Expansion Tank size for Secondary closed loop

[russgs]

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.

 

[ChrisConley]

Maximum pressure would be system fill plus the discharge pressure of the circulating pump in the closed loop. Of course the entire loop won't be at the discharge pressure of the pump, so you've got some safety built in to that number.

 

[walkes]

I would think the maximum pressure would be the relief valve setting for your pressure vessel on the secondary side (heat exchanger), less the static height of the system if the vessel is at the bottom.
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.

 

[introengineer]

Hello Everyone,

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.

 

[walkes]

Static head difference between the fill point and the highest point in the system (converted to psi) plus 5 psi to push air out. Pre-charged expansion tank may be around 12 psig. So a minimum fill pressure of 12 psi.

 

[ChasBean1]

For tank size, compare specific gravities (or specific volumes or densities) at the two extreme temperatures of your system. For example a 30% glycol heating system SG would be 1.05 at room temperature and 1.013 at 200°F, about a 3.5% difference. So if you have a 1,000 gallon system, the ACCEPTANCE volume of the tank (not tank volume) should be at least 35 gallons. You should make it quite a bit more however for error, future tie-ins, etc.

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:

http://tacohvac.reachlocal.net/uploads/FileLibrary/402-015.pdf

CB

 

[DuctHunter]

I dont' typically see expansion tanks on the secondary loops if there is one present in the primary loop. Is it even needed ?

 

[ChasBean1]

Water to glycol HX and closed secondary loop might indicate isolation from primary. The point is moot however as our original poster may have lost interest...

 

[sspeare]

An example might help. Expansion Tank on bottom floor, say Elevation = 0 feet. Assume the bldg is 3 stories, at 10 feet floor-to-floor, so Elev of 3rd floor is 30 feet. Assume the highest point the water is piped to is 4 feet above the 3rd floor. Therefore the total Elev difference is 30 ft + 4 ft = 34 feet. You need at least 4 PSIG residual at the highest piping point. The pressure equivalent of the total Elev difference is 34 feet/2.3 = 14.8 PSIG. Adding 4 PSIG residual to 14.8 PSIG elevation gives you 4 + 14.8 = 18.8 PSIG. This will get you started, but you should check the link below for what happens when you get up to operating temperature. Watch out you don't exceed the boiler's T/P valve setting when you heat up. A boiler with 30 PSIG rating will mean you pop open the T/P valve if you don't size the expansion tank correctly.

 

[Waramanga]

Chasbean, the origanl poster may have lost interest but rest assured that the rest of us are!

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??

 

[sspeare]

You need at least 4 PSIG residual at the highest piping point. The idea is to keep air from getting into the piping system. The 4 PSIG "residual" at the highest point is just a way of making sure you have a slight positive pressure in the piping, so the piping never goes into a vacuum. Taco, Bell & Gossett and other exp tank mfrs typically call for 4 PSIG at the highest point. Bell and Gossett has an expansion tank sizing calculator; see the link below. Did I answer the question?

 

[sspeare]

Re: Expansion Tank Sizing
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!