How to Calculate Pressure Increase from Water Thermal Expansion in a Closed Vessel
How to Calculate Pressure Increase from Water Thermal Expansion in a Closed Vessel
(OP)
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.
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.





RE: How to Calculate Pressure Increase from Water Thermal Expansion in a Closed Vessel
http://www.eng-tips.com/faqs.cfm?fid=1339
RE: How to Calculate Pressure Increase from Water Thermal Expansion in a Closed Vessel
Independent events are seldomly independent.
RE: How to Calculate Pressure Increase from Water Thermal Expansion in a Closed Vessel
Independent events are seldomly independent.
RE: How to Calculate Pressure Increase from Water Thermal Expansion in a Closed Vessel
RE: How to Calculate Pressure Increase from Water Thermal Expansion in a Closed Vessel
http://laser.cheng.cam.ac.uk/wiki/images/2/2b/SAPV...
RE: How to Calculate Pressure Increase from Water Thermal Expansion in a Closed Vessel
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).
RE: How to Calculate Pressure Increase from Water Thermal Expansion in a Closed Vessel
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.
RE: How to Calculate Pressure Increase from Water Thermal Expansion in a Closed Vessel
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"