Expansion Tank size for Secondary closed loop
Expansion Tank size for Secondary closed loop
(OP)
Hi,
Can anyone explain what value to use for the maximum pressure, when sizing an expansion tank. This is on a closed secondary loop. If it were the primary side it would be determined by the boiler PRV, but what is the determining factor for a secondary loop with a water to glycol heat exchanger?
Thanks in advance for any help.
Can anyone explain what value to use for the maximum pressure, when sizing an expansion tank. This is on a closed secondary loop. If it were the primary side it would be determined by the boiler PRV, but what is the determining factor for a secondary loop with a water to glycol heat exchanger?
Thanks in advance for any help.





RE: Expansion Tank size for Secondary closed loop
RE: Expansion Tank size for Secondary closed loop
Say your vessel or piping/fittings are designed for 125 psi and at the bottom, if the top is at 125 psi then the pressure at the bottom will exceed the design.
RE: Expansion Tank size for Secondary closed loop
Im new to the forum. Here is my problem. I suppose it is similar to the initial problem posed in this forum. I have a closed loop system with 30% acqueous prop. gylcol. The expansion tank is located near the pump inlet (about 22 feet of 4" NPS PVC pipe seperate the pump inlet and the tank). According to the P&ID and the Isometric there is no PRV or PSV anywhere on the system. Does anyone know how to calculate the fill pressure?
Some more input that may be necessary is there is no elevation change greater than 20 feet in the system. My initial guess at solving for the fill pressure is to take the pump discharge pressure and subtract the losses (piping valves etc.) up until the expansion tank as well as adding in the static head from the highest point in the system to the location of the expansion tank. Does anyone know if this is correct? Thanks so much for the help.
RE: Expansion Tank size for Secondary closed loop
RE: Expansion Tank size for Secondary closed loop
The pre-charge pressure should generally be equal to the system fill pressure at that location. So if the tank is at the ground floor of an 80' high building, pre-charge to ~35 psig (weight of the water column). Ref. attached:
ht
CB
RE: Expansion Tank size for Secondary closed loop
RE: Expansion Tank size for Secondary closed loop
RE: Expansion Tank size for Secondary closed loop
RE: Expansion Tank size for Secondary closed loop
Ok, so if the tank is at the bottom of the building then the pressure in the tank equals the water column plus 4psig. Where is that 4psi coming from??
RE: Expansion Tank size for Secondary closed loop
RE: Expansion Tank size for Secondary closed loop
I hope I'm not beating this thing to death, but the link below explains the fluid mechanics more clearly than I can. Good luck!