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Is there an error in NFPA 58 calculation of equivalent SCFM of air regarding PSV relieving rate?
2

Is there an error in NFPA 58 calculation of equivalent SCFM of air regarding PSV relieving rate?

Is there an error in NFPA 58 calculation of equivalent SCFM of air regarding PSV relieving rate?

(OP)
The equation that I see, based on an earlier version of the pamphlet, is:

SCFM (air) = (3.09)(W)[(ZT)/(M)]^0.5

where

SCFM (air) is equivalent air flow
W is relieving capacity of product vapor at relieving conditions in lb/hr
Z is compressibility factor of product vapor at relieving conditions
T is absolute temperature of product vapor at relieving conditions (degrees R)
M is product vapor molecular weight

I suspect - but have not confirmed - that "W" should be in lb/min. Otherwise, my answers are rather disturbing.

Any insight or update or errata note that people can share would be helpful.

RE: Is there an error in NFPA 58 calculation of equivalent SCFM of air regarding PSV relieving rate?

not an engineer and far from knowing anything about math

From the 2014 NFPA 58

same as in the 08 edition



SCFM (air) = equivalent airflow in standard ft3/min
W = relieving capacity of product vapor at relieving
conditions in lb/hr
Z = compressibility factor product vapor at relieving
conditions
T = absolute temperature of product vapor at
relieving conditions in °R
M = product vapor molecular weight

RE: Is there an error in NFPA 58 calculation of equivalent SCFM of air regarding PSV relieving rate?

The equivalency of SCFM for air is from which medium? Is it SCFM of air from steam in lbs/hr? or is it SCFM of air from air at ACFH? The only equivalency formula that I know is cfm of air from steam in lhs/min or hr.

RE: Is there an error in NFPA 58 calculation of equivalent SCFM of air regarding PSV relieving rate?

Snorgy:

I recommend you talk to this gentleman. Bruce has been with NPGA for years and authors the NFPA 58 handbook:

Senior Technical Advisor: Bruce Swiecicki, P.E., bswiecicki@npga.org, 21200 S. LaGrange Rd., Suite 353, Frankfort, IL 60423, 815-806-9035

RE: Is there an error in NFPA 58 calculation of equivalent SCFM of air regarding PSV relieving rate?

(OP)
Thanks folks.
No need to pursue further, the equation of interest is (1) out of date and (2) wrong.

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