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Thread: Relief Valve

  1. #1
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    Question Relief Valve

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


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    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.

  3. #3
    Forum Member SCOOBY14B's Avatar
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    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.

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    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.

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    Forum Member SCOOBY14B's Avatar
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    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.

  6. #6
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    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
    wonít 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 wonít change significantly when
    the flow rate changes. Usually the net pump pressure
    wonít 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 wonít 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.

  7. #7
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    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.

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