Page 500 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

466 Applied Process Design for Chemical and Petrochemical Plants

Since actual P2/P1 < critical ratio 0.528, the flow is Use Std. pipe size: 5 in. (cross sect. area= 26.01 sq in)
sonic. disk, or check manufacturer, Use inlet and discharge pipe
Critical pressure =Per= 108.2 (0.528) = 57.12 psia size = 6 in. std.
Flow area required to relieve: [33a]
Example 7-8: Liquid Overpressure, Figure 7-34
Q. ( MTZ) o.so
A=----- (7-47) A check of possible overpressure on a heat exchanger
6.32 CKdPh
handling 95% aqua ammonia in the tubes indicates that
where Q, = 12,000 SCFM tube failure is probably the condition requiring maxi-
M = 28 mum relieving capacity. The aqua is being pumped by a
C = 356, Figure 7-25 positive displacement pump. Thirty psia steam on shell
K,i = 0.62 (rupture disk) side heats (not vaporizes) the aqua. In case of tube failure
Pb = 108.2 psia the aqua would flow into the shell and soon keep the
Z = 1.0 steam from entering. Relief must prevent the shell from
T = 385 + 460 = 845°R failing. The shell is designed for a working pressure of 210
psig.
12000 [ ( 28) ( 845) ( 1.0)] OS The calculations are shown on the specification sheet,
A=----------
6.32 ( 356) ( 0.62) ( 108.2) Figure 7-34.

A= 12.22 sq in. Pressure--Vacuum Relief for Low Pres-sure Storage Tanks
Area of I.D. of 4 in. std. sch. 40 pipe = 12.7 sq in. This
should be adequate. In order to accommodate "breathing" of tanks and
other equipment due to temperature changes, pumping
Example 7-7: Liquids Rupture Disk in and out, internal vapor condensation and other situa-
tions, adequate safety vacuum relief must be provided. In
Determine the rupture disk size required to relieve the many cases both pressure and vacuum relief are needed
following operating condition: ( see Figure 7-35) . For the average product storage tank
Back pressure: 0 psig the API Guide For Tank Venting RP 2000 [26] serves to
Toluene flow: 1800 gpm, SpGr = 0.90 set minimum venting quantities for various tank capacities.
Pressure vessel: MAWP = 25 psig In addition to these tabulated values, calculations are
Relieving pressure: set pressure; use 25 psig made lo satisfy each condition of operation to insure that
Actual relief pressure = 25 + 10% = 27.5 psig there is no situation requiring more than this vent capac-
ity. Emergency vent capacity is also required to supple-
ment the normal requirement in case of external fire or
other unusual condition.
The normal venting Lo be provided must not allow
pressure or vacuum conditions to develop which could
SpGr = 0.90 cause physical damage to the tanks [26].
Fluid: Toluene, flow - 1800 gpm
Basic Venting For Low Pressure Storage Vessels
gpm ( SpGr) o.s
A = __:c.:.._ _ _ Usually reference to low pressure venting is associated
__:_
)0·3
38.0 K (P - P <l
with large storage tanks of several thousand gallons capac-
ity (ranging from a few thousand to a million); however,
� = 0.62 per API [33a], or from manufacturer
P 1 = 27.5 + 14.7 = 42.2 psia small low pressure tanks can be handled in the same
P 2 = 0 + 14.7 = 14.7 psia manner. The usual operating pressure range for the typi-
cal tank is 0.5 oz./in. to about 1.5 psig. Since low pres-
2
sure vessels have pressure ratings expressed in various
A _ 1800 I 0.90 units, Table 7-13 can be useful for conversion.
- 38 ( 0.62 )'\ ( 42.2 - 14.7) Typical large storage vessels are illustrated in Figure 7-36.
Operations associated with storage tanks should be
A= 76.40 ( 0.180) = 13. 75 sq in. required carefully analyzed, since there are several factors which