By the end of the Article, You will be able to understand:
- Purpose of flare systems
- How flare systems operate
- Different types of flare
- Safety when operating flare systems
- Source of Flare gas
- Flare System Components
Introduction
Flares are essentially special burners designed to safely dispose of large volumes of flammable vapours. The vapours are normally generated intermittently as a result of abnormal process conditions occurring during plant start-up, shutdown or emergencies.
Flares are mandatory items of safety equipment installed at crude oil and gas producing fields, natural gas plants, oil refineries, petrochemical plants and indeed at any plant capable of producing flammable vapours which cannot be stored during emergencies.
The combustion efficiency of earlier designed flares was normally very poor due to inadequate air inspiration. These flares were characterised by excessive smoke emission, long red unstable flames and noisy operation.
On the other hand, modern design of flares is characterised by clean and stable flame patterns. These characteristics are achieved by using multiple burner assemblies, to give improved air mixing in the combustion zones and by the injection of smokeless fuel gases such as methane and ethane to special smoke suppression burners.
Greater attention has also been paid to the siting of flare installations in modern processing plants. Proper siting ensures that flares are remote from the process units and have a minimum disturbing impact in terms of noise and light intensity on neighbouring residential and amenity areas.
Because of the vital and very special role flares play in a processing plant it is essential that all plant personnel should have a good understanding of the equipment in a flare installation and how a flare operates in emergency situations.
Purpose of Flares
The purpose of flares is to provide a method whereby unwanted gases and volatile liquids are disposed of or burnt in a safe manner and at a safe location.
During normal operations, only small quantities of unwanted gases and liquids are normally produced. Occasionally, very large quantities have to be disposed of quickly and safely when process equipment is depressurized.
Flaring of vented gases is necessary in order to avoid the following two problems if a gaseous emission is not burnt:-
- The possibility of hydrocarbon vapours reaching the ground in sufficient concentrations to cause a fire or explosion hazard with the possible devastating consequences for plant personnel, the plant itself and surrounding areas.
- The possibility of toxic vapours reaching the ground in sufficient concentration to cause potential health hazards.
Thus, unflared gaseous emissions consisting of hydrocarbons and toxic components create environmental pollution problems.
Flaring provides a ready solution to these two problems, but creates its own problems of heat, smoke, light and noise. Therefore, any flare design must necessarily consider the inherent problems associated with burning of gases.
Design consideration for Flares
The initial consideration for any flare design must be the vapour sources which obviously will determine the ultimate flare system required.
Vapours are typically released automatically from process equipment by various forms of pressure protection devices such as relief valves and rupture discs.
Design consideration for Flares Other design considerations must include the following: –
- Intensity of heat radiated from a flare which constitutes a potential danger to personnel and equipment in close proximity.
- Hydrogen sulphide (H S) if present in the vapours forms sulphur dioxide (SO ) and as such can cause damage to vegetation, if present in sufficiently high concentrations at ground level.
- Hydrocarbon vapours issuing from a plain open-ended tip tend to burn with a very unstable flame, accompanied by large volumes of smoke, mainly due to inefficient combustion air distribution.
- Continuous source of ignition is required.
- Vapour flows can vary from zero to very high rates and vice versa in a short space of time.
- Vapour compositions can vary due to emanations from different sections of a plant.
- Blowdown header system from which the vapour emanates is basically a hydrocarbon vapour pipeline open to the atmosphere at the flare.
- Expansion of high pressure hydrocarbon vapours through pressure relief devices results in cooling and partial liquefaction of vapours in the blowdown header system. Liquid carry -over into elevated flares must be eliminated by a knockout device.
- This particular installation is for an elevated flare used for the disposal of hydrocarbon vapours.
- The vapour source is shown to originate from a multiple number of sections within a plant and discharging into a blowdown header. Intermediate equipment between the blowdown header and the flare is for the purpose of liquid knockout and a liquid seal.
The followings are the areas which needs to be considered for designing a flare system:-
– Heat radiated by flares
– Problems of combustion
– Explosion hazards
– Liquid carry-over
Heat Radiated by Flares
The location and height of a flare stack has to be carefully selected in order to protect plant personnel, critical process equipment and vegetation from the dangers of exposure to the thermal radiation from the flame.
For these reasons and other environmental considerations, flare stacks are usually located downwind of residential areas and remote from process units or plant equipment. Wherever possible, flare stacks should be approximately 90 metres (300 feet) from critical process equipment.
The plot plan also shows a dotted circle round the flares stacks within which no process equipment is installed or access allowed by non-authorized personnel.
The radius of this circle is called the flare sterile radius or safety radius. Its significance is that at any point within the circle, a person could be suddenly exposed to the intense heat liberated from a large flame at the flare tip.
Design consideration for Flares
It is a sterile radius diagram for a flare stack in a derrick structure. Normally, the sterile area is flat and circular with a low bund wall at the perimeter.
A sterile radius is generally determined by considering the flare height and heat liberation capacity. By taking into account the maximum flame length, the height of the flare stack and the prevailing wind it is possible to draw a radius around the flare, outside of which one can work safely for an unlimited time. Conversely, working inside a sterile radius must be restricted.
The flare auxiliary equipment including knockout drums, seal pots and flame front generator panels must be sited outside the sterile radius.
Problems of Combustion
Flare vapours generated in natural gas plants will burn naturally with a lazy red smoky flame, except in the case of methane rich vapour which burns naturally with a blue, relatively, colourless flame.
The main cause of smoke emission or smoking is insufficient mixing of air in the combustion zone at the flare tip.
The modern method of achieving smokeless flaring is by installing a ring of special burners round the top edge of the flare tip and burning a low molecular mass gas at a controlled rate. The burner configuration is arranged so that additional air is drawn into the larger combustion zone and the overall effect is to reduce smoke generation.
Explosion Hazards
Explosions can occur within the flare stack or the blowdown system, if an explosive mixture is formed by the ingress of air and the mixture is subsequently ignited at the flare tip.
Air can enter a flare system in several ways:-
- By venting equipment, containing air, into the flare system during plant start-up operations or after maintenance periods.
- By allowing air to enter the system through vents or drain points which have not been isolated.
There are, in general, two ways of preventing air ingress into a flare system during non-flaring periods: –
- Purge continuously with a hydrocarbon gas into the upstream end of the blowdown system to ensure that the quantity of air which may be drawn in during periods of no flaring is not sufficient to create an explosive mixture. Since it requires approximately 13 volumes of air to 1 volume of light hydrocarbon vapour to produce an explosive mixture, a relatively small flow of purge gas is sufficient.
- Install seals in the flare stack and immediately upstream of the flare stack. Two types of seals are commonly used. One is a gas seal called a molecular seal. The other is a liquid seal containing a mixture of glycol and water which has a relatively low freezing point.
The gas molecular seal is installed in the flare stack near the flare tip and the liquid seal is installed immediately upstream of the flare stack.
Liquid Carry-over
The fourth design consideration is that of liquid carry-over into a flare. From a vapour flow sequence this consideration precedes any other and is critical since flares are designed for vapour disposal, not liquids.
Hydrocarbon liquids can enter a flare system either as slugs of liquid or as droplets entrained in the blowdown vapours, in the following ways:-
- Release of liquid and vapour from process vessels with a defective level control.
- Formation of liquid in the blowdown system by excessive expansion and cooling of hydrocarbon vapours vented at very high rates from low temperature process units.
- Operation of the flare knockout drum with a high liquid level or a defective level control.
- Build-up of liquid in the knockout drum due to failure of the liquid disposal equipment.
- Build-up of hydrocarbon liquid in the liquid seal drum.
If hydrocarbon liquids enter a flare stack either as slugs or as droplets they will be ignited at the tip and then will shower over the surrounding area as “burning rain”. Although there is no process equipment in the sterile area, there is a very real danger of burning liquid damaging and weakening the support structure near ground level.
The most practical way to prevent carry-over of liquids to the flare is to install a properly sized knockout drum as near as possible to the sterile area.
It is equally important to provide a reliable means of detecting liquid level and removing accumulated liquids from the knockout drum.
The drain line from the knockout drum to the liquid disposal device is susceptible to blockage either by ice or other solids. No water or steam should ever be allowed to enter a flare blowdown system in which cold vapours and liquids are present. Electrical tracing is normally provided at the bottom of the knockout drum and along the drain line to prevent accumulation of frozen liquids.
Types of Flares
The requirements for flaring in the process industry has led to the development of three general types of flares in order to ensure high operating performance and efficiency, namely:-
- Non-smokeless Flares
Non-smokeless flares are generally used for flaring hydrocarbon or vapour streams which burn naturally with a smokeless flame such as methane, hydrogen, carbon monoxide and ammonia.
- Smokeless Flares
Smokeless flares are used for the clean disposal of hydrocarbon streams, which normally burn with a dense smoke emission. Most hydrocarbons fall into this category. Flares are made smokeless by providing an adequately distributed quantity of air into the combustion zone. Sufficient air is induced into the combustion zone typically by using methane or ethane gas as an inspirator.
The key factors affecting the smokeless quality of flares are:-
– Quantity and distribution of air in the combustion zone.
– Temperature in the combustion zone.
– Type of hydrocarbon being burned
- Fired or Endothermic Flares
Endothermic flares or incinerators are used for clean disposal of waste liquids. Most liquid streams which are released from a process can be recycled for re-processing. However, certain waste streams are of no commercial value and in fact may be pollutants. Such waste streams are sometimes incinerated in endothermic flares.
Flare System
A flare system should provide an integrated combination of equipment which will receive volatile liquids and gases that are disposed of or burnt by smokeless means.
The special considerations for the flare equipment sizing, location and operation will normally have recognised the problems of combustion, heat radiation, explosion hazards and liquid carryover.
The flare system receives both high and low pressure vapours from different plant sources by means of a high pressure blowdown header and a low pressure blowdown header. The vapours are discharged through a knockout drum to avoid liquid carryover and a liquid seal to avoid associated explosion hazards by ingress of air during non-flaring periods.
Components Parts & their functions
The flare system will now be considered in terms of the component parts and how the various parts operate or function as an integrated part of the flare system.
A flare system has the following component parts:-
- Liquid knockout drum 2. Flare liquid seal
- Flare stack structure
- Flare stack molecular seal 5. Flare tip
- Ground flare combustion cabin 7. Ground flare multi-burner
- Ignition system
Flare Knockout Drum
This listing excludes the various valve and piping arrangements which necessarily comprise the blowdown and flare header assemblies. Since the knockout drum operates as a conventional gas/liquid vertical separator, we are not going to discuss in detail its operation.
The critical function of assure that no liquid is either of the flares.
the liquid knockout drum to entrained and carried over to
Flare Liquid Seal
The first component to consider which is a specific application to reduce the hazard of explosions is the liquid seal.
The liquid seal is simply a vertical vessel located upstream of the flare header which receives low pressure vapours and functions as a minimal liquid block between the blowdown header and the flare stack.
The vapours enter the seal drum through a vertical dip pipe which is normally submerged in the seal liquid. The dip pipe is enclosed in a stilling tube to prevent excessive turbulence and foaming in the seal drum during flaring.
When flaring commences the flare vapours bubble through the seal liquid and out of the drum. However, at high flaring rates there is no positive liquid seal in the drum, since the flare vapour displaces all of the liquid in the stilling tube. The relatively high pressure of the seal drum compared to atmosphere during flaring periods eliminates any concern of air ingress other than via the vapour source. When the flaring stops the dip pipe is automatically refilled and prevents the movement of air from the flare stack back into the blowdown system.
Flare Stack Structure
Flare stacks may be erected and supported in one of the following ways:-
– Self support
– Guy support
– Derrick support
The derrick support structure is a totally separate structure from the flare stack lending to simplicity of design and erection.
The other two types of structures, self supporting and guyed, require a more complex design construction. From a long term operational point of view these two structures have the obvious disadvantages of accessibility for future maintenance requirements and/or modifications.
Flare Stack Molecular Seal
A second means of sealing the flare stack against ingress of air is by the use of a gas molecular seal mounted near the top of a flare stack.
The sealing fluid is a hydrocarbon gas with a molecular mass less than that of air, hence the name molecular seal. In natural gas plants, a methane rich gas with a molecular mass of approximately 19 is used as the sealing fluid (molecular mass of air is 29).
The seal gas is continuously injected as a purge into the flare stack below the seal section. Whilst flaring is taking place there is no seal because the high velocity flare vapours drive the seal gas out of the seal cap and flare tip.
When vapour flow stops, the seal gas fills the stack up to point T and then overflows to fill the seal cap before flowing up to the flare tip. At the same time atmospheric air, of higher molecular mass, slowly diffuses downwards from the flare tip. The heavier air displaces the light seal gas until it reaches point BS on the outside of the seal cap.
Thus, a gas molecular seal is achieved since the air will not displace the lighter seal gas in the annular space which forms a molecular seal cap.
Flare Tip
The flare tip as shown in Figure takes the form of three to five metres of pipe at the upper end of the stack. The flare tip is normally flange mounted to the flare stack for easy maintenance and renewal purposes. The tip is made of special high-temperature steel alloy to withstand the flame temperatures.
A smokeless flare tip design as shown in Figure eliminates flame lift-off at high exit velocities ensuring flame stability and efficient combustion, even in high winds.
Flame stability is ensured by utilising flame stabiliser tabs on the end of the stack tip and by injecting a stream of high pressure flare vapour to an annular flame retention group of burners integral with the flare tip.
The ring of auxiliary flames encircles and continuously ignites the main flare vapour streams preventing flame lift-off.
In addition to the flame retention burners a smokeless flare tip utilises a ring of special smokeless burners also mounted round the top edge of the flare tip.
Four groups of smokeless fuel burners are located symmetrically round the main flare tip. Four pilot burners are installed with one between each group of smokeless burners.
Thus, the arrangement of smokeless burners aids in additional air being drawn into the combustion zone with the overall effect of reducing smoke generation.
Ignition System
The most common remote ignition system is the flame front generator type. The flame front generator is normally used for both ground flares and flare stacks.
The flame front generator utilises a small bore line from the ignition console to the tip of the pilot burner. Fuel gas and air are mixed, in a combustible ratio, at the ignition console and allowed to fill the small bore line. When the line is full of the correct gas/air mixture, it is ignited at the ignition console by means of a spark plug. The flame front thus generated rapidly travels through the line to the top of the flare stack where it ignites the pilot burner.
As shown in the illustration this type of ignition system provides a controlled supply of fuel gas to the pilot burner where an air venturi is used to assure a combustible mixture for ignition.
Surveillance of the pilot burners is by use of thermocouples located in the flame zone of each burner. The thermocouple is connected to a panel mounted alarm to give warning of the flame failure of a pilot burner.
General Operation
The use or operation of a flare system is normally very infrequent. Therefore, the first operating consideration is to maintain a flare system in a ready state to receive combustible vapours.
A typical check list for inspecting a flare system in a ready to operate state would include: –
- Assuring operability of relieving system into blowdown header or system. 2. Checking blowdown header for closed drains and liquid.
- Inspecting knockout drum for ability to discharge any collected liquids. 4. Inspecting liquid seal drum for proper operation and liquid seal level. 5. Confirm continuous supply of fuel gas for pilot and smokeless burners. 6. Confirm operability of flame front generator ignition system.
- Observe that all pilot burners are ignited.
- Enforce personnel restrictions as regards safe distances and proximities of flares.
When a plant is shutting down in emergency conditions and relief valves are relieving at high flow rates, the hydrocarbon vapour can reach low temperatures, in the order of -50°C. The following effects on the operation of a flare system should be considered:-
- Flare Feed Line Contraction
Considerable axial contraction occurs and can result in the line supports moving out of the guide shoes. Expansion bends or bellows units are normally installed to avoid pipeline damage when sudden contraction occurs followed by line movement.
After a low temperature flare operation, the operator should walk the blowdown and flare inlet lines to ensure that the line supports have returned to their normal positions on the support plates.
- Ice Formation on Exterior of Flare Stack
Thick layers of ice can be formed if the flare operates under cold vapour conditions for a prolonged period. When the feed vapour temperature returns to normal, the sudden detachment of large ice fragments presents a potential hazard to personnel.
- Hydrocarbon Liquid Entrainment
Liquid level control of flare knockout drums generally becomes difficult under cold conditions and high vapour loads. If liquid level becomes too high, entrainment of liquid will occur and the subsequent release of “burning rain” from the flare tip can damage the support structure members near ground level. For this reason, the lines from the knockout drums to the flare stack are usually sloped back towards the drums, preventing liquid accumulation in the lines.
Flares are sometimes provided with a water drenching systems to protect the lower parts of the derrick from flare fires caused by liquid entrainment.
- Flare Stack Support Protection
Tall flare stacks are normally supported by a section of skirt which is bolted to a concrete foundation block.
The concrete can become damaged by frequent exposure to very cold operating conditions and this is generally prevented by adequate ventilation of the skirt section to maintain the metal at or near ambient temperature.
- Personnel Access to Flare Stack
It must be emphasized that access to the flare stack is absolutely forbidden except when the flare is out of commission and positively isolated from the flare header.
