Page 489 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Process Safety and Pressure-Relieving Devices 455

T1 = gas temperature, absolute, in degrees Rankine, at cubic feet of vapor at stated conditions of burst pres-
the upstream pressure, determined from the fol- sure, or both.
lowing relationship: 2. Preferred material of construction, if known, other-
wise state service to obtain recommendation.
3. Type of hold-down arrangement: flanged (slip on,
weld neck, screwed, stud) union, screwed, or special.
4. Material of construction for hold-down (flange,
P� = normal operating gas pressure, in pounds per square screwed) arrangement. Usually forged carbon steel
inch absolute. is satisfactory, although aluminum or other material
T� = normal operating gas temperature, in degrees Rankine. may be required to match vessel and/or atmosphere
surrounding the disk assembly.
The recommended maximum vessel wall temperature 5. Temperature for (a) continuous operation and (b)
for the usual carbon steel plate materials is ll00°F. Where at burst pressure.
vessels are fabricated from alloy materials, the value for Teo 6. Required relief or burst pressure in vessel and the
should be changed to a more appropriate recommended back pressure on the disk, if any.
maximum [33a]. 7. Disks to be ASME Code certified.
The relief load can be calculated directly, in pounds
per hour [33a]: When the flow capacity for relief can be given to the
l disk manufacturer, together with the conditions at burst-

( Tm - TI ) 1 .25 , lbs/hr (7 - 34) ing pressure (including temperature), the manufacturer
T I.1506 can check against a selected size and verify the ability of
1
the disk to relieve the required flow.
Rupture Disk Sizing Design and Specification
Size Selection
Rupture or burst pressure of the metal disks must be
specified at least 25% to 40% greater than the normal Rupture disks are used for the same purpose as safety
nonpulsing operating pressure of the vessel or system valves and, in addition, serve to relieve internal explo-
being protected. For low pressures less than 5 to 10 psig sions in many applications. If the pressure rise can be
operating, the differences between operating pressure anticipated, then the volume change corresponding to
and set pressure of a valve or disk may need to be greater this change can be calculated by simple gas laws, and the
than that just cited. capacity of the disk at the relieving pressure is known. The
For mild pulsations, the disk bursting pressure should system must be examined and the possible causes of over-
be 1. 75 times the operating pressure; and for strong pulsa- pressure and their respective relief capacities identified
tions, use two times the operating pressure [18]. Non- before a reliable size can be determined. See Figure 7-14.
metallic impregnated graphite disks may be used to burst
at 1.34 times operating pressure as these are less subject to Calculation of Relieving Areas: Rupture Disks for Non-
fatigue. The bursting pressure must never be greater than Explosive Service
the maximum allowable working pressure of the vessel, and
proper allowance must be made for the possible pressure The vessel nozzle diameter (inside) or net free area for
variations, plus and minus, due to the manufacturer's rup- relief of vapors through a rupture disk for the usual
ture pressure range. See Table 7-2 and specific manufac- process applications is calculated in the same manner as
turers' literature. The ASME Code [l] Par. UG-127 for a safety relief valve, except that the nozzle coefficient
requires disks to burst within 5% ± of the stamped bursting is 0.62 for vapors and liquids. Most applications in this cat-
pressure at a specified disk temperature at time of burst. egory are derived from predictable situations where the
flow rates, pressures and temperatures can be established
Specifications to Manufacturer with a reasonable degree of certainty.
For rupture disk sizing the downstream pressure is
When ordering rupture disks, the following information assumed to reach the critical flow pressure although the
and specifications should be given to the manufacturer. down stream pressure initially may be much lower. Under
these conditions the flow through the "orifice" that the
l. Net inside diameter of opening leading to the flange disk produces on rupture is considered to be at critical
or holding arrangement for the disk, inches; or the flow. The assumptions of critical pressure do not apply