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Does a pressure regulator maintain constant pressure drop?

  1. Jan 10, 2013 #1
    I wanted to know the function of a pressure regulator..
    Am using one to control liquid flow rate.

    Here is my understanding-

    A pressure regulator can be connected across a valve.

    The pressure regulator maintains a constant "pressure drop" across the valve
    so if i set it to give a drop of 10 kPa, whilst the pump provides 800 kPa, the pressure of liquid coming out from the valve will be 790 kpa ("Always maintain a drop of 10 kPa").

    The valve is used to control the liquid flow rate - that is the valve is open or closed (by hand revolutions) which changes the area inside the valve changing the liquid mass flow rate.

    How correct is this?
  2. jcsd
  3. Jan 10, 2013 #2


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    Every pressure regulator I've ever dealt with maintains a constant outlet pressure.
  4. Jan 10, 2013 #3
    not constant "pressure drop"?
  5. Jan 10, 2013 #4

    jim hardy

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    Most regulators act as boneh3d described.

    A spring acts on one side of a diaphragm, fluid pressure on the other.
    Usually the spring side of diaphragm is vented and fluid side of diaphragm is connected to outlet port, so diaphragm is balanced at a particular outlet pressure.
    That's a traditional pressure regulator like he described.
    The ones for home air compressors only cost about $5, disassemble one and figure it out its mechanism.

    If instead the fluid side of diaphragm is connected to inlet port, you have a 'backpressure', or 'relief ' regulator.

    There exist differential pressure regulators that operate as you want, but they are a bit less common. I've used a small one exactly as you describe.
    For that application you connect one side of diaphragm to inlet, other side of diaphragm to outlet. Spring then balances the differential pressure.

    Here's a manufacturer's site with technical literature:

    Look for their series 95LD and 95HD - it can be made to do what you want by pressurizing the spring case.
  6. Jan 10, 2013 #5
    Getting high pressure relief was a 2nd part of the problem.
    So here's a schematic with names of components.

    Explanation :

    Liquid comes out from the pump.
    The "differential pressure regulator" is connected across a metering to maintain constant pressure drop as desired.
    The metering valve can be opened or closed to change the flow rates.
    If something goes wrong, or if the shutoff valve (much ahead in the system) is closed, the high pressure relief valve opens after a specific amount of pressure, sending fuel back into the inlet.

    Just verify me if i am going in the right path =)

    Attached Files:

  7. Jan 10, 2013 #6


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    That's not what you called it before. A "differential pressure regulator" would keep a constant differential pressure while a "pressure regulator" would keep a constant outlet pressure.

    The difference is where the reference pressure is. If you reference to atmospheric pressure, you can maintain a constant outlet pressure whereas if you reference to the upstream pressure, you maintain a constant differential.
  8. Jan 10, 2013 #7


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    Hi sgvaibhav,
    I'm not sure what you're doing with the differential pressure regulator but I think your idea is to have the regulator create a constant pressure drop so you also have a given dP across your metering valve. I think you're envisioning this constant dP across the metering valve will give you a constant flow rate. Is that right? Are you trying to get a constant flow rate? This might give you a constant flow rate across the metering valve (assuming you have a liquid) but the flow through the regulator will vary depending on flow in the system.

    Regulators work by opening and closing depending on pressure. If you take a simple pressure reducing regulator with a given upstream pressure and a set downstream pressure, and then you try to increase flow rate downstream of the regulator, the regulator will open in order to provide more flow to keep the pressure downstream of the regulator constant. A differential pressure regulator is going to do the same thing. If you try to draw more fluid, the valve will see a drop in downstream pressure and open to try to maintain a given dP. If that's what you're trying to do, then you're obviously not going to get a constant flow rate.
  9. Jan 10, 2013 #8

    jim hardy

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    Russ and Q_goest are correct.

    In the schematic given you'll need to control either position of metering valve or speed of pump .

    I withdraw my earlier comment and apologize for mis-speaking.
  10. Jan 11, 2013 #9

    jim hardy

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    I need a fresh start, if only to clear my conscience..
    Back to basics:

    In that Emerson document i linked above,

    click the first link, "Regulator Control Theory"
    and read it carefully.
    Then take a look at Fig 1 which is a traditional pressure regulator.
    Observe that its loading pressure is applied to BOTTOM of diaphragm,
    which means it's an "Up to Close" valve. That keeps feedback negative.

    Fig 1 can be modified slightly for your application.
    If you have read the document (it's only two pages)
    you see that to do what you want, you must make "loading pressure" be proportional to the differential pressure across your "load", which would be your metering valve.
    To that end you'd add a connection from downstream side of "load" to top of diaphragm.
    Select "up to close " as in fig 5 so that feedback is negative.

    hope this helps you with the basics.
    I have no idea what size stuff you're dealing with but rest assured there's an industrial gizmo made for it.

    Note the 95*D valve i mentioned accomodates differential loading of its diaphragm.

    Good luck,

    old jim

    here's the link to that '...theory' document, but it's so lengthy i was afraid it might not copy&paste. Easier to get at it through the first link, i think.

    Last edited: Jan 11, 2013
  11. Jan 11, 2013 #10
    Yes i wanted a "constant different pressure".
    So the component that does that would be called "differential pressure regulator" i guess.
    I do not require adjusting this differential pressure value, so i guess any differential pressure regulator would do the job.

    one of my reference book states this.

    "A flow of fluid may be metered by keeping the pressure drop or difference across the metering valve constant while varying the valve orifice. Most modern fuel controls meter fuel by this method, so an additional refinement would consist of a device to maintain a constant pressure drop across the metering valve, regardless of the pressure on either side of the valve or valve opening."

    Are there any equations responsible for controlling flow rate containing dP or dA?

    Thus im trying to make a schematic as per the above mentioned description.
    I will change the flow rate by opening closing the valve (http://img.directindustry.com/images_di/photo-g/metering-valve-62028-2476555.jpg).
    So the schematic i attached is correct? {Do i need an external relief valve to bypass backflow of excessive liquid back to the pump, or a pressure regulator can do this?}

    Umm, i am little confused. Basically i wont change anything with the pump, leave it as it is so that it gives me a peak pressure output depending upon the flow rate and pump curves.
    So i need to make changes to the shown schematic or add/remove any components?
  12. Jan 11, 2013 #11

    jim hardy

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    Yes, see my post immediately prior to yours , where i tried to recover from yesterday's blunder.

    it starts out: ""
    I need a fresh start, if only to clear my conscience..
    Back to basics: ""

    My face is a bit red....

    Probably i posted while you were typing....

    old jim
  13. Jan 12, 2013 #12
    Hi sir,

    Its ok. I had typed the post long ago, and not posted, thus our posts had overlapped.

    Sorry for the delay from my side.
    I read your prior post and the pdf file.

    i think i understood what you meant, but i just prefer verifying my understanding to assure myself.
    new schmatic attached.

    i also checked out 95*D series and 95HD (pressure reducing, high pressure differential regulator) which looked very suitable for my purpose.
    however i checked another category called 98HD (high pressure differential backpressure regulator)... i wonder if that is of any use for my purpose..

    Attached Files:

  14. Jan 12, 2013 #13

    jim hardy

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    don't apologize - i figured you were working on it.

    well - to make it less ambiguous:

    Look at your regulator:
    The upper part of it,
    the part that looks like two bowls clamped together with a column on top,
    is actually two dished pieces of metal joined at that wide spot about its middle.
    That wide spot is a flanged joint. Usually the flange has a circle of machine screws holding the two halves together, sometimes it's just a clamping ring on real small ones.
    The diaphragm is in the plane of that flanged joint and is held between the two lips of the flange. It separates the upper dish from the lower dish.
    That results in two chambers, one with a spring in it,
    as shown in fig 3 of our "theory" link.
    But the spring in ours is above the diaphragm, not below it as in fig3.
    The spring in ours is physically inside that column atop the regulator.

    It is necessary that the bottom of the diaphragm be connected to upstream side of load
    and the top of diaphragm be connected to downstream side of load.
    You've only shown one connection, downstream, and you've drawn it to bottom of diaphragm.

    So fix that.

    Then talk yourself through its action like this:
    IF pressure on downstream side of your load drops,
    diaphragm sees less pressure on top so it moves UP ,
    which closes regulator, lowering pressure on upstream side of your load,
    restoring desired dp across load.

    Do same talk-through for an increase of downstream pressure, and for a change in load valve's area, and all permutations. Then you are working it in your head and understanding it.

    Have you fallen asleep reading this yet?
    Reason for my long, drawn out, simplistic explanation is I wanted to see you grasp the concept better and i think you're nearly there.
    Please dont feel i am talking down to you - i'm just trying to be exact in my communication. So i keep it simple for my sake. I confuse easily. And i often criss-cross my words and say the wrong thing.
    Feedback systems are NOT intuitive and your first few require this kind of study.

    I'm not good with Paint and couldn't make it edit your drawing - that's my weakness not yours. Otherwise i'd have drawn it , but you are perfectly able.

    Be aware that some valves are UP to CLOSE and others are UP to OPEN.
    You'll have to be aware which you have when figuring how to connect the sensing lines.

    You'll have to decide that from the characteristics of your process fluid and the flows and pressures of your process.
    As i said i only did this once, that was on an air system using very small regulator and valve that came as a unit.

    old jim
  15. Jan 13, 2013 #14
    Read that a few times and tried to grasp it all...
    getting excited as i feel im reaching the eureka stage...

    To verify this time - i drew a regulator this time on paper (plz bear my drawing :P)

    Im trying to simulate the situation of what happens as load is increased (just the vice-versa of what you described - for load being decreased).

    So did I finally get it? Is the attached pdf file correct??? (The diagram and simulation of situation 1)

    Also another tiny question - the high pressure relief valve would have to be placed just after the pump? It would be required as a safety element when the shut-off valve is closed muuuch ahead in the downstream section.

    I need this regulator and this metering control system to control the fuel flow rate as a part of my project.
    The flow rate values are relatively small - ranging from 0.3l/min to 0.95l/min.
    However fuel pressures are expected to go about 100 to 150 psi after the fuel pump.
    Need to calculate the required dP now...

    Attached Files:

  16. Jan 13, 2013 #15

    jim hardy

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    Well now--- I see real progress !

    Congratulations !

    Your sketch might accept these two refinements -

    1. Complete the connection of upstream pressure to bottom chanber, perhaps just below diaphragm, and downstream to top chamber perhaps just above the diaphragm..

    2. Move the regulator body left over to where your question mark is, and draw the regulating valve in the line immediately below. Add a rod to connect regulating valve to diaphragm..

    In your real regulator there's a seal around that rod so regulator body is airtight.
    In your regulator there's also a rod protruding out the top which allows adjustment of spring force.

    As i suggested - get a cheap regulator and take it apart. They're ingenious, and doing that will make your thought process even clearer.

    Usually in el-cheapo regulators the valve is simply a hole covered with a plastic disc, moved directly by the diaphragm. The spring pushes directly on the other side of diaphragm.

    That's what we strive for - "eureka's are a joy forever".
    old jim
  17. Jan 21, 2013 #16
    I was looking out for ready-made regulators to use them.
    But whilst looking out for regulators, I got confused since different configurations are used.
    I found three configurations being commonly used, which I attached in this post.

    -I cannot understand the difference in these configurations, since all these configurations are using differential pressure regulators.
    -Which one would be a good suggested configuration for my purpose.

    Something perfect would be a pressure regulator with a inbuilt valve to control the fuel flow, so a single component does it all.

    In the attached picture, the black box is the pressure regulator, and the trapezoidal body is the metering valve.

    I understand how the regulator works [thanks to your efforts and explanation sir], but i want to know which one suits best for my application.
    I'll briefly mention my system, which will give an idea what kind control system is required for my application.
    Fuel system - Fuel comes from the tank, gets pressurized at 100 psi (max) using the fuel pump. The range of flow rates is from 0.3l/min to 1l/min. Then it flows to the control system (in which I am having confusion). After passing these components, it will finally move to the atomizing nozzles, where the fuel (Kerosene) will get atomized into tiny droplets to be burnt in the combustion chamber.

    I hope my question is little more clear.
    Sorry for so much text :P

    Attached Files:

    Last edited: Jan 21, 2013
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