Discrepancies Between Api 521 Add 2008 And Jan 2014 For Unwetted vessels in fire case

[hianbotech]

I found big discrepancies between our relief valve sizing and the vendor sizing for fire scenario in unwetted vessels when F' is less than 0.01. The discrepancies are based in the version of the API 521 used for the calculation. Our calculation were finalized last year and based in API Std 521 Addendum May 2008 and vendor used the new API STD 521 Jan 2014. I would like to know why the new API 521 give a completely different results for unwetted vessels when the F' is less than 0.01. What happened with all the vessels already installed and protected with relief valves with sizing based on the old API 521.

Following you can find the differences between the old and new version of the API for fire scenario in unwetted vessels:

1. API Std 521 Addendum May 2008
For fire in unwetted vessels, the F' factor is calculated using the equation (9) pag 42, if F' is less than 0.01, then use a recommended minimum value of F' = 0.01.
The relief load is calculated using the equation (12) pag 43

2. API Std 521 January 2014
For fire in unwetted vessels, the F' factor is calculated using the equation (9) pag 42. When the F' factor is less than 0.01 the relief load is calculated using the equation (13) pag 43. This equation was not indicated in the old API std.

As a summary for our PSV:

Our Calc based in old API
PSV-XXXX
F =< 0.01
A cal = 0.348 in2
Orifice size = G
W Required = 5,748 lb/hr
W rated = 8,306 lb/hr

Vendor calc based on the new API
F' < 0.01 (0.003)
A cal = 0.089 in2
Orifice size = D
W Required = 23,102 lb/hr
W rated = 26,608 lb/hr

As conclusion the new API gives for this PSV smaller size, D versus G, but and larger flow, 26,608 lbhr versus 8,306 lb/h.

I really appreciate your help on this issue.

 

[Latexman]

I see nothing about F' in API Std 521 Addendum May 2008, at least a Find F' does not come up with any hits. Can you clarify.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[hianbotech]

see section 5.15.2.2.2 Vessels containing only gases, vapours or super-critical fluids, pag 42
F' can be determined using equation (9). If calculated using equation (9) and the result is less than 0.01, then use a recommended minimum value of F' = 0.01

 

[Latexman]

The API Std 521 Addendum May 2008 I'm looking at only has 11 pages. It starts with Section 5.15.7.4. It's here Link .

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[hianbotech]

The file that you checked only shown the changes in the latest revision. The API STD 521 fifth edition, January 2007 (includes errata June 2007), addendum May 2008 has 206 pages. This is the standard that I checked.

The new API STD 521 is the 6th edition,January 2014.

 

[Latexman]

All we have is the 2014 update. Sorry.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[don1980]

The equation in the older edition of API 521 had a derivation error.