Page 485 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Process Safety and Pressure-Relieving Devices 451

started from static discharge around flammable vapors, down of the process. Usually this is found to be about
flammable liquids released into an area drainage ditch, 20%-30% above the maximum expected peak operation
combustible gas/vapors released through flange leaks or pressure. Again, recognize the requirements for the
ruptures, over pressure, or many other potential hazards. relieving device set pressure not to exceed the actual ves-
The codes [13] suggest typical lists of potential hazards sel NIAWP at the expected relieving temperature (by cal-
and some approach to determine the types of process con- culation or pilot plant test data).
ditions that can cause a fire. A suggested extended list
(Figure 7-14) is presented earlier in this chapter and in the Heat Absorbed
following paragraphs. There are no "pat" formulas to
establish a code capacity for the volume of vapors to be The amount of heat absorbed by a vessel exposed to an
released under any one of the possible plant "failure" con- open fire is markedly affected by the size and character of
ditions. Therefore, the codes assume, based on evaluation the installation and by the environment, These conditions
of test data, that fire is under or around the various vessels are evaluated by the following equivalent formulas, in
and that absorbed heat vaporizes the contained fluid. The which the effect of size on the heat input is shown by the
informatior.. presented is taken from API-RP-520/521 lat- exponent of Aw, the vessel wetted area, and the effect of
est editions [ 10, 13, 33] and the ASME code [1]. The other conditions, including vessel external insulation is
designer should be familiar with the details in these codes. included in a factor F [33]:

Set Pressures for External Fires q = 21,000 FA"'-o.is (7-29)

The maximum allowable working pressure (MAWP) Q = 21,000 FAw+os 2 (7-30)
(discussed earlier) for the vessel or each vessel should be
the maximum set pressure for the rupture disk. Further- where q = average unit heat absorption, in BTU per hour
more, estimated flame temperatures of usually 2500°- per square foot of wetted surface.
35000F should be used to establish the reduced vessel Q = total heat absorption (input) to the wetted sur-
metal wall temperatures (recognizing the benefits of code face, in BTU per hour.
recommended fireproof insulation if properly applied to Aw = total wetted surface, in square feet.
The expression A"'-o.is is the area exposure factor
prevent dislodging by fire water hose pressures impacting or ratio. This recognizes that large vessels are less
on the insulation). The MAWP should then be re-estab- likely to be completely exposed to the flame of an
lished by calculation using the metal wall Code allowable open fire than small vessels. It is recommended
stresses at the new estimated reduced metal temperature. that the total wetted surface (A in the foregoing
This should be the maximum set pressure for the rupture formulas) be limited to that wetted surface includ-
disk provided the new lower value does not cause it to be ed within a height of 25 feet above "grade" or, in
below or too close to the usual expected process operat- the case of spheres and spheroids, to the elevation
ing temperature. Ir. that case, this author suggests that the of the maximum horizontal diameter or a height of
set pressure be 25% above the operating condition, exclu- 25 feet, whichever is greater. (A more conservative
sive of fire, not exceeding the MAWP values. approach is recommended.) The term "grade" usu-
ally refers to ground grade, but may be any level at
When a rupture disk relieves/blows/ruptures, it cre- which a sizable area of exposed flammable liquid
ates a rapid depressuring of the process system and a like- could be present [10, 33].
ly discharge of some or all vapor/liquid in the vessel(s), F = environment factor, values of which are shown in
discharging to the properly designed disposal system. Table 7-8 for various types of installation.
Therefore, great care should be given to setting the rup- Surface areas of vessel elliptical heads can be esti-
ture disk pressure because it does not have an accumula- mated by 1.15 X cross-sectional area of vessel.
tion factor, but bursts at the prescribed pressure of the
disk, taking into account the Code allowed manufactur- These are the basic formulas for the usual installation,
ing tolerances. Two-phase flow will most likely occur when with good drainage and available fire-fighting equip-
the disc (or safety-relief valve) blows (see later references ment. These formulas are plotted on Figure 7-30 showing
to explosions and DIERS work [51, 67]. curves for Q for various values of factor F. The approxi-
For unexpected runaway or process overpressure not mate amount of insulation corresponding to the factors
subject to external fire, the rupture disk set pressure, which is indicated.
is the bursting pressure, should be sufficiently higher Referring to the wetted surface, A,,., t.he surface areas of
than the expected "under acceptable control" conditions ASME flanged and dished head, ASME elliptical heads,
for the operation to avoid the frequent burst and shut hemispherical heads, etc., are often the end assemblies on