Managing the Transition...

Making the transition from operating off the booster tank on your rig to receiving a supply from a pressurized source; such as a hydrant or nurse tanker, can be a careful balancing act. Preventing over pressure to the attack lines is possible with proper technique and equipment, but there is one situation where pressure relief protection will not work.

The two primary means to control the desired discharge pressure during the transition are the discharge relief valve and the electronic pressure governor.

A Hale TPM Model Discharge Relief Valve Control

Discharge relief valves monitor pressure on the discharge side of the pump by use of a pilot valve/sensing chamber. When the hand wheel or crank is set, any spike in pressure causes the actual valve to open, resulting in the flow of water back to the intake manifold in the pump or dumping it to the ground. This is how the system attempts to relieve the over pressure condition. Discharge relief valves require a differential between intake and discharge pressures. This differential can be from about 25-50 PSI. At high flows, it is possible to overtake the relief capacity of a discharge relief valve.

Many new rigs come with electronic pressure governors, which work well when functioning within their design parameters. These devices use a pressure sensor ( transducer) on the discharge side of the main pump to adjust engine throttle, thereby maintaining the desired pump pressure. This is accomplished in the "PSI" or "pressure" mode. When the sensor detects the increasing pressure as you open the intake, it will progressively ramp the engine down until it reaches idle. Herein lies the problem.



Fire Research Electronic Pressure Governor



Many departments run smooth bore or low pressure nozzles. These nozzles, coupled with high performance attack hose can result in pump pressures below 100 psi. For example, a 200' 2" preconnect with a 15/16" tip will have a pump pressure of approx 90 PSI. If the incoming pressure is over 90 PSI, the governor cannot lower the engine RPM enough to alleviate the pressure surge. Correspondingly, there fails to exist enough differential between pressures for a standard discharge relief valve to work. In fact, since the discharge pressure is lower than the intake pressure, neither pressure protection device will work, and the line(s) will be over pressurized, unless you anticipate and take additional action.

In the example we just reviewed, you must prepare to gate back the discharge(s) as you make the transition. With a situation where incoming pressure is higher than outgoing pressure, there is no other way to deal with the over pressure to the line(s).

You have little control over this when operating from hydrants, but you do if operating from tanker nurse. The best option is to have the tanker driver start pumping to you at 50 PSI when being nurse fed.

As you can see, having an extra hand can be necessary. You must prepare to gate back the discharges when making the transition in the event the pressure protection devices cannot work as intended. As a final note, always make the transition SLOWLY!