quetsion fatigue and failure analysis

One reason I think of relates to compressibility. A liquid, being less compressible, stores much less energy upon pressurization than a gas.

The more I think on that, the more I wonder if my conjecture is correct.

First off, I've never been involved with pressure testing of an entire pipeline. But at work, when repairs or any hot work is done to piping details (a piece of pipe between flanges) we have to do a hydrotest before it can be returned to service.

I've only seen water used for this purpose. Some of the reasons that come to mind are:

1. Water is always available, and it isn't dangerous to use. If used properly, it will not corrode the line you are testing.
2. Disposal of the water after the test isn't usually a problem. (if the line is new, then the water will be easy to dispose of. If the line had been in crude or product service, then it will have to go into a proper waste treatment facility).
3. It's easier to pump a liquid that it is to compress a vapor.
4. Vapor is compressible. liquids aren't. What that means - once you have your pipe detail full of water, a stroke or two on a hydraulic hand pump will be all you need to get up to your test pressure. If you were using gas, increasing the pressure would be quite a job. Just ask anyone who has ever tried to inflate an automobile tire using a hand "pump".
5. Depending on various factors, you have to hold the test pressure in your system for various lengths of time. The sun beating down on a line will cause "sun pressure" - the contents inside the line will heat up and expand during the day (or cool down and contract at night). Vapor does this a lot more than liquid, so the results for vapor over time are harder to interpret than for liquid.
6. Finding leaks using water for your test is a lot easier than finding vapor leaks. Also, water leaks won't catch fire or hurt anyone.

Hope this helps.

That is the short answer. Gasses expand upon depressurisation, and if a high pressure pipeline filled with gas fails, the result can be exsplosive, and dangerous, liquids lack that quality.

There are several reasons for using water:

1. It's cheap and easy to work with.
2. It's relatively inert.
3. Gas takes a long time to de-pressurize if you do it slowly.
4. If you depressurize any gas quickly you get a rapid temperature drop and freezing that could damage pipes.
5. No explosive hazard with water
6. Finding a leak in the pipe you are testing results in a leak that is easily seen visually or with pressure gauges.
7. Gases under pressure, if there is a leak, could result in injury.
8. A pump required to pressurize the pipe with water can be small and cheap, whereas a compressor required to pressurize the pipe with air or other gas would be large and expensive.