Relief Valve

[koziol41]

CAN ANY ONE TELL ME WHAT A RELIEF VALVE DOES AND WHEN NOT TO USE IT? THANKS.STAY SAFE OUT THERE.

[GreggGeske]

The two most common types of relief valves used on a fire apparatus are the discharge relief valve and the intake relief valve. Both are used to prevent excessive pressures caused by changes in the conditions under which the system is operating. For example if two discharge lines are deployed and flowing, and one nozzle is closed, the relief valve opens by-passing some of the discharge pressure to the intake side of the pump, so that the other hoseline does not realize the pressure. The only time a discharge r.v. should not be on and set is when only one line is being used. Engine governors have become more popular and use engine speed to control pressure changes. An intake relief valve will dump water to atmosphere to bring the pressure down to a pre-set pressure on the intake side of the pump.

[SCOOBY14B]

I disagree with GreggGeske. A relief valve should be set EVERYTIME a change is made on the discharge of the pump. It SHOULD be used with only one line. What if you are operating one line until you get a positive water source. When you make your changeover and youare sitting on a very good hydrant. Where is all that pressure gonna go?? To the ONE line. Our SOP states that the relief valve will be set after the FIRST line is put into operation and reset EVERYTIME ANY change is made.

koziol41

The relief vavle works on the discharge of the pump. Therefore it has nothing or does nothing to incoming water. Also, the relief valve should NEVER be used to "control" pressures. It is only for an instance where a "surge" encounters the pump. If your relief valve is activated for more than a few seconds at a time, then you need to reset it.

[GreggGeske]

Scooby14B

You are correct. I should have phrased that differently. If pressures over 300 PSI are required some relief valves need to be shut off. When pressures over 300 PSI are used only one line should be used. When switching from tank line to hydrant line the operator should not depend totally on the relief valve. In your example, lets say that the hydrant pressure was 120 PSI, the lowest the relief valve could lower the discharge pressure to would be 190 PSI because of the 70 PSI differential required. Care should be taken to reduce engine RPM when switching from tank to hydrant.

[SCOOBY14B]

GreggGeske

I dont understand your statement about the 190 psi thing? Could you please clarify. It has been my experience that if I am flowing one line @ 120 psi and make my changeover the relief valve will catch about 130-150psi. I would have it set at 130-140 psi anyway.

[GreggGeske]

Scooby14B

"It has been my experience that if I am flowing one line @ 120 psi and make my changeover the relief valve will catch about 130-150psi. I would have it set at 130-140 psi anyway."

In this example for the relief valve to control the pressure down to 130-150 PSI the hydrant pressure could not be higher than 60-80 PSI. The discharge relief valve will operate between 70-300 PSI net pump pressure. (NFPA requirements are 90-300 PSI with pressure surges no greater than 30 PSI) The lowest the pressure the relief valve will operate on a 120 PSI hydrant is 190 PSI (hydrant pressure + 70(worst case) PSI).

I do not know if you want to read all of this but maybe it explains it better.

When operating from draft or from a booster tank, it is
necessary only that the relief valve bypass the same
flow that is shut off, so that the torque load on the engine
will be kept the same; the rpm then will stay the same,
so will the net pump pressure, and as the intake pres-sure
won’t rise significantly, neither will the discharge
pressure. In this case, as long as the flow that must be
bypassed can flow through the relief valve without caus-ing
more friction loss than the net pump pressure, the re-lief
valve will maintain the discharge pressure at the set-ting
of the pilot valve. The larger the relief valve and the
higher the net pump pressure, the more water can be
bypassed before the friction loss gets too high. The relief
valve does not open fully until the maximum flow is being
bypassed; at lesser flows the pilot valve modulates the
pressure on the relief valve piston so that it opens just
far enough so that the friction loss through the relief
valve and piping is equal to the net pump pressure.
When operating from a hydrant connected to a high ca-pacity
water distribution system, the intake pressure
may be quite high, but it won’t change significantly when
the flow rate changes. Usually the net pump pressure
won’t be as high as when operating from draft, so less
flow can be bypassed without causing friction loss
through the valve system equal to the net pump pres-sure.
With a intake pressure of 50 psig, the minimum dis-charge
pressure which the relief valve will hold with a
given reduction inflow is 50 psi higher than when operat-ing
at draft. As an example, if a relief valve will hold 90
psig discharge pressure when operating at draft, with a
750 gpm reduction in flow, then it will hold only a dis-charge
pressure of 140 psig or higher when operating
with 50 psig intake pressure with the same reduction in
flow. It will not hold 90 psig under these conditions be-cause
the friction loss through the relief valve will be
more than the net pump pressure of 40 psi. Instead, the
flow will be reduced, the engine will speed up due to de-creased
torque load, and the net pump pressure will be
increased. The system will stabilize when the flow
through the relief valve and any discharge lines still
open, and the net pump pressure (equal to the friction
loss through the relief valve and piping) causes a torque
load equal to the torque transmitted to the pump by the
engine.
When operating in relay, or from a hydrant at which the
residual pressure will vary appreciably with different flow
rates, the problem becomes more severe. When the
flow through the pump discharge valves is reduced,
then flow through the hydrant is also reduced and the re-sidual
pressure at the hydrant is increased. Conse-quently,
the intake pressure at the pump is also in-creased.
In order to hold the discharge pressure, then,
the net pump pressure must be reduced. The only way
this can be accomplished is to increase the flow through
the pump. This will tend to decrease the net pressure
even if the rpm is not reduced, but it will also increase the
torque load on the engine and thus reduce the rpm and
likewise the net pressure.
If the reduction in flow through the discharge valves is
relatively small (one line shut down), the increase in
intake pressure won’t be very high, and if the net pres-sure
was high enough originally, the relief valve can by-pass
a sufficient flow to increase the load on the engine,
slow it down, and decrease the net pressure by an
amount equal to the increase in intake pressure and
hold the discharge pressure constant. But if, for in-stance,
with a discharge pressure of 120 psig and a
intake pressure of 20 psig, 750 gpm is shut off and the
intake pressure increases to 50 psig, the net pressure
must be reduced from 100 psi to 70 psi. To do this, the
relief valve would have to bypass considerably more
than 750 gpm with a friction loss of 70 psi. This would
take a large valve, as probably at least 1000 gpm would
have to be bypassed at 70 psi friction loss.
Under almost all operating conditions, a relief valve
does a completely satisfactory job of controlling dis-charge
pressure. The following general rule should be
remembered: The higher the net pump pressure, the
greater the reduction in flow can be without increas-ing
the discharge pressure. This means that:
1. When operating from a hydrant or in relay,
when the intake pressure is 10 psig or more, a
relief valve will not control the discharge pres-sure
with as great a reduction in flow as when
operating from draft.
2. If the intake pressure can vary appreciably, a
relief valve will control the discharge pressure
with only a moderate reduction in flow.

[Scoop422]

I was taught to use the pressure relief valve every time including with only one line because of the hydrant pressure. Where I work the hydrants have a pressure of about 70 psi so if you don't throttle down right, or if you have a mid-ship pump panel and can't throttle down and open the intake together, then the guys at the end are going from 100 psi at the nozzle to 170 psi at the nozzle. So in short I guess we use it to protect the guys from the hydrant pressure too.