WARNING – This material is currently in development. Please do not rely on any of the calculations on this page or interpret any of the explanatory text on this page as final or accurate until it has all been thoroughly tested and reviewed. Everything on this page is subject to change.

Creep

When pressure testing polyethylene gas mains with a maximum operating pressure of 2 bar or more, it is important to take ‘creep’ into account. Creep is the expansion of the pipe when under pressure, which depends on several different factors. When you pressurise a polyethylene pipe, it starts to expand. The expansion is begins rapidly, then gradually slows down, eventually stopping. As the pipe expands, its volume increases, and the pressure of the gas contained within the pipe will therefore drop. The drop in pressure over time could look like a leak when in reality there is no leak, so this is why it is necessary to take creep into account.

Here are the main factors that affect creep:

• The type of polyethylene from which the pipe is made
• The temperature of the pipe’s environment
• The pipe’s SDR
• The amount by which the pipe’s expansion is constrained by its surroundings, which can be either backfill or a larger pipe into which the pipe to be tested has been inserted
• The pipe’s history of being subjected to stress

Conditioning

When pressure testing polyethylene pipe, the pipe must undergo ‘conditioning’ before the test is performed. After the polyethylene pipe is pressurised, the pipe is left alone for some time to allow most of the creep to occur before the test begins. This time is called the conditioning period. It is not practical to condition the pipe for the length of time that would be required for creep to completely stop. Therefore there will be a small amount of creep during the pressure test, and it is necessary to allow for that.

Additional creep allowance

Most of the creep occurs during the conditioning period that happens before the pressure test starts. The relatively small amount of creep that happens during the pressure test is called additional creep, and it is dependent on the material used to make the pipe and the ratio of the test period to the conditioning period. Normally it would be necessary to work out what is called the additional creep allowance by taking manual measurements from the appropriate curve on a rather small graph. However, this online calculation eliminates the potential inaccuracy of that method and provides more consistent results.

Input values

Conditioning curve to use SDR 11, PE 80, test pressure 7 bar, MOP 5.5 bar SDR 11, PE 80, test pressure 6 bar, MOP 4 bar SDR 11, PE 100, test pressure 7 bar, MOP 7 bar SDR 26, PE 80, test pressure 3 bar, MOP 2 bar SDR 21, PE 80, test pressure 3 bar, MOP 2 bar SDR 17, PE 80, test pressure 3 bar, MOP 2 bar SDR 21, PE 100, test pressure 3 bar, MOP 2 bar SDR 11, PE 80, test pressure 3 bar, MOP 2 bar

Test period / Conditioning period (unitless)

Calculate Results

Results calculated by Breckland Utility Solutions Limited