Pressure Drop against temperature during hydro test 1

[Vinodadoor]

Good Morning,

I am carrying out hydro test of 10 above ground pipe line using water with a test pressure of 18 bar at temperature of 22 degree (day time) . What will be pressure drop of the line at temperature of 11 degree (early morning)?. Time required to reach the temperature from 22 degree to 11 degree is 16 hours. Please provide me formula for calculate the pressure drop against temperature.

Thanks

With Regards

Vinod. K

 

[LittleInch]

For the calculation see

http://www.eng-tips.com/faqs.cfm?fid=1339

Your issue will be carefully measuring the volume of water you need to keep putting in to maintain your test pressure. If you don't add water, your pressure will be zero after about three hours....

See also PD 8010 pt 1 which has some very useful calculations and graphs to show that the test is satisfactory.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[zdas04]

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

 

[LittleInch]

zdas04 - the FAQ allows you to input diam and wt so the pressure rise isn't as bad as you state.

I do agree that it is better to bleed off water as opposed to top it up, but I've often worked to +/- 5% of the test pressure as a goal when doing pressure tests so an ability to take of and also add water in a measured quantity (usually strokes of the pump) is a good one in my opinion.

Given the low pressure of this test (not sure what code the OP is working to) even a few degrees C drop will drop to virtually zero or burst the pipe.

Vonod - A good approved test procedure is vital to achieve a good test and for everyone to agree BEFOREHAND, how you plan to account for any changes in temperature to demonstrate you have a tight pipe (or nota s the case may be). 16 hours test for an A/G sounds very odd and wherever you are you will suffer some quite big temperature swings which makes a successful test much more difficult to achieve. Why so long??

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way