Prevention and Control of Fires
Preventative measures are the most effective means of minimising the probability of equipment failure and its associated risk. Protection systems are not substitutes for well-designed and well-maintained detection, warning and shutdown systems. However, they can protect the structure and process equipment, limit damage to these facilities and prevent escalation of fire.
Design features can be provided, such as shielding which can reduce the likelihood of vessel failure. The choice between active and passive systems (or their combination) is influenced by the protection philosophy, the fire type and duration, the equipment or structure requiring protection, water availability and the time required for evacuation. In all cases, the specification must be matched to the fire type and exposure. The various types of protection system are considered in more detail below.
- Identify the most cost effective prevention and control measures that have the most significant benefit to personnel and process plant;
- To identify areas of uncertainty in analysing the effects of fire;
- Initiate research to increase knowledge and understanding in ill-defined areas of fire effects, and the corresponding prevention and control measures; and
- Promote the use of a consistent methodology for evaluation of cost effective prevention and control measures.
Current knowledge of prevention and control measures
There are two categories of prevention and control of fires; passive protection and active control and protection.
Passive fire protection (PFP) is defined, in the recently issued ISO standard (ISO, 1999), as 'a coating, cladding or free-standing system which, in the event of a fire, will provide thermal protection to restrict the rate at which heat is transmitted to the object or area being protected'. These materials are used to:
- Prevent escalation of the fire due to progressive releases of inventory, by separating the different fire risk areas, and hence protect personnel until safe evacuation can take place,
- Protect essential safety items and critical components such as separators, risers and topside emergency shutdown valves,
- Minimise damage by protecting the critical structural members, particularly those which support the temporary refuge, escape routes and critical equipment.
Spray applied epoxy intumescent and subliming coatings are most frequently used now, although cementitious materials were extensively used in the past.
Active 'protection' consists of several systems that may require human intervention to initiate. These include ESD and blowdown mechanisms, water deluge and foam systems, monitors, inerting systems, fire extinguishers etc. The installation and activation of these systems is well understood, although the physics of water droplet size and mists, on explosions hazards may require further work. The primary form of active fire protection for hydrocarbon processing areas is fixed deluge. Such systems may be provided to:
- Control pool fires and thus reduce the likelihood of escalation;
- Provide cooling of equipment (except that impinged by jet fires);
- Provide a means to apply foam to extinguish hydrocarbon pool fires; and
- Limit effects of fires (e.g. radiation, smoke movement) to facilitate emergency response and evacuation, escape and rescue (EER) activities.
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Areas of uncertainty
There is an HSE cross-divisional need (preliminary steps are being taken) to develop guidance on the use and operational requirements for passive fire protection materials. At present, there is a lack of consistency in the requirements for protection of pressure vessels. This is reinforced by the recommendation in the HSE report on the Associated Octel incident that 'HSE, in conjunction with the industry, should consider what guidance, if any, should be published on the provision of passive fire protection on vessels.' This should include;
- A 'standard' fire test. This is required because furnace-based fire tests do not relate to conditions in 'real' fires. There was a requirement for fire tests with a manageable, reproducible, well-characterised flame which is used in conditions which can be related to those in a 'real' fire;
- Investigation of the effects of water streams on the performance of PFP,
- The effect of the size and nature of damage or penetrations, and the effect they have on the performance of PFP,
- The effect of blast overpressure on PFP,
- The effectiveness of protective systems for flange connections (possibly a variant of the Jet Fire Resistance Test),
- Consideration of secondary smoke and toxic gas emissions in the context of those from the primary fire.
Industry practice in prevention and control of fire hazards
PFP is generally applied using ISO 13702, where the following functional requirements are given:
- PFP shall be provided in accordance with the Fire and Explosion Strategy,
- PFP of essential systems and equipment, or enclosures containing such systems and equipment, shall be provided where failure in a fire is intolerable;
- Where PFP is required to provide protection following an explosion, it shall be designed and installed such that deformation of the substrate caused by an explosion will not affect its performance;
- Selection of the PFP systems shall take into account the duration of protection required, the type and size of fire which may be experienced, the limiting temperature for the structure/equipment to be protected, the environment, application and maintenance, and smoke generation in fire situations.
Active prevention systems are also based on the FES, but with significant input from QRA studies. Active systems are considerably more expensive to install and maintain and hence further justification is required.
QRA studies usually take some degree of credit for operation of active fire protection systems. These systems are usually designated as Safety Critical Elements (SCE's), pursuant to the requirements of the DCR Regulations and have associated performance standards, in accordance with the requirements of PFEER.
Strategic development issues
Passive fire protection
- To develop effective guidance on the use and operational requirements for passive fire protection materials including,
- The assessment of the size and nature of damage or penetrations that would lead to a significant reduction in performance,
- Establishment of a standard system of assessing the effect of blast overpressure on PFP,
- Determination of the effects of water streams on the performance of PFP.
Active fire protection
- To develop accurate guidance on the design of emergency depressurisation systems that consider the recently identified higher fire loading;
- To ensure that a specific hazard identification profile for each vessels and pipework is carried out;
- To produce improved information on the response of pressurised systems to fire to allow proper assessment of failure times and development of improved models/designs for fire loading.
- Review of the detailed findings of the EU FOAMSPEX project and assessment of their significance for the adequacy of foam systems used on offshore installations.
- To investigate improved design of directed deluge systems;
- To updated guidance on the use / replacement of halon systems.