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# API 520 Rupture Disk Liquid Sizing Question

## API 520 Rupture Disk Liquid Sizing Question

(OP)
Hello,

BACKGROUND
-need to size Rupture Disk
--nonfire case
--liquid is discharge fluid (BRINE aka salt water)
--vent to atmosphere
--using API 520, 9th edition, July 2014
--able to use the Coefficient of Discharge Method where Kd=0.62

QUESTION
-Section 5.11.1.2.1 says to use Equation 28 to size the Rupture Disk for liquid case. (the same equation for PRV in liquid case)

-HOWEVER, for Rupture Disks, do i just use Equation 28 with Kv=1 (viscosity correction factor of 1)??
OR
-Continue to Section 5.8.1.4 where you choose an Area that is the next orifice size larger than this A calculated by Equation 28 by using the API 526 Table 1 (standard orifice size).

-my company offers an approved computer program named INSTRUCALC so before accepting its integrity, I wanted to check it against my previous calculations mentioned in the Background.
-the answer i got was the area, A, provided by Equation 28 . . . but my required area included the parts after Section 5.8.1.4 where i found the next Area larger than A from Equation 28, plugged that into the Reynold's equation (Equation 31,32), then plugged the Reynold's into Correction Factor (Equation 30), which ultimately changed the REQUIRED area (Equation 37).
-i found this resource on FIKE since i began to second guess myself but it shows that for viscous fluids, you DO need to continue and find the next Area that is larger than your A and so on. http://fike.com.br/public/upload/produto/produto_5...

OVERALL
-i understand the steps pretty well and would like some clarification . . . perhaps my understanding is flawed . . . if anybody could help that would be fantastic! thanks!

EQUATION 28
A = [Q/(38*Kd*Kw*Kv)] * sqrt of [G/(P1-P2)]

A = Required discharge area (in^2)
Kd = Effective coefficient of discharge (0.62)
Kw = Correction for back pressure (do not need)
P1 = Pressure 1 (set relief + 10% overpressure)
P2 = Pressure 2 (vented to atmosphere so will be 0 psig)
Kv = Viscosity correction factor
G = S.G. of liquid at the flowing temp referred to water at standard conditions
Q = Flowrate (gal/min)

### RE: API 520 Rupture Disk Liquid Sizing Question

API 520 Pt I section 5.8 is for sizing a relief valve. You calculate the required orifice area and then choose a valve that has the necessary area (orifice that is > the size that you calculated, i.e "next orifice larger").

If you're going to use a disk, then choose one of the two listed procedures for sizing disks. See 5.11.1.2 for the coefficient of discharge method or 5.11.1.3 for the flow resistance method. I prefer the flow resistance method which treats the disk as any other type of flow resistance element which can be represented using a K value (e.g. elbow, tee, valve, etc.). Go to the vendor's website, or the Redbook, to get the K for the disk (referred to as Kr). As for the question about Kv, using a value of 1 for this service is OK because this fluid is nonviscous.

BTW, I think you're making a bad decision by choosing to use a disk for this application (liquid brine line). If you don't have a vapor-pocket in the system, you probably won't even be able to start the pump without bursting the disk. Even if this system does have a vapor pocket, the risk of a brine spill and downtime (caused by premature disk failure) is pretty high. Use a PSV and save you client some money and misery.

### RE: API 520 Rupture Disk Liquid Sizing Question

What is the application in question? Or are you referring to a general procedure for sizing rupture disc?

I agree with don1980 with regards to preference for PSV over rupture disc, but you need to be 100% positive if a PSV is suitable replacement. The rate of pressure buildup during an overpressure event is normally the governing factor in selecting between PSV or non-reclosing devices. In many cases PSV would do the job effectively, but there are instances in which it would not protect the vessel/system from overpressuring.

Dejan IVANOVIC
Process Engineer, MSChE

### RE: API 520 Rupture Disk Liquid Sizing Question

(OP)
don1980 and EmmanuelTop

Thank you both for your input. To clarify, I was referring to a general procedure for sizing rupture disc.

Quoting 5.11.1.2 Rupture Disk Sizing Using Coefficient of Discharge Method (Kd = 0.62)
5.11.1.2.1 The required discharge area, A, in in^2 (mm2^), can be calculated using the appropriate equation for the flowing fluid. See Equation (2) through Equation (7) for critical gas or vapor flow; Equation (12) through Equation (17) for subcritical gas or vapor flow; Equation (21) and Equation (22) for steam; Equation (28) or Equation (29) for liquid; and Annex C for two-phase flow.

Looking at Equation 28 . . . it is for "Sizing for Liquid Relief: PRVs Requiring Capacity Certification". The equations for sizing a PRV or RD in liquid case, from what I see, are the same EXCEPT for the coefficients of course. Where I got confused was whether or not sizing an RD was dependent on the liquid being viscous or non-viscous as the PRV is. (the answer is yes, RD is dependent on liquid being viscous or non-viscous).

The reason why I got confused was I had defined in my calculation that Brine was viscous (I know it is non-viscous now, thanks for pointing that out) . . . and when I used the computer program INSTRUCALC, it calculated the fluid as if it were non-viscous, even though it did not know what the fluid was. I think my situation has turned into more of a question for the Computer Software developers.

Now to answer some of the process questions that you guys have.
-I prefer the Flow Resistence Method as well but I wanted to check something quick and easy for this computer program so I defined it as Coeff of Discharge Method. The computer program DOES have an input for pipe losses and I have not used it yet but I would think it asks for a Total K or maybe it has a table for Type on Left and K value on right.
-This scenario supported a small-scale lab (0.25" lines, 48cc samples, etc) so using an RD was decided acceptable by the stakeholder. If there were a brine spill, the area would be contained and very minimal clean up required. Furthermore, the relief design was overkill. System was rated for 10k psi, relief set at 8k psi, operation at 6k psi (in vague terms). Ambient conditions so overpressure would have only been created via the manual injection pumps.
-The system had preventitive measures against vapor pockets as well.

With this said . . . I think my case is closed. Thanks folks.

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