Heat is a common enemy. It attacks without discrimination and can damage your fire pump as surely as fire damages wood. What's more, a brand new high-dollar pumper with a minor operation error and without the proper accessories could be damaged as easily as a generations-old reserve pumper.

All fire pumps regardless of brand, size or design can overheat in standby operation. There are simple reasons why and even simpler fixes for potential pump overheat problems.

What and how

When a pump is running in a standby mode, maintaining pressure but not producing any flow, energy is still being consumed by the pump. Because no pump is 100% efficient, some of this power is converted to heat. As long as the same water is running through the pump impeller without leaving the pump, this energy will cause the water temperature to rise. The majority of this energy is friction from bearings, seals and rotating components. Horsepower is still going into the pump even if the water isn't flowing out; this horsepower is converted to heat.

With the increasing demands for more expansive cab space (which limits engine radiator area) and higher horsepower engines, auxiliary engine coolers that transfer heat from the engine to the fire pump are quite common. Heat exchangers for accessory hydraulic systems and pump transmissions also can add considerable amounts of heat to the fire pump.

Packing seals are water-cooled because they create friction that causes heat as the packing material rubs and seals on the pump shaft. When the pump becomes hot enough, the cooling water can no longer cool and protect the packing. The packing material can then deteriorate quickly requiring a repacking and sometimes replacement of a scored shaft.

Some mechanical seals can be damaged by heat or thermal shock. If you get a mechanical seal hot enough then cool it quickly by opening a pump valve and admitting cold water, the seal material may crack, requiring seal replacement.

Heat also causes thermal growth of critical parts in the pump that affects running clearances and causes wear, scoring or binding.

Various stages

Both single- and two-stage fire pumps suffer the same heating problems. The two-stage pump in series mode is more efficient at low and zero flow and therefore will last a little longer before it overheats; however, it will still overheat quickly in certain situations. In fact, tests show that a two-stage pump will overheat faster when run in parallel or volume mode than the same size single-stage pump. In this situation, the single-stage pump is more efficient and the two-stage pump overheats. There are good reasons to buy a two-stage fire pump, but overheating alone is not one of them.

It almost makes sense to argue that a big two-stage pump holds more water than a little single-stage pump, so it should take longer to overheat. Again, actual tests show that a 1,000gpm single-stage pump lasted longer without overheating than a 2,000gpm two-stage pump in parallel mode. This relates again to efficiency. The smaller pump is more efficient at zero flow than a bigger pump, therefore there's less heat load. The point here is that big or little, single- or two-stage, all fire pumps can overheat. The key is to prevent overheating with a combination of appropriate hardware and operator procedures.

Tips to avoid overheating

All fire pumps will overheat in standby mode without any water flowing. How long it takes to overheat is determined by the actual pump, installation and accessories. All pumps can be protected from overheating through various means. Most pump operators will remember to crack the tank fill valve or open a pump cooler line or keep a nozzle cracked open during training, but it isn't always so easy with multiple tasks at a real emergency.

Here are a few suggestions:

1. Make it easy for the operator to do things right. The pump cooler line is normally a 3/8-inch tube with a 1/4-inch ball valve returning pump discharge water to the tank. This valve should be installed so that it is normally open. The post-run checklist should include placing this valve in the open position. A reliable check valve such as a 1/4-inch NPT bronze swing check with Teflon seat should be installed so that priming the pump during draft operations isn't compromised. This is a simple, inexpensive item that every pump should have.

2. Have a thermostat. What happens when the pump cooler line isn't enough or the booster tank water becomes warm? The pump heats up. This can happen relatively quickly and on a regular basis when the booster tank is low on water. Here's where a thermostat can help the operator realize the problem while protecting the pump. Electric warning lights for pump overheat have been used for years; however, they are not usually tied to a valve to dump water to cool the pump.

3. Install a temperature relief valve that opens when the pump temperatures climb above the 120- to 130°F range, warning the operator as well as protecting the pump. Mechanical thermal relief valves are available that discharge enough water (10- to 12gpm) to keep the pump cool. As the pump cools down, the thermal relief valve closes so water is not wasted. Water should discharge through a tube near the operator's area or have a warning indicator so the operator can take appropriate action. If the valve is equipped with a warning lamp, the cooling water can be directed back to the water tank.

4. Make operational considerations to protect equipment. Discharge pressures can be backed down from up to 150psi to 80psi when on standby with a car or truck rescue/salvage without fire showing. This is especially important when working with the remaining tank water because there's probably no extra water available to cool the pump. Obviously a third-full booster tank will heat up three times as fast as a full tank.

5. Never, for any reason, run any fire pump for extended periods without water. This mistake can cause major problems very quickly. Almost every fire pump in the United States can run dry for short periods while the hoses are connected without damage. As long as the operator doesn't allow excessive dry run times, there usually isn't a problem. Wet pumps are sometimes run for this reason, but remember a wet pump will not be “wet” very long if you are attaching a hard-sleeve suction hose without a master intake valve to keep the water in the pump.

6. Remember that high-idle switches are spinning the pump faster. While these switches are great for keeping battery banks charged, if the pump is dry while the hoses are being deployed, the chance of heat damage is much greater at the high-idle speed. While high-idle mode should be automatically disengaged when the pump is in gear, this is sometimes not the case.

7. Pay attention to how much heat is being rejected to pump water when specifying accessories. Nice, cool pump water is a tempting cooling source until it heats up or runs out.

Some accessories like hydraulic drives and large air compressors generate a great deal of heat. Using water-cooled equipment will heat up the fire pump much more quickly. Make sure the builder/manufacturer can guarantee the fire department adequate run time without overheating when the booster tank is low. Make this one of your acceptance tests; for better reliability, specify air-cooled accessories whenever these accessories have high power consumption or they run for extended times.

Before it starts

Pump overheating can affect all makes and types of fire pumps, leading to increased maintenance costs and reduced reliability. Luckily it only takes a little planning, a little training and possibly a minor modification to trucks to prevent such problems.

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Michael A. Laskaris is the director of engineering of the fire suppression product division of Hale Products Inc., Conshohocken, Pa. A licensed PA, he also is a member of a volunteer fire department.