Page 493 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Process Safety and Pressure-Relieving Devices 459

the metal or composite temperature stresses for the disk critical flow will occur, and the appropriate equations
finally supplied. Higher temperatures reduce the allow- should be used [33a].
able working stress of the disk materials. Reference [37]
shows that temperature has an effect on metals in decreas-
ing order, with the least effect on the lowest listed metal: When P 1 is increased, the flow through an open disk
increases and the pressure ratio, P 2/P 1, decreases when P2
does not change, until a value of P 1 is reached, and there
aluminum
stainless steel ( changes after 400°F) is no further increase in mass flow through the disk. The
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nickel value of P1 becomes equal to P and the ratio is the criti-
Inconel cal pressure ratio, and the flow velocity is sonic (equals
the speed of sound).

When specifying the material at the disk temperature, the
heat loss at the disk/ disk holder as well as in a flowing Maximum velocity (sonic) of a compressible fluid in
pipe must be recognized and the assembly may need to be pipes is [9]:
insulated. This is important as it relates to the actual tem-
perature at the bursting pressure. Establishing this burst
temperature is an essential part of the system safety and (7- 35)
must not be guessed at or taken lightly.
Table 7-10 presents a temperature conversion table for �
various metals from one manufacturer for conventional or, v, = kgP 'V ( 14 4) (7- 36)
pre-bulged, tension-loaded disks with pressure on con-
cave side (not prescored) as an illustration of the effect of or, v, = 68.1 � KP'V (7 - 37)
lower or elevated temperatures referenced to 72°F on the
burst pressure of a stamped disk. For other types of disk
designs and from other manufacturers, the specific data where v, = sonic or critical velocity of a gas, ft/ sec
for the style disk must be used to make the appropriate k = ratio of specific heats, cp/ c,
g = acceleration of gravity= 32.2 ft/sec/sec
temperature correction. R = individual gas constant = MR/M = 1544/M
M = molecular weight
Rupture Disk Assembly Pressure Drop MR= universal gas constant= 1544
T = absolute temperature, R = (460 + r'F)
0
The ruptured or burst disk on a vessel or pipe system t = temperature, °F
presents a pressure drop to flow at that point, and it can P' = pressure, psia, at outlet end or a restricted location
be estimated by assuming the disk is a flat plate orifice in pipe when pressure drop is sufficiently high
[37] with a discharge coefficient, Kci, of 0.62. As an alter- V = specific volume of fluid, cu ft/lb
nate, the disk assembly can be assumed to be the equiva-
lent of a section of pipe equal to 75 nominal disk diame-
ters in length. For sonic floio [33a]:

Actual pressure ratio, P 2 /P 1, less than critical pressure
Gases arid Vapors: Rupture Disks [33a, Par.4.8]
ratio, flow is sonic or critical.
The sizing is based on use of the ASME Code [ 17] flow
coefficient:
Pc/P1 = [2/(k + l)]kl(k-l), critical pressure ratio (7-7)
K.J = 0.62 [11 (Par. UG-127) for standard metal disks, but use
K.J = 0.888 for graphite disks [70]
K.J = actual flow/theoretical flow = coefficient of discharge where Pc = critical flow throat pressure, psi a
Pb= st.amped bursting pressure, psia = burst pres-
sure + overpressure allowance (ASME Code of
To select the proper sizing equation, determine
whether the flowing conditions are sonic or subsonic 10%) plus atmospheric pressu,-e, psia
from the equations. When the absolute pressure down-
stream or exit of the throat is less than or equal to the crit- Important note: when actual system ratio, P2/P1, is less
ical flow pressure, P then the flow is critical and the des- than critical pressure ratio calculated above by Equation
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ignated equations apply [33a]. When the downstream 7-4.0, flow is sonic. When actual P 2 /P 1 ratio is greater than
pressure exceeds the critical flow pressure, Pc, then sub- critical pressure ratio, flow is subsonic.