Different Gauge Reading for Hydrostatic Pressure Test for Hot Water Piping

[mahara7a]

Hi! I'm currently conducting a hydrostatic pressure test for hot water piping which consisted of a return and a supply line. The covered areas for the tested pipe is from Level 1 to Level 3. For this leak test, cold water is used as its fluid. Attached below is the basic pipe network of our building to give you the idea. At water pumping point is at 175 psi (Gauge No 1)while Gauge No. 2 at the end of Level 1 is at 170 psi. At Level 2 Gauge No.3 is at 145 psi and the Gauge No. 4 at Level 3 is at 140 psi. The specification stated that it should be tested at 175 psi. All 4 gauges have been calibrated. My question is, are there any explanation of whats going on here? I'm expecting the whole pipe network to be at 175 psi. Is it acceptable? If yes, whats the basis of accepting this test? Lastly, is there any formulae of calculating allowable leakage for pressure test with duration of 24 hours?

Thanks in advance.

 

[LittleInch]

I suggest you look up the concept of hydrostatic head.

The specification is meaningless without staying at what elevation this pressure is to be held. Highest point or lowest point.

Change of temperature is the only valid reason for a loss of pressure.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[mahara7a]

LittleInch, thanks for your reply. As a fresh grade engineer sometimes I overlooked the basic of fluid mechanics. Its the elevation that cause the pressure difference even within inside the pipe. The specification just mentioned the 175 psi for sectional test (which we have done earlier) and doesn't state at which elevation shall be held. So we'll just ensure that 175 psi to be at the lowest point of the test point while at the top most floor gauge will act as a monitor gauge. If the water was discharge from the top most floor at controlled discharge the bottom gauge would reflect a proportion of the same drop.

Thanks again.