Why does pressure drop during a hydrotest that has air trapped at the high point?

[jeevesme]

So I've found a couple threads that relate to my situation but didn't really answer it.

So . . . . We are getting ready to perform a 250psig hydrotest on a set of 2" half-pipe coils welded on the bottom head of a vessel. The pipe inlet starts near the bottom center and spirals around and up until it gets near the knuckle where it gets to the outlet. Because the outlet nozzle is vertical(~6" )pointing down and not horizontal, we will not be able to remove all of the air from the piping. Leaving trapped air at the high point. I was speaking with the area engineer about not getting a proper hydrotest because of the trapped air. I told him that if all of the air is not removed, the pressure will not hold. When he asked why, I had no answer and told him if I found out i'd let him know. I've been involved with hundreds of hydros but never really thought about the "why" part until now.

My question is, why does the pressure drop in a system that is seemingly water tight if there is air trapped at the high point?

 

[Compositepro]

The air will slowly dissolve in the water after the pressure increases during the test. This will appear to be a leak. You could keep repressurizing until all the air dissolves and then test for pressure loss, but that can take a long time.

 

[racookpe1978]

Or, if outdoors, testing a tank or large bore piping, I have had the metal heat up, the water then heat up, and the pressure increase. If an air bubble is present, the pressure increase will not be as great, nor the drop during the evening or if rain cools the surfaces as great.

Either way, pressure decrease, pressure increase or the actual change be moderated in either direction, you are not getting a valid (true) test of the new metal.

You can pull a slight vacuum of that top line, or over-fill with water from lower fill points until it is forced out, then secure that vent with a temporary cap or plug.

 

[jeevesme]

Just an update. . . We were able to get good hydros. After removing all of the air possible,we pumped up the pressure to 250psig and dropped to 245psig in 3 minutes and then to 238psig after 8 minutes and held. We pumped it back up to 260psig and it dropped to 257psig in 3 minutes and then to 253psig after 10 minutes and held.

I was surprised that is equalized that quickly. We must have been able to get more air out than we initially thought.