flare systems

FLARE PROTECTION SYSTEMS

Introduction

Most people look at flares as simply a fire on top of a structural support pipe to burn gases. To the public, they often appear as a source of smell, smoke, noise, fall-out, and light. This discussion covers factors in the location of a flare, the equipment involved and the design factors. In addition, it explores the most common factors which are involved in accidents.

No single element involved in the safe and satisfactory operation of a process plant is more important than the flare system for operational or emergency relief of quantities of flammable substances in either the liquid or the gaseous phase.

Emergency releases originating from safety valves, vapor blowdowns, process stream diversion and equipment drainage, which cannot be discharged directly to the atmosphere for reasons of safety or pollution control, are routed through closed systems to a blowdown drum where liquids and vapors are separated.

A flaring system consists of facilities to safely combust vented hydrocarbons. The ideal operating condition would be to eliminate the need for flares as this wastes hydrocarbons which could become products and thereby improve profits. However, facilities to recover large amounts of released hydrocarbons under emergency conditions are not economically justified at this time. The equipment to recover daily leakage rates of hydrocarbons is justifiable.

The flare provides a means of safe disposal of the vapor streams from these facilities, by burning them under controlled conditions to ensure that adjacent equipment or personnel are not exposed to hazard. In addition, pollution control and public relations requirements must be met.

A typical flaring system consists of collection piping within a unit, a flare line to the site, a knockout drum to remove liquid hydrocarbon from the gas stream, an optional liquid seal to provide positive header pressure without surging and protect against flashbacks, a flare stack with flare tip, an optional steam system to maintain smokeless burning, a fuel gas system for pilots together with igniters and instrumentation.

A typical refinery flare will use several utilities when in operation; power, steam, fuel and gas. The careful design, operation and maintenance of the flare system can minimise the costs of these expensive utilities.

Integrated Training Program / Phase B – Flare Protection Systems of 33Copyright © 2004 International Human Resources Development Corporation

LESSON 1

Flare Types and Application

In general there are three types of flares available for onshore use

1) The elevated flare

2) The groundflare

3) The burn pit flare

Although the three basic designs differ considerably in required capital and operating costs, selection is based primarily on pollution and public relations considerations, i.e. smoke, luminosity, air pollution, noise and available space.

1. Elevated Flares

Elevated flares are the simplest and most widely used, offering safe and efficient combustion of waste gases with varying degrees of smokeless burning. By the use of steam injection and effective tip design, heavy hydrocarbons can be burnt smokelessly.

Steam injection, used to reduce smoke pollution, introduces a source of noise and a compromise between smoke reduction and noise is usually necessary. If correctly designed the elevated flare provides the best dispersion characteristics for malodorous and toxic combustion products and is the general choice for either total flare loads, or for handling over-capacity releases in conjunction with a groundflare. For most applications the elevated type is the only acceptable means of flaring "dirty gases", i.e. gases high in unsaturates, hydrogen sulphide or those which have highly toxic combustion products.

Elevated Flare


2. Groundflares

Various designs of proprietary groundflare are available.

Smokeless operation can generally be achieved (with or without assist media depending on design), with essentially no noise or luminosity problems, provided that the design rate to the flare is not exceeded. However, since the flame is near ground level, dispersion of stack releases needs to be carefully considered.

The groundflare is suitable for "clean" gases (i.e. where toxic or malodorous concentrations are unlikely to be released through incomplete combustion or as combustion products), offers very low noise characteristics and reduces the visual effect of a flame, which is concealed at all times.

It should not be used in locations upwind of adjacent residential areas. Generally, it is not practical to install a groundflare large enough to burn the maximum release load and the usual arrangement is in combination with an elevated relief flare. The latter is normally provided with steam injection, but smoke may be accepted during the-small number of major releases.

Enclosed Ground Flare


3. Burn Pit Flares

The burn pit is of simple construction, with low capital and operating costs, and can handle liquid as well as vapour hydrocarbons.

The sizing of pit flare systems is essentially the same as for pipe flares without the knock-out drum. The flare header should slope down to the pit to allow full drainage of liquids.

The flare pit will be sized for the largest flame length, taking account of thermal rise and the predicted volume of liquids to be held. The pit should slope away from the flare tip and the pit orientation should minimise wind blowing into the flare tip.

Remotely ignited pilot burners are essential for the protection of personnel due to the possibility of unburnt hydrocarbons remaining within the pit bund.

There is no means of controlling emission from a low pressure flare and as such their use should be limited.

Burn Pit Flare


LESSON 2

Flare Components

The basic components of an elevated flare system can be summarized as follows:

1) Flare tip

2) Air ingress seal

3) Stack riser and structure

4) Flashback protection

5) Knock-out drum

6) Ignition system

1. Flare Tip

There are a number of different designs of flare tip available:

Pipeflare tips

Steam flare tips

High pressure sonic flare tips

Air blown flare tips

Pipeflares are the most commonly used general purpose tips, but do not provide any degree of smokeless combustion unless the gas is predominantly methane and has a molecular weight less than 20. For smokeless combustion the simplest and most common type of tip, which uses steam as a smoke suppressant, is the generic 'Crown of Thorns' tip which injects steam through a number of nozzles located on a manifold positioned around the circumference of the tip.

Other types use the ejector principle to premix air into the steam through a manifold at the base of the tip. The pre-mixed phase then flows through a number of internal tubes within the tip, emerging to mix co-currently with the flare gas. This type of tip is more efficient than the 'Crown of Thorns', operates with lower noise characteristics and provides a greater extent of smokeless capacity.

Where steam is not available, air blown flares will provide a percentage of smokeless burning. The tip incorporates a series of flow vanes designed to maximise the mixing of flare gas and primary air provided by a blower / fan included as part of the flare system.


Where the relief gas is at high pressure (mainly available on offshore oil and gas production platforms) the driving force of the gas may be used to promote smokeless combustion at sonic velocities. For turndown conditions, consideration is given to the design of a variable slot tip, which will ensure smokeless combustion at relief rates from maximum to purge.

Standard Barrel Flare Tip


Section A-A

Standard Barrel Flare Tip


Steam InletDIA

Section A-A


Air Assisted Tip

Pinecone

Blades

Retention Ring


Multi Points Flare Tip

2. Air Ingress Seals and Purging

There is a danger of severe explosion in the flare system if the flare pilots are ignited before the flare system has been purged from the beginning of the system all the way to the flare. To assure low or zero oxygen levels, a volume of non-condensable gas equal to ten or more times the volume of the flare system is used. The flare system includes all piping from the relief valves to the stack and rising to the elevation of the flare at the burning point.

The pilots should be ignited only after the system has been purged and preferably while purge gas is still being admitted. If the purge gas is combustible, the burning of the purge gas at the flare will be proof of pilot ignition.

Flare systems are subject to potential flashback and internal explosion since flammable vapour / air mixtures may be formed in the stack or inlet piping by the entry of air. The pilot constitutes a continuous ignition source. Flares may be provided with flashback protection, which prevents a flame front from travelling back to the upstream piping and equipment, or may be positively purged with hydrocarbon or inert gas to ensure a non-flammable atmosphere within the stack. The most common cause of a stack explosion is where air has entered the plant and has passed through the flare header as an explosive mixture.

Gas purging is used to protect flare systems from explosions which would result from ignition of a hydrocarbon mixture with air which backflows into the stack. Most hydrocarbons are considered safe and nonflammable with 6% or less oxygen in the mixture. However, when large amounts of hydrogen are present, a lower oxygen level is required. To make allowances for the effect of hydrogen, the minimum oxygen concentration is a function of the molecular weight of the purge gas.

Any gas or mixture of gases which cannot reach dew point at any condition of ambient temperature normal to the jobsite can be used as a purge gas for flare systems. This gas may also be referred to as "sweep" gas.

Steam as a purge gas is not recommended for two reasons. The first is that the steam is at an elevated temperature and the steam content of the flare will shrink as the steam cools and condenses. The second is that as the steam condenses, water will be left in the flare system which presents a freezing hazard and by its “wetting” action encourages accelerated corrosion

The purge gas should enter the flare system immediately downstream of the relief valve so that the purge gas will "sweep" the entire system. If there is more than one header feeding into the flare each header must be purged.

It is recommended that there be a pressure switch immediately upstream of the orifice which regulates purge volume so that an alarm will sound if the purge gas pressure upstream of the limiting or regulating orifice falls below a set point. It is further recommended that the purge gas pass through a strainer in which the mesh openings are not more than one-quarter the diameter of the limiting orifice for purge gas regulation.


This is achieved by the use of a continuous minimum flow of gas designed to prevent air being drawn into the flare system via the flare tip, or otherwise. This is known as the purge gas flow. Without a special flare seal device fitted, the purge gas flow would need to have a velocity of between 0.3 to 0.6 m/sec in order to be effective

Recommend Minimum Flammable Gas Purge for Tall Flare Stacks

A purge gas volume which will create an upward velocity in the flare riser at 0.03 meter per second is normally recommended where the molecular seal is used for the flare. If a fluidic seal is used the purge gas velocity would be 0.012 meter per second.

Depending on the application and client preference elevated flare stacks maybe fitted with a molecular seal (also known as the labyrinth seal) or fluidic seal.


Molecular Seal (Labyrinth Seal)

The Molecular Seal works by relying on the density difference between the purge gas and air. When the purge gas is lighter than air it forms a gas rich zone at the top of the seal that air cannot penetrate, conversely when the purge gas is heavier than air the seal is formed at the base of the device.

In this way only a very low continuous purge flow is necessary to maintain conditions within the seal.

A unique advantage of the molecular seal is that it will maintain safe conditions in the upstream riser for several hours in the event of a loss of purge gas.


Gas Inlet

Gas Outlet

Gas Inlet

Gas Outlet

Fluidic Seal

The Fluidic Seal (ALS) is a frustro-conical device which is located as an integral part of the flare tip, welded within the main body of the tip just above the main flange.

With all flare tip operations, under low relief conditions, air will slowly diffuse down the inside walls of the tip. The Fluidic Seal design acts to locally increase the velocity of purge gas through the seal, thereby moving any air back out of the tip. The Air Lock Seal is of simple rugged construction and has no moving parts, requiring the absolute minimum of maintenance.

Comparison of Molecular and Fluidic Seals

1. The Molecular Seal prevents the ingress of air into the main flare system for a period of 2-4 hours (in the event of purge gas failure) due to the buoyancy effect discussed earlier. The Air Fluidic Seal has no hold-up capacity therefore if purge fails then the system is rapidly exposed to air ingress.

2. The Molecular Seal requires a purge rate of 0.003 m/sec. The Fluidic Seal requires a purge rate of approximately 0.012 m/sec (these are both based on flare tip exit area).

3. Whilst the Molecular Seal requires a lower rate, the decrease could result in the flame burning within the flare tip reducing life time.

3. The Air Lock Seal has the following advantages:

Simple, open free path to atmosphere

No plugging

Easy to install

Offers no wind loading to the support structure. The Molecular Seal is heavy and adds considerably to the overall system weight increasing structural loads and increasing costs of the riser.

No maintenance. If the Molecular Seal corrodes or is blocked, it has to be replaced requiring complete system shutdown.

No drainage or corrosion problems. The Molecular Seal has the potential to corrode at its base and within its drain line, especially with sour gas relief.

Very low capital and installation costs. The Molecular Seal is expensive due to its size and complicated fabrication of the internal baffle arrangements. An extra drain line is required to grade. A full 360° inspection platform is also required for access to the drain and hand holes at the base of the Molecular Seal.

Can be used in a horizontal position i.e. burn pits and angled flaring for offshore applications. The Molecular Seal can only be used vertically.


The Fluidic Seal is a simple low cost device with significant technical and commercial advantages over the Molecular Seal as described above. The use of Molecular Seals is quite uncommon now, as industry has recognised that they create more problems than they solve. Indeed the offshore oil production industry (North Sea - offshore UK/Norway/ Denmark) without exception uses Fluidic type seals instead of Molecular Seals due to structural and weight saving advantages of great significance in the design of offshore production facilities where weight and cost is at a premium.


Flare Tip

Fluidic Seal


Fluidic Seal Behaviour

Diode Pine-Cone

Located integrally at the base of the flare tip the Diode Pine -Cane avoids air entry inside the flare system. It is built with conical spoilers in order to create a gas flow ring by presenting a smaller cross-sectional area to the rising gases, thereby reducing the volume of gas needed to maintain the fixed purge velocity. A continuous flow of purge gas causes air flow reversal. The gas seal is positioned at a number of nominal diameters from the stack exit.

When a flare is filled with a gas which is normally lighter than air, there is a natural tendency for such gas to decant, being replaced by air, consequently a flammable mixture will result. A flow of gas avoids the decanting action and prevents air from penetrating deeply into the sack. The depth of air penetration is a fraction of the gas velocity. For this reason, the device is effective only using the adequate purge gas flow rate (N2, CO2 or other oxygen free gases). This flow will create a minimum gas velocity through the smaller spoiler.


3. Stack Riser and Structure

For most elevated flare systems, the greatest cost item is the support structure.

Several criteria need to be considered in order to determine the support mechanism :

Flare relief rates and duration

Thermal reduction

Smoke emissions and pollutants noise

Location of other plant and proximity to the flare personnel access regulations

Structures

Guyed

Free standing derrick

Guyed derrick

Flare tip removal equipment

Guyed - this type of structure is usually the least expensive to build but in some cases the guy wires result in restrictions on the use of adjacent land in addition to normal spacing restrictions.

Derrick - this type of structure is well suited for tall structures subject to strong winds or where large thermal ranges are expected. The structure can be designed such that the flare stack may be demounted for maintenance purposes, removing the requirement for plant shutdown if the flares are arranged as duty/standby.

The height of this type can be 200 m.

Self-Supporting - this type of structure is designed so that the flare riser pipe has no lateral structural support. For short flares this is the least expensive system to erect and maintain.

This system is applied for flares with a height less than 50 meters. The self-supporting flare is economical and easy to erect, and requires relatively less installation space.

The most common is the guyed stack, which is generally the lowest cost option.Heights of up to approximately 150m have been successfully employed, Although these are few, most refinery stacks being in the 60-100m range. A limitation for guyed stacks is the range of process temperature encountered when in service. This variation in temperature will cause the stack to expand and contract with resultant stretching or loosening of the guy wires. A service range of 200 to 300°C is usually limiting in this case.


In the event of an excessive temperature variation, a guyed derrick can be used or even a free-standing derrick structure.

A structure offering great operational flexibility is the jack-up derrick. This allows flares and risers to be dismounted for replacement and / or repair while a second flare system remains on-line. No plant downtime is necessary. This is a system much favoured by certain operators.


Flare Tip removal Equipment ( Retractable Davit


Horizontal Seal Drum

Try Cock for Checking Hydrocarbon

4. Flashback protection

Water seals are used to provide a positive seal against air ingress and flashback and also to maintain the upstream header at a positive pressure.

Water seal drums can either be horizontally or vertically mounted and must be correctly sized to prevent water carryover through the flare stack under normal operating conditions. Under emergency conditions it must be expected that the water will be carried away by the high flare gas velocities. Fast water makeup is therefore important to maintain the seal integrity.

A common problem with water seals is one of pulsation caused by water moving from side to side, causing the gas flow to vary periodically with time (the period is generally about 1 second). This causes the flare flame to rise and fall and also the flare noise to fluctuate.

The Water Seal vessel is fitted with a special saw tooth dip leg and anti-pulsation baffle to minimise pulsing. The water level is preferably maintained by a constant overflow weir, in combination with a suitable 'S' bend drainpipe. Filling rates will be sufficient to reestablish the seal within 5 minutes if the seal is broken.

The seal vessel may be equipped with an internal steam coil I sparger for winterisation purposes as required.


Vertical Seal Dram ( Palseless Type )


5. Knock-Out Drums

Knockout drums are used to prevent hazards associated with flaring gas containing liquid droplets. Which called carryover (flaming rain ) the drum must be large enough to effect the desired liquid-vapor separation, and have a holding capacity to contain any anticipated slug of liquid. Knock-out drums are designed to remove liquid droplets of excessive size from the gas stream and to return the collected liquid to the process/drain.

Most flares can handle a liquid mist up to the point where the oil droplet settles to grade faster than it is consumed by the fire surrounding it. Generally, this is considered to represent 600 micron particles. In truth, different flare tips can handle different liquid rates. The kinetic energy flare tips, because they take a significant pressure drop at the tip, can handle higher liquid loads than an open pipe flare. "Flaming rain" is a real design case for flare with liquid potential.

Sizing to API RP-521 recommendations is generally adequate but the knockout drum should be sited as close as practically possible to the flare stack and should not possess any internals liable to blockage.

Horizontal K.O. Drum



Vertical K.O. Drum


Integrated Vertical Water Seal & K.O. Drum

6. Flare Pilot and Ignition Systems

One of the main considerations for flare ignition is reliability of operation. An ignition system must be capable of fast performance and repeatability of use over and over again, under all environmental and operating conditions.

Ignition Panels

A complete range of ignition panels is available, designed for manual or automatic operation or a combination of both.

These systems will ignite the flare tip pilots from remote locations either through:

1) conventional Flame Front Ignition techniquesor

2) High Energy ignition.


Ignition Panel

Pilots

The number and position of the pilots depends on the flare type and diameter.

The pilot ignitor nozzles have been developed over many years of operational experience and offer maximum reliability of ignition and stability in winds in excess of 120mph (200 km/hr). The pilot ignitor nozzle and venturi mixing assembly is fabricated from alloy steels to ensure a long service life. For cases where pilot fuel gas has a high sour content, specialised alloys are used.


Pilot with Flare Front Ignition


Pilot with high Energy Ignitor

Flame Front Generator

Almost without exception, flare pilot ignition is performed by using a flame front generation system.

This method involves filling a small bore pipe, which runs from the flame front generator panel to the flare tip, with a combustible gas / air mixture. The mixture is ignited by a spark in an ignition chamber on the panel, generating a flame front which travels to the pilot and lights it at the tip.

This technique is well known, and established throughout the industry. However its performance is affected by a number of factors which combine to present problems in the field making it unreliable i.e.

(i) Flame front lines always collect large quantities of water, which require draining before ignition

(ii) Changes in fuel gas compositions and the use of wet air conspire to defeat operators

(iii) Long term pipe corrosion and lack of maintenance reduce the probability of a good ignition.

The major safety problem with the ignition system is the use of a high hydrogen content fuel for air-gas ignition.

Flare pilots can be serviced through either individual flame front lines or via a splitter manifold located on the flare tip.

Fuel gas and instrument air are supplied to .the ignition chamber in the correct quantities via an on / off valve, needle valve and restriction orifice. The mixture is then ignited using an electric spark.

The resulting flame front will travel down ignition line(s) to light each pilot either separately or through a splitter manifold. This flame front may be transmitted for distances of up to 1,000 meters along standard small bore pipe work.


The panel will continuously monitor the pilot burner flames via the installed thermocouples and should a failure be detected a visual alarm will be raised in the FFG and at the same time an alarm will be activated in the control room via remote contacts.

The Flame Front Generator (FFG) is of free standing easel type construction fabricated from carbon steel. The framework will be open to atmosphere onto which are mounted the instrument and electrical enclosures certified for the specified area classification and weatherproof to IP65 (minimum). The panel will provide the functions of pilot ignition and monitoring of pilot status via thermocouples located in the pilot nozzle heads. The End boxes will be housed in steel enclosures that will be fitted with a heater (if required) to maintain the internal temperature at acceptable levels.

The FFG is offered as a standard proprietary item of equipment and can be supplied for either manual or automatic operation or a combination of both.

Pilot fuel gas and purge supply can be accommodated as a modification to the system if required.

Natural Draft Flame Front Generator

In situations where compressed air is not available, the Natural Draft Flame Front Generator can be used.

The principle of the Natural Draft FFG is straightforward. Fuel gas at moderate pressure is ejected through a small drilling forming the jet of a venturi inspirator. The action of the gas jet passing through the throat of the venturi causes a local drop in static pressure, which causes air to be drawn into the venturi intakes and mixed with the gas. The resulting gas/air mixture passes through an ignition chamber via a length of 2" / 3" piping to the flame front connection of the flare pilot.


In this way a continuous length of piping is filled with a flammable mixture which when sparked in the ignition chamber will ignite and send a flame front through the 2"/3" line to light the pilot.

This is similar to a conventional FFG, which uses compressed air in lieu of an inspirator to achieve the same result.

The other main advantage that the Natural Draft FFG has over the compressed air type is in its ease of use and its wide tolerance of set pressures.

The Natural Draft FFG is normally set up to operate at a certain fuel gas pressure e.g 25 psig. Experience has shown that typically the unit will still function correctly over about a 16 psi range therefore providing you set the gas pressure within the range 17-33 psig the system will work reliably In addition, it is extremely repeatable, when set up in the above manner it will work first time every time. This is certainly not true of the compressed air type where air and gas pressure are critical to within a few psi and repeatability is difficult to achieve.

The design of Natural Draft systems can be up to 170m pipe run incorporating bends, fittings and splitter manifolds.

The Natural Draft FFG is of freestanding easel type construction, fabricated from carbon steel. The framework will be open to atmosphere onto which is mounted the instrument and electrical enclosures certified for the specified area classification and weatherproof to IP65 (minimum). The End boxes will be housed in steel enclosure(s) that will be fitted with a heater (if required) to maintain the internal temperature to acceptable levels.

Flare pilots can be serviced either through individual flame front lines or via a splitter manifold located on the flare tip.

The Natural Draft FFG is offered as a standard proprietary item of equipment and can be supplied for either manual or automatic operations, or a combination of both.

Pilot fuel gas and purge supply can be accommodated as a modification to the system if required.

Electric Ignition Pilot

This pilot is a direct electric ignition flare pilot that eliminates the need for conventional flame front generation systems.

Using this system the pilot flame is directly lit by a High Energy spark generated adjacent to the pilot nozzle.

The term "High Energy" is used to denote ignition equipment which feature sparks formed by the rapid discharge of large capacitors at relatively low voltage across the semi-conducting layer of a surface discharge spark plug.


The spark produced is so powerful that no accumulation of moisture, dirt or oil can prevent ignition occurring. This makes the High Energy system particularly suited to flare pilots where exposure to contamination is always likely.

The spark plug forms the upper part of an ignitor rod, which extends from the pilot nozzle to a point near to the flare tip base flange. At this point a connection is made with an ultra high temperature cable (rated at 600°C) which is run down the flare stack to a point where the thermal radiation has reduced to an acceptable level. This distance is typically 10m. At this point a shielded junction box is used to connect with a suitable multi-core cable which is then used to run down the flare stack and to the control panel.

Within the control panel is mounted an advanced Thyristor Switched High Energy Pulse Ignitor unit designed to provide a rapid series of powerful sparks at the ignitor head. A key advantage of this technology is that the interconnecting cable can be virtually any length enabling the control panel to be located outside the flare sterile area at any convenient location.

The voltage used for the spark is limited to 2.5 kV. This is substantially less than high tension ignition systems and is markedly less liable to tracking / shorting out.

The ignitor unit can be powered from any available mains AC supply or even from low voltage DC supplies. Either standard or explosion proof versions are available.

Pilot flame monitoring is achieved using thermocouples mounted in the pilot nozzles. The thermocouple is run within small bore conduit and is therefore supported over its entire length. This simple technique has greatly extended thermocouple service life by preventing failures due to mechanical fatigue caused by vortex shedding in windy conditions.

The thermocouple signals are run back to the control panel where temperature switches are used to determine the pilot status. This is displayed via red and green lamps on the panel front and volt free contacts are provided for client use.

Using this system it is very straight forward to incorporate automatic re-ignition upon detection of a pilot flame-out.


7. Wind Deflector

WIND DEFLECTOR (Patented)

Deflection of flare burner flames by wind effects frequently results in serious problems during the operation of a flare. Damages on the accessories of the flare unit, such as utilities piping. cables, ladders, etc. can become so heavy that on interruption of the flare operation may become necessary.

This problem can be solved by the installation of the patented wind deflector as on auxiliary element at the flare tip: In a cylindrically bent grid, flat plates of certain sizes and at certain spaces are provided.

On the deflector side. which is exposed to the wind, an air compression, immediately upstream of the plates, will take place due to the energy of the wind stream. After the air stream has passed the free vertical spaces between the plates. a through vortex is formed by the sudden increase of the cross section so that the wind energy is completely absorbed.

This, no air pressure and accumoulotion will build up on the flare stock side exposed to the wind. and no vacuum con form on the lee-side.

In view of the above. a down-deflection of the flame is safety prevented.


flare systems

Documents

flare tip

flare types

flare stack

flare line

typical refinery flare

smoke pollution

optional steam system

fuel gas system