How does a pressure reducing valve work?

  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. FredGarvin

    FredGarvin 5,087
    Science Advisor

    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. 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. I think it would be more appropriate to say that it is impossible to NOT lower the pressure when there is flow.
  6. 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. 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. 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. 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. stewartcs

    stewartcs 2,279
    Science Advisor

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

  11. Hi,

    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.

    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. 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.
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