Flare and Burn Pit

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COURSE OIL AND GAS TECHNOLOGY CODE

SUBJECT FLARE AND BURN PIT P/TM/TRG/P.FB/001

Objectives: Upon completion of the unit the trainees should be able to:

• Describe the purpose of flare system.

• Explain the function of relief devices in a flare system.

• Explain different types and fields of application of flares.

• Describe the main parts of a flare and its accessories.

Contents:

1. Introduction.

2. Simplified description of a flare system and accessories.

3. Detailed description of a flare system.

3.1. Relief device.

3.1.1. General definitions

3.1.2. Pressure relief devices.

3.1.3. Blow down valves.

3.1.4. Pressure control valves.

3.1.5. Remotely operated valves.

3.2. Segregation of fluids to be flared.

3.3. Flare drums 4. Different types and fields of application of flare devices.

4.1. Flare tips

4.1.1. Pipe flare

4.1.2. Sonic flare

4.1.3. Ground flare (multiple tips)

4.1.4. Ground flare with combustion chamber

4.2. Cold vent

4.3. Burn pit

4.4. Burner 5. Associated equipment

5.1. Flare drum

5.2. Headers

5.3. Hydraulic and gas seals

5.3.1. Hydraulic seals

5.3.2. Gas seals

5.4. Purge gas system

5.4.1. Mechanism of gas ingress

5.4.2. Purge gas

5.5. Pilot and ignition system

5.6. Fuel and power supply

Prepared by Ziabari

Checked by Yeganeh Larijani

Approved by

Date 10 July. 2000 Date 19 July. 2000 Date

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SUBJECT FLARE AND BURN PIT CODE

P/TM/TRG/P.FB/001

THE FLARE SYSTEM

1. INTRODUCTION

A flame burns day and night at the top of a high chimney on the plant site. This is the flare

and the flame is produced by burning gases or vapors released during the various

processes carried out on the site. These could be:

- Gases, which must be continuously removed as waste.

- Gases which occasionally have to be discharged from equipment for repair,

maintenance

or depressurization purposes. - Gases escaping through leaks or safety valves blowing off.

- Gases, which have to be removed because they are dangerous, e.g. fire hazards. All these gases and evaporated liquids may not simply be released into the air. They would

pollute the atmosphere and be dangerous for man, animals and plants (fire, poison,

explosion). They are therefore all brought to one point high in the air and burnt there.

The flare system plays a vital role in a factory. It is very important that there should be very

good communication between this factory and all others connected to the flare.

2. SIMPLIFIED DESCRIPTION FOR A FLARE AND ACCESSORIES

Fig. 1 is a diagram of the flare and includes everything necessary for efficient and safe

working.

The flare itself is a high chimney. Waste gases are supplied to the bottom of the chimney

and burnt in the nozzles at the top, with the aid of the air present there.

On the left of the diagram the waste gases enter a knockout drum. The liquid particles

suspended in the gas stream (water and hydrocarbons) are separated and remain in the

drum while the gas escapes upwards.

The liquid is maintained at a certain level by a level controller with an alarm (LCA) and

drained by a pump to the slop tank. Two pumps are installed here, one electrically driven

and the other turbine driven. The first is used for normal operation while the other takes

over if the electric current should fail.

The gases now enter the water seal with two or more submerged pipes, each with a

different insertion length. The liquid level in the water seal is held constant by continuously

supplying water through orifices and allowing it to drain to the sewer through a siphon. A

siphon breaker and a sight glass (SG) are fitted to the siphon. This is done to prevent a

vacuum in the flare up to the water seal.

A steam coil holds the temperature in the water seal at 65°C to remove solvent gases from

the water so that these can be carried along with the flare gas.

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Fig. 1- Diagram of flare system.

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A funnel is fitted at water level to skim off any hydrocarbons floating on the water; these are

then drained to a special drain tank.

The gas exhaust pipe is usually fitted with a flow indicator (FI), a quality register (QR) and a

pressure indicator (PI). The QR works as follows: A thermocouple is heated electrically and the temperature is indicated on a strip of paper.

When the thermo-couple is surrounded by the gas the temperature of the couple will drop

so that this is a gauge of the gas flow.

The drain tank is partly filled with gas oil, which has to be renewed periodically. The

temperature is kept at about 90°C by a steam coil to evaporate any H2S particles in the oil.

These gases disappear to the flare through an extinguisher.

The Flare Head (Tip)

When large quantities of gas have to be burnt the chance of soot forming is very real. This

can be prevented by adding steam and/or water, which will reduce the amount of soot

considerably, probably even to an acceptable minimum.

One disadvantage of steam injection is the very loud noise that it causes. Noise must be

kept below a certain level. This is the reason why water injection is frequently used, which

is much quieter (see Fig. 2)

The water injections take place in circular chamber near the flare mouth and in a central

area somewhat lower. The water is supplied to the chamber through a FRC, controlled by

FI in a branch of the water seal gas supply pipe. The water supply is automatically

increased when the gas flow increases.

The water supply to the central injection passes through an orifice. This common supply is

controlled by a PC.

In the injection nozzles the water is:

a. Divided into very small drops (mist).

b. Sprayed at a certain angle. It is possible that the flame is blown out by the water if the gas supply is too low. No water

may be injected if the gas stream is below a certain level.

If a certain department wishes to get rid of gas and has to flare then the personnel working

with the flare must first be warned that extra water should be supplied to prevent air

pollution. Fig. 3 shows the effect when more or less water is injected.

The pilot flame is supplied by the central fuel system and can be ignited by electric ignition

devices.

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Fig. 2- The flare burner head.

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Fig. 3- Results of supplying water to the flare.

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3. DETAILED DESCRIPTION OF A FLARE SYSTEM

A flare system consists in:

- Relieving devices (pressure relief valves, rupture discs, blow down valves, control

valves, …) - Headers and Sub Headers

- Knock out drum to separate the different phases (water, liquid and gaseous

hydrocarbons) - Sealing devices to prevent air from entering the system (purge gas, water seal, gas

seal) - Disposal devices (cold vents, burning tips, liquid burners, burn pits, …)

3.1. Relief Device

A relief device releases to the atmosphere (via a flare system, or not, excess or unwanted

products).

3.1.1. General Definitions

MAXIMUM ALLOWABLE WORKING PRESSURE (MAWP)

This is the highest pressure for which the safety valve may be set (API RP 521).

Generally, in design the MAWP is taken equal to the design pressure.

The design pressure and the maximum allowable working pressure are normally

applied to the top of the vessel. Any increase in pressure at the bottom, caused by

hydrostatic head or pressure drop from the bottom to the top must be considered in the

vessel design. If a safety device is located at the bottom of a vessel (this is usually not

recommended) the set pressure must be increased by this same amount.

SET PRESSURE

This is the pressure at which a safety devices is adjusted to start to open under service

conditions. As stated above, it may be either the design or the maximum allowable

working pressure.

The set pressure shall not exceed the maximum allowable working pressure of the

protected equipment at operating temperature except where the required capacity is

provided by more than one device. If more than one pressure relieving device is used,

only one device need be set at a pressure not exceeding the maximum allowable

working pressure at operating temperature. The additional relieving devices may be set

at a pressure not exceeding 105 percent of the maximum allowable working pressure.

The pressure drop between the protected equipment and the inlet side of relieving

devices should be checked at relief flow rate to ensure that it is not greater than 3

percent of the set pressure.

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ACCUMULATION

This is the maximum allowable pressure increase in the protected equipment above the

maximum allowable working pressure during discharge through the safety device. It is

determined by the applicable code for any project. In most cases a maximum

accumulation of 10 percent is allowed but 21 percent is permissible when the excess

pressure is due to an external fire (API RP 521).

BACK PRESSURE

This is the pressure on the discharge side of the safety device. Two types of back

pressure have to be considered:

- Pre-existing back pressure: the pressure in the discharge side of a safety device

before it discharges. It can be (semi) permanent when a control valve releases in

the same system or pressure created by the discharge of another near-by relief

device. - The built-up back pressure developed on the discharge side of the safety device as

a result of flow after the relief devices has opened. The maximum allowed built-up back pressure depends on the type of pressure safety

valve. 3.1.2. Pressure Relief Devices

This covers:

- Pressure safety valves including:

• Conventional type for which the set pressure depends on the back pressure. Its

use is limited to a maximum built-up pressure less than 10% of the set pressure. • Balanced type for which the set pressure is independent of the back pressure. The

built-up pressure should be limited to 50% of the set pressure; this back pressure

could be exceeded but that is not recommended and requires a special care.

- Pilot-operated pressure relief valves for which the set pressure is independent of the

back pressure. The built-up pressure should be limited to 50% of the set pressure; this

back pressure could be exceeded but that required a special care.

- Rupture discs

For description and information see API RP 520 part1

- Thermal relief valves: this is a spring loaded relief valve conventional type actuated by

the static pressure upstream. The valve opens in proportion to the pressure increase

over the set pressure. They are used primarily with incompressible fluids.

3.1.3. Blow Down Valves

They are on-off valves connecting the process equipment to the flare header. They are

actuated remotely by the operator or automatically by the emergency shutdown

system.

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3.1.4. Pressure Control Valves (PV)

They are control valves actuated by electronic, pneumatic or hydraulic systems and

releasing, permanently or intermittently an excess of fluid to the flare system. It occurs

mainly during transitory situations like start-up, controlled shutdown, and production of

off-spec products.

3.1.5. Remotely Operated Valves (HIC, XV, ROV,…)

They are actuated by a push button, or through a DCS command. They are either

on-off (XV’s) or controlling valves (HIC). They are used during transitory situations or

for specific purpose like start-up, pipeline depressurisation.

3.2. Segregation of Fluids to be Flared

The number of flare or cold vent system depends on:

- The process scheme

- The characteristics of the fluid to be flared or vented.

The main criteria are:

- Different level of pressure, which could lead to excessive back pressure in other

parts of the headers. It must be avoided to have high pressure products relieving

in the same header as low pressure products, if there is a scenario when the two

reliefs are likely to occur at the same time. - Depending on the operating system wet or dry and cold systems. Normally wet

products and off-spec. dry products should not be relieved in a cold and dry products

system to avoid ice and (or) hydrates formation which could plug the relief system. - Corrosivity of the different gases leading to different pipe materials e.g. sour or

sweet gases, if the price of the different material is significantly different, several

systems have to be considered. - For H2S concentration above 10% mole a separate flare system is required.

- Economical cost considerations of a single (large diameter) header versus several

branches and headers. - Relieving capacity, which could determine the number of flare system.

- Plot plant, which can as well dictate the number of relief headers.

- Maintenance and operation philosophy, which can dictate to have always one flare

on line, hence a duplication of the last portion of the header, flare drum, header to

the flare and the flare tip has to be envisaged for maintenance.

3.3. Flare Drums

A flare knock-out drum is provided to drop out and collect the liquid content of the fluid

flowing in the flare system.

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4. DIFFERENT TYPES AND FIELDS OF APPLICATION OF FLARE DEVICES

To burn or release the gas and (or) liquid, different systems can be used, they are:

- Flare tips,

- Vents,

- Burn pit,

- Burner.

4.1. Flares Tips

The flare tip is the last device of the flare system. It is used to burn the gas without liquid,

except the small quantity due to the carryover from the associated drum.

Different types can be used and they are:

- Pipe flare,

- Sonic flare,

- Ground flare (multiple-tips),

- Combustion chamber.

4.1.1. Pipe flare

A simplified diagram of a conventional pipe flare tip is shown in Fig.4.

A pipe flare tip is always installed vertically and the gas velocity through it is limited at a

maximum of 0.5 to 0.6 Mach for the emergency flow rate (non continuous flow) and 0.3

Mach for the continuous flow.

The maintenance of a conventional pipe flare is very low.

A pipe flare should not be regarded as a simple pipe from which released gas is burnt.

Any flare tip must offer reliable ignition and give flame stability under the most adverse

weather conditions.

Flame stabilization is achieved by the use of specially designed flame retention ring

built into the flare tip. This device stabilizes the flame front by creating vortexes

downstream of the stabilizer to ignite the main gas stream and so help prevent flame

lift-off.

There are many variations of the simple single tip pipe flare and some of these are

discussed below and generally not recommended for production except in special

cases mainly to solve the smoke problems.

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Fig. 4- Pipe flare

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- FORCED DRAUGHT (AIR ASSISTED) TIPS

In principle stoichiometric combustion of relief can be achieved by installing blowers to

deliver the required air. However, for large flares this air requirement is substantial and

the space and weight requirement coupled with the additional complexity and related

maintenance costs has led to very few installations.

This type of arrangement is not recommended except to solve the smoke problems.

- PIPE FLARE WITH WATER INJECTION

Smoke emission and radiation levels are reduced by the addition of a water spray to

conventional pipe flare tips. However, that complicates the system and increases the

cost and maintenance. The water shall be available and for offshore only sea water is

available in quantities required and this leads to accelerated corrosion of the flare tip by

salt deposits and chloride.

This type of arrangement is not recommended except to solve the smoke problems.

- PIPE FLARE WITH HIGH PRESSURE GAS INJECTION

The kinetic energy in high pressure gas can be used to entrain additional air by piping it

separately to the flare tip and injecting it into the flame via a manifold and jets.

However, that complicates the system and increases the cost and maintenance. The

use of HP gas increases the radiation levels and this gas could not be sold.

This type of arrangement is not recommended except to solve smoke problems.

- PIPE FLARE WITH STEAM INJECTION

Smoke emission and radiation levels are reduced by the addition of a steam injection to

conventional pipe flare tips. The steam injection eliminates the black carbon. However,

that complicates the system and increases the cost and maintenance. The steam shall

be available and for cold countries the freezing protection shall be provided.

This type of arrangement is not recommended except to solve smoke problems, it is a

common arrangement in refinery.

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4.1.2. Sonic Flare

The exit gas velocity is at least 1 Mach.

Fluid jets discharging into the atmosphere induce air and tend to mix the induced air

with the fluid. Air when premixed with gas in a gas burner improves combustion and

gives a clean efficient flame, which reduces the emissivity and radiation. The sonic flare back pressure for the design flow rate could reach 4 to 10 bars (normal

4 to 5 bars), when properly designed, however the downstream equipment have

smaller sizes due to the lowest gas volume but a highest design pressure. Each manufacturer has its own design.

The tip ∆P shall be given in the process data sheet at the basic engineering stage.

ADVANTAGES AND DRAWBACKS

The advantages and drawbacks of the sonic flare with the conventional pipe flare are:

Advantages

- Better combustion (good mixing with air) and consequently lower radiations and

lower flare height. - Higher back pressure and consequently smaller headers, sub-headers and flare

drum. - Could be installed inclined, but not recommended and not accepted by all

manufacturers due to the possible tip damage at low flow rate.

Drawbacks

- More maintenance (replacement every to 2 or 3 years, depending of

manufacturer).

- More weight of flare tip.

- More noise.

- Cost (more expensive).

- Low pressure flare system could not be connected to sonic flare due to

back pressure. In this case a separated low pressure flare system is required or

combined with the sonic tip, the LP flare tip being installed in the center of the sonic

tip.

The installation of sonic flare tip is recommended mainly offshore in order to save

weight and space due to the smaller headers, sub-headers and flare drum and flare

length.

4.1.3. Ground Flare (Multiple Tips)

The ground flare (multiple tips) consists generally of 3 rows of several pipe flare

installed vertically and having a height of about 1.5m, they are also known as finger

flare. They could be installed only onshore because they require a large area far from

the plant due to toxic gases at low/ground level.

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The use of multiple tip burners provide more efficient combustion through improved air

entrainment and reduced flame length by the use of smaller burner nozzles. The improved combustion of multiple tips results in lower smoke emission and radiation

levels than a comparable large single tip. However, the use of multiple tip flares is not recommended. Operating experience has

not been particularly good. The direct flame impingement on some of the tips,

particularly at low flaring loads, strong winds has a serious effect on life of these tips

and consequently maintenance costs have been high. Design of multiple tip pipe flares is particularly difficult. To obtain a correct distribution

of the flow through each tip for any flow rate which requires the installation of control

and restriction orifices (that is not recommended on flare system) and by experience

the design is never correctly done. This type of ground flare could not be used for a low pressure flare system due to the

necessary back pressure for the flow distribution through each tip.

4.1.4. Ground Flare With Combustion Chamber

This type of ground flare consists of a burner forced draught installed inside a chimney

Fig. 5.

It is only installed:

- Onshore when the local environmental regulations do not permit to have a visible

flame or when there is no space to install another type of flare. - Offshore when it is not possible to install another type of flare.

The advantages are:

- Low radiation level and less space

- No visible flame, that is valid only for environmental regulations but not for operations. - Less noise.

The drawbacks are:

- High cost due to the need of air blower and chimney with fire resistant internal

material - Weight due to installed devices

- Space of the chimney

- Limited to a relatively small flow rate (limited by the chimney size)

- No visible flame for operations point of view

- Air blower to be continuously run in case of emergency

- Maintenance of air blower, burner and fire resistant internal material of the chimney

The sizing of this ground flare with combustion chamber can be performed only by the

venders.

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4.2. Cold Vent

The cold vent is similar to a flare tip but the gas is released to the atmosphere and not

burnt.

Fig. 5- Ground flare with combustion chamber

The cold vent height is governed by dispersion calculations and in any case admissible

radiation levels, in case of accidental lightning, shall be performed to finalise the cold vent

height.

The cold vent is fitted with:

- Either a sonic tip (as for flare tip)

- Or a pipe flare tip (as for flare tip) but in this case the device for the flame stability is

not required. The gas velocity through the tip is to be about 0.8 Mach in order to

achieve a good dispersion of the gas in the atmosphere.

4.3. Burn Pit

A burn pit is installed only onshore and generally for manual intermittent operations.

The burn pit consists of:

- A pit,

- A pipe feeding the pit by gravity,

- Pilots.

It is generally used to burn liquid coming from well clean-up and off-spec. liquid products. It

is not used to burn liquids in continuous for a long time. The combustion of the liquid is very bad with a lot of smoke because it burns at the surface

of the pit practically without air mixture.

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In the rainy countries, the pit shall be equipped with a water drain.

When the space is not a problem, the burn pit could be used to replace an elevated flare.

That has been used in the past in the desertic countries.

The advantages and drawbacks of the use of burn pit instead of an elevated flare are:

Advantages

- No need of flare drum if the plant layout permits to have a gravity flow with a

continuous slope to the burn pit. - Easier to install and to do the maintenance of the tip (about at the ground level)

Drawbacks

- Very large pit and a lot of civil work, that could be envisaged only in the countries

where civil works and manpower cost are low. - Refractories (fire resistant) shall be installed on the pit part where the burn pit is fed,

that could generate a heavy maintenance because the installation of these

refractories is difficult. - The dispersion calculations are subject to discussion because that is not very well known and could impose to install the burn pit at a relatively long distance from the plant.

The installation of a burn pit to replace an elevated flare is not recommended.

4.4. Burner

The burner is generally used for well clean up and also sometimes for off-spec liquid

products burning (e.g. LNG plant).

5. ASSOCIATED EQUIPMENT

5.1. Flare Drum

As stated in section 2 a flare drum is provided to drop out and collect the liquid content of

the vapors. The drum ensures liquid separation for the following reasons:

- To prevent liquid accumulation at the base of the flare boom or tower and obstruction

of the flow of gas. - To minimize the risk of burning liquid “golden rain” emerging from the flare tip and

endangering personnel and equipment. - To recover and reclaim valuable product materials from the flared gas.

Design Considerations

- Generally it is necessary to provide one flare drum for each flare pressure-level. For

example, where a sonic tip (high pressure) is used in conjunction with a conventional

pipe flare (low pressure) it is necessary to provide independent flare drums for each

system.

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- All flare drums shall be located at the low point of the system.

- Internals on gas side (e.g. mist eliminators) in the flare drums shall be forbidden.

- Generally flare drums should be installed horizontally. Vertical flare drums could be

installed in case of low vapor flow containing a small quantity of liquid or unless space

allocation is critical.

Horizontal flare drums with an appropriate L/D ratio are much more effective than

vertical flare drums.

- When warm moisture bearing vapors are segregated from cold dry vapors, both wet

and dry flare drums may be required. However, every effort is made to minimize the

number of items of equipment and to avoid the need for a cold flare drum. One solution

is to introduce the cold vapor source into the main flare header immediately upstream

of the flare drum inlet nozzle or with a separate inlet nozzle. For this type of design a

number of precautions are necessary, including:

* Avoid operation of the main flare header at a temperature below the minimum allowed by

the material specification of the header.

* Avoid overcooling of the flare drum inlet nozzle. * Check minimum temperature achieved by the flare drum vessel wall.

- The flare drum design pressure shall be at least 1 bar higher than the maximum back

pressure at the design flow rate with a minimum design pressure of 3.5 bar g. (API

RP 521).

- The liquid level in a flare drum when the flare starts to below should be taken as the

high high level. This is important for calculating the vapor space for liquid droplet

separation.

* For large diameter horizontal drums a spilt flow arrangement (1 inlet and 2 outlets or 2

inlets and 1 outlet) can be effectively used to minimize drum dimensions, in this case the

diameter is then calculated using half the vapor flow rate.

* Heating coils should be considered for the flare drums. The purpose of the heating coils

are to avoid possible freezing and to maintain any oil carry-over at an acceptable margin

above its “pour point”

* At least a high high level (LSHH) is always installed on a flare drum and initiates an ESD

level (total process shut down). The location of this LSHH shall such be that the inlet gas

does not create a liquid vortex and gives a wrong information.

5.2. Headers

Two types of lines shall be considered:

- Lines upstream the relief devices (PSV’s, BDV’s, PCV’s).

- Lines downstream the relief devices which form sub-headers and headers.

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- Lines upstream the relief devices

- These lines shall be self-drainable to the protected capacity without pocket and low

points (Fig. 6) - The isometric shall be the simplest with the minimum of fittings (e.g. elbow)

- For the PSV’s the diameter of this line is at least equal to the inlet nozzle of the PSV,

it is as short as possible and the pressure drop in this line shall not exceed 3% of the

PSV set pressure for the maximum discharge flow rate to avoid opening and closing at

high frequency (“chattering”). Chattering may result in lowered capacity, damage to the

seating surfaces and could even lead to heavy vibrations of piping and connected

instruments.

- For the BDV’s the diameter of this line shall be at least 2”.

Fig. 6- Sketch showing the slope for lines upstream and downstream relief devices

- A heating device (electrical or steam heating) will be installed on the lines upstream

the relief devices with a no continuous flow for the liquid or gas with liquid having a

pour point higher than the minimum ambient temperature and for liquid and LPG with

free water if the ambient temperature is lower than 0°C.

- The upstream lines of safety devices in cryogenic service will not be insulated on at

least 1 m upstream of the devices to avoid the risk of their freezing.

- Lines downstream the relief devices

- These lines and also all sub-headers and headers shall have at least a continuous

slope of 2mm per m to the flare drum and from the flare stack to the flare drum (see

Figure 6)

- All relief devices shall be installed in high point.

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- Flare headers should be large enough to avoid excessive back pressure which would

prevent safety valves from operating properly. - In general, the discharge piping from a PSV should be at least one line size greater

than the PSV outlet nozzle. - In general, block valves (except full bore and locked in fully open position downstream

the relief devices), check valves, flow orifice, flow meter (expect annubar (1) type or

equivalent) are not installed on flare headers and sub-headers. - If one flare header receives different effluents, (hot, wet, cold, … ,) a segregation of

sub-headers must be performed to avoid mixing the different fluids and in this case the

connection could be done just upstream the flare drum nozzle or different nozzles on

the flare drum could be also envisaged. - The number of headers and sub-headers will be also function of the plot plan, the fire

zones and the material selections. In case of relatively low temperature the heating due

to the ambient temperature can be taken into account. - The lines downstream the relief devices are not insulated and heat traced except for

liquid having a pour point higher than the minimum ambient temperature. In fact, only

the discharge line will be heat traced, the good engineering practices admit that the risk

of freezing is practically null then the self-drainable line size is relatively important.

5.3. Hydraulic and Gas Seals

When the flow of gas through a flare or cold vent stack reduces to very low or no flow, air

may ingress from the mouth of the stack to form an explosive mixture in the stack or flare

header. It is normal practice to avoid this hazard by continuous bleeding a small flow of

oxygen-free hydrocarbon or inert gas (purge gas) through the stack to deter air ingress. To

reduce the purge gas flow, a seal system can be installed. Two main systems of seals exist:

- Hydraulic seals,

- Gas seals.

5.3.1. Hydraulic Seals

- The hydraulic seals are water seals and they are not often installed today because

they are more expensive than the gas seals. - They could be installed at the bottom of the flare stack or on a water seal drum

horizontal located not so far from the flare stack. - Water seals require a constant water flow.

(1) ANNUBAR = A gas measurement device that consists of a multiple-ported pitot

tube installed inside a pipe through which gas is flowing; it is installed perpendicular

to flow of gas; the length of the annubar is equal to the diameter of the pipe in

which it is installed, it sences the difference between total flowing pressure and

static pressure, gas volume is calculated from this difference.

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- Water seals can be used only for a conventional pipe flare tip because they cannot

work with a high back pressure in order to avoid to discharge gas to the sewer and

through the vent used as a vacuum breaker on the seal leg. - Water seals cannot be installed if a cold gas relieves to the flare due to the risk of

water freezing and the blocking of the gas to the flare. - The installation of water seals is not recommended if the minimum ambient

temperature is lower than 0°C to avoid also the risk of freezing. - Water seals being installed at the bottom or near the flare stack, the air ingress

problem in the flare stack itself is not solved. - The advantage of water seals is to protect in any circumstances the upstream

headers and flare drum against the air ingress mainly when the purge gas is stopped

and particularly during the maintenance.

5.3.2. Gas Seals

- Three types of gas seal are in common use:

* Static inverted gas seals,

* Dynamic baffle seals,

* Double purge baffle seals.

All types of seal are intended to reduce the rate of air ingress and the necessary

purge flow. Their use for sonic flare tip in case that the gas could content some

liquid (case of sealing with possible condensation) shall be checked with the vendor

from a mechanical viewpoint. - These gas seals are installed immediately upstream the tip.

- They are designed and supplied by the manufacturers of the flare tip.

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Fig. 7- Inverted gas seal

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5.4. Purge Gas System

- When the flow of gas through a flare or vent stack is below a minimum value, air may

penetrate from the tip of the stack to form an explosive mixture in the stack or flare header.

5.4.1. Mechanism of Air Ingress

- STEADY STATE CONDITIONS

No complete model for the process of air ingress at the stack tip has been proven but

the likely mechanisms are as follows:

* Wind action across the tip of the flare stack accelerates the emerging flow of gas

resulting in a low pressure region in the tip. Air is then drawn into the low pressure

area at a rate dependent on the wind speed.

* When purging with a gas lighter than air the air tends to penetrate the stack

displacing the lighter gas from the stack. It has been postulated that when laminar

flow conditions exist in the stack the purge gas will tend to flow through the central

core whilst the air penetrates at the circumference of the stack.

A variety of tests have been carried out to determine the factors which influence the

rate of air ingress:

* Gas density

The rate of ingress increases as the purge gas density falls. Heavier than air purge

gases are extremely efficient as they tend to behave like a piston and flush the air

out of the stack. * Stack diameter

The required purge velocity to maintain similar oxygen concentrations in the stack

increase with stack diameter. * Wind speed

As wind speed increases the low pressure area in the stack tip becomes more

marked and the air ingress increases. Furthermore, the size and strength of wind-

induced eddies is increased, and as a result there is enhanced air penetration. * Sub-atmospheric pressure within the flare system

If hot or low molecular weight gas is used to purge at low flow, the buoyancy of the

gas may reduce the pressure below atmospheric at the base of the stack.

- TRANSIENT CONDITIONS

There exist process conditions which occur only intermittently which result in rapid air

ingress and these must also be considered when designing and operating a flare or

vent stack:

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* Rapid temperature change

If hot gases are discharged form a stack and the flow reduced suddenly in certain

weather conditions the gas remaining in the stack will cool rapidly resulting in

contraction in gas volume and the drawing in of air from atmosphere. * Recommissioning of stack after plant maintenance

During plant shutdowns and maintenance operations equipment may become filled

with air. On resumption of operations caution must be taken to ensure the discharge

of air does not form explosive mixtures in the stack, flare system should be inerted

before re-start-up wherever possible. * Vacuum within the flare headers

If a water seal is installed and hot gas is relieved, when the flow ceases the gas

remaining will cool resulting in a partial vacuum in the headers upstream of the

water seal. To ensure against this occurrence vacuum breakers are normally

installed so ensuring against air leakage and breakdown of the water seal.

5.4.2. Purge Gas

The flow of purge gas to the stack is normally a waste of valuable purge gas or

product. For economic reasons the purge gas flow is normally minimum to ensure safe

conditions within the stack and extended tip life.

When using a purge gas with a molecular weight below the molecular weight of air

(29), a large part of the flare piping can be under vacuum if the purge gas flow is not

sufficient.

It is recommended to use the heaviest gas molecular weight as the flare purge gas.

Nitrogen with a purity at least of 95%, if available in sufficient quantities can be used as

an alternative source. Indeed, the quantity of nitrogen required is slightly less than the

amount of low molecular weight associated gas required. However, nitrogen may be an

expensive commodity and, although preferred, may be economically unattractive.

Nitrogen could be used as a purge gas back-up.

The purge gas must be injected at the beginning of the largest flare headers or sub-

headers in order to purge the whole flare system.

5.5. Pilots and Ignition System

Pilot burners are provided to ensure a continuous ignition source at the flare stack. The

system consists of multiple continuous pilot burners arranged around the flare tip,

supplemented by remote controlled pilot ignition system to ensure against flare failure. The

prime consideration of the pilot system is reliability. Its design must incorporate features,

which allow it to function under adverse weather conditions, particularly high wind speeds.

The system is usually specified by the vendor based on his own proprietary equipment and

purchased as part of the flare tip package.

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- Pilot burners

The number of pilot burners distributed around the flare tip is function of the tip diameter

and the predicted pilot flame deflection due to wind. Typical Figures are given below,

although the actual design should be made by the flare vendor.

Tip diameter Nb. Of pilots

8” and below 2

10” -36” 3

Greater than 36” 4

For sonic tip, the tip diameter is taken as the equivalent diameter corresponding to the exit

gas area.

Pilot burners are typically 1” diameter stainless steel tips protected by a small rain

protection head. Pilot burners must be supplied by a reliable source of fuel gas. A low

pressure alarm with remote indication in the control room should be provided on the pilot

fuel gas system.

Gas consumption per pilot burner is typically 8-10 Nm3/h

Each pilot is fed by an individual gas line, which is blinded from the ignition system.

- Pilot ignition

A pilot ignition system is provided to allow ignition of continuous pilot burners during

start-up or inadvertent flame failure of burners. Igniters are generally of the flame

propagation type in which a premixed flammable mixture of air and fuel gas is electrically

sparked and the resulting flame front is propagated to each pilot burner through a small

open ended pipe, the length of this pipe shall be limited at about 100-150m, with a longer

pipe, the system does not work correctly. Some other concepts are under development and

the high energy sparks igniters seem to have a good record.

Ignition is normally performed manually through a local push button, although automatic

ignition systems actuated on receipt of a flame failure alarm signal are available. However,

manual systems are recommended.

The flame failure alarm signal does not work properly due to the difficulty to have proper

instruments, thermocouple or IR/UV detectors are rapidly out of service because they are

burnt and replaced on many places by a camera.

The igniter is weatherproof and must be located to allow easy and safe access. The system

is a vendor package and is purchased integrally with the pilot burners and flare tip

assembly.

Ignition could be also performed with a special gun, that could be used as a back-up when

it is authorized by the local regulations (example it is not authorized in Indonesia), in any

case the design shall also include this solution.

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5.6. Fuel and Power Supply

Flare pilots and ignition systems require high integrity fuel gas (in any case above the water

and hydrocarbons dew points to avoid liquid) and power supply. It is recommended that

these systems be provided with a back-up propane bottle supply to supplement the normal

plant fuel gas system. Ignition systems should be powered by D.C. batteries.

Flare and Burn Pit

Documents

adverse weather

remotely operated

pour point

vertical flare

low pressure

pressure control

conventional

cold vent