Hydrocarbon Process Safety – Fire Hazards, Risks and Controls

This article covers the following learning outcome: outline the fire hazards, risks and controls in Oil and Gas Industry.

Lightning

A lightning strike is a massive discharge of electricity from the atmosphere, where the electrical charge has built up, to the earth.
The threat from lightning cannot be entirely eliminated, particularly with floating roof tanks where vapour is usually present around the rim seal. In these circumstances, measures to mitigate the consequences of a fire should be provided, including automatic rim seal fire extinguisher systems.

Threats from a lightning strike include:

• Sparks which can cause a fire or explosion;
• Power surges to electrical equipment, particularly monitoring and safety devices which can render them inoperable.

Protection from lightning strikes is a specialist area requiring expert knowledge as to what systems are suitable for each facility.

However, in general they include the following:

• A “dissipation array system” which reduces the potential between the site and any storm cloud cell that might be in the vicinity;
• A grounding system called a “current collector”. This provides an electrically isolated area within which the facility will be located. This is normally made up of wire buried to a depth of about 25 centimetres and which surrounds the protected area. This wire is also connected to rods which are driven into the earth at about 10-metre intervals. Finally, the enclosed area is integrated by a net of cross-conductors which are also connected to any structures within the area, as well as the grounding system itself. This allows any current to discharge to earth safely;
• Electrical surge suppression devices. These devices have two distinct functions to perform. First, to stop direct strikes within the facility, and second, to prevent fast-rising, high current surges.

In general, the necessary precautions are:

• To keep the lightning channelled far away from the immediate neighbourhood of flammable and explosive materials;
• To avoid sparking or flashover in joints and clamps, and at nearby components;
• To prevent the overheating of conductors;
• To prevent flashover or sparking due to induced voltages;
• To prevent raising the potential of the earth termination system;
• All metal containers to be of sufficient thickness (usually 5 mm minimum);
• Down-conductors fitted to all other metal structures and in sufficient numbers as to subdivide any current surge adequately;
• All earthing systems to be interconnected to a single earth termination system. This usually takes the form of a mesh or grid pattern around the site.

Fire triangle and potential consequences of explosions and thermal radiation

Fire can be defined as ‘the rapid oxidization of a material or substance’. This is known as combustion, which
releases light, heat and various reaction products such as smoke and gas.

Fire is made up of three interdependent elements known as the fire triangle. These are:

• Heat or a source of ignition;
• Fuel;
• Oxygen.

This is known as the fire triangle.

The fire triangle

The fire triangle is a way of understanding the way in which these elements, which are necessary for most fires to burn, interrelate. The triangle shows that for burning to start (and to continue to do so) requires three elements: heat, fuel and an oxidizing agent (this is usually oxygen – but not always).

A fire naturally occurs when these elements are brought together in the right proportions.

A fire can be prevented or extinguished by removing any one of these elements.

Similarly, fires can be prevented by the isolation of any one of these elements, particularly fuel and ignition.

It is important to remember that it is only the vapour from a fuel that actually burns. Before combustion takes place, a solid or liquid must be heated to a point at which vapour is given off and that vapour can ignite.

Explosions

An explosion is a type of fire but one which combusts with such a rapid force that it causes an effect known as over-pressure (explosion). Under certain conditions, the speed of the front of the flame may move to a supersonic level, resulting in a significantly more powerful explosion.

There are three types of explosion that are associated with the oil and gas industry. These are:

1. Boiling Liquid Expanding Vapour Explosion (BLEVE);
2. Confined Vapour Cloud Explosion (CVCE);
3. Unconfined Vapour Cloud Explosion (UVCE).

We touched on these types of explosion in Hazards, Risks and Controls available for Safe Containment of Hydrocarbons this article but, because of their significance in the oil and gas industry, it’s worthwhile reminding ourselves of what these types of explosion are.

Boiling Liquid Expanding Vapour Explosion (BLEVE)

We will now look at a scenario whereby a Liquefied Petroleum Gas (LPG) storage vessel is exposed to an external source of heat – possibly a fire. The LPG is kept in a liquid state inside the vessel because it is held at a temperature below its boiling point, which means it takes up substantially less space than in its gaseous state. Anything which changes that state from a liquid to a gas, such as the external source of heat (fire), will increase the pressure inside the storage vessel to a potentially unsustainable level.

At first the vapour will be vented out via the pressure relief valve on the top of the vessel. However, the rate of increase in pressure under these circumstances is likely to be unsustainable and the vessel is likely to eventually fail, with a consequential loss of containment. The resulting instantaneous release of LPG vapour will likely make contact with a source of ignition, resulting in a Boiling Liquid Expanding Vapour Explosion (BLEVE).

Should a situation occur whereby a source of heat (fire or otherwise) begins to radiate itself onto an LPG storage vessel, the following action should be taken.

Apart from removing or extinguishing the source of heat, the storage vessel, and any other storage vessels nearby, should be deluged with copious amounts of water to keep the metal cool.

Confined Vapour Cloud Explosion (CVCE)

A confined vapour cloud explosion is an explosion following a leak of vapour which occurs in a confined space, such as a building or a tank.

Unconfined Vapour Cloud Explosion (UVCE)

An unconfined vapour cloud explosion is an explosion following a leak of vapour which occurs in an unconfined space, outdoors.

Thermal radiation

Thermal radiation is the transfer of heat from one source to another. This can be a structure or a person. Where the recipient source is a person, the consequences can be severe.

The initial effect of exposure to a source of heat (fire) is to warm the skin. This then becomes painful as the amount of energy absorbed increases. Thereafter, second-degree burns begin to take effect, with the depth of burn increasing with time for a steady level of radiation. Ultimately, the full thickness of the skin will burn and the underlying flesh will start to be damaged, resulting in third-degree burns.

When plant, including pipework and vessels, is exposed to thermal radiation the effect is the transfer of heat to the product inside the plant. This can change the characteristic of the product and make it less stable. These characteristics include the potential to make the product expand and/or increase the amount of vapour given off, amongst other things. This can result in loss of containment, with an ensuing vapour cloud explosion, jet fire, pool fire or running liquid fire.

Electrostatic charges

Whenever a liquid moves against a solid object, such as the inside of a pipe, it generates a static electrical charge. This is caused by ions (charged atoms) being transferred from the liquid to the surface of the pipe or vessel.

The most common cause of static electricity build-up is where there is a flow (transfer) or movement (mixing process) of liquid within a process.

The amount and rate of static generation can be dictated by a number of factors. These factors, or their elimination or reduction, can also be used to control the risks associated with static electrical generation.

These include:

• The conductivity of the liquid;
• The amount of turbulence in the liquid;
• The amount of surface area contacts between the liquid and other surfaces;
• The velocity of the liquid;
• The presence of impurities in the liquid;
• The atmospheric conditions. Static build-up is enhanced when the air is dry.

Let’s look at some typical areas within a process where static electricity is most likely to occur, as well as some simple control measures.

Electrostatic charges – piping systems

As we’ve mentioned, the flow of liquid through piping systems can generate a static charge. However, there are factors which can influence the amount of charge generated. These include the rate of flow and the velocity of the liquid.

Control measures include keeping the rate and velocity of the liquid low. This can be achieved by ensuring pipe dimensions are appropriate for the volume of liquid flowing through them; and also ensuring the length of pipe is as short as possible.

Electrostatic charges – filling operations

Filling operations, which involve large flows of liquid and splashing, generate turbulence. This turbulence allows the large amounts of liquid to pass against the vessel surfaces which in turn generates a static charge. If the liquid has already passed through piping to get to the filling operation, this will only serve to increase the accumulated charge already generated.

Control measures include:

• Ensuring filling operations do not involve the free-fall of liquids. This will reduce the amount of splashing taking place;
• Lowering the velocity of the liquid being filled;
• Ensure fill pipes touch the bottom of the container being filled;
• Tanks which have been filled with products that have a low conductivity, i. e. jet fuels and diesels, should be given time to relax before the process continues;
• Tanks which have been filled with product should not have any ullage (vapour space) for a set period of time. Nor should any dipping of the product take place, again for a set period of time.

Electrostatic charges – filtration

By their very nature, filters have large surface areas, and this can generate as much as 200 times the amount of electrostatic charge in a piping system that has a filtration system within it, as compared with the same piping system without filtration.

Control measures include ensuring good bonding and grounding is in place (see below).

Electrostatic charges – other issues

• Liquids which have particles within them are more susceptible to the generation of static charge than those without;
• Static can be generated when liquids are mixed together;
• Piping or vessels which allow a space for vapour to accumulate are a particular concern as any spark generated from a discharge of static electricity may cause an explosion inside the pipe.

Methods of controlling static charges

Although the generation of static electricity cannot be totally eliminated, the rate of generation and its accumulation can be reduced by the following control measures, what you can read below.

Methods of controlling static charges – additives

In some instances, anti-static additives can be introduced to reduce static charge build up.

Methods of controlling static charges – bonding and grounding

Bonding and grounding techniques are a very effective means of minimizing the risk of spark generation from a build-up of static electricity.

A bonding system is where all the various pieces of equipment within a process system are connected together. This ensures that they all have the same electrical potential, which means there is no possibility of a discharge of electricity, by way of a spark, from one piece of equipment to another.

Grounding is where pieces of equipment (which may be bonded together or not) are connected to an earthing point. This ensures any electrical charge in the equipment is given the means to constantly flow to earth, thus ensuring there is no potentially dangerous build-up of charge which could lead to a sudden discharge of electricity, by way of a spark.

All equipment which is involved in processing or storing flammable liquid, gas or vapour should be bonded and grounded.

Some other considerations are:

• Incidental objects and equipment, such as probes, thermometers and spray nozzles, which are isolated, but which can become sufficiently charged to cause a static spark, may need special consideration.
• The cables used for bonding and grounding cables should be heavy duty cables. This is to ensure that they can cope with physical wear and tear without compromising their grounding ability. It is also to ensure that their electrical resistance is as low as possible.
• The bonding of process equipment to conductors must be direct and positive.
• Using an inert gas, such as nitrogen, within the ullage space of a storage vessel will prevent an explosion or flash fire occurring if an electrostatic spark does occur. The inert gas lowers the oxygen content of the gas in the ullage space, thus ensuring there is insufficient oxygen to support a burning process (oxygen being part of the fire triangle).
• Operators should wear anti-static clothing.

The identification of ignition sources

Fire hazards, risks and controls

In the oil and gas industry, the severity of any incident involving fire and/or explosion is likely to be very grave, possibly involving loss of life, severe damage or destruction of plant, as well as having a potential impact on local communities. Consequently, any type of fire or explosion is unacceptable and controls must be put in place to prevent such an occurrence. These controls fall into two main categories.

First, any product should remain contained or under control throughout the process it is undergoing. In simple terms this means that any leak of product is regarded as highly undesirable. However, if a leak does occur there should be systems in place to detect it immediately and for appropriate action to be taken to control it and/or mitigate any consequences.

Second, all sources of ignition should be eradicated as far as possible in areas where product is processed and has the potential to escape.

Where it is necessary to introduce an ignition source into such an area, such as maintenance involving hot work, then an appropriate risk assessment should be undertaken to identify and Risk Management Techniques used in the Oil and Gas Industriesevaluate the risks, as well as introducing a permit-to-work regime. These measures may well be accompanied by other appropriate controls, such as temporarily shutting down the process and having fire-fighting equipment to hand.

Identifying sources of ignition

We will now look at potential ignition sources which need to be considered when conducting a risk assessment. Some of the sources of ignition have had basic control measures added.

• Smoking and smoking material.
  • A total ban on smoking and the taking of smoking materials into controlled areas should be enforced.
• Vehicles.
  • Vehicles may be totally prohibited or restricted to only specially adapted vehicles.
• Hot work such as welding, grinding, burning, etc.
  • Implement a permit-to-work regime.
• Electrical equipment.
  • The equipment should be suitable for the zone it is intended to be used in. It should also be properly and regularly inspected and maintained.
• Machinery such as generators, compressors, etc.
• Hot surfaces such as those heated by process or by local weather (hot deserts).
• Heated process equipment such as dryers and furnaces.
• Flames such as pilot lights.
• Space heating equipment.
• Sparks from lights and switches.
  • Use only electrical equipment and instrumentation classified for the zone in which it is located.
• Impact sparks.
• Stray current from electrical equipment.
  • Ensure all equipment is bonded and earthed.
• Electrostatic discharge sparks.
  • Bond and ground all plant and equipment.
• Electromagnetic radiation.
  • Make the correct selection of equipment to avoid high intensity electromagnetic radiation sources, e. g. limitation on the power input to fibre optic systems, avoidance of high intensity lasers or sources of infrared radiation.
• Lightning.
  • We have covered the control measures for lightning earlier in this section. There should be measures in place which reduce the potential of a lightning strike, as well as a grounding system to disperse any charge that may affect the installation. A further consideration is to look at weather windows (i. e. to not work during electrical storms).

Other control measures include:

• Controls over activities that create intermittent hazardous areas, e. g. tanker loading/unloading.
• Control of maintenance activities that may cause sparks or flames through a permit-to-work system.
• Precautions to control the risk from pyrophoric scale. This is where a substance can ignite spontaneously in air, particularly humid air, and is usually associated with formation of ferrous sulphide.
• Where control and/or detection equipment is regarded as critical, such as smoke and flame detectors, then a back-up or secondary system may be considered appropriate.

All of these control measures are supplementary to the main control and fire-fighting systems such as emergency shutdown systems, fire deluge systems, sprinkler systems, etc.

Zoning/hazardous area classification and selection of suitable ignition-protected electrical and mechanical equipment and critical control equipment

Introduction

Gases and vapours can create explosive atmospheres. Consequently, areas where these potentially hazardous airborne substances present themselves are classed as hazardous areas so that appropriate controls can be implemented.

However, how often these substances present themselves is also a factor in determining the appropriate level of control. For example, if the presence of a flammable vapour only happens once every three months, it would not be sensible to apply the same level of control to an area where a flammable vapour is present all day, every day.

Read also: Safety Critical Equipment Controls in Oil and Gas Industry

The answer is to apply a classification to areas – called zoning – which places appropriate controls on the type of equipment that can be used in that area and which potentially can create a source of ignition, particularly electrical equipment, which reflect the risk involved.

This zoning is determined by the frequency and extent of explosive atmospheres being present over a fixed period of time and the likelihood of an explosive atmosphere occurring at the same time as an ignition source becomes active. All of these parameters are established through a rigorous risk assessment.

Zoning

A place where an explosive atmosphere may occur on a basis frequent enough to be regarded as requiring special precautions to reduce the risk of a fire or explosion to an acceptable level is called a “hazardous place”.

A place where an explosive atmosphere is not expected to occur on a basis frequent enough to be regarded as requiring special precautions is called a “non-hazardous place”.

Under these circumstances, “special precautions” means applying measures to control sources of ignition within an area designated as a hazardous place.

Determining which areas are hazardous places, and to what extent, is called a “hazardous area classification study”. A hazardous area classification study is a method of analysing the extent and frequency to which an area is subject to having an explosive atmosphere. The main purpose of this is to facilitate the appropriate selection and installation of apparatus, tools and equipment which can be used safely within the environment, even if an explosive atmosphere is present.

A hazardous area classification study involves giving due consideration to the following:

• The flammable materials that may be present;
• The physical properties and characteristics of each of the flammable materials;
• The source of potential releases and how they can form explosive atmospheres;
• Prevailing operating temperatures and pressures;
• Presence, degree and availability of ventilation (forced and natural);
• Dispersion of released vapours to below flammable limits;
• The probability of each release scenario.

Consideration of these factors will enable the appropriate selection of zone classification for each area regarded as hazardous, as well as the geographical extent of each zone. The results of this work should be documented in hazardous area classification data sheets. These sheets should be supported by appropriate reference drawings which will show the extent of the zones around various plant items.

Hazardous areas are classified into zones based on an assessment of two factors:

1. The frequency of the occurrence of an explosive gas atmosphere;
2. The duration of an explosive gas atmosphere.

These two factors in combination will then facilitate the decision-making process which will determine which zone will apply to the area under consideration.

• Zone 0: An area in which an explosive gas atmosphere is present continuously or for long periods of time;
• Zone 1: An area in which an explosive gas atmosphere is likely to occur in normal operation;
• Zone 2: An area in which an explosive gas atmosphere is not likely to occur in normal operation but, if it does occur, will only exist for a short period of time.

As the zone definitions only take into account the frequency and duration of explosive atmospheres being present, and not the consequences of an explosion, it may be deemed necessary, because of the severe consequences of any explosion, to upgrade any equipment specified for use within that area to a higher level. This will be a discretionary option open to the analysis team.

Selection of equipment

As we inferred earlier in this section, the whole idea of zoning is to determine what apparatus, tools and equipment may be installed or used in a particular zone. The issue with electrical equipment is that it normally creates sparks, either as a result of the brushes coming in contact within the rotating armature, or when a switch is activated. Either event can ignite any flammable gas present in the atmosphere in the vicinity of the equipment.

Consequently, manufacturers have designed specialized equipment which overcomes, in various ways, the issue of having sparks which are exposed to the local atmosphere. The particular solution which is incorporated into each piece of equipment is signified by a code which is marked on the equipment’s product identification label. For example, “d” signifies equipment which has the motor and switch enclosed in a flameproof enclosure, or “q” powder filled. Both pieces of equipment are safe to use in zones 1 and 2, as indicated in Table 1 below.

The guide to what equipment is appropriate for each zone is the ATEX equipment directive and, whilst this is not a legal requirement outside the EU, most of the electrical standards have been developed over many years and are now set at international level.

Apparatus, tools and equipment are categorized in accordance with their ability to meet the standards required when used within each zone, as shown in Table 1.

As well as taking into account the sparks that electrical equipment can generate, consideration also needs to be given to the potential surface temperature of all equipment, not just electrical equipment, although most electrical equipment does generate heat as a matter of course.

In order to facilitate this, temperatures have been categorized into six classes: T1-T6. The bigger the T-number, the lower the allowable temperature of any equipment used. The temperature class will be determined by the auto ignition temperature of the substance involved (see Table 2).

The T-number of each piece of equipment will also be marked on the equipment’s product identification label. If several different flammable materials may be present within a particular area, the material that gives the highest classification will dictate the overall area classification of any equipment used.

Questions and Answers