That FAQ is a challenge to get your head around. If I make the assumption that a pipe is infinitely rigid, then pressure in a water-full line will change about 100 psi/°F (12.40 bar/°C). If you fill your line the day before and add water as needed to accommodate degasing, then when you start in the morning with ambient temp 11°C and water temp equal ambient and pressure the line to 18 barg. If it is a sunny day then the pipe will heat up much quicker than the air, and you should expect a 1 bar increase every 5-10 minutes. If you don't drain water off, you will be at something like 144 barg (or really 0 barg since the pipe is likely to fail) by the hottest part of the day.
If you start your test at the hottest part of the day, you will have to add a lot of water by dark.
You didn't say how long a test you are doing. I've found that testing above ground piping outside for more than 2 hours it is best to start at sunset and deal with ambient heat loss, the sun is really a challenge.
When I write a test procedure I am careful to specify acceptable actions during the test and the criteria for a successful test. For example, I generally allow draining water off the test without limit, but don't allow water to be added after the start of the test, then I define "success" as a final pressure above MAWP. With that criteria a 3 hour test that starts in the morning will nearly always pass if the mechanical integrity is as-designed, but if there is actually a problem it will show up. I've had tests that we drained water for the first hour, then clouds came in and we saw the pressure dropping, but it generally didn't cause the pressure to drop below MAWP. Having some room on the downside for temp changes is a really good thing.
David Simpson, PE
MuleShoe Engineering
In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. —Galileo Galilei, Italian Physicist
