How to Calculate Pressure Increase from Water Thermal Expansion in a Closed Vessel 3

[BronYrAur]

I am trying to dust the equations out of my brain on how to determine the pressure increase in a closed system as the water temperature increases. I tried an internet search, but I am only finding information related to hydronic expansion tank sizing calculations. I want to know how the pressure changes without an expansion tank.

Let's say I am filling a hydronic system with 40 deg F to a 12 psig initial fill. This is typical for a 2-story building. Now, let's say I close the valve to the expansion tank leaving me no spare volume for expansion. Now I heat the water up to 180 deg F. I realize that the thermal expansion of the piping system will give some relief, but I also know that the expansion rate of the water will be much higher, which will result in a sharp increase in pressure. Let's assume there is no relief valve.

How do i calculate the new pressure? I am trying to make this calculation to prove the someone that a relief valve is necessary.

Thank you for your help.

 

[ione]

Have you given a glance at this?

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

 

[BigInch]

You don't have to prove it with calculations. Seriously! Show them the design code for any pressure vessel!

Independent events are seldomly independent.

 

[BigInch]

BTW, you can't buy a "ready-to-install" water heating tank anywhere in the world I know about that doesn't come with a pressure relief valve already installed on the tank. If you find one without a relief valve, move to another country.

Independent events are seldomly independent.

 

[desertfox]

Faq391-224

 

[desertfox]

See example 3.2 in this link:-

http://laser.cheng.cam.ac.uk/wiki/images/2/2b/SAPV.pdf

 

[DSB123]

BronYrAur,
For a closed system with liquid contents the pressure will always increase with a rise in temperature. Just use P1V1/T1 = P2V2/T2 where V1 & V2 are the volumes at start and finish, which are the same(ignoring pressure distention), T1 & T2 are the temperatures and P1 & P2 are the pressures. So you have P2 = T2P1/T1. So if T2 increases then P2 also increases.

Also ask the "someone" why thermal relief valves are required on most closed in systems with significant inventory (if the container is not designed for the increase in pressure).

 

[Latexman]

DSB123,

In general you are right, but your methodology is wrong. You used a combination of Boyle's and Charles's GAS laws. This is a liquid filled vessel.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[zdas04]

The FAQ's have a lot of math, but most potable water systems use more rules of thumb. The rule that I use (that actually matches the math and field observations pretty closely) is that pressure will increase about 100 psi for every 1°F (i.e. your 140°F increase would be increasing pressure towards 14,000 psig as the pipe failed)

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"