Page 540 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

506 Applied Process Design for Chemical and Petrochemical Plants

_ TYPICAL
ELBOW MOUNTED
HAD EXTINGUISHER
ON DUCT

SAFETY DISCONNECT SWITCH
ON ACCESS HATCH

TYPICAL EXTINGUISHER
MOUNTED ON RAISED
HOUSING (INSTALLER
FABRICATED)

Figure 7-62. Typical Fenwal explosion suppression system. By permission, Fenwal Safety Systems, Inc.

relates to explosion prevention systems and the "design, P; = the maximum initial pressure at which the com-
construction, operation, maintenance and testing of sys- bustible atmosphere exists, psig
tems for the prevention of deflagration explosions" and is R = the ratio of the maximum deflagration pressure to
a valuable document for designers. One significant the maximum initial pressure, as described in
requirement of this code relates to establishing the design Code Par 5-3.3.l
pressure of a ASME code pressure vessel if the vessel is to Fu = the ratio of the ultimate stress of the vessel to the
allowable stress of the vessel
possibly be subject to and contain a deflagration and its
explosion pressure internally: F, = the ratio of the yield stress of the vessel to the
allowable stress of the vessel
Note: 1 psi = 6.89 kPa.
From NFPA-69 [55), extracted by permission. Design
Pressure: 5-3.3.1 The dimensionless ratio R is the ratio of the maxi-
mum deflagration pressure, in absolute pressure units, to
1.5[R(P. +14.7)-14.7] the maximum initial pressure, in consistent absolute pres-
I , psig (7- 64) sure units. For gas/air mixtures, R shall be taken as 9.0;
F u for dust/air mixtures, R shall be taken as 10.0.
1.5[R(P. +14.7)-14.7] Exception: A different value of R may be used if appropri-
pd = I > psig (7-65) ate test data or calculations are available to confirm its
FY
suitability.
where Pr = the design pressure to prevent (vessel) rupture 5-3.3.2 For operating temperatures below 25°C, the value
due to internal deflagration, psig of R shall be adjusted according to the following formula:
Pd = the design pressure to prevent deformation due to
internal deflagration, psig R' = R [298/ (273 + T;)] (7-66)