costs for Flare System

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6. INSPECTION (MAINTENANCE), SAFETY AND RECOVERY

6.1 Inspection

Flare systems are typically custom designed units consisting of common equipment. Because of the nature of the materials handled and the conditions under which components operate, the flare system is subject to corrosion, erosion, thermal stress, cracking, spalling and plugging. Most of the maintenance costs and problems, however, arise from instrumentation and process control devices. Daily, monthly and annual inspections are recommended.

On a daily basis, the auxiliary fuel, pressure seals, knockout drum, and monitoring and electrical devices should be physically inspected to verify that they are clean, functioning and calibrated. Pressure seals should be tight and intact. Gas jets should be free of corrosion and cleaned of deposits and blockages. Valves and electrical devices should be checked for proper position and condition. Such things as dirty contacts, moisture leaks, deteriorating insulation and plugged drains should all be repaired. Pressure gauges, thermometer and/or thermocouples and level indicators should all be inspected for physical integrity and calibrated as necessary.

On a monthly basis moving parts such as fans and blowers, solenoids, check valves and dampers should be lubricated and cleaned of any foreign matter that may interfere with operation.

Annually or during each equipment shut down, structural components including anchors, straps, foundations and guy wires, should be inspected for integrity. Refractory lining should be checked for cracks and spalling. The outer shell of the stack and flare system components should be checked for cracks and fatigue caused by over pressurization or temperature stress. Flares are typically utilized in harsh environments and corrosion/erosion problems should be carefully monitored and attended whenever found.

As always, inspection forms should be tailored to system specific components (e.g., electric arc ignition system) and operational and regulatory requirements. All maintenance activities and inspections should be recorded and studied for trends and variances from design and/or normal operating conditions. (EPA, November 1991)

6.1.1 Maintenance

Most of the problems that occur with flares have a direct impact on emission rates. Flares are not immune to physical problems caused by overloading. Excessive flow rates may cause explosions, uncontrolled fire, and ventilation of toxic or obnoxious gases. Hence routine maintenance to assure that safety devices are intact and that process controllers are functioning properly is critical. Fouling and plugging is the deposition of foreign material on the exterior and/or interior of nozzles, valves, monitors, controllers and burner heads. Cleaning of deposits is generally performed only during major shutdowns. (Buonicore, 1982)

6.1.2 Spare Parts

Generally flares are moderate maintenance systems. A facility should maintain a ready supply of antifouling agents, gauges, valves, floats, and gasket material.

6.1.3 Malfunctions

Operational failures and malfunctions include both equipment and personnel induced accidents. A brief discussion on problems associated with flares is provided in Table 5 to alert readers of issues that may cause safety problems and/or excessive emissions. (Stone, March 1992. )

Table 5: Malfunction Mechanisms, Symptoms and their Corrections.

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6.1.4 Operator Training

Similar to any piece of equipment, a flare will not receive proper maintenance without management's support and the willingness to provide its employees with proper training. Efficient operation of a flare, promoted by adequate inspection and maintenance procedures, is important. Management and employees must be cognizant of proper procedures necessary to prevent equipment malfunctions or failures.

System training should be received from the manufacturer when a new system is commissioned. The manufacturer’s start-up services will generally include introductory training for facility operators and maintenance personnel. The field service engineer involved in startup procedures will instruct plant personnel in the methods to ensure proper assembly and operation of the system components and instrumentation and controls. Training should also include procedures to perform simple troubleshooting.

Following start-up training, regular courses should be held by in-house personnel or through the use of outside expertise. The set of manuals typically delivered as part of a new installation will include manufacturer-recommended maintenance procedures. Annual in-house training should at a minimum include a review of these documents and confirmation of the original parameters. Training should include written instructions and practical experience sessions on safety, inspection procedures, system monitoring equipment and procedures, routine maintenance procedures, and recordkeeping.

6.2 Safety

The availability of a flare or a vent is absolutely necessary in oil and gas production operations. It ensures that safe disposal of the hydrocarbon gas inventory in the process installation is possible in emergency and shut down situations. Where gas cannot be stored or used commercially, it is essential that the risk of fire and explosion can be reduced by either flaring or venting.

Even where associated gas is being sold or re-injected, small amounts of gas will still need to be flared or vented for safety reasons. Oil and gas processing and storage equipment is often operated at high pressures and temperatures.

When abnormal conditions occur, the control and safety systems must release gas to the emergency flare or vent to prevent hazards to the employees or public. Good maintenance and operating strategies are the main mechanisms used to keep this already small volume as low as practicable.

Another safety issue in the application of flaring and venting is the toxicity of the gases being disposed. In some situations, the toxicity of the gas relative to the toxicity of its combustion products may need to be considered when choosing between flaring and venting as a means of disposal. An example would be where gas containing hydrogen sulfide is being produced. Hydrogen sulfide is a gas that can be fatal if inhaled even at low concentrations. However, its combustion product, sulfur dioxide, is relatively less toxic.

Since flaring is still a major refining process in most of the countries, oil and gas operators will have to find improved ways to control and manage their flaring safely. (J.Loss.Prev, 1996) (John Bellovich, Jim Franklin, Bob Schwartz, 30 August 2006)

6.2.1 Emergency planning

Oil and gas companies operate flares to safely burn excess gases in facilities, as such it's important they invest in emergency planning and preparation to protect workers. Employers should have a plan of what to do in case flares are unable to function correctly. This includes how they may have to evacuate the facility in case of an equipment malfunction or the necessary personal protective equipment workers will have to don to guard against any harmful exposure.

6.2.2 Maintenance

During an emergency, flare systems will need to release gas through piping to flare stacks to relieve pressure in piping or equipment. However, employers may be unaware of potential problems with this equipment if they have not checked it recently. Since there is the risk of malfunction in flare and ignition systems at oil and gas facilities, employers should consider implementing regular maintenance and testing for these systems to ensure they are working properly. Flares that are not regularly checked may pose a greater danger to facilities as this could increase the chance of fires and explosions.

6.2.3 Ventilation

Companies may not expect workers to inhale gases emitted from flares, outside containments have a high likelihood of mixing with indoor air. The gases that are being burned may be toxic depending on the type of gas and concentration. Companies should assess whether the gases that are being burned are more dangerous in their combustion form. With the risk of poor indoor air quality resulting in negative health effects for workers, employers should consider improving their ventilation systems as well as installing air purification equipment in case there is a high concentration of hazardous gases in the workplace.

6.2.4 Noise

When flaring natural gas occurs, it often produces a loud noise. Employees who work near flares may be at risk for hearing loss or other auditory problems. According to the U.S. Occupational Safety and Health Administration, almost 125,000 members of the national workforce have reported serious or permanent hearing loss since 2004. Employers could provide their workers with hearing protection like ear muffs or plugs to prevent loud noises from affecting their hearing.

6.3 Recovery

Several steps may be help to reduce the flared gas losses such as: proper operation and maintenance of flares systems, modifying start-up and shut-down procedures. Also, eliminating leaking valves, efficient use of fuel gases required for proper operation of the flare and better control of steam to achieve smokeless burning all contribute to reducing flare losses. Recovery methods may also use to minimize environmental and economic disadvantages of burning flare gas.

Recently, several technologies in flare tip design offers the greatest reduction in flare loss. Even in most advanced countries only a decade has passed from FGRS, thus the method is a new method for application in refineries wastes. Of such countries active in FGRS are USA, Italy, the Netherlands, and Switzerland. Most FGRS has been installed based primarily on economics, where the payback on the equipment was short enough to justify the capital cost. Such systems were sized to collect most, but not all, of the waste gases. The transient spikes of high gas flows are typically very infrequent, meaning normally it is not economically justified to collect the highest flows of waste gas because they are so sporadic. However, there is increasing interest in reducing flaring not based on economics, but on environmental consideration.

There is a range of methods to reduce and recover flaring, it is summarized as the followings

1. Collection, compression, and injection/reinjection

- Into oil fields for enhanced oil recovery.
- Into wet gas fields for maximal recovery of liquids.
- Into of gas into an aquifer.
- Into the refinery pipelines.
- Collection and delivery to a nearby gas-gathering system.
- Shipping the collecting flared gas to treatment plants before subsequent use.
- Using as an onsite fuel source.
- Using as a feedstock for petrochemicals production.

2. Gas-to-liquid (GTL)

- Converting to liquefied petroleum gas (LPG).
- Converting to liquefied natural gas (LNG).
- Converting to chemicals and fuels.

3. Generating electricity

Burning flared gas in incinerators and recovering exhaust heat for further use (generation and co-generation of steam and electricity).

7. FLARE GAS IMLICATIONS

Oil companies find it more economically expedient to flare the natural gas and pay the insignificant fine than to re-inject the gas back into the oil wells. Additionally, because there is an insufficient energy market especially in rural areas, oil companies do not see an economic incentive to collect the gas. From a social perspective, the oil-producing communities have experienced severe marginalization and neglect. The environment and human health have frequently been a secondary consideration for oil companies and the local governments. However, although there may be reasons for the continuous gas flaring, there are many strong arguments suggesting that it should be stopped.

Corporations’ accountability to the people and environment surrounding them imply that oil companies should be required to re-inject the gas, to recover it, or to shut down any extraction facilities in which the gas flaring is occurring. The ramifications for human health, local culture, indigenous self-determination, and the environment are severe. As is the case in most oil producing regions of less developed countries, the economic and political benefits are given significantly more weight by the government than the resulting damage to the environment and human health therefore a series of implications and problems affected by gas flaring is summarized below.

7.1 Environmental Implications

People outside the oil and gas industry sometimes express concerns about the environmental impacts of flaring and venting. One such concern relates to the potential for global climate change. Both carbon dioxide and methane (the major component of natural gas) are known as greenhouse gases associated with concerns about global warming.

Flaring produces predominantly carbon dioxide emissions, while venting produces predominantly methane emissions. The two gases have different effects, however.

The global warming potential of a kilogram of methane is estimated to be twenty-one times that of a kilogram of carbon dioxide when the effects are considered over one hundred years.

While there are still many uncertainties in our understanding of the complex issue of climate change, it makes sense to avoid the unnecessary release of carbon dioxide or methane into the atmosphere, where practicable. This points to a need to reduce emissions in a reasonably practicable way. However, it is important to recognize that other environmental impacts also need to be managed.

Sometimes those needs can conflict with managing greenhouse gas emissions. This conflict may take a variety of forms, but usually relates to the need to manage potential contributions to local environmental impacts, such as air quality, alongside global issues, such as climate change. Although the global warming potential of methane when compared to carbon dioxide usually suggests that flaring is a more environmentally attractive option than venting, neighbors of onshore oil and gas developments sometimes prefer venting because it is less visible and produces less noise due to this a lot of regulations and limitations have been developed, table 6 below shows the typical allowable waste gas stream of plants that’s also required by EPA standards. (Standards, April 2012) (EPA, November 1991)

Table 6 : Allowable Waste Gas Composition.

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7.1.1 Climate Change

Gas flaring contributes to climate change, which has serious implications for both local and the rest of the world. The burning of fossil fuel, mainly coal, oil and gas (greenhouse gases) have led to warming up the world and is projected to get much, much worse during the course of the 21st century according to the Intergovernmental Panel on Climate Change (IPCC). This scientific body was set up in 1988 by the UN and the World Meteorological Organization to consider climate change. Climate change is particularly serious for developing countries. Gas flaring contributes to climate change by emission of carbon dioxide, the main greenhouse gas. Venting of the gas without burning, a practice for which flaring seems often to be treated as a synonym, releases methane, the second main greenhouse gas. Together and crudely, these gases make up about 80% of global warming to date.

7.1.2 Acid Rain

Acid rains have been linked to the activities of gas flaring. Structures and buildings may be corroded by the composition of the rain that falls as a result of flaring. The primary causes of acid rain are emissions of sulfur dioxide (SO2) and nitrogen oxides (NO) which combine with atmospheric moisture to form sulfuric acid (H2SO4) and nitric acid (HNO3) respectively. Size and environmental philosophy in the industry have very strong positive impact on the gas-flaring-related CO2 emission.

Acid rain acidifies lakes and streams and damages vegetation. In addition, acid rain accelerates the decay of building materials and paints. Prior to falling to the earth, SO2 and NO2 gases and their particulate matter derivatives, sulfates and nitrates, contribute to visibility degradation and harm public health.

7.1.3 Agriculture

The flares associated with gas flaring give rise to atmospheric contaminants. These include oxides of Nitrogen, Carbon and Sulfur (NO2, CO2, CO, SO2), particulate matter, hydrocarbons and ash, photochemical oxidants, and hydrogen sulfide (H2S). These contaminants acidify the soil, hence depleting soil nutrient. Previous studies have shown that the nutritional value of crops within such vicinity are reduced. In some cases, there is no vegetation in the areas surrounding the flare due partly to the tremendous heat that is produced and acid nature of soil pH.

The effects of the changes in temperature on crops included stunted growth, scotched plants and such other effects as withered young crops. Reference concluded that the soils of the study area are fast losing their fertility and capacity for sustainable agriculture due to the acidification of the soils by the various pollutants associated with gas flaring in the area.

7.1.4 Thermal Emissions and Luminosity

As in the case of thermal radiation, it is probable that most of the visible radiation is the result of radiation from hot carbon particles. Electronic transitions, such as in the formation and recombination of certain radicals: CH, C2, HCO, NH, and NH2 are also accompanied by emission in the visible and near ultraviolet, but probably contributes only a small fraction of the total luminous radiation. The distribution of radiation frequencies from hot carbon particles is predicted from Planck's radiation law and requires a knowledge of the flame temperature.

7.1.5 Noise Pollution

Noise pollution from flares has for too long been an inconvenience, accepted in petrochemical plants as an inevitable byproduct of flaring process. It has been established that major individual source of noise from flare is usually at the flare tip itself. This is especially true when the flare tip is of the type used for smokeless flaring of hydrocarbon gases utilizing steam injection.

Basically, noise is created because of two reasons, 1- steam energy losses at the high-pressure steam injectors, and 2-unsteadiness in the combustion process.

Ground flares are normally quieter than elevated flares. This is probably due to the fact that the flame contained inside a box is protected from wind effects and the stabilizing effect of the heat re-radiated from the refractory walls reduces then random characteristics of combustion. The walls themselves will absorb some of the sound energy.

Sophisticated design of flare tips has greatly reduced the noise pollution. In some designs, combustion efficiency has been greatly increased by remixing of air with gas before they are combusted. Steam is also premixed with air and gas before gases leave the flare tip. Some of the turbulent noise energy is thus shielded by the tip itself.

Environmental consequences associated with gas flaring have a considerable impact on local populations, often resulting in severe health issues. Generally, gas flaring is normally visible and emitted both noise and heat.

Thermal radiation and noise level is calculated as a function of distance from the flare for flare systems. The results are presented in Table 7. (J.Loss.Prev, 1996) (McDaniel, July 1983)

Table 7: Thermal Radiation & Noise Level as a Function of Distance.

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7.2 Health Implications

7.2.1 Radiation Effect on Humans

Numerous investigations have been undertaken to determine the effect of thermal radiation on human skin. It is understood that with an intensity of 5425 kcal/m2/h the pain threshold was reached in 8 s and blistering occurred in 20 s. On the bare skin of white rats, an intensity of 5425 kcal/m2/h produced burns in less than 20s. The same report indicated that an intensity of 20 350 kcal/m2/h caused burns on the skin of white rats in approximately 6s.

The following data give exposure times necessary to reach the pain threshold as a function of radiation intensity. These experimental data were derived from tests given to people who were radiated on the forearm at room temperature. It is stated that burns follow the pain threshold very quickly. Since the allowable radiation level is a function of length of exposure, factors involving reaction time and human mobility must be considered. In emergency releases, a reaction time of 3-5s may be assumed. Perhaps 5-10s more would elapse before the average individual could seek cover or depart from the area, which would result in a total exposure period ranging from 8-15 s.

Table 8: Exposure Times Necessary to Reach Pain Threshold

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7.2.2 Adverse Effects

The implication of gas flaring on human health are all related to the exposure of those hazardous air pollutants emitted during incomplete combustion of gas flare. These pollutants are associated with a variety of adverse health impacts, including cancer, neurological, reproductive and developmental effects. Deformities in children, lung damage and skin problems have also been reported.

7.2.3 Hematological Effects

Hydrocarbon compounds are known to cause some adverse changes in hematological parameters. These changes affect blood and blood-forming cells negatively. And could give rise to anemia (aplastic), pancytopenia and leukemia.

The technology to address the problem of gas flaring exists today and the policy regulations required are largely understood. Global emissions from gas flaring stand for more than 50 % of the annual Certified Emissions Reductions (624 Mt CO2) currently issued under the Kyoto Clean Development Mechanisms. However, flaring is considered as much safer than just venting gases to the atmosphere. Pollutants of flare and their health effect are summarized in Table 9.

Table 9: Pollutants of Flare and Their Health Effect

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7.3 Other Implications

7.3.1 Economic Loss

Aside from the health and environmental consequences of gas flaring, the nation also loses billions of dollars’ worth of gas which is literally burnt off daily in the atmosphere. Much of this can be converted for domestic use and for electricity generation. By so doing the level of electricity generation in the country could be raised to meet national demand. Nigeria has recorded a huge revenue loss due to gas flaring and oil spillage. Though more than 65 % of governmental revenue is from oil, it is estimated that about $2.5 billion is lost annually through gas flaring in government revenues.

7.3.2 Pollution

Drilling mud and oil sometimes find their way to the streams, surface waters and land thus making them unfit for consumption nor habitable by man or animal. This problem has been produced by a range of international oil companies which have been in operation for over several decades. The economic and environmental ramifications of this high level of gas flaring are serious because this process is a significant waste of potential fuel which is simultaneously polluting water, air, and soil.

7.3.3 Social

Although the plant operation has complied with the environmental regulation, sometime the outcome resulting flare system may not meet the expectations of the plant's neighbors.

Example: A smokeless flame may meet the regulatory requirements, but the neighbors may complaint due to light and noise from flare system causing instability of the local community and sometimes aggression.

CONCLUSION

Energy demand and consumption is rising day by day which have a significant side effect or even a main effect that influences the lives around it and the atmosphere of the entire planet. Gas flaring is one of the main sources to cause environmental problems through greenhouse gases and other emissions. These emissions have high global warming potential and contribute to climate change. Improving the equipment and process design of flared gas and its emissions are very important and has been a challenging approach.

Due to this the best design and favored methods have been approached, taking into account the environmental and economic considerations. While working to improve these two, it also leads to reduce noise, thermal radiation, operating and maintenance costs, air pollution, gas emission and reduces fuel gas and steam consumption.

It is well known that there are many economical ways to achieve flaring minimization and gas conservation in oil and gas refineries. In order to find these ways, a detailed study and reliable designs of the flare must be provided of comprehensive process evaluation of plants, comprehensive monitoring of flow and composition of flare gases, investigation of existing flare systems and finding alternative choices for reusing and recovering flare gases. Based on process evaluation and estimation, alternatives can be also derived to reduce gas flaring.

REFERENCES

1. API. (JANUARY 2000). Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries, API RECOMMENDED PRACTICE 520, Seventh Edition. 1220 L Street, N.W., Washington, D.C. 20005: API Publishing Services.

2. Buonicore. (1982). A. Air Pollution Control Equipment: Selection, Design Operation and Maintenance. Prentice Hill .

3. Charles E. Baukal, JR. (2014). The John Zink Hamworthy Combustion Handbook, Second Edition: Volume 3 – Applications., Boca Raton, Fort Lauderdale, USA: Taylor & Fransic Group LLC.

4. Cheremisinoff, N. P. (1998). PRESSURE SAFETY DESIGN PRACTICES FOR REFINERY AND CHEMICALOPERATIONS. USA: Noyes Publications.

5. Cheremisinoff, N. P. (Apr 1, 2013). INDUSTRIAL GAS FLARING PRACTICES. New Jersey USA: John Wiley & Sons Cooperation.

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8. EPA. (November 1991). Control of Volatile Organics Compound Emissions from Reactor Processes and Distillation Operations Process in the Synthetic Organic Chemical Manufacturing industry. USA: U.S. Environmental Protection Agency, QAQPS. EPA-450/4-91-031.

9. Hossein Shokouhmand, Shahab Hosseini. (October 24, 2007). Optimally Economic Design of Flare Systems. San Francisco, USA: World Congress on Engineering and Computer Science.

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14. Leslie B. Evans, William M. Vatavuk, Diana K. Stone, Susan K. Lynch, Richard F. Pandullo. (September 2000). VOC Destruction Controls. North Carolina, Durham: Office of Air Quality Planning and Standards, U.S Environmental Protection Agency.

15. Ling, A. L. (July 2007). FLARE SELECTION AND SIZING (ENGINEERING DESIGN GUIDELINE). Johor Bahru: KLM Technology Group.

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Title: The Gas Flare Stack Process