Page 450 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition
P. 450
416 Applied Process Design for Chemical and Petrochemical Plants
3. Pressure relief must be adequate to prevent internal
pressures from rising over 10% above the maximum
allowable working pressure, except when the excess
pressure is developed by external fire or other
unforseen heat source. See design details in later
paragraph. Papa [85] proposes an improved tech-
nique for relief sizing. Also see Figures 7-7 A and 7-7B.
4. When a pressure vessel is exposed to external heat
or fire, supplemental pressure relieving devices are
required for this excessive pressure. These devices
must have capacity to limit the overpressure to not
more than 21 % above the maximum allowable work-
Assembly UA-2M ing pressure of the vessel. (See Figures 7-7 A and 7-
7B.) A single relieving device may be used to handle
the capacities of paragraph UG-125 of the code, pro-
Figure 7-SP. Typical union type disk holders. They are not all avail- vided it meets the requirements of both conditions
able. By permission, B.S.&B. Safety Systems, Inc. described.
pressure devices. There are exceptions covered by
paragraph U-1 of the code, and in order to omit a
protective device this paragraph should be exam- 5. Rupture disks may be used to satisfy the require-
ined carefully. For example, vessels designed for ments of the code for conditions such as corrosion
above 3000 psi are not covered; also vessels with <120 and polymer formations, which might make the safe-
gallons of water, vessels with inside diameter not over ty/relief valve inoperative, or where small leakage by
6 inches (at any pressure), vessels having internal or a safety valve cannot be tolerated. They are particu-
external operating pressures not over 15 psig larly helpful for internal explosion pressure release.
(regardless of size), and a few other conditions may
not be subject to this code.
6. Liquid relief valves should be used for vessels that
2. Unfired steam boilers must be protected. operate full of liquid.
