Subdivision waterline 2 hour pressure test calculation

  1. When performing a 2 hour pressure test on a waterline, the formula for calculating the allowable water loss over the 2 hours is given on the testing form. But, the form asks for the actual water loss. Of course, you know the pressure loss, but it asks for the water loss in gallons. This can be accomplished by using a pump to force water back into the waterline until you reach the starting pressure you had when the test started and having a way to measure the amount of water you used to re-fill the waterline. I have came across several contractors that can't do this, so I thought I would calculate the actual water loss knowing the pressure loss over the 2 hours. I believe the calculation would incorporate the temperature of the water being tested, the initial and final pressure, the initial and final density of water, and the bulk modulus of water. With a little research, I found formulas to calculate the change in density of the water per the change in pressure. With the change of density known, I then can calculate the change in volume and thusly the gallonage. But, the gallonage values I am getting seem very low and not in accord with the allowable water loss quantity. I am wondering if these principals of water are based on a closed system in which the change in density is calculated per the change in temperature and pressure, and doesn't account for the case where water escapes the system. I am also wondering if with the loss in pressure being usually small, the calculation of gallonage loss is highly inaccurate. Anyway, I came to this forum to ask if any one else knows of a formula for accurately calculating the water loss in gallons, or if you would just tell me I am wasting my time calculating it and to just have the contractor figure out a way of re-filling the waterline to physically see how much water it takes. Thank you for your time.
     
  2. jcsd
  3. mfb

    Staff: Mentor

    A different pressure might also lead to a different volume, if the walls are not perfectly rigid (nothing is perfectly rigid).
    Air in the system can influence the system significantly, too.
     
  4. SteamKing

    SteamKing 8,853
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    Changes in water density due to changes in temperature or pressure are minute, as long as the water remains liquid. That's why water is effectively incompressible and has a high bulk modulus.

    Since you are doing testing to obtain approval from some authority, there must be an approved test method or methods. I would try to find out more about these methods.
     
  5. SteamKing

    SteamKing 8,853
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  6. Thank you for your reply, mfb. I am sure there is air in the system. The pressure test will have to be performed again after bleeding the system to try and get the air out. I don't think the pvc pipe itself would expand under pressure enough to effect the water loss calculations, but movement of fittings could expand the volume enough to create a new volume.
     
  7. Thanks for your reply, SteamKing. There are test method procedures for doing a pressure test. My research has found several around the country and they all list pretty much the same procedure for doing it. The calculation for getting the ALLOWABLE water loss over the 2 hour test is the same for all cases. There is no stated calculation for getting the ACTUAL water loss based on the pressure change over the 2 hour test. Most of the procedures I have found say to have equipment that can put water back into the system to get it back to the original test pressure and be able to measure the amount of water it took. My question on this forum was to ask if any other engineer had a formula to calculate the ACTUAL water loss over the test. I was just wondering if I am wasting my time try to calculate it and just go with the pumping water back into the system. Thanks again for your help.
     
  8. SteamKing

    SteamKing 8,853
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    Yeah, I think you're wasting your time. I think the authorities want a measurement of the actual amount of water used to re-pressurize the system. The approach in the OP didn't make much sense to me.
     
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