Throttle or Pressure mode?
There's good logic to both sides of Throttle mode or Pressure mode question, HFD, but on the balance, I lean toward pumping relay in Pressure, partly for the reasons you give. The argument that has been advanced for pumping in Throttle mode is that in Pressure mode, under some conditions, a "hunting" situation could begin. That's where one engine reacts to a change, then the next one reacts and so on down the line. Supposedly you wind up with everyone is out of sync with one another. The few times that I've been in relays like that and we were all in Pressure mode, that didn't occur. The units responded to changes at the nozzles and all worked well.
Stay safe out there, everyone goes home!
Pressure Governor answers
I am the North East Regional Manager for Fire Research Corporation. All of the information that is provided in this post will be specific to the Fire Research product line. How ever the basic theory of operation for any pressure governor is the same, but each manufacture uses their own concept, formulas and values to make the governor function. My descriptions are based on the governor being in “PRESSURE” mode unless other wise stated.
The first thing that I will address is a 2 stage pump. A pressure governor does not know if a pump is a single stage or a multi stage pump. The pressure governor will increases or decrease the engine RPM to maintain the discharge pressure that is selected by the operator. The discharge pressure is supplied to the pressure governor by the discharge pressure sensor in the form of an analog electrical signal that changes proportional to discharge pressure in the pump. The pressure transducer if correctly installed will be in the discharge manifold of the fire pump. Some fire pump manufacturers provide a port in their casting for the main discharge pressure gauge line or transducer. There are some advantages for operating a two stage pump in volume that will affect the performance of a pressure governor. The first is when operating with a high intake pressure if the transfer valve is in pressure/series mode the discharge pressure at idle will be higher thus decreasing the window of operation for the governor. The second advantage to operating in volume/ parallel mode is the engine RPM’s will increase more to accomplish the same pressure change compared to operating in pressure/series. This provides for a little better resolution for the governor’s control. How ever an operator will probably not be able to tell a difference anyway.
The next thing to discuss is the pressure transducer detecting air in the pump. A pressure transducer can not detect air. The pressure governor reacts to air in a pump because a fire pump is a centrifugal pump which means it can not pump air. If air is introduced into a pump the discharge pressure in the pump will drop and the governor will increase the engine speed attempting to maintain the discharge pressure selected by the operator. The pressure governor will not drop to idle from a small amount of air in the pump. If the water supply is not sufficient for the amount of water that you are trying to discharge and the discharge pressure drops below 45 PSI the pressure governor will limit the engine speed to a 1100 RPM’s, if the pressure drops below 15 PSI the engine will be returned to idle. If a water supply is reestablished and the discharge pressure rises above 45 PSI the governor will increase the engine RPM and return to the pressure selected by the operator.
The pressure governor does not monitor the intake pressure of the fire pump so a rapid 30 PSI change on the intake pressure would only be detected by the governor because as a result of the intake pressure changing the discharge pressure will drop. If the governor will increase the engine RPM to return the discharge pressure to the pressure selected by the operator. This is assuming that the discharge pressure did not drop below 45 PSI.
The reason that it is recommended to establish a draft in “RPM” mode is to reduce rapid changes in the engine RPM by the governor due to air pockets in the hard suction and pump. After you have established your prime and have a steady flow of water from the pump it is recommended that you switch to “PRESSURE” mode and allow the governor to do its job.
When switching the water supply source from tank to hydrant, draft to hydrant, or draft to relay, water flow through the pump can become turbulent and the positive pressure from these sources may generate a sudden pressure surge. It is recommended that the governor be set in “RPM” mode before changing the water supply source. After you have completed changing your water source, switch back to “PRESSURE” mode of operation.
When relay pumping or charging a large diameter hose with water it is recommended that the pressure governor be operated in the “PRESSURE” mode. If you are operating in “RPM” mode you have very limited pressure protection and the operator must manually control the pump to protect against water hammers and maintain a consistent discharge pressure. If your pressure governor is dropping to idle when you charge Large Diameter Hose you may be opening your discharge valve too rapidly and the pump discharge pressure is dropping below 45 PSI. Open your valve a little slower and do not open the valve completely until the hose is completely full of water. (This will also protect your hose from damage caused by a water hammer.)
I hope the information that I have provided has answered some questions. If you would like additional information about the operation of a Fire Research product feel free to e-mail firstname.lastname@example.org. Please provide a way for me to contact you. You can also post your question here and allow others to learn the answer to your question as well. Additional information is available from our web page fireresearch.com.