Page 542 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

508 Applied Process Design for Chemical and Petrochemical Plants

For vent area limited or restricted to one end of an From Table 7-27:
elongated enclosure (vertical tank, silo, etc.), the venting
equation is limited to: C = 0.17 (psi) i/s

Pred = 0.40 = 0.40 psi
(7-68)
1
A,.= 0.17 (16,000)/(0.40) 1 2 = 4,300 sq ft
where L3 = longest dimension of the enclosure, ft
A = cross section area, sq ft This will require roof bursting panels, side wall burst-
Per = perimeter of cross section, ft ing panel, or an end panel that could be hinged to blow
out. The panel relieving pressure should be set for 0.40
For highly turbulent gas mixing in an enclosure and psi - 0.35 psi = 0.05 psi to burst or relieve per code.
the vent area is restricted to one end of the elongated
enclosure, ratio of length-to-diameter should not exceed High Strength Enclosures for Deflagrations
2,or

This section and Chapters 6 and 7 of the code [27)
(7-69)
apply to vessels and equipment capable of withstanding
more than 1.5 psig internal pressure. These design pro-
For other conditions, refer to the NFPA-68 Code. For
the above relations to apply, the constant, C, should be cedures do not apply to a detonation that is not believed
referred to fuels having the characteristics of one of those to be capable of being vented successfully [27).
in Table 7-27. The vent devices used to relieve the overpressure from
Ref. [27) presents a thorough discussion of limits to the deflagration must be structurally sound, low in
structural components strengths, and these should be weight, and should not fragment to form missiles when
observed. Ductile design practices should be used. The the force hits the device.
maximum allowable design stress should not exceed 25% The discharge from high pressure vessels must be vent-
of the ultimate strength. The strength of the enclosure ed out of the building to avoid fires and explosions and
should exceed the vent relief pressure by at least 0.35 psi. overpressure in the building, and the backpressure or
The vent design must provide at least the area required pressure loss through the vent duct/pipe must be recog-
to satisfy the volume of the enclosure (see NFPA-68) [27). nized as affecting the relieving pressure of the vessel.
Never locate a vent duct discharge to atmosphere in an
Example 7-17: Low Strength Enclosure Venting area where the discharge might be drawn into (a) fresh
air intake to a building or, (b) fresh air intake to a com-
Design to protect a large warehouse containing plastic pressor or gas fired engine.
materials that can emit ethylene and propylene vapors. Rupture disks when properly sized and located on the
The dimensions on the rectangular building with a flat potentially overpressure vessel have been shown to pro-
roof are: vide the best protection for a deflagration but not a deto-
nation [54).
100 ft long X 50 ft wide X 20 ft tall.
Determination of Relief Areas for Deflagrations of
The building design has been selected as good for 0.4
psi overpressure. See Table 7-23 for glass window shatter- Gases/Vapors/Mists in High Strength Enclosures
ing. Use known design figures when available.
Areas are: The nomographs of Figures 7-63 A, B, C, and D [27)
were developed by Bartknecht [54) for the conditions of:
Floor = 100 X 50 5,000 sq ft
Roof= same 5,000 sq ft • no turbulence in vessel at time of ignition
2 ends = 2 X 50 x 20 2,000 sq ft • low ignition energy of 10 joules or less
2 sides = 2 X 100 x 20 = 4,000 sq ft
16,000 sq ft • atmosphere pressure

Vent area: To utilize the charts Figures 7-63A thru D, enter vol-
ume, read up to selected Prcd value and across to vent
(7-67) pressure P,ta,, and down to vent area required.