Regarding hydrotest pressure and temperature relationship

[purode333]

Dear sir/all,

We have 2.5 KM of C.S. pipe line, and we have start the hydro test at 5pm with 18kg/cm2 and as per holding time of two hours , it is completed at 7pm , without any leakage or seepage.

But after completion of 2 hrs., pressure decrease by 1hrs. and it found 17kg/cm2.
We stated that due to decrease in temperatures, pressure also decrease. but client not ready to accept.

Pls help.

Design pressure is 12kg/cm2 and hydro test pressure is 18 kg/cm2.

(Sorry to members, wrongly posted in BPV forum, so posted in this, piping).
Regards
Prasad

 

[LittleInch]

See also this FAQ

http://www.eng-tips.com/faqs.cfm?fid=1339

designed for expansion, but applies to decrease as well.

See my previous response.

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

 

[bimr]

It is possible. Calculations on the effect of temperature can only be meaningful if proper temperature measurements are taken. Precautions should be taken to ensure that the effects of sun, wind, rain, and other factors do not invalidate the measurements and that the test section has reached thermal stability before the hold period commences.

 

[BigInch]

What pipe diameter?
Was the pipe buried?
What was the temperature at the beginning and at the end of the test?

 

[davefitz]

It might not be a help, but if you did document the temperatures at the start and end, then you could use maxwell's relationship to find
dP/dT|v = dA/dV|T= -d2F/dT/dV, and you would likely need to use a steam table based on a canonical equation of state such as the NBS/NRC steam tables for these partial derivatives.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[BigInch]

White Rabbit!

 

[rconner]

A drop of a kg/cm^2 (nearly a full bar or 14+ psi?) sounds like quite a bit to attribute to e.g. some un-quantified change in pipe/contents temperature in only an hour of testing - was curious what is the length, size and pipe material/joints tested? Your case would however appear to be strengthened if you and the inspector have examined every square cm of piping in the test extent and see no leaks. In any case, how about pumping up again and timing results?

 

[BigInch]

Exactly. That 14 psi is a 5% drop in test pressure. It does seem excessive after only 2 hours. That's why I asked for the begin and ending temps and if the line was buried. Even than it may be too much. Perhaps, if no leaks were observed, there was air in the line???

 

[purode333]

Dear all,

Thanks for response.

First of all, Dia of pipe is 8".

Pipe is painted with MIO and joint painting only balance.

There is no air in the line.

Temperature is app. 36 degree in starting and 34 degree at end time.

Length of line will affect it or not.

Is there any standard chart between temperature and pressure, or any thumb rule for such cal.

Regards
Prasad

 

[BigInch]

The formula is independent of length of the pipeline, as it is in terms of volume per unit length, however it is important to recognize possible differences that can occur between pipelines and vessel containers. A buried pipeline is typically considered fully restrained along the longitudinal axis of the pipe, so only transverse changes in volume are relavent, whereas the formula given in the FAQ describes a container free to expand in all dimensions. Unburied pipes subject to little frictional restraint in the axial direction would act more like an unrestrained vessel.

 

[LittleInch]

The calculation for a BURIED pipeline in BS PD 8010 gives a figure of 4.7 bar per degree C (assumes an 8" line with 8mm wt)

If this is above ground ( sounds like it) then you should be able to inspect the entire line, even in 2 hours and show no leakage?

If this is an above ground line, it would take longer than 2 hours for such a small ambient air temperature drop to affect the entire volume of water, even assuming the water was at 36 degrees to start with ( that's rather warm)

So what is it? an A/G line on supports where you can visually check EVERY joint / length of pipe or a buried line where you haven't actually got proper temperature measurments of the pipe/water temperature and didn't give it enough time to stabilize before the test?



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

 

[purode333]

Dear sir,

Thanks for reply.

Regards
Prasad

 

[racookpe1978]

How do know that your isolation valves (which includes drain and vent valves) or isolatioin blanks for that section of the pipe did not leak? What is the total volume enclosed for the test?

 

[Tuckabag]

If your pipe has been sitting in the sun all day full of water, I'd wager the water temp was a lot higher than the 36deg C ambient when you started the test.
I'd expect if it was now out of the sun during the test period, the water will have fallen more than the 2deg C ambient change.

 

[1gibson]

If they refuse to acknowledge the temperature reduction can cause a reduction in pressure, and you have 100% verified no leakage, then start the test cold (right when sun starts to hit the pipe) and finish at a higher temperature?

 

[bimr]

Refer to Pipeline Rules of Thumb: A change in the water/pipe temperature may give the appearance of a leak. If the temperature of the pipe/water decreases, the test pressure will decrease. An increase in water/pipe temperature will cause the test pressure to increase. The effect of a temperature change may be estimated using the equations and data contained in Appendices A and B. To achieve any degree of accuracy in these calculations, it will be necessary to have accurate temperature and pressure readings. See the attachment for the calculation procedures.

 

[BigInch]

And the pipe should be buried.

 

[rconner]

Good point. I noticed also in the quite detailed procedure bimr provided that it shows pictured quite specific placements of the thermometers or other temperature probes alongside the buried pipeto be monitored during the duration of the test (it would appear not ambient or air temperature i.e. trying to capture as close as one can the actual temperature of the "pipe and water").

 

[BigInch]

It's very important to have as uniform a temperature as possible. Long line, unbuired, or 25% unburied, and exposed to sun is asking for trouble.