Polyethylene piping pressure testing and creep

[templar113]

Hi Everyone,

I may be listed as an Electrical Engineer, but at the moment I'm working as a graduate in Gas Distribution (Natural Gas) in Australia for 4 months or so.

Anyhow, someone recently gave me a little software tool relating to PE pipes, creep and pressure testing. I would like to convert (or create a new one) in Australian units and most commonly used PE sizes and test pressures. The issue is Australian standards currently don't take into account creep when pressure testing, however I heard that they do in Britain?

It takes the following parameters:

1. Test pressure (in bar)
2. Pipe length (in metres)
3. Diameter (outer diameter in mm)
4. SDR
5. Choice between PE80 and PE100

From this it first of all tells me the pipe volume in m^3.

The more important thing however is that it takes into account creep (which is when during the pressure test the pipe will expand and then relax and hence drop a small amount of pressure) and then tells me how long the pressure test must last (in hours) during which the pressure is not allowed to drop by more than 3 mbar.

The issue is I don't have any of the equations or tables behind these calculations, because the tool must have come from the UK. (It was dug up from some old archive of files).

I figured out how it determines volume because I have a lookup table that gives me all the internal diameters for the different SDR values of PE pipe from a local pipe manufacturer. The rest is just simple cross sectional area times the length.

I also converted units to kPa instead of mbar or bar, which is also a linear relationship (1mbar = 0.1kPa).

The issue outstanding now is how it determines the pressure test period during which no more than 3 mbar (or 0.3 kPa) may be dropped.

I've so far figured out that given a certain test pressure, the test time is linearly proportional to the volume in the pipe. In other words if we set a test pressure of say 7 bar (700 kPa) (a.k.a. IP - Intermediate Pressure) then the ratio between volume and the number of hours is constant.

The problem is I don't know how this ratio was determined in the first time and how it relates to the test pressure!

Does anyone out there either:

a) Know about any equations to do with polyethylene creep and relaxation given a test pressure and what sort of drop could be expected over time?

b) Know about any specifications or tables that could provide anything like this?

I have a bad feeling that these "magic" values were determined through experimental means...

Also is anyone out there familiar with British Standards or procedures related to taking into account creep when pressure testing a PE pipeline?

Thanks a bunch!

 

[vzeos]

http://plasticpipe.org/publications/pe_handbook.html

 

[templar113]

Thanks for that. Very informative document, however I couldn't find what I was looking for.

I'm starting to think that the calculations in the software were originally written from the results of an experiment rather than a set of equations. I found a pattern in the numbers the program generates, but no luck finding how these numbers came to be in the first place!

I've given up on this for now until I get more info.

 

[BeardedBob]

The UK Institution of Gas Engineers and Managers (

http://www.igem.org.uk)

standard TD/3 gives some limited guidance on pressure drops and creep effects in the testing of PE pipe, although no equations. It's possible your progam might be based on this.

Bob Andrews

http://www.fleetech.com

 

[stanier]

Australian Standard AS 2566.2 Buried Flexible pipelines-Construction gives clear guidance on themethods used to establish the hydrotesting of a thermoplastic pipe.