Page 490 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

456 Applied Process Design for Chemical and Petrochemical Plants

where a fixed downstream side pressure into which the bursting diameter will actually burst on test and then in
disk must relieve is greater than the critical pressure. actual service.
The coefficient of discharge, K 0, is the actual flow To specify a rupture disk:
divided by the theoretical flow and must be determined
by tests for each type or style and size of rupture disk as • identify the desired bursting pressure, Pb
well as pressure-relieving valve. For rupture disks, the • list the required MR
minimum net Jww area is the calculated net area after a
complete burst of the disk, making allowance for any
structural members that could reduce the net flow area of For example, a system requiring a bursting pressure, Pb,
the disk. For sizing, the net flow area must not exceed the of 150 psig at 400°F, would have an MR range of -4% to
+7% at the operating temperature with a burst pressure
nominal pipe size area of the rupture disk assembly [l]. tolerance of ±5%. The disk supplied by a specific manu-
The bursting pressure, Pb, of the conventional tension-
loaded disk is a function of the material of which the disk facturer (MR varies with manufacturer and pressure
ranges) could have a bursting pressure as low as - (0.04)
is fabricated, as well as its thickness and diameter, and last- (150) = -6 psi, or 144 psig; or as high as ( +0.07) (150)
ly but not least in any manner, is the temperature at which = + 10.5 or 160.5 psig. If the disk is stamped at the oper-
the disk is expected to burst, and not just the temperature ating temperature at 144 psig it could burst at ±5 % or 7 .2
corresponding to the disk set pressure [37]. This type disk psi or 136.8 psig or 151.2 psig. On the other hand, if the
is best suited to be set. on Pb at least. 30% above the system disk were stamped at operating temperature to burst al
operating pressure. The reverse-buckling disk with knives 160.5 psig (its highest) then it could actually burst ±5% of
to aid the bursting are compression loaded because the this, or ±8.03 psi, giving an actual burst pressure of 168.5
dome of the disk faces the internal vessel pressure. The psig to 152.5 psig. Adapted from [37] by permission.
bursting pressure, Pb, of this disk is dependent on the
dome's geometrical shape and the characteristics of the The Code requires that the disks be burst on test by one
knife blades, but it is essentially independent of thickness of three methods using four sample disks, but not less than
and generally does not need a vacuum support [37]. It is 5% from each lot. Figure 7-32 illustrates test results for
often used when the operating pressure is as high as 0.9 burst pressure versus temperature of a disk design, all fab-
Pb. There are some potential problems or even hazards ricated from the same material, and of the same diameter.
with this design if the knives fail, come loose, or corrode, It is critical that such an examination be made to be
and the use must be examined carefully. This disk like any certain that the bursting or set pressure at this tempera-
other disk, should never be installed upside down from its ture does not exceed the lVIA,i\TP of the vessel at the
original design position. It should not be used in partial operating temperature per the ASME code [l] (see Fig-
or total liquid service. ures 7-31A and 31B).
The reverse-buckling disk, without knives but with a As allowed by code [ l ], the average of the manufactur-
pre-scored disk surface, offers some features that do not er's disks burst tests could be stamped, for example, (144
depend on the knives being in place because the thick- + 160.5)/2 = 152.3 psig with an actual ±5% of 152.3 psig
ness of the metal disk dome along the score line deter- allowed for actual burst pressure of any disk at the operat-
mines the bursting pressure of the disk. ing temperature.

The Manufacturing Range (MR) Selection of Burst Pressure for Disk, Pb (Table 7-3)

The ASME code [l] requires that a ruptured disk must It is essential to select the type or style of rupture disk
be stamped with a bursting pressure that falls within the before making the final determination of the final burst
manufacturing range. This range identifies the allowable pressure, and even this selection must recognize the pres-
range of variation from a specified burst pressure to the sure relationships between the disk's manufacturing
actual burst pressure provided by the manufacturer and range and the vessel's maximum allowable working pres-
as agreed upon with the disk user. The stamped burst sure. (Also see Figures 7-31A and 31B.)
pressure of a lot of rupture disks is the average burst pres- Table 7-9 summarizes the usual recommended rela-
sure of all the destructive tests performed per code tionship between the operating pressure of a process
requirements. The average of the tests must fall within the (should be maximum expected upper range level) and
manufacturing range (see Table 7-2). the set pressure of the rupture disk. Recognize that the set
The thickness of one material for manufacture of a pressure of the disk must not exceed the MAWP of the ves-
disk, along with the specific disk type, is the key factor in sel. (See Figures 7-31A and 31B.) The burst pressure, Pb,
establishing at what pressure range a disk of a specified can now be defined. The use of the manufacturing range