Pressure Drop Observed in Piping During Hydrotest When Temperature Drop

How much is acceptable and test timing (and duration) are things that your local regulator or customer will need to confirm.

We run hydro pressure test on domestic plumbing for 24 hours, so start and end time are same. We run pressure test (vacuum) on sewer pipelines to check there is no ingress of groundwater for around 15 minutes (Actual time depends on calculated total pipe volume). We run hydro tests on water mains (over over 199mm diameter) for 48 hours but on reticulation mains (smaller than 200mm) for only 24 hours.

I suspect that it would also depend on the line charge fluid. For us its either water or sewage with the related environmental impacts. If the fluid was oil, LPG, or some other liquid then the consequences of leaks are significnatly different.

You are supposed to correct for temperature changes. Then, the allowable pressure drop depends on local piping code(s) and/or designer/purchaser acceptance criteria. There is no universal standard.

Yes pressure test in the night is allowed if a 24h test is required. What else could you do since you can not change night into day or can you?

The drop has to be justified with some basic PVT calculations.

If you had 20 bar in the beginning and it goes back to same temperature and pressure at the next day, what would you call the test (regardless of what it did in the night)?

Well, that depends upon the test protocol document, which cannot be seen from here.

Rise during the day and drop overnight? A function of temperature variations? One wouldn't expect anything else. What this is saying is that the system is leak-free.

Is there an Engineer at the facility that can advise locally?

Holding time is only for 2 hrs. Its a Process Piping as per ASME B 31.3

Kiddo, what is it? Temperature rise in the afternoon by holding pressure for 2 hours?

If you was just to get up early you could have this done at dawn just before the sun rises.

I would like to see the test procedure with a statment "test to be conducted before sunrise to eliminate temperature changes during test!"

Or "test to be conducted during highest typical ambient temperatures" to be on the high side of safety margin, especially if the contents are dangerous if released.

If you pressure test during the dawn hours or mid-morning and there is no leak the pressure will increase slightly. That is not a normal thing for a pipe that is leaking. If the pressure remains the same it is because either there has not been any temperature change or the leak is so small that it has little effect if any. A loss will be due to a leak.

Test the system with water if possible if there is no specified material. Water is not compressible so any leak will cause a dramatic lowering of the pressure. Gases, such as air are compressible so they will leak with very little change in pressure. With water most testing pumps used are manual or small electric pumps since it is uncompressible. Fill the pipe with water with no air present and pump it up.

Make sure all personnel present are wearing the proper PPE. Too close and they could get hurt.

Good Luck, Old Salt

Hydro testing can be a yo yo sometimes. Typically a test procedure is provided by EOR or your company's engineering dept. Hopefully you are following it. Also if you are not getting all the air out upon initial water fill, this can also fluctuate pressures. If you don't have high point vents and low point drains you'll chase your tail trying to get the test off. Need to install the high point vents at the minimum. Depending on your pipeline contents or code, you may have to seal weld the plugs into the valves after the test. Avoid testing pneumatically. It can be done but there are immediate dangers along with a test procedure that builds in relief valves and you have to account for temperatures/pressure drops/rises during the process. If it is a natural gas pipe line or any pipeline that cannot be evacuated with drains you may have to apply pneumatic test. This requires extreme attention to caution and safety. The destruction and aftermath of improper pneumatic testing is devastating. People don't realize how exponential the force of compressed air is in large volumes. This requires execution planning and review so everyone within the boundaries knows what's going on and when to get the heck out of there. Don't discount the safety precautions.

Water testing is certainly the preferred method for natural gas lines. In order to do it, instead of pressurized air, prudent operators construct their piping from the onset to be able to do so with water. One of the transcontinental pipelines will never design one without this provision. They run 30 ---> 42" pipes with approvals to operate at in excess of 1,000psi.

Briefly, isolation valves are closed, piping filled with water, test pressure obtained, test pressure and time achieved, pump/drain water out, numerous pig runs to remove most of water, manpower inserted/removed to clean up residuals and then seal line for operation. If the line fails the repair work is done without removing all the water if possible. If not, drain/pump off water, make repair and retest.

Testing with air is similar to playing Russian Roulette with a Claymore Mine.

Good Luck, Old Salt