Module 11B Flare System

Training Program on Basic Process Engineering Practices By

September 13, 2012

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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

Flare K.O Drum

Water seal

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Purpose of Flare SystemProcess 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 protectionSeptember 13, 2012 5

Flare Network ComponentsProcess (Unit 1)Fuel Gas Flare Tip Mol Seal

Pilot Burner

Unit Flare HDRPC

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

Pump6

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.

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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 ComponentsSteam Assisted Flare

Safety Relief and Flare Header

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Steps on Designing Flare SystemIdentify Systems For Relief Protection Determine Controlling Load For Each Relief

Identify Cases For Over-pressure

Select Set Pressures

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

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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 loadSeptember 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.

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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. Runaway chemical reaction- should be specially evaluated from licensor information. Usually this case or fire case controls the PSV sizing.

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Over-pressure: Blocked Discharge CaseThis 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

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Heat Exchanger Tube FailureWhen 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

Shell Side

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Utility Failure Example Cooling Water Failure

When there is a sudden failure of cooling water in overheadCondenser

condensersTop product

of

distillation

column, the column pressure 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 theReboilerBottom product

additional quantity of vaporgenerated to flare.

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Control Equipment Failure- Oversupply of Heat

When the control of fuel supply or steam supply to reboilerCondenser

fails, there could be excessive heating resulting in rise inTop product

column temperature and overpressurization.

Feed

Distillation Column

To overcome this, a relief valve is required that can vent theReboilerBottom product

additional quantity of vaporgenerated to flare.

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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.

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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 StacksFlare stacks are of three types:Self Supported Derrick Supported Guy Supported

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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 dispersioncalculations.

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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 inthe 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.

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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 tankFlare stack

Atmospheric storage tank Vent stackSeptember 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, FourthEdition, 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