Multiple PSV Sizing at very low pressure setting 3

[devaxrayz]

Hi all,

Currently i'm doing some PSV sizing. The data is as follow :
- vapor relieve capacity is 3 MMSCFD (12220 lb/hr)
- setting pressure 0.05 psig
- Z = 0.98, Mr = 36, Cp/Cv = 1.3
- The backressure is estimating at 0.01 psig

While sizing i found that this is a SUBCRITICAL FLOW, and i'm run the sizing with subcritical sizing formula as in API 520.

I come up with calculated effective discharge area is around 72 sq in, while the largest standard orifice size as in API 526 is type T with 26 sq in area.

1. First question is simple. Does it means i need 3 PSV type T to cover the required 72 sq in orifice area?? If you intended to have a 3 PSV on one vessel does it mean you have to divided the relieve flow required by 3?? in my case i design a 1 MMSCFD PSV??

2. I cross-checked my calculation using a CRITICAL flow formula and come-up with a result of 9.32 sq in area. This area is far more lower than the first calculation, and i'm a little confusing because i though that at CRITICAL flow, you required more area since the flow will not increase even you add the source pressure. Can I use CRITICAL flow formula for SUBCRITICAL flow condition? or I can't because the formula is not valid??

Actualy the INSTR enggineer also calculated the PSV size with his software (INSTRUCALC)and come-up with the result of 9.32 sq in area (or the same with calculating with CRITICAL flow formula.

Thanks for any comment before...

and regards,

-bay-

 

[xcrosby]

Your set pressue of 0.05 psig does not look right. You will not find a spring loaded valve with such a low set pressure.

 

[EGT01]

devaxrayz,

As Xcrosby suggests, something doesn't look right. I think you need to first identified the type of equipment or class of vessel you are trying to protect then follow the appropriate practices for them.

The recommended practices of API RP 520 are intended as a supplement to the requirements of ASME Section VIII pressure vessels. If the class of vessel you are trying to protect has a pressure rating in the 0.05 psig range, that sounds like a low pressure storage tank not a pressure vessel and you should be following the recommendations in API 2000 which covers API 650 and API 620 storage tanks. Also, you need to be looking at selecting the type of relief devices designed for that type vessel, i.e. conservation vent type relief devices.

 

[devaxrayz]

Thanks for the response.

for a little background: the PSV will be installed at a KO Drum (design pressure 50 psig) prior of flare stack. However, the PSV intention is NOT to protect the KO Drum. It is installed because the KO Drum may NOT operate more than 0.05 psig.

Why?? Because the source of vapor to the KO Drum is an ATMOSPHERIC separator tank design at around 0.1 psig. It was considered that if the KO drum operate at 0.05 psig, the backpressure to the atmospheric tank will be more than its design pressure.

I have seen the relief device (breather valve) that equip in the atmospheric tank, and it doesn't use a spring loaded device but use some weighting disc to it. I have not look to API 2000, but just a for a clue, is there a procedure for sizing this low pressure relief device on it??

O ya, i still looking forward for my number 1 question above?? Just to satisfy my desire of understanding

thanks and regards,

-bay-

 

[Montemayor]

devaxrayz:

Xcrosby and EGT01 are giving you the right advice. You don't have a heavy problem. You're simply not attacking the problem with the right tool. API 2000 is the most correct recommended practice to follow. InstruCalc is not suited for this problem because you're not dealing with a Section VIII pressure vessel. Go to:

http://www.protectoseal.com/

there you will find the answer to what you are seeking - according to your description of the application. Don't try to calculate the "correct" orifice size for the application. Just get your worse credible scenario identified as to capacity and pressure/temperature conditions and the supplier like Protectoseal will do the rest.

 

[EGT01]

devaxrayz,

In response to your number 1 question, neither API 520 or API 526 applies to low pressure storage tanks. So, as long as that is part of the question
"Does it means i need 3 PSV type T to cover the required 72 sq in orifice area??"
the answer is "No".

Do as Montemayor suggests and define the vessel code requirements, the required relief rate and conditions and then select the appropriate relief device (or devices) for your requirements such that their combined capacity is equal to or greater than your needs. And yes, you may need more than one relief device. The flow through each device will be a function of their type, size, and set pressure.

In addition, with the latest information you provide about your system, I would also question what you propose as being feasible. If I understand correctly, you have an atmospheric tank designed to only 0.1 psig venting to a KO drum designed to 50 psig and which then vents to a flare stack.

In my experience, I don't think it would be uncommon for the pressure at the base of a flare stack to be more than 0.1 psig even without a flaring event happening. During a flaring event the pressure will likely be much greater, the 50 psig design pressure on the KO drum makes sense. So the PSV you add to the KO drum vent, where does it vent, to the atmosphere??? Obviously, it won't do any good to vent it to the flare. So, that means that an equivalent 3 MMSCFD capacity for that relief device won't be going up the flare, right?

 

[devaxrayz]

Montemayor and EGT01, first thanks for your post. It realy helpful.

Actualy what i want to ask is if someday i have to size a PSV for pressure vessel using API 526 and got a required orifice area 72 sq in, does it mean i should use 3 type T orifice??

And YES, the pressure at bottom of flare stack and also pressure loss at flare burner is also be my concern due to lack of experience. Honestly, i'm intended (assuming the flare pressure as atmospheric) to vent the PSV outlet to the flare. And what you wrote here base on your experience make me reconsider about it again. I give you two star for it.

Regards,

-Rayz-

 

[EGT01]

devaxrayz,

When sizing relief valves for pressure vessels, if you determine that you need 72 sqin for the required relief area of your largest relief event, using 3 relief valves each with a T size orifice would be one possible approach but that may not be your best choice. I would suggest that you study the sections in API Recommended Practice 520 Part I that cover Mulitple and Supplemental Device Installations.

One of the problems when all of the valves are very large is there may be a small relief event that can not be properly handled by a large valve. For a small relief event, a large valve will have excessive capacity and the valve will be prone to chatter which can result in damage to your equipment. You may find that 2 "T" size orifices (2*26=52 sqin) plus 1 "P" orifice (6.38 sqin) plus 1 "R" orifice (16.0 sqin) would better suit your needs.

Also, when using multiple relief valves you should consider staggering their setpoints so they all don't start to relieve at the same time. In the example above, the smallest valve would be set to open first, followed by the next largest valve and then the 2 largest valves. API RP-520 gives details about the limitations in selecting the set points for multiple device installations.

As another possibility for valve arrangement, some relief valve manufacturers offer valves in extra-large sizes. For example Farris offers a size "W" which is 57.26 sqin.

 

[devaxrayz]

EGT01

thanks alot, this kind of advice that i would never get at school

cheers,

-Rayz-