Page 470 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

436 Applied Process Design for Chemical and Petrochemical Plants

release of overpressure is required. Once the disk has rup- when bursting pressure is reached, the disk
tured, the process system is exposed to the environment being in compression reverses with a snap action
of the backpressure of the discharge system, whether at which time the four knife edges, Figure 7-SG
atmospheric or other. The process system is depressurized cut the metal and it clearly folds back without
and the disk must be replaced before the process can be fragmentation. There is another version of the
restarted. Typically, the types of disks available are same concept of reverse buckling, but it uses a
pre-scored disk and thereby omits the knife
1. Solid Metal Rupture Disk (Figure 7-8). This is the blades. These types of disks do not need vacuum
original type of rupture device, available in various supports, unless there is unusually high differen-
metals and non-metals. It should normally not be tial pressure across the disk.
used for operating pressures greater than 70% of 2. Graphite Rupture Disk (Figure 7-9). There are spe-
rupture pressure in a non-corrosive environment. cial designs of disks and disk assemblies for specific
The metal disks use a domed or hemispherical shape, applications, and the manufacturer should be con-
with pressure on the concave side. As the pressure sulted for his recommendation. Disks are available
internally increases, the metal wall thins as the metal for pressure service, pressure-vacuum applications,
stretches to achieve a smaller radius of curvature. high temperature conditions, and close tolerance
After the wall has thinned sufficiently, it will burst to bursting conditions. The bursting accuracy of most
relieve the pressure and tension loading on the designs is ±5% for rated pressures above 15 psig and
metal. The accuracy of metal disks is ±5%, except for ±0.75 psig for rated pressures 14 psig and below. It
the Reverse Buckling Disk Assembly, which is ±2%. should be noted that these ratings are not affected
The usual recommended maximum operating tem- by temperature up to 300°F.
peratures for metal rupture disks is given in Tables 7-
4A and B. Also see Figures 7-SG and SN. A new concept in graphite disks includes addition of
a fluorocarbon film barrier between the process and the
(a) Solid metal disk with vacuum support: When vacu- disk, and is termed a duplex disk. These disks are suit-
um can occur internally in the system, or when able for temperatures to 392°F, with accuracies as just
external pressure on the convex side of the disk mentioned.
can be greater than the pressure on the concave Graphite disks are normally used in corrosive services
side of the disk, a vacuum support is necessary to and/or high temperature situations where metal wall
prevent reversal of the disk, Figure 7-SD. thickness and corrosion rates make the metal units
(b) Solid metal disk with rating near minimum Jar size: impractical because of unpredictable life cycles. The disks
With a rating pressure near the minimum avail- are available down to 1 psig ±0. 75 psi, and are not affect-
able for the size and material of construction a ed by fatigue cracking. An interesting feature is the use of
special thin disk is attached to the under and pos- standard ASA (ANSI) flanges rather than special flanges.
sibly the upper sides of the rupture disk to ensure See Figure 7-9C.
freedom from deformity caused by the condition It is important to recognize here also that once the disk
of the disk holding surfaces, Figure 7-SE. There bursts the system is depressured, and there will be frag-
are several versions of what to include under such ments of graphite blown out with the venting system. Spe-
conditions, therefore it is advisable to clearly cial discharge designs are often used to prevent plugging
explain the installation conditions and applica- of discharge pipe and fragments from being sprayed into
tion to the manufacturer. the surrounding environment.
(c) Composite rupture disk: This type consists of a
metal disk (not necessarily solid, it may have Calculations of Relieving Areas: Safety and Relief Valves
slots) protected by an inner and/or an outer
membrane seal, Figure 7-SE. There are several References to the ASME Code [l] and the A.Pl Code
possible arrangements, including vacuum sup- [10] [33] are recommended in order that the design
port, as for the styles of paragraph (b) above. engineer may be thoroughly aware of the many details
This general class has the same use-rating limita- and special situations that must be recognized in the final
tion as for the solid disk. sizing and selection of a pressure relieving device. All
( d) Reverse acting or buckling disk assembly: This design details of these codes cannot be repeated here; however,
allows the disk to be operated in a system at up the usually important requirements are included for the
to 90% of its rated burst pressure. The pressure typical chemical and petrochemical application for the
is operating on the convex side of the disk and guidance of the engineer.