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Is my pressure calculation correct?

  1. Sep 26, 2015 #1

    ISX

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    I built a spreadsheet to determine the flow and everything through a pump. I won't get involved with the first part just assume it to be correct and the gallon per hour/minute calculations are correct.

    https://docs.google.com/spreadsheets/d/1OXQPKwTy6vsKELy3Aa4Tzj9lit4ZlKMdsAl6CQaT_Lk/edit?usp=sharing

    After that I have a hose with a 1/4" I.D. I was able to calculate velocity with hose diameter and GPM. Then pressure.........I used the formula found here but the result seems very low. It doesn't seem impossible but I just want to know it isn't bogus. I mean does 2.85 gallons really flow through a 1/4" hole in a minute with only 1.3psi?

    That brings up the dynamic vs static pressure thing that I don't completely understand. I am simply trying to find how much pressure is required to satisfy the required flow rate... so I'm not sure where static/dynamic ties into that.

    Any help is much appreciated. Thanks
     
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  3. Sep 26, 2015 #2

    SteamKing

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    No, the total pressure drop will be much more than 1.3 psi. It will depend on the length of hose used and the material used to make the hose.

    The dynamic pressure is the energy equivalent of a unit amount of fluid flowing at a certain velocity.

    You need to study Bernoulli's equation for starters:

    https://en.wikipedia.org/wiki/Bernoulli's_principle

    Dynamic pressure is that phenomenon which allows the use of a manometer to measure pressure differential:

    https://en.wikipedia.org/wiki/Dynamic_pressure

    Static pressure is pressure produced by a fluid simply sitting in one spot.
     
  4. Sep 26, 2015 #3

    ISX

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    That's the problem. It is a supply pump feeding another pump which means it has a constant pressure for the most part. It is basically the same as having your water heater at your house.. So the city water is 80psi but my shower head flows 1GPM. I realize there is a pressure drop between the city water and the drop after it comes out of the shower head, but if I only need 1GPM, then I certainly don't need 80psi to replenish the water in the tank, I would need much less, but at what pressure is required to maintain that? Problem is I cannot measure the "shower head", I only have the "city water" and the "tank", both of which are at the same pressure, so you see my problem. I can put a flow meter on the city water line (for example), and now I have the flow, but what pump output pressure would actually be required to accomplish the same output flow.. Obviously I can put whatever pressure I want to a system but mathematically, how much pressure is actually needed given a certain pipe size..
     
  5. Sep 26, 2015 #4

    SteamKing

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    Well, you certainly wouldn't want 80 psi coming out of your shower head, at least, not if you want to keep your skin.

    It's important to know where the pressure of 80 psi is maintained. Is it the water main in the street? Is it under a water tower?

    It's still not clear what it is you want to do. You say you want to "replenish the water in the tank". What tank? Your water heater tank?

    It would be best if you started explaining at the beginning and skip the spreadsheets and whatnot for now.
     
  6. Sep 26, 2015 #5

    ISX

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    Alright I'll explain exactly what I'm doing without using all these examples. I have a diesel truck and it has an injection pump. It shoots off a set amount of fuel, say 1000 cubic mm's of fuel per injection. There's 6 barrels/plungers in this pump and they each fire when it's their turn in the firing order. The entire injection pump internals are submerged in fuel, which replenishes the barrels after each injection. So a plunger goes up, shoves fuel out, goes down, sucks fuel around it in the gallery. That gallery is maintained at a set pressure regulated by an overflow valve with a spring that opens at 20psi-ish. Now there is a lift pump that pumps the fuel to the injection pump gallery, maintaining the fuel supply and enough pressure to maintain 20psi. I know how much fuel gets injected and with that I can add all the injections up and come up with an average flow rate in gallons a minute or whathaveyou. But the 20psi thing is not a set in stone. I want to know how much pressure is actually needed to keep the gallery filled at that flow rate. It has to be topped off, there are no air gaps so the whole system should be maintained at a set pressure. Ignore the overflow valve, just assume a constant flow of X gpm and the pressure needed to maintain that. I know it isn't a perfect world but I'd like to see how it works out on paper. This injection pump has been ran at 5psi and yet I've seen the overflow valve tweaked up to 60psi and on the test bench I don't see any difference in injection pump output, so be it 5psi or 60, its still filling the gallery completely so how much is really needed to shove enough fuel through the smallest point (the port on the pump) of 1/4" diameter ID.. Thanks for the help.
     
    Last edited: Sep 26, 2015
  7. Sep 26, 2015 #6

    SteamKing

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    If I'm understanding you correctly, you have a diesel fuel reservoir from which the injector pump draws a supply of fuel. The pump supplying the reservoir only needs to generate enough pressure to pump the diesel fuel from the storage tank, thru the supply line, and into the reservoir. The pressure at which the supply pump operates has nothing to do with the pressure at which the injector pump operates.

    The injector pump is designed to operate at high pressure so that a set amount of diesel fuel can be sprayed into the combustion chamber of the engine thru some really tiny holes, which atomizes the fuel to allow it to burn more completely.

    You could use a small boy with a bucket to keep the fuel reservoir filled with fuel and it shouldn't affect how the injector pump operates.

    As with any pump, if you want to know how this supply pump performs, you have to figure out the pressure drop in the attached piping system for a number of different flow rates and plot that piping system curve on the pump performance curve to see what flow the pump is capable of producing.

    http://www.engineeringtoolbox.com/pump-system-curves-d_635.html
     
  8. Sep 26, 2015 #7

    ISX

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    You are correct. As long as the pump has fuel in it, it would work, but given the setup of having a supply pump, I'd like to know the pressure it needs to maintain in order to supply 2gpm or whatever it was through that 1/4" fitting. The supply pumps performance is not my concern. It could be a tank 1000ft in the air giving me an endless supply of pressure and I just put a regulator on it. So if I turn the regulator down, at what pressure will it stop supplying enough fuel to keep the gallery full. That is all I am trying to find out.
     
  9. Sep 26, 2015 #8

    SteamKing

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    You won't know that unless you either run an experiment to find out, or you know the construction of the supply system. You need to know more than just the pipe or tubing size to figure the system curve for the supply line.
     
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