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March 2013: An Updated Look at Cooling Towers

On a regular basis we look at our previous articles and, when sufficient changes occur, we update them for our readers. Sometimes it's due to new technology, sometimes to loss history and sometimes to look at the bigger picture. This month we've chosen cooling towers, last discussed in September 2004.

Cooling towers are very common in commercial/industrial occupancies. They may be used to for air conditioning needs or for process cooling for certain types of production equipment. Towers used for process cooling can be an integral part of production and their loss could result in significant business interruption.

A tower's function is to cool water. Warm water is distributed in the tower by spray nozzles, splash bars or filming-type fill in a manner that exposes a very large water surface area to atmospheric air, thus cooling it. The air is circulated either by fans, convective current, natural wind currents or the induction effect from sprays. Via a closed loop piping system, the cooled water is pumped back to the process equipment, extracts heat and returns back to the cooling tower.

Fire protection in the form of automatic sprinklers is most often viewed as unnecessary since the tower is normally wet. However, a high percentage of commercial towers are filled with wood or plastic splash bars and when the tower is not in use, this combustible fill material will burn. In fact, depending upon the construction method, there can be parts of a combustible cooling tower that remain dry even during operation.

There are several types of cooling towers currently in use such as Mechanical/Natural Draft, Counterflow or Crossflow, Induced-Draft or Forced-Draft, or a Hyperbolic Crossflow and Counterflow (For a description of the various types, see FM Global Data Sheet 1-6, Cooling Towers). Regardless of the type of tower, automatic sprinkler protection may be needed based on the construction material.

Materials commonly used for tower construction are selected to resist the generally corrosive conditions. The material can be wood, plastic, concrete or metal. From a fire protection standpoint, wood and plastic construction are the main concern since there would normally be no fire exposure from a metal or concrete tower.

Wood can be used for all static components, not only the splash bars. Wood can be used for framing or for fan support.

Fiberglass-reinforced plastic materials have a broad use. They can be shaped into complex components such as a piping, fan cylinders, fan blades and structural connecting members. Polypropylene, ABS and other plastics are normally used as fill bars.

There are combustible cooling towers available that have been approved by Factory Mutual Research that can be used without sprinkler protection. They fall into two categories: those being Limited Combustibility, or Multi-Cell. Limited Combustibility towers are constructed such that the fire does not continue to self-propagate beyond its point of origin. Multi-Cell towers have individual cell units within the tower. A fire in one cell will have no effect on the others.

Although sometimes a cooling tower is too small from a property or operational standpoint to be protected internally, it still needs to be looked at from an exposure standpoint. At the very least, a 20 ft. space separation is needed between unprotected towers and from structures or processes that generate airborne ignition sources (chimneys, incinerators or flare stacks), or from materials and processes with a severe fire hazard (petroleum tanks, processing and piping). Where space separation cannot be provided, either sprinklers should be installed within the tower or a one-hour fire resistance should be provided between the tower and the exposure.

In addition to fire protection, cooling towers need to be examined for mechanical and structural failure. Mechanical failure points include the fan motor, speed reducer, fan wheel, drive shaft, cold-water basin, distribution system, sump and screen, float valve and overflow connections. Structural examination should be done to prevent an ensuing structural collapse of wind damage to the tower, with a focus on deteriorated structural members, corroded fire protection components (heads, piping, hangers, fittings and braces), basin, drift eliminators, fan blades, etc. The above examinations should be done by qualified contractors or experienced site personnel on a regular basis.

The tower should also be examined to determine how it fits in to the site's operations. Contingency plans should include a loss of the cooling tower, or the largest tower if more than one is provided for the site/process. To facilitate the use of temporary towers, the arrangement of the supply and return lines should be arranged to allow the temporary towers to be connected to your system, as well as arranged to individually isolate each tower. As an extra step, if the process that is supported by the tower(s) is of a high value (from a property and/or operational standpoint), consider having N+1 redundancy to the cooling tower system.

If your operation utilizes cooling towers that are critical to production, the fire protection, maintenance and business interruption associated with them should be reviewed. Risk Logic can assist in inspecting and determining the need for the proper protection and maintenance required, as well as the need for redundancy.