In looking at diaphragm expansion tanks for a hydronic chilled water system (glycol), the standard tank precharge (air side) is 12 PSIG. 

What happens when the pumps are active at, say 40 PSI?  Does the tank bladder collapse? 

The customer wants to build some reserve into the tank to allow for possible leaks.  However, won't the extra fluid remain in the tank until the pressure drops below the precharge pressure?

I'm not sure I understand the concept here.

the purpose of expansion tanks (with or without bladders) is to accommodate the thermal expansion/contraction of the fluid due to temperature changes. without expansion tanks, the piping system is subject to over/under pressurization, which leads to major maintenance problems.

expansion tanks are precharged with a gas (air, nitrogen, etc.) that depends upon the piping system volume, temperature, etc.

where is expansion tank located? upstream/downstream of pumps? the bladder will compress/expand depending upon system pressure and any fluid temperature changes. generally, expansion tanks are located in piping system such that pressure changes are minimal (i.e. upstream of pumps). if there are large pressure differences, then this will likely be a design consideration for determining the expansion tank size.

the piping system should also have a water make-up system to maintain proper fluid levels. if there are leaks within the system (i.e. pump packing, leaking joints, etc.), then the water make-up system maintains system pressure.

There are some differences between a bladder tank with a pre-charge pressure and an expansion tank without a bladder.

In a simple expansion tank there must be an gas volume above the liquid level (otherwise there is no point in having the tank).  As the liquid and the gas are in direct contact, the gas pressure must always equal the liquid pressure.  The pressure is determined by the system equipment only, and the volume of the liquid it determined by its PV=nRT.  The relationship between gas and liqued volumn in the tank by P1V1 = P2V2.  Tank serves only as a place for the liquid to expand into.

With a bladder tank things are different.  The liquid side should be full (no air in the liquid side of the tank).  Pressure of the gas and of the liquid again must be equal, but now it is because of a force balance across the surface area of the bladder.  The pressure of the liquid is now determined by the system PLUS the precharge pressure.  The liquid pressure can never fall below the precharge pressure.

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