Active and Passive Fire Protection

Active fire protection systems such as water sprinkler and spray systems are widely used in the process industries for protection of storage vessels, process plant, loading installations and warehouses. The duty of the fire protection system may be to extinguish the fire, control the fire, or provide exposure protection to prevent domino effects. For some applications foam pourers or fixed water monitors may be a more appropriate method of delivery than sprays or sprinklers. Other more specialised systems using inert gases and halogen based gases are used for flooding enclosed spaces.

Passive fire protection can provide an effective alternative to active systems for protecting against vessel failure. This generally consists of a coating of fire resistant insulating media applied to a vessel or steel surface. It is often used where water or other active protection media supplies are inadequate, such as in remote locations, or where there are difficulties with handling fire water run-off. Fire walls are another form of passive fire protection that are used to prevent the spread of fire and the exposure of adjacent equipment to thermal radiation. An important criterion in deciding which system is most appropriate for fire exposure protection is the likely duration of the exposure to fire as passive fire protection is only effective for short duration exposure (1-2 hours).

General Principles

The operator should be able to demonstrate that it has an effective and practical plan for the containment and fighting of fires on its process installations. The following site factors should be considered in determining whether active and passive fire protection measures are required:

• fire hazard posed by substance;
• toxicity of substances and the smoke produced;
• inventory size;
• frequency of hazardous operations;
• distance to other hazardous installations;
• available access to fight fire;
• fire fighting capability of on site emergency response team;
• response time of nearest fire brigade;
• resources available to fire brigade.

Design of system

Active fire fighting systems need to be reliable and the design of the system should demonstrate this. The design of fire fighting systems should conform to specified standards and Fire Offices Committee ‘Tentative rules for medium and high velocity spray systems’.

The location of items such as the foam and water sources should be a safe distance from any hazardousinstallation. Critical valving and instrument cabling located on the protected installation should be capable of withstanding the effects of fire and heat.

The system should be supplied by a secure water supply which should include items such as backup diesel pumps where appropriate. The design must ensure that the active fire protection system is not starved of water due to other demands on the water supply system during a fire.

Choice of fire fighting media

The selection of media will depend on the required duty. This may be to extinguish the fire, control the fire, or provide exposure protection. Types of fire fighting media are:

• Water;
• Foams;
• Inert gases;
• Chemical powders;
• Halons.

Water is not recommended as an extinguishing media for low flash point liquids, but it is used widely throughout industry for fire control and exposure protection.

Foam is a more effective extinguishing media for low flash point substances and is widely used against liquid fires. There are various types of foam available, but the most widely used is protein foam. Alcohol resistant foam is used for application on polar solvents where the foam stability is affected. Other more specialist foams have been developed to give improved extinguishing properties such as fluoro-protein and aqueous film forming foams. Foam can be delivered as low, medium or high expansion depending on the required duty.

Other agents such as inert gases, chemical powders and halogen based gases (Halons) can be delivered by active fire protection systems, but these tend to be installed where process equipment is contained within an enclosure such as a gas turbine enclosure. A common use for these systems is in the protection of switch rooms and control panels. There has been movement away from the use of Halons over recent years due to their potential effect upon the ozone layer and other undesirable environmental effects.

Choice of Passive Fire Protection

For the protection of vessels from fire exposure there are a number of types of passive fire protection that can be applied.

• mortar based coating
• intumescent coating
• sublimation coating
• mineral fibre matting
• earth mounds

The protective systems based on coatings are normally sprayed onto the surface following mixing of the required components. A reinforcing glass fibre scrim or steel wire gauze is applied to prevent cracking and peeling of the coating under fire conditions and to provide additional strength to resist the impact of high pressure water jets. The fire protective coating is further protected by a weather protective top layer. The fire resistant performance of the coatings is dependent on the thickness of the coating. Fibre matting systems consist of fireproof mineral fibre matting clad with a protective galvanised steel sheet. The protective capability of the system is provided by the poor heat conductivity of the system.

Earth mounds are commonly used in the LPG industry, where vessels are either fully or partially buried in an earth mound. The presence of the earth mound effectively prevents a fire from developing around the vessel.

Fire walls are sometimes employed in process and storage areas to prevent the spread of fire and protect adjacent equipment from thermal radiation. These may be an integral part of a process building or warehouse structure or may consist of a free-standing wall specifically built for the purpose. Firewalls are normally built of brick, concrete or masonry and the number and size of openings should be kept to a minimum.

Performance of the protective system

For active fire protection systems required delivery rates and durations for various types of application. For fire engulfment protection a water rate of 9.81 litres/min/m2 over the exposed vessel surface and its supports is standard. For protection from lower levels of thermal radiation from fires on adjacent units lower rates of water application are allowable.

For passive fire protection systems the operator should have supplier or manufacturer information demonstrating that the fire protective system employed meets defined performance criteria based on standard tests that replicate the fire conditions likely to be encountered in the work place. Typically the criteria will be that a protected surface will not reach a certain temperature in a defined time period during a standard test.

Maintenance requirements

Active fire protection systems require to be well maintained to ensure reliability. In particular systems using water and water based foam are prone to rust deposits which can block sprinkler heads and spray nozzles. Procedures should be in place to ensure regular maintenance and testing of systems. Maintenance contracts are often placed with the supplier of the fire protection system. Records of these activities should be kept by site operators.

The performance of passive fire protection systems can deteriorate in time due to weathering and corrosion. Plant operational and maintenance activities may damage or remove the fire protection. Additionally the protected surface itself can corrode beneath the fire protection. Procedures should be in place to ensure that both the passive fire protective system and the protected surface are regularly inspected and repaired as appropriate.

Containment of firewater

Foam and water based active fire protection systems can generate considerable amounts of effluent with significant potential environmental damage. Where active fire protection systems are installed the overall design of the facility should cater for the collection of fire fighting effluents. Operating sites should have effluent disposal plans in place as part of their emergency plans.

Supporting measures

Where active or passive fire protection is installed, these systems should be supported by hydrants at suitable locations. Suitable portable fire fighting equipment should also be located on the plant.

Mortar based fire protection fire protection is commonly used to protect load bearing steel work from collapse under fire exposure. The application of this to vessel supports and supporting structures for process equipment is standard where flammable substances are handled in quantity.

Industry applications

LPG industry

The use of water deluge systems for the protection of bulk LPG storage vessels and loading bays is standard in the industry for all but the smallest installations. Passive fire protection is used as an alternative and in particular earth mounding of LPG vessels is an established practice. Large LPG cylinder compounds covered by canopies are normally provided with either fixed water monitors or a sprinkler system.

Flammable liquids / solvent bulk storage

Whilst active fire protection is not a standard requirement for vessels containing flammable and highly flammable liquids, site factors such as inadequate separation distances from other plant or the proximity of occupied buildings may necessitate the use of active or passive fire protection to prevent escalation of a fire event. Where protection of remote storage tanks is required, passive fire protection is commonly used. However, it is not normal practice to protect storage tanks in locations that do not represent a hazard to people directly or by domino effect.

Process operating units

Both the material handled, the size of the flammable inventory and the local fire fighting capability will influence the requirement for active fire protection on a process structure. In particular, where process equipment handling significant quantities of flammable material are located inside a building and fire fighting access is poor, then fixed fire protection systems should be provided.

Warehousing

Some significant fires have occurred in chemical warehouses. The considerations are much the same as those for process operating units. For the storage of high hazard materials such as organic peroxides in warehouses, fixed sprinkler systems using either foam or water should be provided. However, it should be noted that the effectiveness of sprinkler systems in warehouses may be limited if stocking densities are high. Particular care is required in designing such systems.