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# Pressure Changes Due to Ambient Air Temperatures

## Pressure Changes Due to Ambient Air Temperatures

(OP)
I am trying to develop a calculation excel sheet and SOP for our hydro-test team. A problem we keep running into is our customers (and us) want to know how the temperature drop and increase throughout the day affect the the pressure during out hydro test. I read through the FAQ's and found how to calculate pressure increase/drop based on temperature increase/drop, but there are a couple more things that I would like to know:

1) Can this equation also tell me how much fluid would need to be drained/added during a hydro test in order to maintain the same pressure with the temperature variances?

2) How to find the temperature change of the fluid based on the temperature change of the ambient air? Most of our pipelines are outside and the temperature varies greatly during the even here in Southern California and many times a customer will not have a temperature gauge on the pipeline and/or we may not have a temp gun on hand.
I am assuming we can use the thermal resistivity approach, but I think there are too many unknowns since I don't know the temperature of the fluid or the heat transfer. Only thermal properties of fluid and pipe, and the temperature of the air.

I apologize for the beginner type questions, but I have been out of practice for awhile and have to bush-up the subject.

Thanks

### RE: Pressure Changes Due to Ambient Air Temperatures

There is a methodology on how to determine the effects of temperature change on hydrotest pressure. I've seen a discussion on this which I believe was in this subforum (Pipeline).

Depending on the length of pipeline it will become more problematic to determine appropriate temperatures to perform any calculations on. If it was largely below ground it may be easier to calculate but having mostly outside, which I'm taking to mean above ground, could make it complicated due to variation in temperature due to things as simple as changes in cloud cover along many miles of pipeline. And the possibility that you have no temperature gauge or even a temp gun I don't know how you intend to perform any analysis. You'll need some data other than the weather channel.

I'd recommend first discussing/presenting some of your approach but the first item should be feasible. Regarding actually using this data in a meaningful way, you have to determine how accurate any calculations or methodology you come up with is. Largely below ground, short lengths, temperature readings both above and below ground, and full inspection on above ground components, sure, it's feasible and possibly appropriate. 500 miles of varying lengths of below and above ground piping while a hurricane passes over with limited temperature probes, well it gets complicated.

What you're looking for is possible just a lot of variables should/need to be taken into account and it depends on the complexity of it. If I find that thread I'll try to point you to it.

As one last note, depending on code, you should review any explicit requirements in its pressure test requirements.

Thanks,
Ehzin

### RE: Pressure Changes Due to Ambient Air Temperatures

(OP)
Sorry I should have given a bit more detail. The hydro testing we do deals with on average 200 feet or so of pipeline (usually above ground); we occasionally get pipe that is 2 miles long, but nothing longer (our hydro test unit can't handle anything longer). Cloud coverage is usually not a problem since we operate in the valley of California and clouds are about as abundant as water.

As for the not having a temperature gauge or temp gun, we usually do have one or both of them; I was just wanting to know so in case we don't have either I could calculate the temperature drop in the pipe in order to run the pressure drop calculations.

And the calculations I am performing do not have to be spot on, it is just a way for us to tell our customers that the pressure drop/increase or the amount of fluid added/removed is a reasonable number based on the temperature drop/increase and the calculations from it.

Thank you for your time and response!

### RE: Pressure Changes Due to Ambient Air Temperatures

The fact that it's above ground, typically short length, and consistent cloud cover you may be able to develop an approach on this. Without knowing the temperature it'll leave you in the dark. You could measure pressure drop and back calculate the approximate required temperature change to elicit that pressure drop although you run into the scenario of an unverified data point. I would consider reading some of API RP 1110 as I believe it discusses the approach you've mentioned although it doesn't really give any calculations.

Having it all above ground and short could make performing a 4 hour hydro more problematic in some cases. Long pipelines that are largely underground are less influenced by aboveground temperature changes to the point where it may be largely unnoticed on a chart recorder.

Have you put your approach on paper yet? I don't see why it shouldn't be possible but at this point I would be more curious what your calculations look like. Still haven't managed to find a specific thread that discussed this in the past.

Thanks,
Ehzin

### RE: Pressure Changes Due to Ambient Air Temperatures

Will the test fluid expand more than the internal volume of the pipe (which will also expand due to temperature)?

### RE: Pressure Changes Due to Ambient Air Temperatures

(OP)
I have not put anything to paper yet. This is a bit of a side project for me right now and I was just trying to figure where I should start and what equations I will need. Looking back at the process and reading our discussion, it looks like I may be able to perform all the needed calculations with the pressure drop equation provided in the FAQ section, but I do need to double check if I can get volume addition/remove of test fluid from these calculations.

The test fluid is water, so yes the test fluid will expand more than the internal volume of the pipe in many cases where the temperature increases by more than a few degrees in a short amount of time.

### RE: Pressure Changes Due to Ambient Air Temperatures

1) probably, but the equations aren't quite set up for that.

2) that seems a bit odd, since I would think that your pipe is outdoors, and in the central valley, the sun beating down on darkish pipe probably changes its temperature way more than whatever the ambient temperature might be.

TTFN (ta ta for now)
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