The layout and general design of a petroleum refinery or bulk storage installation should be optimised with respect to safety, operational efficiency and environmental protection.
National regulations (e.g. COMAH) and local regulations including petroleum-licensing conditions, building regulations and local bylaws, may have specific layout requirements and should be consulted at the design phase of an installation. For example, the preparation and submission of a pre-construction safety report can be a requirement under Seveso II-type legislation. Discussions should be held at an early stage with all authorities responsible for these and any other requirements. Formal approval should be obtained before construction work commences.
Some petroleum companies have in-house standards for site layout and minimum separation
distances may be specified. These may be based on experience and can in many cases be used as a starting point for layout purposes. However, for optimisation purposes, there should be additional considerations.
Under a goal setting (i.e. risk-based) legislative framework, detailed layout studies and fire protection analyses should be carried out. Their purpose may be to optimise installation layout, whilst considering necessary FEHM measures. For example, fire protection such as water spray systems might be considered (depending on potential fire exposure and emergency response, etc.) if land use needs to be optimised and storage tanks or plant are to be situated close to each other. For a typical study, the following should be considered:
— Credible fire scenarios at the installation (e.g. pool fire, pressurised gas jet or liquid spray fire, etc.).
— Fire probability and consequences (e.g. potential for asset damage).
— Potential fire exposures, including personnel and buildings and implications for life safety (e.g. is the flame from the fire likely to impinge on adjacent equipment, vessels etc. or will nearby items and personnel be exposed to high radiant heat levels?).
— Potential risk reduction options or mitigation measures (e.g. fixed water spray systems or foam systems), including the extent of spacing/separation required between items or areas of plant.
Generally, spacing between tanks and other items of plant can be relaxed with a higher degree of fire protection. For example, if PFP (i.e. a fire wall) is provided between two critical product pumps, then greater separation may not be required.
In some cases, appropriate fire detection backed up by a rapid fire response (whether by fixed fire-fighting systems or by manual means) can allow relaxation.
In all cases, criticality of plant and equipment and implications of loss for asset damage, business interruption and reputation should be considered, as well as those for life safety.
For areas where personnel are normally present (e.g. loading and unloading areas) there may be considerations for access and emergency egress. Also, appropriate areas should be set aside to allow safe vehicle movement, and features such as crash barriers should be installed to prevent collision with plant and structures.
In heavily built-up areas, a risk assessment should be carried out to determine both personnel and societal risks (see section 1.7.2) arising from potential fire or gas release events. The use of fire and explosion modelling
and other scenario analysis tools such as event tree analysis can assist in this purpose.
For buildings and other occupied structures, potential for external fire spread should be assessed.
Fire could start and spread because of exposure to fires within plant areas or it could propagate due to fire spread from adjacent or adjoining buildings. Generally, a 'clear' area should be provided around buildings where possible to minimise fire spread.
4.8.2 Boundaries
Installations should be surrounded by a suitable security fence or wall of minimum height of 2 m. Where petroleum installations are situated within a fenced or controlled area, such as dock or harbour premises, the requirements for fencing may be relaxed by agreement with the local controlling authority.
4.8.3 Storage tank layout/secondary containment Installations intended for the handling of only Class II(1) or Class III(1) petroleum products present a lower level of risk than those handling Class I, Class II(2) or Class III(2). However, safe separation distances of storage and handling installations from boundaries should still be observed for these products having regard to the installation's location and the nature of its surroundings.
Normally, good tank design and operations good practice should prevent large product releases.
Catastrophic tank failure is one possibility, but is usually considered a low probability event. Although considerable research has been aimed at the subject of bund overtopping, good bund design and minimising potential for large releases in the first instance should significantly reduce the probability of such an event.
Tank inspection practices aimed at identifying potential corrosion points well before a leak could develop should be implemented as part of a site pre-planned inspection and maintenance programme.
Above-ground tanks should be provided with a form of secondary containment, which will serve to contain any releases that may occur. Bunds or walls may be constructed from earth, concrete, masonry or steel, or a combination of these. They should be substantially impervious to liquid and capable of withstanding the hydrostatic pressures to which they may be subjected. The floor of the bund area should be substantially impervious to petroleum and its products in order to safeguard groundwater quality.
Environmental regulations and water protection standards should be observed in the design of compounds, drainage systems and impounding systems.
Intermediate walls of up to half the height of the main walls, but normally not more than height 0,5 m may be provided within a bund area to control losses of containment and avoid the spread of substance to the vicinity of other tanks sharing the same bund. Such walls should divide the tankage into groups of a convenient size.
When planning tank bunds and bund walls, the bund should be capable of holding a volume equal to 110% of the maximum capacity of the tank.
As an alternative to these designs lower walls may be employed in conjunction with systems to direct the lost product to an impounding basin at a convenient, safe location.
The maximum total capacities of tanks within a single compound should be:
(1) Single tanks, all classes, including
(3) Two or more fixed roof tanks
60 000 m3
(4) Crude oil tanks Not more than two tanks of greater individual capacity than 60 000 m3 The data for (2), (3) and (4) may be exceeded provided that an assessment indicates no significantly increased risk of environmental impact or to people. Such assessment may take account of developments in floating roof seal technology and practice and should consider the design of appropriate fire protection and extinguishment measures.
For guidance on storage tank separation distances in relation to fire risk reduction options (including bunding), see annex C.2 and Table C.1.
4.8.4 Process plant layout
Process areas should include access-ways for fire-fighting, as well as routine inspection and maintenance.
Some guidance on process plant layout includes:
— Access-ways should be arranged in a rectangular grid pattern, so that fire-fighting can take place from two opposite sides.
— To limit fire spread, low walls or kerbs should be provided and each should be connected to a drainage system (but not any storm water system).
These can assist foam blanketing and limit fire
spread caused by low flash point products floating and burning on the surface of the water (carry-over). However, during fire-fighting, it should be recognised that the drainage capacity of kerbed areas may be exceeded and flooding may occur under full fire-fighting water application rates.
— Fixed water spray or foam systems should be considered for high-risk equipment where fire-fighting access is poor or if items are vulnerable to fire exposure. PFP should also be considered.
4.8.5 Fire-fighting access
Pre-fire plans (see section 8.7) should identify emergency vehicle access points, including means of gaining entry where unattended or remotely-operated secure entry systems exist. Roads and crossings, as well as overhead pipe rack clearances, should allow emergency vehicles easy access to all areas of the site.
Main roads should also be suitably surfaced and drained. Speed bumps, which could limit response times, should not be provided on emergency routes.
Roads or access over firm ground should be provided to allow fire appliances to approach within reasonable operating distance of the hazard. Access should be kept free of obstruction. In certain circumstances, railway lines may impede access for fire appliances. Each case should be considered separately, but for initial guidance, access should be provided within 20 m to 45 m of the hazard. Water supplies should be available at these places.
A subsidiary road should be provided in large installations for general access and fire-fighting purposes around the perimeter. This road may be sited within the safety distance specified for the spacing of tanks from the boundary and should have access to the public road system at two points at least. Secondary access to the site should normally remain secure or locked and with well-defined arrangements for opening in emergencies. Connecting roads should normally be arranged to permit approach from two directions to all major fire hazards onsite.
Roadways should be provided with passing spaces for fire vehicle access if they do not permit two lanes of traffic. Recommended widths for two-directional traffic and for single directional traffic should not normally be less than 6 m and 4 m respectively. Cul-de-sacs should be avoided, but, if necessary, should be provided with adequate turning areas. Road junctions and curves should be constructed with sufficiently large turning circles to ensure easy vehicle manoeuvring. It may be necessary to provide one or more vehicle turning points and to cater for emergency vehicles, such as by providing hard standings at strategic locations.
Each large storage area of flammable substances or major process plant unit should be accessible from at least two sides. If access is only possible from two sides, these should, wherever possible, be the longest opposite sides.
The design of the road layout should be influenced by plant complexity and the type(s) of fire appliances likely to be employed in fire-fighting. Road widths, gate widths, clearance heights, turning circles and axle loadings for the various types of vehicles likely to be called to the hazard should be considered. These could include vehicles other than fire-fighting appliances such as heavy bulk foam and/or carbon dioxide (CO2) carriers.
One or more hard standings should be provided beside each open water source to enable fire-fighting appliances to be positioned at strategic points, where this is necessary to prevent blocking roadways. A waiting area should be allocated near each main entrance to the site as a rendezvous point for emergency vehicles where this is warranted by the size or nature of the installation.
These aspects should be considered in consultation with the local government Fire and Rescue Service (FRS).
4.8.6 Drainage systems
Due to their flammability and classification as dangerous to the aquatic environment, sewerage companies and environmental agencies generally do not allow entry of petroleum and its products into drainage systems and natural watercourses under their respective control. In addition, fire-fighting water is likely to be highly polluting, posing a threat to watercourses, groundwater and sewage treatment facilities. Therefore, the capacity of site drainage systems should be carefully evaluated and the management of fire-fighting water should be included in ERPs (see 8.8).
Adequate drainage for storm water should be available and special provisions such as pumps, run-off areas, etc., may be necessary for the disposal of water used in fire-fighting operations. Increasingly, there are controls on the release of fire-fighting foam due to concerns over the toxicity of fluorochemicals used, and some environment agencies may require catchment and specialist disposal of foam run-off.
To avoid flooding during fire-fighting, the drainage system should be designed to cope with the fire-fighting water available to that area, including cooling water.
Generally, this would comprise at least 90% of the flow, assuming some 10% evaporates in the fire.
Area drainage or alternative disposal systems for the large volumes of water that may be used should be
adequate to avoid flooding, which can introduce other hazards. Consideration may be given to installing recycling facilities for oil-free water. The system should be designed to prevent carryover of petroleum, its products and other pollutants into the sea, rivers, or other environmentally sensitive areas. Contingency plans should be discussed with the relevant environment agency.
Consideration should also be given to the possible danger from the mixing of incompatible effluents.
Flammable vapours can arise if hot fluids, e.g. steam condensate, mix with petroleum and its products in drainage systems. Also, flammable substances may be carried offsite by drains and precautions should be taken to prevent this possibility.
4.8.7 Fire protection and other safety critical equipment
Fire protection and other safety critical equipment should be located in safe and non-hazardous areas.
Consequence modelling should be carried out to determine placement of such items as they may constitute sources of ignition. Consideration should also be given to locating such equipment so as to enable access at all times during incidents. In addition, such equipment should be capable of withstanding the effects of fire and explosion if its use is required during emergency conditions. For example, fire pumps should be located at a safe distance away from any possible fire consequences.
4.9 BUILDINGS FIRE PRECAUTIONS