Large pressure swings in secondary heating loop?

[russgs]

We have a school with a primary heating loop off the boilers and a secondary glycol loop via a converter. Our plumbers are telling me that in the shoulder seasons when the temperatures are warmer they have to add fluid beacause the pressures in the glycol side are too low but when the outdoor temp gets very cold the loop gets to full heat (180F) the pressure gets too high and they have to drain some fluid out of the system.
My first thought was too small an expansion tank, but I checked and rechecked and the sizing is OK. What else could cause such large pressure swings. Perhaps the initial charging of the expansion (diaphragm) tank was incorrectly done. Is this a possibility? I don't think incorrect pump sizing would be the culprit?

Thanks for any tips.

 

[zekeman]

Depends on what those pressures are. If you have a closed glycol loop, the expansion tank should be sized to take care of expansion from cold to 180 F. If you have substantial pressure swings then you have either an inadequate or failed expansion tank. BTW make sure that the valves surrounding the pressure tank are open. A properly sized tank will show moderate swings over a range of temperatures.
What is the total volume of fluid in the secondary loop? And what is the maximum volume expansion? From the literature I found that the volume expansion of a 50% aqueous solution from 50 F to 180 F is approximately 4%
You can check for a failed bladder in the expansion tank by draining the secondary loop and checking the pressure at the tank as it drains. Bladder integrity is evidenced by slowly falling pressure during the draining. A ruptured bladder will will bwe immediately evident after a short loss of fluid-- pressure will drop to 0.

 

[zeusfaber]

Another possibility is that the charging valve on the air side of the expansion tank is leaking. This allows the tank to go solid with symptoms identical to those zekeman describes for a ruptured bladder

A.

 

[russgs]

Thanks very much. I will check these possibilities out and let you know.

Regards,

 

[russgs]

BTW
3782 U.S. gallons (50/50 mix) is the closed system volume.
Diaphragm tanks are 264 U.S. gallons each (quantity of 2) for a total of 528 U.S. gallons. 12 psi precharge and acceptance volume is 81% of tank volume.

 

[btrueblood]

"12 psi precharge and acceptance volume is 81% of tank volume"

IF that means that at system fill pressure the expansion tanks have only 100 gallons of capacity "left" (less than 3% of system volume), then the 4% thermal expansion zeke calculated is going to give you grief...

Would be easily remedied with slightly more fill pressure to give better volume balance.

 

[russgs]

I agree. I just found out that there was a roof top unit with a glycol coil installed on the roof along with the new expansion tank. The spec called for a 12 psi pre-charge which would be way too low since the RTU is 3 stories above...would you agree?

One more question. The proper maximum pressure to use for the primary side is about 5 psi below boiler pressure PRV...but on the secondary side, what determines the max operating pressure? You probably wouldn't want to exceed the pumps max inlet pressure..correct?