Query on balanced bellows type relief valve set at lower pressure 3

[chemks2012]

Dear all,
I am checking relief calculation done by somebody and not sure of few things and I would appreciate your help.

I have a reactor with design pressure of 10barg and accumulation of 10% i.e. accumulation pressure of 11barg.
This reactor is installed with an independent balanced below type relief valve with set pressure of 5barg is installed [note: the low set pressure is due to process limitation] for vapour phase release.
Balanced bellows type valve has been selected as high built up back pressure is expected.
The valve size is 3”X4” and is discharges to an atmospheric vessel located away from a reactor.

Now as can be seen, I can allow overpressure of 120% to take advantage of vessel accumulation pressure of 11barg i.e. I can consider relieving pressure of 11barg and consider relieving fluid properties at 11barg.
However, existing calculation are based on 10% overpressure i.e. relieving fluid properties have been considered at 5.5barg and my queries are:

1) How could I justify this oversized relief valve? Is this allowed at all? Please comment
2) As I understand, balanced bellows type relief valve can cop up built up back pressure up to 50% of set pressure
3) If point#2 above is is correct, what is the significance of use of 10% overpressure here? Sorry, I asking this because generally the ALLOWABLE built up back pressure is less than or equal to overpressure allowed? Please comment or correct me if I am wrong.

Thanks in advance.

 

[don1980]

Chemks –

(1) Not a concern. All properly sized PSVs are oversized – they intentionally have more capacity than needed. Excessive oversizing can be a problem, especially in liquid services. At the set pressure of this particular PSV, there’s no reason to suspect that it’s excessively oversized, and this is a vapor service. The fact that this valve is allowed to reach 120% overpressure increases rather than decreases the confidence that it will operate in a stable manner. Suppose a scenario was missed in the original design. Let’s say you have a scenario that requires 100% overpressure in order for the valve to achieve the necessary capacity. The PSV will lift at 5 barg and remain open while the vessel pressure increases to 10 barg. As long as the built-up backpressure doesn’t exceed the overpressure, then the necessary force balance on the valve is satisfied – that is, you have the necessary net lifting force on the valve disk. It’s only when the built-up backpressure exceeds the overpressure that you have to be concerned about the closing force exceeding the lifting force.

(2) From a back-pressure perspective, a balance or pilot operated PSV doesn’t need to be considered unless the built-up backpressure exceeds the overpressure. Possibly your application needs a bellows valve because the discharge is routed to a header or another vessel, and you want to avoid changes in the effective set pressure due to variations in superimposed backpressure. If this PSV is routed to atm then a bellows isn’t necessary.

(3) I don’t clearly understand your question. Most PSVs are set at the MAWP, in which case the allowable built-up backpressure is equal to the overpressure. There are no designs in which the allowable built-up backpressure is less than the overpressure. For conventional valves the allowable built-up backpressure is 10% or 21%, but those values can be exceeded as long as the built-up backpressure doesn’t exceed the overpressure. Ref API 520 Pt1, 8th ed., 5.3.3.1.3.

 

[chemks2012]

don1980

Thank you very much for your reply

1) Understood - thanks.
2) As the valve discharge to the bursting disc vessel which is at atmospheric pressure there is no variable superimposed back pressure. The original design indicates that balanced bellows was recommended as the built up back pressure was 30% which was more than the overpressure allowed which is 10%.
3) Sorry that it was not much clear. you have partially answered my query in your point no.1. However one thing is not clear and it would be great if you comment.

As stated earlier, the original design is based on set pressure of 5barg and overpressure of 10%. Now let's consider two cases.

case:1 if i assume the conventional relief valve, then the maximum allowable built up back pressure is 10%. Am I correct?

case:2 if i assume the balanced bellows valve, then what the maximum allowable build up back up back pressure? I read somewhere it is 50% maximum? Am I missing anything here?

Thanks in advance.

 

[don1980]

The original designer chose to limit the overpressure to 10%. He didn't have to do that. There are code limits on accumulation but there are no such limits on overpressure. The perceived need for a bellows PSV stemmed from the original designer's arbitrary decision to limit the overpressure to 10%.

Commenting on case-1: Built-up backpressure is nominally limited to 10% for conventional PSVs, but this limit (10%) doesn’t apply when the allowable overpressure exceeds 10% (e.g. fire cases or cases in which the PSV is set below the MAWP). In those cases the built-up backpressure is limited to the amount of overpressure. Furthermore, unless the built-up backpressure exceeds the overpressure, there’s no need to consider a bellows or pilot PSV.

Commenting on case-2: As mentioned above, a bellows valve is only needed when the built-up backpressure exceeds the allowable overpressure for a particular installation. Let’s say you have a case that allows 100% overpressure (PSV is set below the MAWP), then the built-up back pressure can be as high as 100% without the need for a bellows.

In the application we’re discussing, the use of a bellows PSV actually creates a problem, because there’s a mechanical limitation to how much backpressure the bellows can tolerate. The following explanation is something that a lot of engineers don’t recognize. I suggest you pull up a cut-away diagram of a bellows PSV because it will help visualize this explanation. The force balance on the bellows is different from the force balance on the valve disk. From a valve stability perspective, a conventional valve can tolerate (and be perfectly stable) with 100% built-up backpressure if the overpressure is that same value (100% in this case) or higher. In this example, the force balance on the PSV disk is the same as that in the case where the built-up backpressure is 10% and the overpressure is 10%. Overpressure and built-up backpressure are competing forces (act in opposite directions). Built-up backpressure won’t cause the PSV to re-close until it exceeds the overpressure. That’s why a conventional PSV is perfectly suited for the application you're discussing (5 barg set pressure, 10 barg MAWP, therefore 120% allowable overpressure). The force balance on the bellows is different because the reference pressure on the bellows is always atm pressure. When the built-up backpressure exceeds 40-50% the bellows starts to deform (bends inward toward the valve stem), and this exerts a closing force on the disk (the valve is no longer balanced). So, for your application the bellows valve is limiting the overpressure to 40-50%, whereas this limit would be 120% if you’d used a conventional PSV. It's a bit counterintuitive to recognize that a conventional PSV is actually better than a bellows PSV for this application.

 

[chemks2012]

don1980

Thank you so much for your reply - such an amazing info!

one thing to ask though.. From a valve stability perspective, a conventional valve can tolerate (and be perfectly stable) with 100% built-up backpressure if the overpressure is that same value (100% in this case) or higher. In this example, the force balance on the PSV disk is the same as that in the case where the built-up backpressure is 10% and the overpressure is 10%.

In the example we are discussing, if i allow 120% overpressure, are we saying that the allowable back pressure for conventional relief valve is 120%.

PS: There is another parallel thread discussion on similar topic but still I am not sure above query.

Thanks in advance.

KS

 

[don1980]

Copying from the second post above.......

For conventional valves the allowable built-up backpressure is 10% or 21%, but those values can be exceeded as long as the built-up backpressure doesn’t exceed the overpressure. Ref API 520 Pt1, 8th ed., 5.3.3.1.3.

 

[chemks2012]

don1980

Yes got it. Thanks!