NFPA 69 vs NFPA 68

[ARenko]

I have a vessel that will be operating at typically 1-2 psig - it is connected to a flare line. The vessel will occasionally handle air and natural gas and will experience mixtures in the explosible range. We intended that the vessel will operate at maximum 5 psi by using a relief device.

In NFPA 69 Section 13 it gives an equation (13.2) for calculating the vessel MAWP required to contain a deflagration. I calculate this to be 150 psi at our minimum operating temperture of 32F. R=9, but is adjusted to 10.4 for 32F operation. Fy = 1.9 (A 516 Gr. 70).

When I talked w/ a company about sizing a relief device they used NFPA 68 and came up w/ a 30" diameter rupture disk. My understanding from NFPA 69 was if the vessel was 150 psi MAWP and the initial pressure (Pi) was 5 psig, then the vessel could contain the deflagration.

When I review NFPA 68, the term Pred corresponds with Pmawp * 2/3*Fy from NFPA 69 and when I use the calcs from NFPA 68 I get the same rupture disk sizing. Now I'm confused - do I need the large explosion vent or will my vessel contain the deflagration? Another confusing thing is how can the NFPA eq. 7.3.3.2 be used when it states initial pressure must be <= .2 bar (2.9 psig)?

Can anyone offer any advice? Am I understanding NFPA 69 incorrectly?

 

[psafety]

One or the other not both, if you design to NFPA 69 you are effectively containing the deflagration forces.

 

[ARenko]

Thanks for the response psafety. My understanding is we just need to make sure the initial pressure doesn't exceed the Pi we used in the calcs, which is 5 psi. A relief valve for this purpose would not be expected to relieve the deflagration, correct?

 

[psafety]

The relief valve would NOT be expected to relieve any of the deflagration, you are designing a vessel to contain.

If this is an ASME Section VIII vessel you may need a relief just to comply with code. In these cases I typically install a 1/2 - 3/4 inch PSV set at the MAWP as stamped. Its only purpose, to meet the legalities of the Code.