Closed Loop Water Make Up / Expansion Tanks

If you are about to perform a test and balance on a Chilled Water Closed Loop system, my advice, don’t start it without knowing the piping system is full of water and free of air, unless you don’t mind wasting time. Go to the roof, open some vents, if you don’t get water, without air, then begin. If you get either, notify the MC to get it ready!!!

Chilled Water loops are primarily a Closed Water Loop. There is an exception, rarely seen anymore, and that is when an open tank (with a float valve inside maintaining a water level) and positioned above the piping system, connected with a pipe, (typically a ¾”), to keep the piping system full. And you only need about 20 PSI of domestic water pressure to get the job done.

Although no one seems to like them anymore, they did the job, as it met the parameters needed for the piping system. Keep it full of water!

Well now, we could have the make up water in a chiller plant on the second floor, with a building above, (for this example we will use 20 stories of an office building, and consider each story is 12-foot slab to slab).

Okay, now this make-up water system needs to have the water pressure needed, and would need a PRV (Pressure Regulating Valve).

Well, it needs more devices, such as a gauge or two, valves, a blow-off, perhaps with a by-pass for fast make up, and definitely should be connected to the “SUCTION SIDE OF THE PUMPING SYSTEM”. This is a must!

So, what is the PRV to be set to, and how much water pressure would we need to do the job?

Okay, Us T&B guys know this, take the height of the building above the PRV station, which doesn’t have to be exact, but close, so 20 stories at 12’ is 240 feet of pipe above it. Since we know there is 2.31’ to a pound of pressure, it is 240/2.31 or 103 PSI. Then we could add about 10 PSI to be safe, and as that would be okay for the rating of the Pipe Valves and Fittings (PVF), the set point would be 110# at the PRV. And guess what? The domestic water pressure needs to be at least 110 PSI.

Note, normal PVF is rated for 150 PSI, the next is 250 and it would be special flanges, valves and devices needed. Everything within the system, must be rated for that kind of pressure. But remember, the higher you go the less the “Standing Pressure”. Let’s say at the 18th floor now at 24’ to the roof or top of the piping system, the pipe pressure is only 10 PSI.

A quick tip, if you ever open a gauge cock and it sucks air, THE PRV AIN’T SET RIGHT!

Expansion Tanks

A requirement for any closed loop system.

Starting with Bladder Tanks, the tank needs to be set for the correct pressure according to the PRV set point. A few pounds less would be my suggestion.

A Bladder normally comes with 12 PSI in the bladder. Well, if we are about to put a 110 PSI in it, the Bladder will be collapsed after about 30 PSI and there won’t be any room for expansion or contraction at 110 PSI. So, before it is connected to the system, it needs a charge of air or nitrogen.

The other type is a “Plain Steel Tank”, normally with a sight glass to see the level, if with the site glass valves open, it is at least 30% air, then you’re good. I’d close the valves again as its usually the site glass that leaks air and makes the tank ineffective.

Hope this helps someone.