Howe does a pressure reducing valve work

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by bobbobwhite, Jan 25, 2008.

  1. bobbobwhite

    bobbobwhite New Member

    Oct 30, 2007
    Need to install a new water pressure reducing valve as the old one failed and I have 100 lbs now coming out faucets like a fire hose. Worried it may blow my clothes and dishwasher hoses if I don't fix it soon.

    My local plumbing house said that "pressure and volume are different; you don't reduce water pressure by reducing the volume". But, he didn't tell me how the device reduces pressure. How does it work?

    Someone will know here, but it seems to me that volume has to have a part in it. Thinking like an inventor, wouldn't aerating the water with 50% air reduce pressure by half without changing any other thing? Or, not?

    What is the answer?
  2. Bob NH

    Bob NH In the Trades

    Oct 20, 2005
    New Hampshire
    Here is a link to a diagram and description of the operation.

    The spring which is adjusted by the adjusting screw acts against a diaphragm that is pressurized by the output pressure. If the pressure is too low, the spring force acts on a valve mechanism to open it, admitting flow from the high pressure source. If the pressure it too high the force acts against the spring so the valve will not be opened.

    Different regulators may have different details but they act on the same principle.

    Increasing the flow often causes the output pressure to be lower because there are pressure losses in the mechanism. The loss of pressure is sometimes called "droop". Regulators are often rated by "droop" versus flow rated.

    Some regulators use a pilot mechanism (more comples) that permits greater flow with less droop.
    Last edited: Jan 25, 2008
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  4. Gary Swart

    Gary Swart In the Trades

    Sep 1, 2004
    Yakima WA
    The difference between pressure and volume is confusing at first. The subject usually comes up when people complaint they have no pressure in a fixture. What they really mean is they can't get enough volume out of the fixture. Frequently this occurs in older homes with galvanized pipes that have corroded inside to the size of a soda straw. If a pressure gauge was put on the fixture, they would find they had plenty of pressure. So, while you do have to have some pressure to push the water, when the pipe size is reduced, the volume of water decreases. Increasing the pressure would help the flow only marginally. I'll yield to Bob's technical knowledge on just how a PRV reduces pressure without effecting volume. By the way, when you install a PRV, you must also install an expansion tank.
  5. bobbobwhite

    bobbobwhite New Member

    Oct 30, 2007
    For Bob from Bob

    As you state it, flow from higher pressure is reduced by the diaphram spring, but when pressure is increased, how does pressure increase without increasing flow? Cannot fully understand these separate functions operating independently. Maybe need a cutaway diagram along with the physical law in action within the valve. The way I initially thought it reduced pressure is through aeration, i.e., 50/50% water/air ratio thus also reducing water volume but not reducing total area of flow. But, no.

    I well understand the concept of gradually reducing pipe size working with sufficient gravity to increase pressure(as with the Roman aquaducts), but not the above concept.
  6. Bob NH

    Bob NH In the Trades

    Oct 20, 2005
    New Hampshire
    The pressure doesn't increase without increasing flow.

    A pressure regulator is designed to have constant output pressure regardless of the flow.

    However, the control pressure in the cutaway view at the link that I provided is the pressure inside the regulator acting on the diaphragm. If there is little or no flow through the regulator the pressure will correspond to the regulator setting.

    In the diagram at the link in my earlier post, when the output pressure is lower than the setting corresponding to the spring position, then the upper diaphragm plate will have less force from the gas and the spring will push the diaphragm plated down on the small spring-loaded rod coming up from the bottom. When that rod is pushed down it opens the orifice at the gas inlet at the bottom.

    When the gas causes the pressure to rise to push the diaphragm up, then the small spring on the top of the rod causes the rod to mpve upward, thereby closing the orifice.

    Those internal parts achieve an equilibrium so that the orifice is opened just enough to supply the flow necessary to maintain equilibrium of the upper spring and the pressure forces on the diaphragm.

    If there is a lot of flow through the valve, there are two things that contribute to the delivered pressure being less than the set pressure (the "droop").

    The primary cause of droop is that for higher flows the valve must open farther to let more fluid through the valve orifice, which requires greater extension of the spring. When the spring extends more it exerts less force against the diaphragm, so it takes less pressure to hold the actuator in equilibrium. Since that control pressure is also the pressure delivered to the outlet of the regulator, the result is lower delivered output pressure.

    The second cause of lower pressure at higher flow is the greater pressure losses in the regulator fittings.
  7. fidodie

    fidodie New Member

    Jan 22, 2008
    new jersey
    Here is an academic description of volume vs pressure

    - because it is academic, the pressure in the system is constant. i know my well-fed house cycles - drops to 45psi, then up to 60psi, but ignore things like that.
    And ignore friction in the pipes, and poorly designed turns and Ts.....

    think of a bowl of water - the pressure is equal across the flat bottom of the bowl and proportional to the depth of the water (doesn't matter how big the bowl is, only how deep, linear proportion)

    put a pin hole in it and the pressure delivered by the water is the same as if you peeled the bottom off at the instant of opening it - the pin hole would only allow a small volume to get by, while the large opening would allow alot more (this geometrically proportional)

    Now consider that there is a water supply to the bowl that matches the outflow, no matter how big or small the opening without changing any other variable (if it were a waterfall, it would impart force on the top of the water - so just say it magically maintains the level) - thus constant pressure, variable flow - you do it everyday with your kitchen sink - the lever makes variable flow!

    we haven't even started talking about velocity (inversely proporional to the hole size) - which is really what people want coming out of their showerhead.
    the pin hole shoots the water because of initial velocity. think of a showhead that passes 2.5 gpm but only has one hole, vs one with many small holes also 2.5gpm. remember pressure is constant (and drops to 0 as soon as it leaves the opening)

    don't think of your supply as being pumped, think of it as a big tank on the hill, it has no velocity, infinite volume, and fixed pressure.

    As an aside: our homes are not an academic exercise, and professional experience puts the theories to work , with all the tweaking they need to work in the real world.....

    any help?

    here is the bridge to electricity - maybe that will also help

    The Water Connection

    I love a good analogy. And the electricity/water one is one of my favorites, but it does have its shortcomings, so bear with me.

    Think of electricity as running through a wire as water runs through a pipe. This can really help it all make sense. Now, lets equate some basic terms:

    VOLTS: This is how fast the water is flowing through the pipe, ("Voltage")
    AMPS: This is how much (volume) of water flowing through the pipe, ("Current")
    POWER: This is force that the water in the pipe can exert (read on, it'll make sense)
    Last edited: Jan 25, 2008
  8. tony17112acst

    tony17112acst New Member

    May 18, 2010
    Harrisburg, PA
    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
  9. hj

    hj Moderator & Master Plumber Staff Member

    Aug 31, 2004
    Cave Creek, Arizona
    Pressure and volume are two sides of the same coin. In a given pipe, if the pressure drops so will the volume, but when you start putting orifices on the opening, such as when you have a faucet, and the orifice size can be varied as by opening and closing the faucet, then you change the parameters, and then you can have the same volume with a lower, or higher, pressure. A pressure reducing valve is like a balance scale if you move the balance point away from the center. Then you can balance a heavy weight, (the incoming water pressure), with a smaller one (the reduced pressure). The volume available will be whatever can pass through the PRV's regulating mechanism and will have NOTHING to do with the sizes of the pipes.
  10. jimbo

    jimbo Plumber

    Aug 31, 2004
    San Diego, CA
    Think of it like this: at the street side, you have a higher pressure than what you want in the house, and within the limits of our discussion, an unlimited available flow rate. Anytime the houser pressure is LOWER than the set point, the valve increases flow until the pressure on the house side IS at the desired PSI. The spring-controlled devices do not control pressure within very tight range, but rather within an acceptabel range for this application.
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