Water spray systems

Water remains the predominant fire suppression medium worldwide. By adapting its method of application, water can even be used to extinguish or control fires in situations where it would not normally be considered suitable.
This article deals with fixed water spray installations other than sprinklers, which were dealt with in other articles.

Definition

A water spray system is connected to the supply through an automatically or manually actuated flow-control valve. Water is then piped to specially designed nozzles, which distribute it over the protected area.
A detection device installed in the protected area actuates automatic valves.

Types of system

The two types of water spray system are:

1. High velocity system (HV) HV systems are mainly used to extinguish fires involving oils or similar combustible liquids. Oil-filled electrical equipment such as transformers or lubrication systems on steam-driven turbines are generally protected by HV systems. Extinguishment of fires by HVs is achieved by a combination of the following:
   • Cooling of the burning oil surface reduces the vaporisation rate.
   • Steam generated in the fire zone causes a smothering effect through
   • Oxygen displacement.
   • Dilution of the burning product – a water miscible product e.g. alcohol can be diluted to a level where it will no longer burn.
   • Emulsification through the bombardment of the surface of the product by the high velocity water droplets. The emulsion formed consists of either globules of oil suspended in water or globules of water suspended in oil. The effect is temporary and the mixture will separate again in time.


 

2. Medium velocity systems (MV)
   MV systems are mainly used to protect structures, plant and storage vessels from radiated heat and direct flame impingement.
   Typical applications include the complete external surface protection of bulk liquefied gas pressure vessels to prevent container failure and the resultant BLEVE (boiling liquid expanding vapour explosion); bulk flammable liquid storage tanks; structures supporting hazardous plant and equipment such as conveyors.

   In addition, fire prevention may be achieved particularly where flammable gas leaks are likely to occur. The action of the spray on the leak will help to dissipate the gas more quickly and prevent concentrations from reaching their flammable limits.

System actuation

Most systems can be actuated both automatically and manually.
In small automatic systems, water is held back from the open heads on the distribution piping by an automatic control, which incorporates a heat-sensing device, such as a frangible glass bulb. Heat from the fire breaks the bulb, which allows the vale to open and water to flow to the discharge nozzles.
In large systems, a deluge valve is fitted between the mains supply and the distribution piping. In addition, a further pipe system incorporating glass bulb detector heads is installed in the protected area and is also connected to the deluge valve. The latter piping is filled with compressed air at a greater pressure than the system water pressure. This keeps the deluge valve closed.
When the detector head actuates, the air escapes and the system water pressure forces open the deluge valve and water flows through the distribution piping to the discharge nozzles.

Choosing a system

When choosing a system for a particular risk, consideration should be given to:

1. Physical and chemical properties of the materials likely to be involved e.g.:
   • flash point
   • specific gravity
   • viscosity
   • solubility/miscibility
   • reactivity of water sensitive or confined high temperature materials

2. Possible equipment damage through the thermal shock of water on high temperature equipment.
3. Clearances between system components and live electrical equipment. Design clearances are available and ensure complete safety.

Design of the system

Specialists should only design water spray systems. All system designs should include working drawings, specifications and hydraulic calculations, which should be forwarded to the manufacturer of the installation components for approval.
Consideration should be given to drainage facilities to handle water discharged from the system and to the likelihood of spilt combustible or flammable liquids, which may be present within the protected area.

Testing and maintenance schedule

Acceptance tests
All newly installed systems should undergo a full acceptance test including:

• flushing of all systems and supply piping
• hydrostatic pressure test of all piping
• a full discharge test, where practicable, to check nozzle positioning, discharge patterns, obstructions to spray patterns and nozzle blockages
• operating test of all detection systems and moving valve parts for correct operation

It is recommended that a test certificate showing the tests performed and their results be obtained from the testing authority.

Water spray systems.pdf

People in Risk

Marcel Wood

Marcel Wood heads Etana’s risk management division. Before starting in insurance he worked for the fire department, rescuing people and (literally) putting out fires. He’s passed through Munich Re and Santam, and has designed the only risk management tool aimed specifically at the insurance industry – ARQ. Marcel is a mean guitarist, a brilliant vocalist and an accomplished soccer player with the personal motto: “Do your best all the time”.