The first step in the design and development of a system to prevent, control and extinguish fires is to identify the various release scenarios that could lead to an explosion and/or fire. This is often done through use of a hazards analysis technique such as HAZOP (Hazard and Operability Study) or a “What If” study.
The hazards analysis will identify the sources of leaks that could lead to a fire. The analysis will also provide a first estimate as to the size of these leaks. Naturally the selection of the size of the actual leak will depend on many parameters, particularly local operating conditions. Some initial guidance as to the types of leak and their magnitude is provided below.
- Full bore (guillotine) ruptures of process lines.
- Full surface fixed roof tank fires.
- Pump seal failures. These failures can be initially represented by assuming equivalent hole sizes of 9 mm diameter.
- Small leaks from process equipment and piping, sampling systems and level glasses. For initial calculation purposes an equivalent hole size of 6 mm diameter can be used.
- Small leaks from flange joints. Representative hole sizes are 1 mm diameter for ring type joints, 2.5 mm diameter for spiral wound gaskets and 7 mm diameter for compressed asbestos fiber gaskets (if asbestos is still being used).
Types of Fire
Broadly speaking there are two types of fire on process facilities: pool fires and jet fires. A pool fire occurs when the flammable material is spilled on the ground and then catches fire. It can spread in all directions depending on ground slope and the presence of drains. The heat and flames can threaten the integrity of equipment, piping and structural supports that are in the area. A jet fire, by contrast, originates at a hole in a pipe or piece of equipment, often at high pressure. The effect of the flame is localized but it can be intense thus creating additional leaks and causing the fire to spread. The United Kingdom Health and Safety Executive provides guidance to do with the modeling of pool and jet fires (HSE Jet Fires, HSE Pool Fires).
Single Fire Concept
The firewater system and firefighting equipment are generally designed to handle just one major fire at a time (this is analogous to the single event scenario concept used in relief valve design). Some parts of the firefighting systems are sized to handle less significant contingencies. For instance, foam concentrate requirements can be determined by a tank fire rather than by the worst contingency, which may be a fire in the process area.
The radiation levels to which the various pieces of equipment may be exposed, as calculated during a fire safety assessment process, determine the quantities of firewater required for exposure protection. As a starting point for more detailed and location-specific calculations the working value for the radiation limit for equipment items is 15,770 watts/m2 or 5,000 Btu/hr/ft2. If the equipment item is not a pressure vessel and contains volatile liquids a corresponding value of 2,365/750 can be used.