RPM Mode Vs. Pressure Mode on a Tanker

[kuh shise]

Change-over Pressure spike

jercvfd:

The original pressure governor designed by hale in the 1950's were all installed on gasoline engines. When the throttle was decreased by the spike in pressure as you changed over from tank water to hydrant water, the throttle plate closing caused a vacuum increase in the intake manifold. This vacuum is applied to the pistons during the intake stroke, while the crank case breather allows atmospheric pressure to be applied to the bottom of the piston. The differential pressure immediately slows a gasoline engine. Since pump discharge pressure is proportional to rpm, you get an almost immediate reduction in pump discharge back to the set point.

A modern diesel engine has a turbo that is stuffing fresh air into the "inter-cooler" and then into the intake of the engine. You might think of the intercooler as an air tank that will continue to push the engine around until the rpm's coast down to the set point. Something like not using an engine brake on a down hill. The diesel will continue to roll, but a gasoline engine will slow down.

The installation of a pressure relief valve would solve your problem if the pump operator had set the relief valve just above the desired set point for the preconnect lines. We have modified several of our engines by adding relief valves in our own shop. Your pump operators need to be instructed how to use the relief along with the Class I pressure governor, because it is very easy to set-up a war between the two different controllers. If your engineers decide to install a relief valve system, I would suggest using a Hale relief, since the Watrous works in a way that could make it fail in the open position thus preventing the engine from building pressure.

Kuh Shise

[nuthead]

Dacron hose, not leather? What is next, getting rid of rotary and piston pumps?

Piston pumps still have their place....for priming the pump.

[bobsnyder]

I have heard from a few people say with the class 1 governor's that if you are drafting water you should use the RPM mode, is this true? or has anyone else heard this?

I prefer it. If you're working in pressure mode and you have a problem pulling a draft for some reason (or a problem develops after you do), the pump can momentarily "run away" on you (until it hits the engine governor and the system shuts itself down). In RPM mode, this won't be an issue.

I've never seen a governor system with a pressure relief valve, let alone an automatic one. What's the point of having both?

I actually know exactly where to find a few. These companies put the pressure relief on the pump to serve as a safety against surges when using handlines. Don't forget...a pressure governor maintains pressure by adjusting engine RPMs to match its pressure setting. If a hydrant or a supply pump operator hit it with enough pressure on the incoming side, the pressure governor can do only so much, so fast to stop it. Once the incoming pressure overcomes the pressure goveror, it's all over and you need a direct mechanical means (gating back on valves, etc.) to control the pressure. An intake relief valve wil do almost the same job by defending against incoming surges or overpressurized lines, but won't help you with runaways during drafting, or similar malfunctions.

[kuh shise]

Pressure relief and pressure governor - Pa. Fire Academy

Back when "Mac" first got the KME for the academy at Lewistown, he encountered this same problem, so both systems were installed to prevent the spike at change-over.

[ccfdblehman]

I thought priming pumps were rotary vane pumps not piston pumps...

Or are there some of both?

[16Scott]

Clarifying pressure vs. RPM for Tankers

. . ."Pump in "pressure" mode. That is the main point. With most set-ups, is the only way that your pump governor is going to work properly."

I disagree but let me explain. If your tanker is supplying an engine also equipped with a governor, the engine should definatelt be pumping in "pressure." However, the tanker (or any other apparatus that is supplying an engine) should pump in "RPM" mode. The tanker has no need to regulate pressure - that is the job of the attack engine. With both governors trying to regulate pressure you may experience constant pressure oscillations as the two pumps try to "battle it out" in an effort to maintain pressure.

. . ."I have heard from a few people say with the class 1 governor's that if you are drafting water you should use the RPM mode, is this true? or has anyone else heard this?"

It is theoretically impossible to draft in PSI mode because the pump is dry. The PSI mode utilizes a pressure sensor (transducer) inside the discharge manifold. When the pump is dry the transducer reads ZERO pressure which will not permit an increase in RPM. Drafting in RPM mode turns off the transducer. Following a successful draft and charging lines, change to PSI.

[16Scott]

Tankers and Governors

. . ."Pump in "pressure" mode. That is the main point. With most set-ups, is the only way that your pump governor is going to work properly."

I disagree but let me explain. If your tanker is supplying an engine also equipped with a governor, the engine should definatelt be pumping in "pressure." However, the tanker (or any other apparatus that is supplying an engine) should pump in "RPM" mode. The tanker has no need to regulate pressure - that is the job of the attack engine. With both governors trying to regulate pressure you may experience constant pressure oscillations as the two pumps try to "battle it out" in an effort to maintain pressure.

. . ."I have heard from a few people say with the class 1 governor's that if you are drafting water you should use the RPM mode, is this true? or has anyone else heard this?"

It is theoretically impossible to draft in PSI mode because the pump is dry. The PSI mode utilizes a pressure sensor (transducer) inside the discharge manifold. When the pump is dry the transducer reads ZERO pressure which will not permit an increase in RPM. Drafting in RPM mode turns off the transducer. Following a successful draft and charging lines, change to PSI.

[MColley]

I have heard from a few people say with the class 1 governor's that if you are drafting water you should use the RPM mode, is this true? or has anyone else heard this?

Yes that is true. We run 2 apparatus out of our station with class 1 governor controls. And you should be in rpm mode to draft. As well as if you are any truck in a relay other than the attack engine.

[txgp17]

As well as if you are any truck in a relay other than the attack engine.

I disagree, there is no pressure protection for the other engines, aside from their own intake relief valve, unless you have a governor and a discharge relief valve, which is extremely rare.

If all the pumps in a series are cranked up to 3,500 RPM (that's pump speed, not engine speed), and the attack engine suddenly reduces his flow, then the friction loss instantly disappears. But the governor keeps the pumps cranked up at a high speed. You go from having a relay to giant 3 or 4 stage pump. That is dangerous.

Centrifugal pumps are known for their ability to take advantage of intake pressure, and their ability to add net pressure to the water. When the friction loss is removed, this pressure compounds with each impeller in the system. Something has to give, and it might be something you were counting on.

When you're in a relay, your governor should be in the PRESSURE mode.