Atmospheric Relief Device 1

[Bill3752]

I posted this on another forum, but received no input. Perhaps somewhere here can help.

I am evaluating a relief case for a client and am confused about how to handle an atmospheric relief device. It is on a suface condenser designed for 25 psig; the condenser is under a turbine. The Graham-Viking literature (based on HEI standard) shows the size of device needed (a) "For protection" and (b) "for maximum non-condensing operation".

Here is the issue. The existing 24" device appears to be slightly undersized (steam flow up to 250,000 pph spec on device - I need 270,000), and installation of a larger device will be extremely expensive, if not impossible. The shell design pressure is 25 psig. I back calculated a Kd for the device (.39), then computed the flow achievable at 25 psig, using a 10% allowable in each case. As you would expect, adequate flow can be had at 25 psig. Does anyone know why the maximum set is 10 psig? I do know that the Graham devices are not built with the same adjustable "spring" type arrangement as a normal RV.

I presume my options are to install a larger device, which may involve increasing the nozzle size (or maybe even installing a new exchanger!). I am supposing that the inlet pressure drop might also be greater than 3% of set (.3 psig) meaning it really needs to be directed mounted to the condenser.

Another option would be to install relief valve to make up the difference.

Can anyone give any guidance. My knowledge of atmospheric relief is obviously lacking.

Thanks, Bill

 

[Latexman]

I do know that the Graham devices are not built with the same adjustable "spring" type arrangement as a normal RV.

Is the atmospheric relief device made by Graham? I've never heard of those and I could not find them on their website. Please give a better description of the relief device.

Good luck,
Latexman

 

[Bill3752]

This is an atmospheric relief device, adhering to the standards established by the HEI - i.e. 250,000 pph steam for a 24" device.
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Made by Graham- Viking. Similar devices made by others have same HEI defined capacities.

 

[Latexman]

Sorry, that still doesn't help me understand it better.

If the condenser is an ASME vessel with a 25 psig MAWP, then you are allowed a 10% accumulation to 27.5 psig, no matter what the set pressure of the atmospheric relief device is. The valve will open at 10 psig. At a relieving pressure of 27.5 psig (assuming it's good for that pressure), it'll have much more capacity than a relieving pressure of 10 (or 11) psig. Sounds like you are okay as long as your calcs are right and the valve is strong enough, but I repeat, I don't have any experience with surface condensers and "atmospheric relief devices". Maybe someone that does will chime in shortly.

Good luck,
Latexman

 

[Chance17]

Some relief devices below 15-psig set require 100% overpressure to be considered fully open. This idea does not exactly match your case -- but it could explane some of the numbers.