I am working on a hydronic heating

[Andrewg945]

I am working on a hydronic heating system with a Minimum relief valve capacity of 90lbs/hr. The present relief valve is 30 psi. The building is 3 stories and the boiler is in the basement making the height over 3 stories that the water must travel via pressure. Is it acceptable to upsize the relief valve to 40 or even 50 psi, in order to have sufficient pressure to reach the top floors?

 

[LittleInch]

You need to be more precise about the height between boiler and the highest point PLUS whatever pressure you maintain in the closed loop you have.

"Three stories" is maybe 12m? So that's 1.2 bar or 17 psi, but then you need to add the pressure you maintain at the top. That's the unknown here. Plus the pressure you need to transport water around the loop which is friction loss, not static head.

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[Andrewg945]

The distance would be close to 40 feet. I’m not concerned with the system pressures. I’m concerned that the boiler minimum relief capacity is 90lbs / hr. PR valves are rated in psi. I want to increase the PR valve to 40 or 50 psi. Does that violate the minimum relief capacity on the boiler of 90lbs /hr? What is the relationship of these two different measurements. System pressure is not a concern because the piping is black iron pipe throughout. I have done this before, but I’m trying to understand the relationship between 90lbs/ hr and psi?

 

[Latexman]

What's the boiler and black iron pipe and fitting MAWP? 150 psi?

Good Luck,
Latexman

 

[LittleInch]

I will stand to be corrected here, but I think there is no relationship.

The pressure upstream the orifice in the relief valve is such that you have choked flow as the d/s pressure is basically atmospheric whereby the mass flow of a liquid doesn't increase if you increase the upstream pressure.

I think.

Can you post details of your relief valve.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Andrewg945]

Ok,
I called the boiler company. The maximum allowable Working pressure is 50 psi. Therefore I can replace the 30 psi relief valve with a 40 psi without any worry. I believe the expansion tank is also no good because the valve was closed and the pressure was reduced in the system. I will install a 40 psi, and a new expansion tank and this should alleviate any problem. Thank for you for your help.

Andrew

 

[LittleInch]

Your static head is about 18 psi. If the RV is downstream of your pump you only have about 15 psi spare for max expansion. That's not a lot so the valve could easily weep in service.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Andrewg945]

I disagree. The system was setup running with a 30 psi RV and working pressure of 15 psi. The valve was weeping at these pressures. The original expansion tank was broken and too small. I am upsizing the expansion tank and increasing the valve capacity to accommodate the 3 psi increase in working pressure for the sake of the additional story of water travel. The new exp tank is sized correctly and is 66% larger than original and designed to spec. This will alleviate any future problems.

 

[LittleInch]

Ah, so there's another floor being added?

3 psi is about 2m additional head. Doesn't sound enough for another floor.

Also whilst the size of the tank is important, so is its minimum pressure when cold ( has to be enough to ensure pressure > 5psi at the highest point) and more importantly what its maximum pressure is when hot. Get this wrong and your relief valve will still drip away. My point was that you don't have much room for this pressure rise from cold to hot.

It's not surprising the pressure valve was leaking if there was no expansion tank available....



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Andrewg945]

With the correctly sized exp rank and a new 40 psi RV it should be fine. Max temp is 180 degrees. If there is a problem I can also drop the temp to 170