Body cavity thermal expansion calculation 1

[TransMech]

Hi guys,
Does someone know how to calculate body cavity thermal expansion pressure?
For example,
30inch ball valve, full port, #600class, how much degree increasing of temperture could cause pressure expansion from 1000psig to 1440 psig.
Very appreciate.

 

[gerhardl]

If you assume you both the existing and the increased pressure are in the gas phase, and the volume (body cavity) is constant, eg the gas is enclosed in a thight vessel, you can use the combined gas law: p1*v1/t1=p2*v2/t2. v1 = v2 and will cancel out, and p1 and p2 are known and in the saame denomination. T1 and T2 are in degrees absoulute (deg. Kelvin ca 273 plus deg C). Solved you will get T2 = 1,44 * T1. This will be the worst case, leakage to a lesser pressure will lower the cavity pressure.

 

[TransMech]

Thanks Gerhardl,
How about on liquid service? Is it same like gas phase?
Thanks.

 

[LittleInch]

Liquid thermakl expansion is much worse that gas.

See

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

which gives you all you really need to know. As the valve is so much stronger than pipe II would ignore any terms or calculations about expansion of the pipe under pressure or temperature and just calculate the expansion nd compressibility of the liquid.

Usually you find a rel small temp increase (20 to 30 C) can be enough to exceed design pressure depending on what pressure you start with. In your case I would guestimate 30C

Of course it then only needs a very small amount to be vented for this pressure to fall away.

My motto: Learn something new every day

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

 

[gerhardl]

I learnt something. Star!

 

[TransMech]

Congratulations LittleInch on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

That is awesome!
Hi LitteInch,
You estimated 30C temp increasing could expand pressure from 1000psig to 1440 psig.
why my culcalation is around 5C.
The factor,
αf=0.83x10-3 °C-1
β=1.35x10-3 MPa-1
Thanks.

 

[LittleInch]

No problem,

Mine was a guess, but if you've got a hydrocarbon liquid, I'm not surprised by that number. The FAQ has a pressure rise of about 60psi per degree C for water and hydrocarbons are much worse in this respect. As said, the volume you need to relieve is very low, but if you've got a liquid filled valve then you need some form of pressure relief or you can damage the valve or seals.

My motto: Learn something new every day

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

 

[Roberto1956]

The cavity relief is a function of inner pressure.
If it is a self relief trunnion ball valve you have to consider the line pressure that is a counter pressure.
In other words, if API 6D stablish as maximum 1,33 design pressure to relief the pressure, when you make your evaluations you have to use 0,33 design pressure 'cause 1,00 will be the counter pressure that is in pipeline.