Page 488 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

454 Applied Process Design for Chemical and Petrochemical Plants

Code Requirements for External Fire Conditions 7. To provide some external protection against the
damage that an external fire can do to a pressure
Paragraph UG-125 (3) of the ASME code [I] requires relief valve or rupture disk, this author recommends
that supplemental relieving capacity be available for an that these devices be insulated after installation in
unfired pressure vessel subject to external accidental fire such a manner as not to restrict their action but to
or other unexpected source of heat. For this condition, provide some measure of reliable performance,
relieving devices must be installed to prevent the pressure even if the vessel is not insulated.
from rising more than 21 % [13] above the maximum
allowable working pressure of the vessel. The set pressure Pressure Relief Valve Orifice Areas on Vessels Containing Only
should not exceed the vessel l\'IAWP. A single relief device Gas, Unioetted Surf ace
may be used for this capacity as long as it also meets the
normal overpressure design for other possible causes of
10 percent. If desirable, multiple separate devices can be Due to gas expansion from external fire, the API code
installed to satisfy both potential overpressure situations. [I 0] provides for calculation of the pressure relief valve
For this condition, the API-RP-521 Code [13] (Figure orifice area for a gas containing vessel exposed to external
7-7A) shows an allowable 16% maximum accumulation fire on the unwetted surface:
relieving pressure above the set pressure. For external fire
conditions on a vessel, the maximum allowable accumu-
lation pressure is 21 % above the set pressure [13] for (7- 32)
both single or multiple relieving devices (Figure 7-7 A).
Based on air and perfect gas laws, vessel uninsulated,
Design Procedure and vessel will not reach rupture conditions. Review for
specific design situations [33a]
The usual procedure for determining relief area
requirements is:
where A = effective discharge area of valve, sq in.
l. Determine the external surface area exposed to fire,
as set forth by F' = operating environment factor, min value recom-
mended = 0.01 when the minimum value is
Q = 21,000 FAw+o.s 2 ( 7-30) unknown, use F' = 0.045. Can be calculated by [33c]
[ 25 :
and Table 7-8 and the paragraph on Surface Area F' = 0.1406 (Tw - T 1) 1. (7 - 33)
.
Exposed to Fire. CK d 1 1 o.6506
2. Determine the heat absorbed, Q, from Figure 7-30. A 3 = exposed surface area of vessel, sq ft
3. Calculate the rate of vaporization of liquid from P 1 = upstream relieving pressure, psiabs. This is the set
pressure plus the allowable overpressure plus the
atmospheric pressure, psiabs.
(7 - 31)
where M = Molecular weight of gas/vapor.
4. Verify critical pressure from Equation 7-7 and estab-
lish actual back pressure for relieving device. A3 = exposed surface area of the vessel, in square feet.
5. Calculate relieving area by applicable equation for P 1 = upsu-earn relieving pressure, in pounds per square
critical or non-critical flow, using the flow rate deter- inch absolute. This is the set pressure plus the allow-
mined in (3) above. (See Equation 7-10 and follow- able overpressure plus the atmospheric pressure.
ing). The area actually selected for orifice of safety C = coefficient determined by the ratio of the specific
type valve must have orifice equal to or greater than heats of the gas at standard conditions. This can be
calculated requirements. For a rupture disk applica- obtained from Equation 2 [33a] in 4.3.2.1 of API
tion, the full free open cross-sectional area of pipe Recommended Practice 520, Pan I, or Figure 7-25.
connect.ions in inlet and exit sides must be equal to K.:i = coefficient of discharge (obtainable from the valve
or be greater than the calculated area. manufacturer). K.:i is equal to 0.975 for sizing relief
6. Select a valve or rupture disk to accommodate the valves.
service application. Tw = vessel wall temperature, in degrees Rankine.