Pressure Protection Bladder Expansion Tank

[uscme1997]

I have a chilled water system with an ASME Section VIII, D1 expansion tank. The expansion tank has an internal bladder, which is the connection to chilled water system (i.e. water inside of bladder). The tank itself is charged with air. Being that this is an ASME vessel, I need to provide pressure protection. Fire case is the only overpressure scenario. So instinctively I would think that the PSV should directly protect the tank and be mounted to the air side of the expansion tank. However the tank is a standard TACO design and has no connection for mounting a PSV. So I am left with mounting a PSV to chilled water connection side. Though I see issues with this design. Anyone have experience with this?

 

[LittleInch]

But in the event of fire won't any excess pressure just result in water being pushed out of the expansion tank?

Must be a big expansion tank....

And choose which forum you want this in and and delete the other one please. Double posting is not allowed.

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[uscme1997]

Thanks - did not know about the double posting policy. I will rectify that situation shortly.

When we evaluate fire case, we assume that the vessel is blocked in, thus there would be no pathway for liquid to exit the vessel. Mounting the PSV on liquid side would temporarily help the situation, by relieving water during fire, but if the fire continues then the pressure in the tank would increase again and there would be no method to relieve pressure a second time since the bladder separates the PSV from the air side of tank.

And should the bladder fail from the temperature of the fire, it does not seem reasonable to assume there is a clear pathway from air side of tank to the PSV inlet.

 

[LittleInch]

Does the tank not have a filling nozzle?

Still think having an expansion tank isolated AND a fire at the same time is pushing the bounds of reality. But what's the design pressure and max operating. It takes a lot of temperature rise to result in a pressure rise of x3 (+600C).

So does it actually need a fire case PSV?

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[uscme1997]

So, in my world (Department of Energy - Nuclear Waste Management), if its an ASME vessel and fire case is plausible - then yes a PSV for fire case is required. Basically, we say that a fire is just as possible to happen during a maintenance event (i.e. vessel LOTO) as during a normal operations event. Also, the locked-in vessel + fire is conservative and taken as normal practice at DOE sites.

The only "filling nozzle" is a schrader valve for charging the tank. Otherwise the only connection is to the chilled water system (i.e. bladder side).

MAWP = 125 psig, Normal operating Pressure = 15 psig @ 107 degF. Relief valve is set at 125 psig. I have not run the numbers with PSV the air side yet, but with the PSV on the water side the temperature only gets to ~366 degF before relieving. I am sure it will get much higher than that for the gas side. May even get high enough that I would have to lower the PSV set pressure for the relief valve to be of any use (i.e. prevent vessel rupture).

 

[TBP]

I've never seen safety or relieve valves installed on these tanks. This is low pressure, low temperature HVAC system tank, that is there for operational purposes only. It's not a compressed air receiver. It allows the system water going from cold to warm to have somewhere to expand, so the relief valves don't lift every time the system cycles on/off - on both hot water heating and chilled water systems.

 

[uscme1997]

TBP - But if the expansion tank is an ASME Section VIII, D1 vessel - then pressure protection must be provided for the vessel or an analysis performed in accordance with UG-140, overpressure protection of a pressure vessel by system design, must be performed. In this case, overpressure by fire is deemed a credible event and a pressure relief is required.

 

[TBP]

What was the code of construction for this system? Perhaps post your question on the HVAC board?

 

[uscme1997]

Piping is ASME B31.3 (2004ish) and vessels ASME Section VIII D1 (2010ish).

 

[uscme1997]

Is there aa acceptable method to get ppl from other forums to review this thread? TO get some additional thoughts?

 

[TBP]

Contact the tank manufacturer with your question, and perhaps post it on the HVAC board with a note saying you've posted it here, and didn't get much in the way of a response.

 

[LittleInch]

What type of TACO expansion chamber is this?

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

 

[uscme1997]

CBX254-125

Note that there is a 1/2" nozzle in the tank that is plugged. This cannot be used for a PSV. Removal of the plug invalidates the manufacturer warranty and can cause bladder damage.

 

[LittleInch]

Well it's an option provided that you isolate the tank and blow the air side down to zero before removing the plug.

If the issue is the air getting too hot and / or the potential of the water boiling (bit difficult to see that myself) then you need to find a way into the air side of the tank. So either use the connection that is available or tap a new one, but that's even more fraught with issues. With the bladder in the way then you don't have a connection from air to water to allow you to put a relief valve there.

If you void the warranty, then so what? It's yours now.

Or buy an expansion tank with a spare nozzle.

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

 

[uscme1997]

Thanks to all for your assistance! Regards.

 

[LittleInch]

So what is your plan?

It's always good to finish a post with a solution or a way forward.

Thanks

LI

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