Dead Weight Tester used in Hydrotest 1

[bonita09]

We need to do a pretest on a pipe at 350psi for 4hrs. Will report the air content at 0.2% of volume by using a dead weight tester to pressurize the system to target pressure.

1) So in the UUT port of my DWT i will attach a pressure transducer (for the pipe pressure reading) and connected in series with the pipe system. Is that ok?

2) However, I only have a DWT with oil medium.
How can I convert it to water?

3) Im obviously no piping engineer but i want to learn more on pipe testing (hydro and air), can you suggest any good reference for starter like myself?

Highly appreciate your generous thoughts.

 

[Compositepro]

Dead weight testers are generally used to calibrate pressure gauges, which have a small internal volume. Then the calibrated gauge is used to pressure test the pipe, using some other source of pressure. A DWT cannot pressurize a pipeline.

 

[btrueblood]

Well, it can...but you'd be pumping a long time.

 

[bonita09]

I only got the idea from California State Lands Commission
A Procedure for the Hydrostatic Pressure Testing of Marine Facility Piping (as per attached, page 11).

So initially my pipe is already filled with water using an external pump and I will just raise the pressure using a DWT.

It seems I cannot use a caibrated pressure gauge as the requirement for the pressure test is quite tight, 0.2% accuracy.

A manufacturer suggested that I can have a 2~3 m stainless pipe and fill it up oil, hence when I do the test, the water will not go into my DWT. But Im not sure if there's any effect in the accuracy of my measurement.

 

[btrueblood]

You can purchase deadweight testers that operate with water as the working fluid. Most oil-based deadweight testers can operate just fine with water, though you would want to recalibrate them more frequently as the lower lubrication properties of water will accelerate the wear of the piston. Any deadweight tester is effectively a gauge, though you have to manually adjust it (by adding/subtracting weights) to get the correct reading. Seems a painful way to get +/- 1 psi accuracy, given that many transducers can give that accuracy too.

If you operate with an oil column, you could correct the reading based on the known height of the column, adding or subtracting the piezo head difference caused by the difference in specific gravity of the oil relative to water. If you do this calculation, you will find that the difference is pretty trivial (about 0.1 psi or so for typical oil s.g. of 0.8) for a few feet of oil column.

 

[bonita09]

I learned that theres a specific DWT just for the pipe pressure test. But Im trying to figure out how I could use my typical hyraulic (oil DWT for pressure gauge calibration) DWT for this application.

Instead of the pressure difference due to specific gravity, will it be a greater concern for me about some possible bubbles or trapped air as I need to compute for the air content.

 

[btrueblood]

I'd think the trapped air in the piping would be of more concern than a bubble in a, (what, 1/4"?) tube run to the DWT? If it bothers you, put a bleed valve in the top of a loop. Fill system, then bleed air until you get water squirting out the bleeder valve.

 

[bonita09]

Thanks so much guys. Youve been a life saver.
Truly appreciate your inputs and patience with me.