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How does a pressure reducing valve work?

  1. Jan 14, 2008 #1
    I'm trying to understand how a pressure reducing valve works. I'm puzzled:
    Water enters one side of the valve. Whatever opening the valve gives for the water to flow through, won't it come out with higher pressure? If the static pressure is less on the outlet side, doesn't that indicate that there's no physical contact between the inlet and outlet side? And as soon as water is drawn, won't this contact be established and high pressure reach the system?
  2. jcsd
  3. Jan 14, 2008 #2


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    It's really very simple on the surface. The fluid pressure is reduced by taking a permanent pressure drop across the valve. This is accomplished by a variable flow area inside the valve. The typical set up involves a spring and diaphragm that are connected to a pintle or other device that regulates the flow opening in the valve. The spring is adjusted to the desired outlet pressure by compression or relaxation. The incoming fluid pressure reacts against the spring/diaphragm force to create an equilibrium of forces. So if the incoming pressure goes up, the force on the diaphragm goes up and causes the spring load to increase. This will move the pintle to close off the flow area. When the inlet pressure drops, the load on the diaphragm decreases and the spring extends to lessen its force and causes the pintle to open the flow area.

    Last edited: Jan 14, 2008
  4. May 18, 2010 #3
    I have wondered this for years and I think I just figured it out. I could never understand that if water can get through, how can pressure change no matter HOW small the opening? The common explanations I find on the web are explanations of a mechanical device, but never of an intuitive nature. Here's a (hopefully) intuitive way (if I'm right).

    If you turn on the valve to your garden hose at a very low setting to where water is just steadly trickling out, there is NO pressure on the hose, but you do have water flowing through. Now, slowly open the valve to where you have your desired pressure in the hose, say 40 psi. AS SOON as it hits 40 psi, then you close it; pressure will drop. You are fast enough that as soon as you detect 39 psi, the you open it quickly a little till you get back to 40. You do the same thing over and over with ultra quick reflexes achieving the 40 psi steadily! That's about what the spring and the diaphram is doing (except constantly).

    So you CAN lower the pressure even though there is a continuous flow of water!

    I hope that explains it intuitively! -tony17112acst
  5. May 18, 2010 #4


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    I think it would be more appropriate to say that it is impossible to NOT lower the pressure when there is flow.
  6. May 21, 2010 #5
    Im not sure how accurate this is, but i think based on the conditions in the outlet, the valve will always drop the pressure to the required level, no matter what the inlet conditions are. If the outlet conditions require a pressure of 10bar, the valve will drop the pressure from 100 to 10 or from 40 to 10.

    But with higher pressure drops you have problems like noise and vibration. Velocity in the valve is another problem which causes damage to the valve internals.

    In the valve industry today, you have valves which have a number of stages to drop the pressure to a given level.
    Its basically like letting water fall from a height. If you let water just fall you would drop the pressure and have an increase in velocity which causes damage to the ground at the bottom, thus most valve companies use stages, its like letting the water flow from the same height but over a number of stairs.Thus the pressure is dropped over a number of stages.

    Another problem with large pressure drops is that you could have the pressure fall below the vapour pressure and recovering to the required level, thus causing cavitation in the valve which damages the valve internals.
  7. May 21, 2010 #6
    If you look at a pressure reducing valve that is laid out in-line then it takes a certain amount of pressure to overcome the spring and plug at the inlet side. The resulting output pressure is inlet pressure minus the pressure that is overcome at the entrance.
    The result, under some conditions, will be chatter (the rapid opening and closing of the valve) but this can be overcome by using pilot pressure to regulate flow - it is a different kind of valve (pilot operated relief) but it is less prone to chatter and inlet pressure changes.
  8. Jul 24, 2010 #7
    Thanks, FredGarvin. At last, an explanation I think I understand. Here, I have a maximum water pressure incoming of 9.7 bar. Our instant water heating appliances call for a maximm of 6 bar. Sometimes the appliances appear to fail after some 6 months, with cold water escaping from the safety valve of the heater. The reducer is replaced and all is well for another 6 months. Should they cut off water flow completely if there is no demand at the output side? Simple physics tells me if there is even a small open path between input and output then full pressure will be applied to the turned off appliance, operating the 6 bar safety valve. The plumbers say No, it is just a(nother!) faulty reducer. We may have to change to a different water heating device than these "instant" ones, as the leakeage costs of water are mounting up.
  9. Nov 26, 2010 #8
    Solved. A "pressure reducer" only works if there is a flow through it. Block the outlet pipework, such as turn off the faucet/basin tap/hose tap/sprinkler tap---there will be full pressure at the outlet unless the reducer springs shut off the valve completely when there is no flow. Basic physics, like I was taught in 1953. No salesman understood this; and as old guys, we thought technology had overtaken us! Sorted out our problem by getting a water heater that handles 10bar. No leaks, dribbles or high water bills. Don't believe the sales blurbs/plumbers on these things.
  10. Nov 26, 2010 #9


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    Just for clarity, a pressure regulator, which is a type of pressure reducer, does not require any flow to reduce the pressure.

  11. Jun 15, 2012 #10

    I just came across this forum after similarly scratching my head. There is a great page on Wikipedia showing a pressure regulator value but it doesn't fully explain the physics. http://en.wikipedia.org/wiki/Pressure_regulator

    Having read this forum a few times I think I understand but wanted to summarise.

    As mentioned, a pressure regulator cannot reduce the outlet pressure in a static system. However, once water/air is flowing it can restrict the inlet such that the flow is reduced. This, in turn reduces the pressure on the outlet side. As the outlet flow drops, the valve closes tighter and tighter until the flow stops completely. At this point the system is static, but with a lower pressure on the outlet than the inlet because they are not actually connected anymore!

    I don't believe that the valve has to be dynamic to work, as suggested in the hosepipe example, but probably has a certain dynamic response which will wear it out over time.
  12. Aug 12, 2012 #11
  13. Aug 13, 2012 #12
    A related item;

    I formerly tested hydraulic valves many years ago, and using a pump to force oil through a spring loaded poppet will produce a pressure drop, and the oil passing through it will warm up.

    As I recall, it was roughly 10 degrees F per 1000 psi. Sticking my finger in the discharge flow, I noted the temperature changed instantaneously, and in proportion to the changes in the compression of the spring affecting the pressure drop across the device.

    Neat demonstration of how the potential energy of the pressurized fluid manifested as heat in the discharge flow.
  14. Jul 2, 2013 #13
  15. Oct 3, 2015 #14
    Time lag? Is there a time lag of maybe only a second or two, or more, between the time the maximum water pressure of say 120psi enters the valve and the time it reduces to 60psi? My water outside spigot gauge has a red and black needle. At night the red shows a spike of 120 but I never see the black move past 50-60 psi. I'm thinking the water spikes briefly to 120 but quickly reduces to the norm. This problem is driving half our neighborhood crazy. We just got our first public water utility, and our first pressure reducing valves. We are worried about our inside house pvc lines holding up under 120 psi instead of 60. I'm thinking there is just a time lag between the spike and the reduction, and that there is really nothing to worry about.
  16. Oct 27, 2015 #15
    When reading those, I can see some Voodoo explanations, which is pretty disturbing :P.
    How does reduction valve work? Well, simply put, it is an element, which has big hydraulic resistance. If you are familiar with electric circuits, and how does work, you should be able to grasp this idea pretty easy - imagine your hydraulic system being electric circuit.

    Flow is current, pressure is voltage. From the most basic ones, we also have resistance. But is there something like this in hydraulic (fluid) systems? Sure there is!
    If you have a straight pipe, which is long enough, it will work as reduction valve as well - even tho it is not easy to see, pipe does have its own hydraulic (fluid) resistance.
    From where it comes from? Were, mainly viscosity, turbulence, energy dissipation and so on. Pipe having many turns can make it have more hydraulic (fluid) resistance (which is turn means higher pressure drop!). In the most easy words, the easier it is for fluid (well, I was using hydraulic all the time, but this applies to all fluids, in fact) to flow, the less resistant the element is, and less of a pressure drop you will get.

    Reduction valve is simple resistor - its construction makes it hard for water to flow through, giving us big hydraulic resistance. It, of course, allows us to set how much "harder" it is for water to flow, by decreasing or increasing the gap (this is done by many different ways).

    It has nothing to do with "time lags" or any other Voodoo stuff. :P
  17. Mar 8, 2016 #16
    can pressure reducing valve can be used to cut off flow at a maximum pressure
  18. Mar 10, 2017 #17
    howdy gents!

    another reason you're seeing pressure spikes is due to thermal expansion when the system is static and the water heater fires up to heat the cooled off water in the tank that was drawn out. so no water is flowing which means that if you have an incoming pressure of 150 psi then the regulator reduces it to 70 psi, so now when the system is static the the incoming 150 psi overcomes the diaphragm and spring which closes the flow of water and now you have no way for the thermal expansion to relive itself until the 70 psi (low side) reaches 151 psi. the problem is that the temperature & pressure relief vale on the water heater (Tank/tankless) is designed to bleed off pressure when it is too high and then is full open at 150 psi. in my experience prv valves start to bleed off the extra pressure at as low as 125-135 psi. this is why it Byan L is watsing water because the expanding/heated water is causing his prv to leak. this easily solved by installing an appropriately sized thermal expansion tank.

    150 psi coming in
    regulator lowers it to 70 psi
    no water is flowing (Like when your sleeping)
    the water heater turns on to heat the cooled off water and the average 40 gallon tank expands approx. 1/2 gallon in volume
    the extra volume has nowhere to go and it has a "block wall" stopping it from returning to the high pressure side of 150 psi
    this causes the prv to leak.

    if you currently do not have a thermal expansion tank. do this:
    first thing in the morning when you first turn on the bathroom faucet pay close attention to the flow and you'll notice it starts out very strong and then fades off once the thermal expansion has relieved itself.

    Stay thirsty my friends!
  19. Feb 19, 2018 #18
    Actually, a PRV is a valve that opens and closes, not precisely like Perq's vodoo comment "an element with big hydraulic resistance". When the valve is completely open there is no friction loss except the valve minor local loss. Of course, when the VRP is somewhere between the open and close position, there will effectively be hydraulic resistance, and you may compare the inlet and outlet gages to verify this. This may also happen with any valve that opens and closes manually: if you have an outlet gage that shows rising head, you manually close the valve to produce energy loss until the outlet gage reading reaches the desired head. The PRV's work in the same way except with a pilot valve sensing the gage readings (the differential pressure) and an actuator instead of your hand; if the desired outlet head is exceeded, the valve actuator proceeds to close the valve until the calibrated outlet head is reached. This is well and good. The problem arises when the outlet pressure has a sudden rise of excessive magnitude, as when a pipeline is shutoff at a low point long way downstream. The shut-off will cause a significant pressure surge due to static rise in the pipe which will greatly exceed the calibrated PRV outlet head, causing the PRV to completely shut down. The conundrum for us hydraulic engineers here is that the excessive high pressure downstream of the PRV is trapped and the pipes may not stand the strain. This is what happens in water distribution networks at night. When all domestic connections are shut down and there is no flow (e.g. 2 or 3 am), high pressure is trapped in the network after any PRV. Thus the leakage and water loss concerns of water utilities world wide.
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