Designing a Foam System

In today’s society oil and its petrochemical derivatives play an ever-increasing role in our day to day life. New materials and chemicals are constantly being developed to meet the needs of an expanding and developing market place. Global economies are affected by the supply and demand of these commodities. In recent years, one major refinery fire incident in America had a massive impact, overnight, on oil prices internationally. With this back­ground it has become even more vital for Loss Prevention Engineers to ensure that flammable liquid assets are protected to the best degree possible.

Indeed, the protection philosophy applied must not only consider the consequences of an incident within a facility but also the effects on the environment and society outside.

In many cases the only effective solution to a major flammable liquid fire or unignited spill incidents is the use of foam – correctly selected and applied. Often, especially with decreasing plant manpower levels, this requires a fixed system. Nowadays there is a greater choice of foam agents and hardware available for this purpose, thus making the task of optimum system design much more complex.

In Control Fire Protection now have access to the most comprehensive range of foam equipment available from both Ansul and Skum, can assist and advise in the design process and make the task easier.

What is the risk protected

Before any system design can be carried out, it is obviously necessary to define the risk to be protected. This does not just mean finding out the dimensions of the hazard. The fuel type, availability of power supplies, area classifications, ambient conditions, water supply and site valve/pipework specifications may all have some bearing on final system design.

The most important point to remember is that the hazard to be protected should not be looked at in isolation. Even when the risk appears to be remote from others, a system designer should consider all relevant outside factors such as the local availability of compatible back-up foam stocks from neighbouring facilities. When several risks are present on one site it is particularly important to look at the worst possible fire scenario and make sure resources are available to tackle it.

All recognised international standards make it very clear that sufficient foam concentrate and application equipment must be available for the largest incident so the fire protection engineer must decide whether a fire in one hazard could escalate to others – or more importantly, can good system design with fast detection and rapid actuation prevent this happening?

Also, of course, it is often possible to provide more cost effective fire protection by serving several hazards from one central proportioning unit and foam concentrate supply. It is in these areas, with their wide range of experience of risk assessment and system design in all industry sectors that In Control Fire Protection can provide the most cost­ effective solutions to fire protection problems.

Although foam systems can be used for protection of combustible solids such as paper, by far the most com­mon use of foam is for the protection of flammable liquids. The type of liquid and its physical properties affect the type of foam to be used and the system running time required to assure extinguishment.

What foam should be used

In recent years there have been many new foams developed and introduced. Different generic types are available such as "protein" and "synthetic" based foams.

Undoubtedly the general trend is towards multipurpose foam concentrates that can be used on all types of fuel, but these may not be the most cost-effective in all cases.

Foam liquid manufacturers have increased the confusion by continuing the debate on what generic type is best. The truth of the matter is that the important thing is not what raw material a foam is made from but how the end product performs!

To perform effectively, several properties are needed from a foam. These depend on the accuracy of proportioning and method of application as well as on the foam concentrate.

The most important properties that a foam requires for effective fire extinguishing are:

  • Cohesion – to form a stable blanket on the fuel surface
  • Heat resistance
  • Burnback resistance – to prevent remaining fire burning off the foam blanket
  • Adhesion – to give sealing against hot objects in the fire
  • Vapour suppression – to prevent vapours rising through the foam blanket
  • Stability – to provide security against re-ignition. Stability is often measured in terms of "25% drainage time" which is the time taken for 25% of the foam solution required to make a given foam sample to drain from that foam. Longer drainage times mean greater stability
  • Flowability – to move rapidly across a fuel surface and around any obstacles in its path
  • Flame knockdown – to quickly control the fire
  • Fuel tolerance – to ensure that the foam is not broken down by contamination with the fuel. This is particularly important in the case of water soluble fuels such as alcohols, gasohols, ketones etc. as they break down standard types of foam.