Relief valve - 1/3 and 2/3 area.

[patrickraj]

Dear members,

In relief valve calculations,to avoid chattering because of flow capacity is small, two valves are choosen when calculated orifice area is high, with approximately 1/3 and 2/3 area. The smaller valve can be set to relieve at 3% higher pressure than the larger one. Why is it so? What advantage is expected? Suppose in a fire case, a vessel with a design pressure of 100 psig, the set pressure will be 134.7 psia. What will be the set pressure of smaller valve in this case?

Thanks in advance.

 

[EGT01]

Patrickaj,

You may want to review the American Petroleum Institute Recommended Practice 520 Part I (API RP-520 Part I).

I believe you will want the smaller valve to open first and therefore should be set at a "lower" set point than your larger valve. This will allow the smaller valve to open and control the smaller relief events without affecting your larger valve.

As for the 3% value you mention, I'm not sure to what you are referring but typically a relief valve may start to leak when the pressure approaches about 90% of set pressure and may start to simmer at about 97% of set pressure. If you want your smaller valve to handle smaller relief events without affecting your larger valve, you should design your smaller valve so that it can achieve required capacity at a set pressure plus overpressure that is below 97% of your larger valve set pressure. However, depending on the amount of excess capacity the valve you select for your smaller valve may have, this approach can be relaxed.

The code to which your system is designed will usually have limitations at to the relief device set pressures and generally is given in relation to system MAWP. For example ASME Section VIII is slightly different than ASME Section I.

Since you have not given the system limiting MAWP, I would assume that 100 psig is the limit and the first relief device must be set at or below 100 psig. Depending on your equipment design code, the second relief device can be set slightly higher than MAWP but you should confirm your system limiting MAWP and not just the MAWP of the vessel to which the relief device is connected.

Without further knowledge of your system, 134.7 psia can not be a set pressure but would be the (approximate) maximum allowed accumulated pressure during relief. Per ASME Sec VIII, you are allowed 21% accumulation allowing 135.7 psia.

If your equipment were designed per ASME Section VIII and the larger relief valve was needed only for fire exposure then you could set the smaller valve at 100 psig (or less) and the larger valve at 110 psig (max allowed per ASME Sec VIII). For the fire case then, the combined capacity of both valves at a maximum relief pressure not exceeding 135.7 psia must be equal to or greater than the required capacity.

 

[TD2K]

You should also review ASME section 8, Div 1 which discusses PSV set pressures. Typically, exerpts of this are included in many PSV manuals.

If you have 1 pressure relief valve on a vessel, it can be set no more than 100% of MAWP and must prevent the vessel from exceeding more than 110% of MAWP (there may be an exception for liquid filled vessels but I don't have the reference material available).

If you have 2 (or more) relief valves, they can be set such that the total relieving pressure can be no more than 116% of MAWP AND the highest valve can be no more than 105% of MAWP. The 105% is, I believe, simply based on the typical 110% accumulation pressure used for sizing a PSV. 110% of 105% setpressure (max) comes very close to the 116% accumulation allowed under the code. Coincidence, maybe, it's just my interpretation of it. In any event, the code rules (catchy phrase). Remember that there is nothing against setting a PSV, should you chose, less than MAWP although that affects your operating flexibility.

The 21% overpressure allowed per code only refers to determining the available versus required relieving capacity in a fire case, it does not allow you to set your PSVs higher than the above constraints since you could have other relieving events (although they may not be the sizing case wrt required area) requiring one or more PSVs to open.

As EG01 mentions, I'm also not sure where the 3% you mention comes from. 3% is the maximum inlet line loss allowed under the code and API 520, I've not seen it used wrt set pressures.

 

[TD2K]

One addition, the 110% and 116% allowable overpressures I mentioned are for non-fire relief cases.

For fire relief, the appropriate overpressure is the 21% allowable overpressure I talk about near the end of my post.

 

[patrickraj]

EDT01 & TD2K
Thanks for your useful replies.
This WAS BASED ON an article in CHEMICAL ENGINEERING by ROBERT KERN in Feb 1977 issue.