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Theory

Equipment

Flare systems

This lesson is entitled 'flare systems'. This refers to an environmentally friendlyand safe way of burning waste gases released in various production processes. Inaddition, this lesson contains sections on other ways of discharging waste gasesand waste liquids.

Safety is a primary concern in flare systems; there is a lot that can go wrong! Forthis reason, two sections are devoted to safety, one on theoretical backgroundand one on the procedure to be followed for fitting and pulling spades (in-lineblinds).

Contents of the lesson

1 Flare with accessories

2 Safety

3 Flame arrestor

4 Blow-down system

5 Flare procedural requirements

The copyright in this material is vested in Shell Global Solutions International B.V., The Hague, The Netherlands and Shell Netherlands Raffinaderij B.V. All rightsreserved. Neither the whole or any part of this document may be reproduced, stored in any retrieval system or transmitted in any form by any means (electronic,mechanical, reprographic, recording or otherwise) without the prior written consent of the copyright owner.

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Lesson

1 Flare with accessoriesA bigger or smaller flame burns from a tall stack on the plant site day and night.This is the flare; the flame is produced by burning gases and vapours releasedfrom various processes at the plant. They may be:- gases that must continuously be discharged as waste during the production

process;- gases that must be discharged occasionally from a device because of repair,

maintenance or depressurisation, as well as during starting and stopping;- gases released due to leakage and via relief or safety valves;- gases that must be discharged because they present a hazard, for instance in

the case of fire.

All these gases and evaporated liquids cannot be released into the atmosphere asthey are. This would pollute the environment and present all kinds of hazards(fire, intoxication, explosion) to humans, animals and plants. For this reason,they are led to a point high up in the air and burnt there.

Question 1What types of gases are flared?

Question 2Why are waste gases burnt?

Figure 1 shows a simple diagram of a flare system.At the various plants on the complex, there is a flare (gathering) pipe, to whichequipment can be connected. The diameter of the pipe must be large enough todischarge the maximum quantity of gas present in the equipment without anappreciable pressure increase. The pipe is laid on a slope to preventaccumulation of liquid.

At plants where the flared gas can contain a lot of liquid or where the flared gascan easily condense in the pipe, a knock-out vessel has been placed to separateliquid and gas. The vessel is usually fitted with a high/low level alarm. Thedimensions of the vessel must be such that in emergencies it can store asufficiently large amount of liquid, which is subsequently removed by means ofa slops pump.

Question 3What are the requirements for a flare pipe?

The water seal is used to prevent underpressure (vacuum) in the flare pipe. Asthe gas inlet dips into the water up to a certain depth (dip pipe), no gas from theflare or air from the atmosphere can flow back up the pipe, for the pressure inthe pipe is higher than that in the flare, due to the pressure of the column ofwater. The height of this column of water which causes the pressure differenceis equal to the insert depth.

- flare

- flare pipe

- knock-out vessel

- slops pump

- water seal

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5590-030-001

Figure 1Diagram of a flare system

The water seal has been fitted with a funnel-shaped skimming pipe to keep thelevel constant and discharge hydrocarbons floating on the water. For theremoval of these substances from the water seal, a drain vessel has been fitted atthe lowest point. Liquid from the flare is also removed via this vessel.

Question 4What is the purpose of a water seal?

Flare headThe flare tip is provided with:- pilot burner;- ring steam nozzles;- central steam nozzle.

This is illustrated in figure 2.The pilot burner has a permanent pilot flame to ignite the outflowing gases andto prevent any unburnt gas from ever being emitted into the atmosphere.

The ring steam nozzles are used to enable the injection of steam into the burningflame to improve combustion, particularly in the case of a sooting flame whenburning gases which do not burn easily, in large quantities. The positive effectcan clearly be seen in figures 3a, 3b and 3c.

- skimming pipe

- pilot burner

- ring steam nozzles

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5590-030-002

Figure 2Flare tip

Figure 3a Figure 3b Figure 3cNo injection of steam Sparing injection of steam Plentiful injection of steam

The central steam nozzle provides a constant jet of steam, which providesimproved draught in the flare due to its injector effect.

Question 5In what way is a sooting flare flame made less sooting?

- central steam nozzle

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2 SafetyIn flare operation there is always a potential explosion hazard.An explosion occurs when the following three conditions are met at the sametime:- the presence of hydrocarbons;- the presence of an ignition source;- the presence of air (oxygen).

In normal operation, two of these conditions, a and b, are always met. Wetherefore have to prevent the third condition, c, from being met, in other words,no air (oxygen) must enter the flare pipe.

Air (oxygen) can enter the flare pipe in the following situations:- during the venting of equipment;- during repairs and maintenance;- due to the water seal not functioning properly and due to leakage;- via vent and drain valves.

To exclude each of these possibilities, it is necessary to observe strictly themethods and regulations laid down for all work related either directly orindirectly to the flare system.

Question 6Which three conditions are necessary to cause an explosion?

We mentioned above that the water seal is used to prevent underpressure in thepipe. There is a situation, however, in which underpressure can be created all thesame. This situation occurs when a hot gas has been passed to the flare for sometime and the gas flow is stopped. Due to cooling, the volume of the gas in thepipe will now be reduced, creating an underpressure. In that case, the flare pipemust be filled with an inert gas (such as nitrogen, N2). For this purpose, aconnection has been fitted on the pipe. See the 'N2' pipe in figure 1. Thisnitrogen purge can also be used to flush the pipe, before work is carried out.

3 Flame arrestorIn some cases, a flammable gas, such as hydrogen (H2) or oxygen (O2) cannot bevented to the flare. This is not done either if a gas contains oxygen.

Hydrogen has a very low specific mass. This means that an underpressure iscreated in the flare pipe, enabling outside air to enter the flare system, whichmust not be allowed to happen.

- explosions

- nitrogen purge

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In all such cases, the gas is led directly to the outside air via a flame arrestor, asshown in figure 4.

5590-030-004

Figure 4Flame arrestor

The flame arrestor consists of a casing in which a corrugated plate is woundaround a solid core. On its way out, the flammable gas must pass through thechannels formed by the coiled plate.

Should the outflowing gas catch fire, the temperature of the flame will dropbelow the ignition point due to the large surface area in the flame arrestor,preventing the flame from penetrating further into the equipment downstream ofthe arrestor. (Obviously, this does not apply if the flame arrestor is hot already,as in that case the flame will pass through the screen).

Question 7What is the purpose of a flame arrestor?

4 Blow-down systemIn addition to the flare system, which is used primarily for the venting ofgaseous products, there is another system used for blowing down hot liquidproducts. This is the blow-down system (see figure 5).The purpose of this system is to be able to rapidly blow down the entire liquidcontents of a plant or unit to a vessel outside the plant in the case of a calamity.

In many cases it is necessary for these products to be cooled, for which reason acooling coil has been fitted. The purpose of the steam heating is to be able toevaporate any liquid and vent it to the flare.

- blow-down

- cooling coil- steam heating

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5590-030-005

Figure 5Diagram of a blow-down system

Blowing down can be started from a location outside the plant, behind a firewall. Here, a normally closed ball valve has been fitted in the pipe, which can berapidly opened with the hand wheel located behind the fire wall.

Question 8What is the purpose of the heating element in the blow-down vessel?

5 Flare procedural requirementsTo carry out work on flare systems for which the system needs to be opened,special, extremely stringent safety measures are necessary. These safetymeasures must be carried out according to stringent procedural requirements.

The following are the requirements for placing and pulling spades or the turningof spectacle blind flanges in flare systems.

Preparation1. Planning is to receive a request for the flare procedure 5 days before the start

of the work, via the flare system manager, on 'checklist for flare work' form(see figure 6) and ensures the work is planned at the time requested.

2. Planning ensures the request is brought to the attention of the operationsdepartment at least 48 hours before the start.

3. The operations department is responsible for making sure a suitable workplatform, provided with at least two escape routes in the form of staircases, isprovided and is inspected 24 hours before the start by the work supervisor.

4. The operations department ensures the work permit is present in the controlroom at least 24 hours before the start.

5. The operations department is responsible for loosening the bolts of the flangeconnections concerned and checks whether the required gasket is present.

6. The operations department ensures that, if necessary, temporary lighting isinstalled.

7. The operations department, through the security department, ensures the rightroadblocks are put in place.Planning, through the security department, ensures the roadblock permit isobtained.

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Implementation1. The work is carried out by two men provided with special underclothing, gas

suit with respirator, heat-resistant gloves and, if necessary, a fire-retardantover-suit.

2. One man is on stand-by at the place of work, dressed in a fire-fighting, heat-resistant and liquid-tight overall, provided with a compressed air mask and ahand-held pressurised water-spray nozzle, plus a portable fire extinguisherwithin reach.

3. On ground level, at a safe distance and on the windward side of the first man,two men are positioned dressed as under 1, in case relief is necessary.

4. The permanent supervisor is responsible for providing the right means ofcommunication between the persons carrying out the work and those incharge of supervision.

5. The dedicated work supervisor in the shift ensures a properly lined up fireengine is on stand-by and a fire pump is started for increased water pressure.

6. The work supervisor is permanently on location during the execution of thework. Before the work starts he checks, together with the plant shiftsupervisor and the flare manager whether all required conditions are metbefore he gives the go-ahead.

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CHECKLIST FOR FLARE WORK

To be handed in to TAS/34 5 days before the flare start of the work DateTo be filled in by the department that issued the order Description of

flangesRe: W.O. No.:

Nature and place of the work (if necessarywith sketch on separate sheet)

Date and time of execution Number Name:

Dim. Dept.:Pressurerating

Initials:

Type ofpacking

Tel. Nr.:

System managerapproved

Name:Initials:

To be handed in to TAS/36 48 hours before the start of the workTo be filled in time by the permanent supervisor Name

InitialsSubject CheckYes No

Explanation if answer is "no"

Flare platform - workable- escape routes in place- inspected by

Flange bolts - loosenedCrane assistance - requiredAir compressor - with buffer vessel

- with air drierPackings - present on work platformSpare packings - present on work platformTemporary lighting - installation OK

- operation OKWork license (category 2) - submitted

To be filled in before execution by the permanent supervisor in charge NameInitialsSubject Check

Yes NoExplanation if answer is "no"

Fire hoses and nozzle - present on work platformWater pressure - pressure increase setFire brigade assistance - presentH2S beeper - present on work platformInternal communication system - tested for proper operationCrane assistance - presentAir filter - replaced by new filterCompressed air mask - present on work platformFresh air mask - air connections (max. 4

lengths) per cap- max. 2 caps per air filter

Wegafzetting (verzorgd door VMBD) - presentWork license (category 2) - present

Environmental code ParticularsWind directionOutside temperature

Figure 6Checklist for flare work

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SummaryThe flare is used for burning 'regular' waste gases and is there for safety as wellas environmental reasons. To prevent explosions, air (oxygen) must be kept outof the flare system under all conditions.For the purpose of proper operation and safety, a number of auxiliary devices(with accessories) have been fitted, such as knock-out vessels, a water seal,steam injection and a pilot burner. Flammable and oxygen-rich waste gases mustnot be discharged via the flare, but must be blown down via a flame arrestor.The blow-down system is an emergency device for the blow-down of liquids inthe case of calamities.The final section gives an example of safety regulations and proceduralrequirements and highlights how stringent they are, with various stand-byfacilities to be provided for.

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TestExercisesDo not send in your answers for correction

1. Which dangerous situations can be created in working with the flare system?

2. Why does air in the flare system present a hazard?

3. How is air that may have entered removed?

Answers to the questions in the lesson1. Types of gases which are flared include:

- waste gases during the product process;- gases that have to be discharged occasionally because of repairs etc.;- gases released due to leakage or venting via relief or safety valves;- gases which may present a hazard.

2. Waste gases are burnt to minimise environmental pollution and to preventfire, intoxication and explosions.

3. The flare pipe must be large enough to prevent an appreciable pressureincrease when venting the maximum quantity of gas present in the equipmentand it must be laid at an incline to prevent accumulation of liquid.

4. The water seal is used to prevent underpressure (vacuum) in the flare pipe.

5. A sooting flare flame is made less sooting by injection of steam.

6. The three prerequisites for causing an explosion are:a. presence of a flammable material (hydrocarbons);b. presence of an ignition source;c. presence of oxygen (air).

7. A flame arrestor is used to prevent flashback of the flame if flammablegases must be blown down directly (bypassing the flare).

8. The purpose of the heating element in the blow-down vessel is to enable theevaporation of liquid and thus make it suitable for discharge via the flare.

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Answers to the exercises1. Air is able to enter the flare pipe and underpressure can occur due to cooling

down of the system.

2. Air contains oxygen and oxygen is usually the third factor required to causefire or an explosion.

3. By injecting nitrogen into the flare pipe; for this purpose, a special supplypipe is permanently connected.

Problems and assignmentsAnswer and send in for correction

1. What is the purpose of the flare system?

2. What is the purpose of the blow down system?

3. What is the purpose of the knock-out vessel in the flare system?

4. How can air enter the flare pipe?

TheoryEquipmentFlare systemsContents of the lesson

Lesson1Flare with accessories2Safety3Flame arrestor4Blow-down system5Flare procedural requirementsImplementation

SummaryTestExercisesAnswers to the questions in the lessonAnswers to the exercisesProblems and assignments