Flare System 151x
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Module – 11B Flare System
Training Program on Basic Process Engineering Practices By
September 13, 2012
1
What is Flaring? Flaring is a combustion control process in which waste gases are piped to a remote, usually elevated location and burned in an open flame in the open air. A specially designed burner tip, auxiliary fuel, and steam or air are used to promote mixing for nearly complete combustion (>98 %). The flaring process can produce undesirable by-products, including noise, smoke, heat radiation, light, SOx, NOx, CO, and an undesired source of ignition. However, proper design can minimize these. Flare Tip
Process Equipment Flare Header
Process Equipment
September 13, 2012
Flare K.O Drum
Water seal
2
Purpose of Flare System Process plant can be subjected to excessive overpressure or under-pressure due to process upset conditions.
Safety Valves or Rupture Discs prevent the equipment from reaching overpressure condition i.e. protects it from exceeding design pressure by releasing the excess gases. The gases released in a process plant is generally hazardous. Primary purpose of flare system is to safely take the released gases to a flare stack and burn it. Flare system is also used for burning gases due to emergency venting. Example of emergency venting- Gas flaring when a consumer shuts down. September 13, 2012
3
Causes of Over-pressure External fire Blocked Valve Process abnormality or mal-operation Equipment or service / utility failure Changes in ambient conditions Runaway chemical reaction Flare system is used to destroy flammable, toxic or corrosive vapors, from relief valves or emergency venting. September 13, 2012
4
Flare System Design Factors Key design factors to ensure flare safety and performance include:
Smokeless operation Flame stability Flare size, capacity, stack diameter Thermal radiation Noise level Reliable pilot and ignition system Flashback protection September 13, 2012
5
Flare Network Components Flare Tip Mol Seal
Process (Unit – 1) Fuel Gas
Pilot Burner
Unit Flare HDR PC
Main Flare HDR
Flare Stack
Air
Process (Unit – 2) Fuel Gas
Unit Flare HDR
Flare Ignition System
Flare K.O Drum
Water seal
Incinerator Fuel Gas (2”) September 13, 2012
Pump 6
Flare Types •
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 of enhancing mixing at the flare tip (i.e., steamassisted, air-assisted, pressure-assisted, or non- assisted). • Elevating the flare can prevent potentially dangerous conditions of high radiation at ground level or operating area of a process unit. The distance and height of the flare stack is set by radiation calculations (API RP 521) • Further, the products of combustion can be dispersed above working areas to reduce the effects of noise, heat, smoke, and objectionable odors. Dispersion and ground level concentration of pollutants from flare also may set the height of the flare stack. September 13, 2012
7
Flare Types, Contd...
Smoke problem • Cracking can occur with the formation of small hot particles of carbon that give the flame its characteristic luminosity. If there is an oxygen deficiency and if the carbon particles are cooled to below their ignition temperature, smoking occurs.Non-assisted flares are more prone to smoking.
Non- assisted flares • The non-assisted flare is just a flare tip without an auxiliary provision for enhancing the mixing of air into its flame. Its use is generally limited to gas streams that burn readily without producing smoke.
September 13, 2012
8
Flare Types, Contd...
Assisted flares • In assisted flares, induction of air for combustion and mixing are enhanced by various means described below.
Steam assisted flares • Steam is injected into the combustion zone to promote turbulence for mixing and to induce air into the flame.
Air assisted flares • Some flares use forced air to provide the combustion air and the mixing required for smokeless operation.
Pressure assisted flares • Gas pressure is kept high at the battery limit of the flare to promote mixing at the burner tip. September 13, 2012
9
Flare Hardware Components Steam Assisted Flare
Safety Relief and Flare Header
September 13, 2012
10
Steps on Deg Flare System Identify Systems For Relief Protection
Identify Cases For Over-pressure
Select Set Pressures
Determine Controlling Load For Each Relief
Select Stack Height, Diameter And Distance
Select Type Of Flare Tip, Seals
Estimate Worst Scenario For the Plant
Line Sizing & P&ID For Flare System
Piping Layout
Equipment Specification
September 13, 2012
11
Determining Flare Load in a Plant The first step is to analyze the causes of overpressure in various equipment and systems and calculate the loads due to safety valve popping. External fire
Process abnormality or mal-operation Equipment or service / utility failure Changes in ambient conditions Runaway chemical reaction
Once the loads are calculated, they are systematically tabulated under above heads. The chances of simultaneously occurring failures dictate the flare load September 13, 2012
12
Examples of Safety Valve Sizing Cases
Fire Case- required to be estimated for vessels 25 feet from ground. Heat flux due to fire is taken as 21 or 34.5 MBtu/Hr/Sq. ft.
Surface up to 25 ft x heat flux x absorption factor x insulation factor.
API RP-521 (1993) gives the equation-
Q= 21,000 x F x A 0.82 Where - Q= Heat absorption in wetted area. A= Wetted area in sq. ft. F= Environment Factor (F=1 for bare surface, 0.15-0.3 for insulated surface) NFPA – Q= 21,000 x F x A 0.84 Where (F=0.3 for bare water sprayed, buried or insulated surface) September 13, 2012
13
Examples of Safety Valve Sizing Cases
Blocked Flow- inadvertently closed block valve, failedshut control valve, power failure, pump failure with upstream vessel level affected.
Tube rupture-differential pressure between shell side and tube side to be evaluated. Control valve failure- due to air failure or other causes.
Power failure – resulting pump failure, instrument air failure, failure of agitator in vessel etc.
September 13, 2012
14
Examples of Safety Valve Sizing Cases Steam related failure- can cause excessive steam pressure due to fail open valve, stoppage of steam supply with low vaporization and rising levels, high vapor load due to excess steam. Reflux failure- causes vapor overload. Since column is at ground level fire case usually controls. Thermal relief- Blocked liquid line with heat load like steam tracing or solar radiation.
September 13, 2012
Runaway chemical reaction- should be specially evaluated from licensor information. Usually this case or fire case controls the PSV sizing.
15
Over-pressure: Blocked Discharge Case This can happen when there is a sudden closure of valve in any flowing pipeline. In this case, the safety valves provided on pipeline or equipment need to be designed on full flow rate
Block discharge from well head
Oil manifold
September 13, 2012
16
Heat Exchanger Tube Failure When there is a wide difference in design pressure between the two exchanger sides and the low pressure side is designed at a pressure less than two-third of design pressure of high pressure side, a relief valve is required at the low pressure side
Tube Side
September 13, 2012
Shell Side
17
Utility Failure Example – Cooling Water Failure
When there is a sudden failure of cooling water in overhead condensers
Condenser
of
distillation
column, the column pressure Top product
starts increasing due to loss of reflux after 5-10 minutes.
Feed
Distillation Column
To overcome this, a relief valve is required that can vent the Reboiler
additional quantity of vapor Bottom product
September 13, 2012
generated to flare.
18
Control Equipment Failure- Oversupply of Heat
When the control of fuel supply or steam supply to reboiler fails, there could be excessive
Condenser
heating resulting in rise in Top product
column temperature and overpressurization.
Feed
Distillation Column
To overcome this, a relief valve is required that can vent the Reboiler
additional quantity of vapor Bottom product
September 13, 2012
generated to flare.
19
Selecting the Set Pressure
Depending on temperature rating of the equipment and material of construction, design pressure or maximum allowable working pressure (MAWP) is decided. The set pressure of safety valve is to be equal or lower than design pressure. It is guided by codes like API 520.
September 13, 2012
20
Relief Line Sizing- Guidelines No PSV inlet line pressure drop should be greater than 3% of the set pressure. PSV discharge side should be at least one size higher than the inlet side. PSV discharge side pressure drop should not be more than 10% of the set pressure.
Back pressure on safety valve should not exceed 10% of set pressure. For bellows type safety valve it can be higher. There should be no restriction on relief lines – full bore LO valves, no Restriction orifice, no flame arrestor etc. Be aware of limitations of sonic flow. Sonic flow limits maximum possible flow in a line. Do not exceed 50% of sonic velocity. September 13, 2012
21
Flare System Hardware and Network Design After completing the design of process systems, a final flare and relief analysis of process system should be done. A comparative study of flare and relief loads should be determined and the worst scenario foreseen. Based on the worst conditions, flare load is designed. Based on the controlling flare load, the flare equipment and system hardware are designed Network of relief lines from numerous equipment with main flare header Flare k o drum Liquid transfer pumps Flare stack are designed. September 13, 2012
22
Flare Stacks Flare stacks are of three types: Self ed
September 13, 2012
Derrick ed
Guy ed
23
Stack Height The height and distance of a flare is determined by the ground level limitations of: • thermal radiation intensity, • luminosity, • noise, • height of surrounding structures, and • the dispersion of the exhaust gases.
API RP 521 sets the guidelines for radiation and dispersion calculations.
September 13, 2012
24
Stack Height Contd..
Solar Radiation • API RP 521 provides guidelines for radiation limits for
estimating stack height. • An industrial flare is normally sized for a maximum heat intensity of 1,500-2,000 Btu/hr-sq ft when flaring at its maximum
design rate. At this heat intensity level, workers can remain in the area of the flare for a limited period only. • If, however, operating personnel are required to remain in the unit area, the recommended design flare radiation level excluding solar radiation is 500 Btu/hr-sq ft. • The solar radiation is in the range of 250-330 Btu/hr-sq ft.) September 13, 2012
25
Stack Height Contd..
• Flare height may also be determined by the need to safely disperse the vent gas in case of flameout. • The height in these cases would be based on dispersion modeling for the particular installation conditions. • The minimum flare height normally used is 30 feet.
September 13, 2012
26
Cold Vent
In cases where the safety relief valves are small in number and venting possibilities are minimal, cold venting of natural gas can be carried out in stead of flaring. The gas should be mainly methane (much lighter than air) so that it goes up and disperses in the air much above operating
level. Cold venting is also done for atmospheric storage tanks or
where adequate back pressure for flare system is not available. September 13, 2012
27
Flaring from Atmospheric Tanks • Atmospheric Storage Tank designed as per API 650 can not tolerate back pressure of flare system. They need to be vented. • Atmospheric Storage Tanks for refrigerated liquids designed as per API 620 (500 mm water) can be connected to flare.
Refrigerated Atmospheric Storage tank Flare stack
Atmospheric storage tank Vent stack September 13, 2012
28
Codes and Guidelines API RP 520 Sizing, selection, and installation of pressure relieving devices in refineries Part I – Sizing and Selection, 1993. Part II – Installation, 1994. API RP 521 Guide for pressure-relieving and depressuring systems, 1997.
API RP 526 Flanged Steel Safety Relief Valves, Fourth Edition, 1995. API RP 527 Seat Tightness of Pressure Relief Valves, Third Edition, 1991. API Std 2000 Venting atmospheric and low pressure storage tanks: Non-refrigerated and refrigerated, 1998. API RP 2521 Use of pressure-vacuum vent valves for atmospheric Loss, First Edition, 1966. September 13, 2012
29
Training Program on Basic Process Engineering Practices By
September 13, 2012
1
What is Flaring? Flaring is a combustion control process in which waste gases are piped to a remote, usually elevated location and burned in an open flame in the open air. A specially designed burner tip, auxiliary fuel, and steam or air are used to promote mixing for nearly complete combustion (>98 %). The flaring process can produce undesirable by-products, including noise, smoke, heat radiation, light, SOx, NOx, CO, and an undesired source of ignition. However, proper design can minimize these. Flare Tip
Process Equipment Flare Header
Process Equipment
September 13, 2012
Flare K.O Drum
Water seal
2
Purpose of Flare System Process plant can be subjected to excessive overpressure or under-pressure due to process upset conditions.
Safety Valves or Rupture Discs prevent the equipment from reaching overpressure condition i.e. protects it from exceeding design pressure by releasing the excess gases. The gases released in a process plant is generally hazardous. Primary purpose of flare system is to safely take the released gases to a flare stack and burn it. Flare system is also used for burning gases due to emergency venting. Example of emergency venting- Gas flaring when a consumer shuts down. September 13, 2012
3
Causes of Over-pressure External fire Blocked Valve Process abnormality or mal-operation Equipment or service / utility failure Changes in ambient conditions Runaway chemical reaction Flare system is used to destroy flammable, toxic or corrosive vapors, from relief valves or emergency venting. September 13, 2012
4
Flare System Design Factors Key design factors to ensure flare safety and performance include:
Smokeless operation Flame stability Flare size, capacity, stack diameter Thermal radiation Noise level Reliable pilot and ignition system Flashback protection September 13, 2012
5
Flare Network Components Flare Tip Mol Seal
Process (Unit – 1) Fuel Gas
Pilot Burner
Unit Flare HDR PC
Main Flare HDR
Flare Stack
Air
Process (Unit – 2) Fuel Gas
Unit Flare HDR
Flare Ignition System
Flare K.O Drum
Water seal
Incinerator Fuel Gas (2”) September 13, 2012
Pump 6
Flare Types •
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 of enhancing mixing at the flare tip (i.e., steamassisted, air-assisted, pressure-assisted, or non- assisted). • Elevating the flare can prevent potentially dangerous conditions of high radiation at ground level or operating area of a process unit. The distance and height of the flare stack is set by radiation calculations (API RP 521) • Further, the products of combustion can be dispersed above working areas to reduce the effects of noise, heat, smoke, and objectionable odors. Dispersion and ground level concentration of pollutants from flare also may set the height of the flare stack. September 13, 2012
7
Flare Types, Contd...
Smoke problem • Cracking can occur with the formation of small hot particles of carbon that give the flame its characteristic luminosity. If there is an oxygen deficiency and if the carbon particles are cooled to below their ignition temperature, smoking occurs.Non-assisted flares are more prone to smoking.
Non- assisted flares • The non-assisted flare is just a flare tip without an auxiliary provision for enhancing the mixing of air into its flame. Its use is generally limited to gas streams that burn readily without producing smoke.
September 13, 2012
8
Flare Types, Contd...
Assisted flares • In assisted flares, induction of air for combustion and mixing are enhanced by various means described below.
Steam assisted flares • Steam is injected into the combustion zone to promote turbulence for mixing and to induce air into the flame.
Air assisted flares • Some flares use forced air to provide the combustion air and the mixing required for smokeless operation.
Pressure assisted flares • Gas pressure is kept high at the battery limit of the flare to promote mixing at the burner tip. September 13, 2012
9
Flare Hardware Components Steam Assisted Flare
Safety Relief and Flare Header
September 13, 2012
10
Steps on Deg Flare System Identify Systems For Relief Protection
Identify Cases For Over-pressure
Select Set Pressures
Determine Controlling Load For Each Relief
Select Stack Height, Diameter And Distance
Select Type Of Flare Tip, Seals
Estimate Worst Scenario For the Plant
Line Sizing & P&ID For Flare System
Piping Layout
Equipment Specification
September 13, 2012
11
Determining Flare Load in a Plant The first step is to analyze the causes of overpressure in various equipment and systems and calculate the loads due to safety valve popping. External fire
Process abnormality or mal-operation Equipment or service / utility failure Changes in ambient conditions Runaway chemical reaction
Once the loads are calculated, they are systematically tabulated under above heads. The chances of simultaneously occurring failures dictate the flare load September 13, 2012
12
Examples of Safety Valve Sizing Cases
Fire Case- required to be estimated for vessels 25 feet from ground. Heat flux due to fire is taken as 21 or 34.5 MBtu/Hr/Sq. ft.
Surface up to 25 ft x heat flux x absorption factor x insulation factor.
API RP-521 (1993) gives the equation-
Q= 21,000 x F x A 0.82 Where - Q= Heat absorption in wetted area. A= Wetted area in sq. ft. F= Environment Factor (F=1 for bare surface, 0.15-0.3 for insulated surface) NFPA – Q= 21,000 x F x A 0.84 Where (F=0.3 for bare water sprayed, buried or insulated surface) September 13, 2012
13
Examples of Safety Valve Sizing Cases
Blocked Flow- inadvertently closed block valve, failedshut control valve, power failure, pump failure with upstream vessel level affected.
Tube rupture-differential pressure between shell side and tube side to be evaluated. Control valve failure- due to air failure or other causes.
Power failure – resulting pump failure, instrument air failure, failure of agitator in vessel etc.
September 13, 2012
14
Examples of Safety Valve Sizing Cases Steam related failure- can cause excessive steam pressure due to fail open valve, stoppage of steam supply with low vaporization and rising levels, high vapor load due to excess steam. Reflux failure- causes vapor overload. Since column is at ground level fire case usually controls. Thermal relief- Blocked liquid line with heat load like steam tracing or solar radiation.
September 13, 2012
Runaway chemical reaction- should be specially evaluated from licensor information. Usually this case or fire case controls the PSV sizing.
15
Over-pressure: Blocked Discharge Case This can happen when there is a sudden closure of valve in any flowing pipeline. In this case, the safety valves provided on pipeline or equipment need to be designed on full flow rate
Block discharge from well head
Oil manifold
September 13, 2012
16
Heat Exchanger Tube Failure When there is a wide difference in design pressure between the two exchanger sides and the low pressure side is designed at a pressure less than two-third of design pressure of high pressure side, a relief valve is required at the low pressure side
Tube Side
September 13, 2012
Shell Side
17
Utility Failure Example – Cooling Water Failure
When there is a sudden failure of cooling water in overhead condensers
Condenser
of
distillation
column, the column pressure Top product
starts increasing due to loss of reflux after 5-10 minutes.
Feed
Distillation Column
To overcome this, a relief valve is required that can vent the Reboiler
additional quantity of vapor Bottom product
September 13, 2012
generated to flare.
18
Control Equipment Failure- Oversupply of Heat
When the control of fuel supply or steam supply to reboiler fails, there could be excessive
Condenser
heating resulting in rise in Top product
column temperature and overpressurization.
Feed
Distillation Column
To overcome this, a relief valve is required that can vent the Reboiler
additional quantity of vapor Bottom product
September 13, 2012
generated to flare.
19
Selecting the Set Pressure
Depending on temperature rating of the equipment and material of construction, design pressure or maximum allowable working pressure (MAWP) is decided. The set pressure of safety valve is to be equal or lower than design pressure. It is guided by codes like API 520.
September 13, 2012
20
Relief Line Sizing- Guidelines No PSV inlet line pressure drop should be greater than 3% of the set pressure. PSV discharge side should be at least one size higher than the inlet side. PSV discharge side pressure drop should not be more than 10% of the set pressure.
Back pressure on safety valve should not exceed 10% of set pressure. For bellows type safety valve it can be higher. There should be no restriction on relief lines – full bore LO valves, no Restriction orifice, no flame arrestor etc. Be aware of limitations of sonic flow. Sonic flow limits maximum possible flow in a line. Do not exceed 50% of sonic velocity. September 13, 2012
21
Flare System Hardware and Network Design After completing the design of process systems, a final flare and relief analysis of process system should be done. A comparative study of flare and relief loads should be determined and the worst scenario foreseen. Based on the worst conditions, flare load is designed. Based on the controlling flare load, the flare equipment and system hardware are designed Network of relief lines from numerous equipment with main flare header Flare k o drum Liquid transfer pumps Flare stack are designed. September 13, 2012
22
Flare Stacks Flare stacks are of three types: Self ed
September 13, 2012
Derrick ed
Guy ed
23
Stack Height The height and distance of a flare is determined by the ground level limitations of: • thermal radiation intensity, • luminosity, • noise, • height of surrounding structures, and • the dispersion of the exhaust gases.
API RP 521 sets the guidelines for radiation and dispersion calculations.
September 13, 2012
24
Stack Height Contd..
Solar Radiation • API RP 521 provides guidelines for radiation limits for
estimating stack height. • An industrial flare is normally sized for a maximum heat intensity of 1,500-2,000 Btu/hr-sq ft when flaring at its maximum
design rate. At this heat intensity level, workers can remain in the area of the flare for a limited period only. • If, however, operating personnel are required to remain in the unit area, the recommended design flare radiation level excluding solar radiation is 500 Btu/hr-sq ft. • The solar radiation is in the range of 250-330 Btu/hr-sq ft.) September 13, 2012
25
Stack Height Contd..
• Flare height may also be determined by the need to safely disperse the vent gas in case of flameout. • The height in these cases would be based on dispersion modeling for the particular installation conditions. • The minimum flare height normally used is 30 feet.
September 13, 2012
26
Cold Vent
In cases where the safety relief valves are small in number and venting possibilities are minimal, cold venting of natural gas can be carried out in stead of flaring. The gas should be mainly methane (much lighter than air) so that it goes up and disperses in the air much above operating
level. Cold venting is also done for atmospheric storage tanks or
where adequate back pressure for flare system is not available. September 13, 2012
27
Flaring from Atmospheric Tanks • Atmospheric Storage Tank designed as per API 650 can not tolerate back pressure of flare system. They need to be vented. • Atmospheric Storage Tanks for refrigerated liquids designed as per API 620 (500 mm water) can be connected to flare.
Refrigerated Atmospheric Storage tank Flare stack
Atmospheric storage tank Vent stack September 13, 2012
28
Codes and Guidelines API RP 520 Sizing, selection, and installation of pressure relieving devices in refineries Part I – Sizing and Selection, 1993. Part II – Installation, 1994. API RP 521 Guide for pressure-relieving and depressuring systems, 1997.
API RP 526 Flanged Steel Safety Relief Valves, Fourth Edition, 1995. API RP 527 Seat Tightness of Pressure Relief Valves, Third Edition, 1991. API Std 2000 Venting atmospheric and low pressure storage tanks: Non-refrigerated and refrigerated, 1998. API RP 2521 Use of pressure-vacuum vent valves for atmospheric Loss, First Edition, 1966. September 13, 2012
29
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