Low Differential Pressure -Conventional Safety Valve

[Chance17]

I am evaluating a safety valve with low differential pressure
Storage drum 10-ft dia by 30-ft S-S.
Contingency = Hexane Liquid overfilling
Safety Valve Activation pressure = 75-psig which equals vessel MAWP
Downstream disposal = 10-psig normal -- nearly closed containment
Containment is protected by 50-psig safety valve very much smaller than hexane vessel safety valve
The containment fill time is about 15-minutes

I have been sizing the hexane safety valve at differential [75-psig * 110%] - 50-psig = 32.5 Delta P
The hexane safety has adequate capacity at the low differential

I understand the disposal system has issues.
Question: Are there any issues if you focus only on the hexane safety valve??

 

[georgeverghese]

A conventional PSV is a plain dp safety relief device and its set pressure floats along with downstream pressure. So in your case, when downstream pressure reaches 50psig, its actual relieving pressure is 75+50=125psig. You should be using a balanced bellows or a pilot operated PSV which can tolerate high backpressures and yet maintain its set pressure.
This is the reason why the API does not recommend a conventional PSV for applications where disposal system pressure is in excess of 10% of its set press.
If you wish to retain the tank PSV as conventional, then the disposal system PSV set pressure should be no higher than 75*0.1/1.1 = 6.8psig, but I would guess you would not be in favour of this since you say this downstream PSV capacity is much less than that at the tank.

 

[Ehzin]

I believe to expand on George's comment (Apologies if this isn't the preferred format), you reach a steady state where the relief valve has 75 psig + 1.1 times overpressure of 82.5 psig to limit overpressure of the vessel. The device, assuming it's a conventional spring loaded relief valve, requires typically 10% of set pressure downstream. In your case the downstream pressure is 50 psig. So as assumed, the applied pressure on the bottom of the hexane relief device reaches is 82.5, the downward force is 75 psig (In terms of spring load, this resolves to a corresponding pound or Newton force). When the downstream system reaches 50 psig, this adds to the downward force on the back of the hexane relief device aiding to seat your hexane relief valve. So under full flow relief conditions, you have 82.5 psig pushing up, the spring load of 75 psig pushing down, and an additional 50 psig from your downstream system pushing down. This downward force that would be applied from your downstream containment system is based on the assumption that it is a conventional spring loaded relief device. If it's a bellows seal this goes higher since the downstream pressure doesn't applied as much of a load on the back of the relief devices disc. If it's a pilot then the impact of any backpressure is even more reduced.

The downstream pressure in a rated flow scenario like full relief won't necessarily stop flow as if the relief valve reseats flow will stop and the backpressure will no longer exists (From a pressure drop and back pressure point of view). Best case would be that the back pressure once flow is achieved will cause the hexane relief valve to settle out in some partially opened position and you'll arrive at a capacity lower than what you've calculated. Your calculations may show that it cane pass say 10,000 lb/hr but since the relief valve is only 75% open it won't pass that full amount. Since this flow region can't be fully quantified it's best to avoid it and prevent back pressure from exceeding set pressure margins by either installing a bellows relief valve or a pilot operated relief device.

I hope I explained this properly but if not feel free to provide response for further discussion.

Thanks,
Ehzin

 

[Ehzin]

To expand on your second section (Too much afterthought) regarding the 32.5 psi for your differential. Assume for a moment your downstream system reached 50 psig and then your hexane relief valve stopped because pressure temporarily declined. If you continue to increase the pressure on your hexane system back up to to 75 psig and further up even 82.5 psig. The hexane relief device sees 82.5 psig on the bottom of the disc and sees 50 psig on the back of the device (Assuming it's a conventional spring devices). Since the spring is sized to apply 75 psig, you have a net downward force. This requires the upstream system pressure to reach 125 psig to begin lifting the device and full flow to occur approximately around 130 psig to achieve full flow. If the vessel/equipment the hexane relief valve is protecting is only rated for 75 psig then this drastically exceeds the design.

The scenario above is just an example of what could be encountered based on the limited information provided. I've made assumptions for the sake of the discussion. Specifics of your system would clarify whether or not the assumptions apply.

Appreciate any further details or added discussion.

Thanks,
Ehzin

 

[LittleInch]

you appear to have a few different issues.

The first to me is that you have a 75 psi system discharging into a "containment" which cannot accept flow for more than 15mins before it will exceed its relief pressure of 50 psi as the relief rate of the "containment" is "very much smaller than hexane vessel safety valve"!!. What happens then?

This sounds like a very bad idea to me.

Then with a simple PRV you have a variable back pressure, starting at 10 psi and ending up at 50.

So your effective pressure relief set point on your hexane system will also vary by 40 psi - Again a truly bad idea.

plus the 110% figure is the maximum allowable during a relief event NOT the allowable set pressure.

We need some more input here. Please.



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