Possible Expansion tank issue, condenser pump cavitating

Mech schematic attached

The only place I've only ever seen that was on a hot water heating system that had the expansion tank installed on the discharge side of the circ pump. The tank pressure gauge read 15 PSI before the pumps started, and read 15 PSI after the pump came on. The suction pressure on the pump dropped into negative. We knew the circ was moving water, because we could feel the piping heating up.

The only thing I can think of, is that while your drawing is correct, your expansion tank is actually connected to the wrong side of the pump.

I have verified the connection. It is on the suction side of the pumps. You can see it on the photo below.

What is really puzzling to me is why is the air side pressure so different from the water side pressure.

Suspect that's because the expansion tank has no water in it. Then the bladder can't expand anymore but will be at a higher pressure than the water.

Have you tried pressurising the system when no pumping until the air pressure starts to go up? Usually the only way to check that there is actually water in the tank is to monitor the air pressure from empty as it fills.

How do you fill this system? The diagram shows a mysterious box saying glycol make up package? Is this on line to maintain pressure?

Your picture might make sense to you but doesn't to me.

Only other thing is that the valve is closed.

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Thanks for the tip Littleinch. I do think that, your suggestion of the tank having no water in it could be a possibility.

The glycol make up package is a simple glycol feeder that draws from a tank and pumps directly into the system. However the feeder has been manually shut down and is currently valved off.


Regarding the picture, the right side of the photo shows two suction lines coming off the header and going to the pump. In the background you can see the expansion tank and a black line connecting the tank to the suction header.

as little inch has said, the expansion tank with the pump running will be empty, but showing the charge pressure above the diaphragm, so there is no correlation between the 2 pressures.
The -7 psi in the inlet pipe work seems odd to me as I would expect for an installation as such the suction pressure would be positive - what are the relative levels between the evap. coolers and the pump centreline?

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

I'm >90% convinced this is your issue.

12psi is the factory pre-charge if you look at your data sheet. Therefore for me the bladder is taking up the full volume of the tank.

Your make up package therefore needs to supply glycol at> 12 psi, probably up to at least 50psi or higher in order to get a decent volume of water inside the expansion tank.

Now whether this tank is big enough and in the 110F situation you loose water through some pressure relief I don't know. I can't see a PRV on your schematic, but all this should have bene thought about and written into the operating procedures??

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Also: If you get a response it's polite to respond to it.

Expansion tanks typically come pre-pressurized to 12 - 15 PSIG. This is normally what is required for single storey buildings with the boiler and associated equipment being in the basement If your make-up feeder is set for 50 PSIG, then that's 115.5 feet of head. Tank should likely be pressurized to 50 PSIG just sitting on the floor, and not connected to anything, before being connected to the filled/pressurized system.

How is the system performing? Any complaints? What pressure are the relief valves set for? Have they lifted - discharging into some type of tank since it's glycol - and no one realizes it?

All, thanks for your tips and advice.

@Artsi, the fluid coolers are actually lower than the pumps. If I had to estimate, about 15' to 20' lower than the pump center line.

@littleinch, there is a PRV its kinda hard to see in the photo but if you look close enough its on the expansion tank connection piping. The operating manual just consists of equipment cut sheets and shop drawings, the contractor didn't really provide any procedures.


@TBP relief valves are set for 30 PSI. They haven't gone off though


We began looking at the system this year when we noticed the pumps were very noisy. The distance between the fluid coolers and the chiller plant is quite a bit so the system contains about 16000 gallons of glycol in it. The operators have over the summer added 2000 gallons of (expensive) glycol but the pressure still hasn't built up in the piping. At first, I thought the glycol is just ending up in the expansion tank and the bladder keeps filling up- but the air side pressure hasn't changed and actually doesn't change whether the system is running or off, which suggests that the bladder is basically fully squeezed and the glycol side pressure is still too low despite the addition of glycol.

There's about 200' of piping that's underground so there is a possibility that there is a leak somewhere but we've done tests where the operators have isolated the fluid coolers and all other equipment (chillers, HX) and used an air compressor to pressurize the pipes but the pressure doesn't actually drop

Then it is leaking out of some of that equipment you shut off for the test.

2000 gals in 16000 is a HUGE amount.

Testing it with air is a bad move though. You won't find a leak that way as the air will mask any small volume change.

If the initial bladder was 12psi when it came from the factory and it's still 12 psi then it means there is no liquid in the tank. If there was glycol in then the air pressure would go up.

But first find the leak.

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

Expansion tanks really have only one function. They give the system liquid a space to expand into when it heats up, so the pressure doesn't increase and lift the relief valves.

That's it.

I suspect that the cause of the problem you're suffering is 200 feet away from the expansion tank - in the other building.

When you figure-out what's causing your troubles, please post it.

Also how do you know whether the system is actually full of liquid?

If you've got no operating instructions it isn't beyond reason that whoever filed the system didn't do it very well.....

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

I'm with TBP, the expansion tank on the inlet pipework has nothing to do with it, the pressure on the underside of the diaphragm can only be the pressure in the inlet pipe ie, minus 7 psi irrespective of the charge pressure - look elsewhere.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)