Diesel Fire Pump Damages
Why Seldom-Used Engines are Often Damaged
Diesel engines are a rugged and reliable choice for propulsion and power generation. In applications where electricity is not available or may be disrupted, a Diesel engine is a good choice for providing an alternative to an electric motor for pumps. Why, then, do so many Diesel engines with seemingly low usage have disproportionate degrees of damage?
Combustion engines, and Diesel engines in particular, can be divided into two categories of use: continuous duty and stand-by. Continuous duty engines are started up and operated for long stretches of time – or at least the application intends for long operation. The engine in your car, for instance, is classified as a continuous duty engine – if enough fuel is supplied, it could run dozens of hours before needing to be shut-down for maintenance. Stand-by engines are operated quite differently – stand-by applications have an engine sit waiting for use for long periods of time, with short cycle operation on an unscheduled basis. Emergency fire pumps are a good example of an application for a standby engine.
To understand damages to fire pump engines, knowledge of how the fire pump engine fits into the fire suppression system is required. A fire pump engine is self-reliant; it has some controls, usually with a back-up power supply, to tell it when to start; a battery for starting; its own fuel supply with gravity feed; and cooling water from the fire water piping through a heat exchanger. The engine drives a fire pump – usually a horizontal centrifugal pump – which draws water from an independent tank and is isolated from the fire suppression piping by either automatic valves or non-return valves. Manual isolation valves are fitted on the supply and discharge from the fire pump and a recirculation loop can be provided for testing.
The method of providing cooling for the engine from the fire water piping is a major root cause of damage to stand-by Diesel-driven fire pumps.
Two case studies illustrate this point:
1. During an annual test of a Diesel-driven fire pump, the technician closed the door to the fire pump room and left to do an errand. Smoke or steam was noted coming from the room, and the fire department was summoned. The room was entered and the engine stopped. Evaluation after the incident revealed the supply and discharge isolation valves were closed and there was no recirculation piping; the water in the fire line did not have the flow required to supply the engine heat exchanger and the engine overheated. Damage was limited to distorted cylinder heads and burnt pistons.
2. At a potable water pumping station, high winds caused a tree to fall on the electric motor controller for the potable water pumps. A Diesel-driven fire pump for the facility is connected into the potable water supply as an emergency pump in case of power loss; when the electric motor controller was damaged, the Diesel-driven pump started. The blocking valves for the supply, discharge, and recirculation lines were closed; the pump could not move the water. Again, no water flow was supplied to the engine heat exchanger and the engine ran until it seized. Examination revealed no coolant in the engine block and the heat exchanger had evidence of coolant in the water side with melted retaining brackets.
As these two cases illustrate, it is important to maintain flow to the engine heat exchanger from the fire line piping for Diesel-driven fire pumps. The heat exchangers on these engines are small relative to air-cooled radiators on other Diesel engine applications; they require constant water flow to cool the engine properly or the engine will overheat and be damaged. It is important when testing these fire pump engines, to ensure proper water flow is available to the heat exchangers. Also, when the testing is complete, the fire pump should be left ready to perform its function – this will ensure the cooling water is there for the engine, too.