Testing water pipes with air pressure

[TSN79]

Picture a piping system made up of threaded steel pipes. It is to hold regular water at 7-10 bars. When testing the system for leaks, air is used due to a frost issue and the consequences of a larger leak with water. But what holds water doesn't hold air, and so it slowly leaks out. The question is how fast the air should be allowed to leak out before being considered "good enough"? I suppose that is dependent on the number of joints and perhaps what air pressure is applied? Usually we never apply more than 1,5 bar of air pressure during such a test.

 

[Simon Bridge]

What is the current accepted value?

 

[TSN79]

A drop of about 0,5 bars after six hours or so will generally be accepted.

 

[Baluncore]

Testing pressure vessels or pipes with a gas under pressure is dangerous because of the compressive energy stored in the air. That is probably why a maximum pressure of 1.5 bar is used for the preliminary integrity test. A liquid has much lower stored energy so is safe when testing higher pressures and volumes.

Most pressure vessels and joints are designed to seal better under pressure than without. For that reason it is not only viscosity of the fluid that makes a difference, but may be the increased pressure of the water that better seals the joints.

The bleed rate will be determined by the number of joints being tested and the method / material used to seal those joints. If the number of joints per volume of pipe is reasonably fixed then the pressure lapse rate will be a good indication of integrity, independent of the size of the network being tested.

Do you ever brush a soap solution onto the joints to find the air leaks? That might be quicker than a long pressure test.

 

[Averagesupernova]

IMO pressure testing with air is nothing more than a preliminary test. A slow leak on extensive piping will take a long long time to show up as an indicated pressure drop. Fill this piping with liquid and pressurize and the pressure may drop in a matter of seconds or a minute. Naturally it is advisable to test with air first in case there was a completely forgotten connection or something of this nature. After all, who wants to make a mess with liquid spilling?

 

[Ketch22]

I have worked in similar testing types for liquid systems. A more conventional approach (I do understand that if temperature is dangerous to the fluid we need to reconsider). Charge the system with water, this is important due to the stored energy issue. Than attach a small test rig that has a air chamber similar to a "reverse" head tank. This allows a small buffer and a place to stabilize the pressure due to pump variations. At this point feel free to use your established criteria and still feel good about a controlled predictable amount of stored energy.