diaphragm expansion tank

[Clymber]

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.

TIA,
bleacher

 

[pmover]

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.

-pmover

 

[MintJulep]

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.

 

[krb]

I would second the comments from pmover and add that when the tank is put into service, the pressure on the gas side of the bladder will rise as water flows into the tank and compresses the the bladder and makes the volume of the gas side smaller. If pressure in the system drops, the higher pressure in the tank will then force water back into the system to maintain system pressure. Bell and Gossett has a manual for calculating the tank size and giving a description of operation of these tanks. This info is available on their website. As pmover mentioned, the tank will need a make up line, and the BG literature shows how all of these items go together. This is a fairly simple and economic way to be sure the system has the correct volume all the time. Without a makeup, you will need some method to add water when necessary, and this could be very problematic. If you have a large system, I would suggest putting a flowmeter on the makeup to alert personnel when makeup is excessive, indicating a large leak in the system that needs repair.

 

[glfrank]

In my experience, bladder style tanks should be located at the point of lowest pressure in the system, usually on the suction side of the system pump(s). The tanks I've used had specific instructions about locating the tank at the point of lowest system pressure. Also, an air separator or automatic air vent should be located somewhere in the piping, ahead of the tank. It is important in bladder systems to keep air out of the bladder.

Look at this link to B&G's bladder tanks

http://fhaspapp.ittind.com/literature/files/572.pdf

Last page of the manual shows suggested piping diagrams. Hope that helps!

 

[walkes]

Clymber,

I just want to add a few comments to the other posts. The expansion tank is typically at the suction side of the pumps. Say the precharge pressure is 12 psig (your system fill pressure), the inlet side of your pumps would be 12 psig, your pump head of 40 psig is added to that so your pump discharge pressure is now 52 psig. The fluid is pumped around the circuit and due to losses will be at 12 psig when it returns to the expansion tank.

If the fluid expands in the system due to temperature change, the acceptance in the tank will allow more fluid in while maintaining the system precharge pressure of 12 psig. If there is a leak in the system, the system pressure will drop to a point where your fill system engages (assuming automatic fill as in a glycol fill tank, or pressure reducing valve on make-up water), extra capacity in an expansion tank won't help you with a system leak.

 

[Clymber]

Thanks for the information. The existing expansion tank is located on the suction side of the pumps. However, the client is converting from water to glycol so tank size must increase. I think I've got it now. If I consider worst case temperature extremes and calculate volumes for each, I can then size the expansion tank accordingly.

From your comments, the only trouble could be if an automatic make-up is not available. This client prefers a manual fill, which means routine monitoring is necessary to ensure no leaks have occurred.