Page 461 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Process Safety and Pressure-Relieving Devices 427

relief valve must be at least ,If in. in pipe size, [1] Par UG- quately known. They should be researched or investigated
128 (see [79]). by laboratory testing for possible runaway conditions and
then the kinetic and heat/pressure rise calculations
Selection and Application should be performed, even if some assumptions must be
made to establish a basis. Refer to later paragraphs and
Causes of System Overpressure the American Institute of Chemical Engineers Design
Institute for Emergency Relief [67]. At the time of aves-
Figure 7-14, Operational Check Sheet [25], lists 16 pos- sel or pressure/vacuum system failure, the calculations
sible causes of overpressure in a process system. There are for the effected pressure-relief devices are always reviewed
many others, and each system should be reviewed for its by plant management and the Occupational Safety and
peculiarities. System evaluation is the heart of a realistic, Health Administration (OSHA) inspectors. A few notes
safe and yet economical overpressure protection installa- on causes of process system failures are noted below, with
tion on any single equipment or any group of equipment. additional comm en ts in API-521 [13, 33a, b, c]
Solving formulas with the wrong basis and/ or data can be
disastrous. The following should be reviewed:
Failure of Cooling Water: assume all cooling mediums
fail, determine relief capacity for the total vapors entering
1. The sources of possible overpressure
2. Maximum overpressure possible from all sources the vessel, including recycle streams. See [3] and [10].
3. Maximum rate of volume increase at the burst pres-
sure, and temperature at this condition. Reflux Failure: (a) At top of distillation column, capaci-
4. Length of duration of overpressure. ty is total overhead vapor [iO], (b) when source of heat is
in feed stream, capacity is vapor quantity calculated in
Capacity Requirements Evaluation for Process immediate feed zone [3], (c) when reboilers supply heat
Operation (Non-Fire) to system, capacity is feed plus reboil vapors [3]. Each sit-
uation must be examined carefully.
Each system and item of equipment should be exam-
ined for operational safety as set forth by specific plant
area (and process fluids) requirements and the codes Blocked Outlets on Vessels: (a) For liquid, capacity is max-
previously cited. The codes particularly [10, 13, 27, 33a, b, imum pump-in rate. (b) For liquid-vapor system, capacity
c] establish guides based on wide experience, and are is total entering vapor plus any generated in vessel [ 10].
sound requirements for design. Relief capacity is based on
the most severe requirement of a system, including possi-
ble two-phase flow [67]. A system is generally equipment Blocked Outlets and Inlets: for systems, lines or vessels,
or groups of equipment which is isolated by shut-off capable of being filled with liquid and heated by the sun
valves. Within these isolated systems a careful examina- or process heat, require thermal relief to accommodate
tion of the probable causes of overpressure is made [6]. the liquid expansion (assuming vaporization is negligible).
Figures 7-15, 7-16, and 7-17 are suggested guides [25].
Capacities are calculated for conditions of temperature Instrument Failure: assume instrument control valves
and pressure at actual state of discharge. Final discharge freeze or fail in open position (or closed, which ever is
pressure is the set pressure plus overpressure. worse), determine capacity for relief based on flows, tem-
It must be emphasized, that the determination of the peratures, or pressures possible under these circum-
anticipated maximum overpressure volume at a specified stances. The judicious selection of instrument failure
pressure and temperature is vital to a proper protection sequence may eliminate or greatly reduce relief valve
of the process system. The safety relief calculations should requirements.
be performed at the actual worst conditions of the system,
for example, at the allowable accumulated pressw·e and
its corresponding process temperature. These can be Equipment Failure: pumps, tubes in heat exchangers and
tedious and perhaps time-consuming calculations, but furnaces, turbine drivers and governor, compressor cylin-
they must not be "glossed" over but developed in a man- der valves are examples of equipment which might fail
ner that accounts for the seriousness of the effort. They and cause overpressure in the process. If an exchanger
must be documented carefully and preserved permanently. tube splits or develops a leak, high pressure fluid will
The situation is just as critical, if not more so, for run- enter the low side, overpressuring either the shell or the
away reactions or reaction conditions that are not ade- channels and associated system as the case may be.