Setpressure and Blowdown in Pressure Relief Valves 2

[Lebpower]

Hello my friends,

[ul]
[li]I'm new designing relief valves, I want to calculate the set presure of a RV, I have read that it has to be lower than the MAWP but ASME Sec. VIII div. 1, allows an overpressure of 10% (in vessels with only 1 RV,
1.-) In which scenario would I want to set the valve with an overpressure?.
2.-) Is there a formula to calculate the setpressure?[/li]
[/ul]


[ul]
[li]And also I want to know about the Blowdown, I have read that is the pressure which the valve close it or start to close (not sure) 3.-) Can anyone clarify this?

4.-) And I also want if I have a setpresure close to the Operation Pressure, is it possible to get Blowdown? If so, can anyone explain me how?

5.-) Is there a formula or a recommendation to calculate Blowdown?
[/li]
[/ul]

 

[Ehzin]

Lebpower,

To start off, there is a specific subforum for relief devices under Chemical Engineers section titled "Safety Relief Valve engineering (PSV)". In the future, they may be able to give a more thorough, informed answer over there.

But to give some feedback on your question and I'm not an expert on the subject but the overpressure you've mentioned doesn't mean you can increase the set pressure some margin over MAWP. You're still bound to keep the set pressure at or below MAWP of your vessel. What it does allow you to do is increase the pressure in which you perform your relief rate calculations.

Say I have a vessel with an MAWP of 100 psig (6.8 barg). I'll put my PSV set pressure at 100 psig but when I do my relief rate calculations I assume that during this event the pressure will build 10% over this value. There's likely a few reasons for this 10% allowable overpressure but here's a few I would imagine account for it. There's margin between MAWP and the original hydro that makes this overpressure acceptable (Current ASME Section VIII, Div. 1 has a 1.3 times hydro) and while the PSV lifts at 100 psig it won't really establish full flow till some margin over set pressure. Based on the two previous comments, you're afforded 10% overpressure during your relief capacities. This provides a larger differential and therefore higher capacity and for gas relief devices, this increase in pressure allows for denser phase therefore more flow.

All of the above allows you to have a smaller relief device that mimics what should be and what would happen in the event of a relief device. Without this 10% allowable overpressure you would have a larger relief device than you need and would provide no additional benefit.

To summarize,

1) Valve isn't set with overpressure. Overpressure is factored into relief capacity.
2) Set pressure can be any value so long as it's at or below MAWP. May be some condition where this statement is incorrect.
3) It's the point at which the valve closes after a relief event. If your PSV lifts at 100 psig it won't necessarily close back down at 100 psig. It requires some margin under set pressure before it'll eventually reseat.
4) Blowdown is related to what happens after a relief event. If you have set pressure close to operating it can prematurely lift which is more of a concern for spring-actuated relief valves. An option if you have operating pressure within 10% of set pressure is to go with a pilot-actuated. They can typically afford closer operating to set pressure scenarios. If operating is 90% or more of set pressure, would definitely check into a pilot. This value may be even lower than 90%, would recommend checking with any respective PSV vendor.
5) Check with respective PSV vendor for blowdown amount. It's tied to how well the valve is designed and at which point it'll begin to reseat after relief.

This isn't an insult but considering I'm not entirely sure in what capacity or to what end you're asking this information I would caution against being the sole person to size and/or specify relief devices without further mentoring or studying on the subject. You've mentioned you're designing relief valves and I believe the above is fairly basic information, it's even defined in API 520 Part I, that shouldn't need to be asked if you're in the process of specifying safety devices.

If you have any more questions or topics to discuss feel free to bring them up(May recommend posting in the PSV subforum versus here)but I would caution against taking on any project in which you're not yet competent without proper oversight considering the importance of these devices.

Thanks,
Ehzin

 

[Danlap]

Hi Lebpower,

In addition of excellent comment from Ehzin, agree with Ehzin that you should connect to either Process engineering from principal or end user side, as well as more a potential mentor from your side.
Why end user? since they are the one who determine the set pressure (they know the vessel or protected equipment MAWP) as well as the blow down pressure.
It is also quite straight forward reading material from API 520 - I & II
Set pressure: is pressure at which the relief valve has been set
The pressure at which the valve reseats is the closing pressure. The difference between the set pressure and the closing pressure is blowdown.
Not to be generalized, depending on the fluid, blowdown can range from 10% to 20% of the set pressure. On some relief valve type, blowdown set can be adjustable as well.

Then on more detail basis, depending on the flow, type (conventional, pilot operated, etc.) and other factors then manufacturer do nozzle sizing, etc.

Goodluck,
regards,
MR

https://nosuchvalve.com

All valves will last for years, except the ones that were poorly manufactured; are still wrongly operated and or were wrongly selected

http://www.eng-tips.com/faqs.cfm?&rat1=2&fid=376