Page 401 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Ejectors and Mechanical Vacuum Systems 369

1 s P < 10 Lorr; wa =, 1tD8P0.3 4 (6-12) flow transition occurs from critical (sonic) to subsonic flow
through any particular leak. These factors can be calculat-
2
10 � P < 100 torr; w, = 1.2 1tD8Po 6 (6-13) ed by conventional fluid flow methods if the size of the
leak is estimated, which then is a real problem, or by using
100 � P < 760 torr; w, = 3.98 1tD8 (6-14) the data of Table 6-8 and applying fluid flow head losses.
The hydraulic effects for the submerged portions of
where D = sealed diameter, in. (estimates of nominal the vessel or system can be ignored when the total static
diameter, acceptable) head on the submerged portion is not greater than 0.53
w, = acceptable air-leakage rate assigned to a times atmospheric pressure plus the hydraulic head [22],
system component, lb/hr i.e., r,
8 = specific air leakage rate, lb/hr/in.
P = system operating pressl!re, torr
W10 = Wr = total calculated air inleakage, lb/hr P, = (p/760) + (hLPd34) (6-16)

3. Calculate the total acceptable air leakage rate, WT, where P, = static pressure, atm
lb/hr by adding D,V' a to the sum of the leak rates P = atmospheric pressure, mm Hg
assigned to the individual system components, w3 hL = liquid height, fl, below liquid surface
PL = specific gravity of liquid, relative to water = 1.0
'Wr = l:W' a + LW lb/hr (6-15)
0,
Acceptable Air /nleakage Rates {24}
To determine the capacity of the vacuum pump, the val-
ues of "\,VT above should not be used. It is necessary to In order to estimate an acceptable air inleakage rate
apply over-design or safety factors to ensure reliability [26] for sizing a vacuum pump for use in the medium to high
because pump capacity decreases with time and wear and vacuum system, consider:
air inleakage surges can occur due to a wide variety of leak
developing situations that result in more air or surges of
air inleakage. The over-design factor should be applied 100 Microns (0.10 Torr) to 1.0 Torr Range
only to the pump inlet throughput specification. The rec-
ommended [22] over-design factor should be 1.5 to 2.0 Estimate W' a:
times the air inleakage rate [22], and should also be
applied to saturated vapors entering the vacuum equip- 0.1 � P < 1 torr; w', = 0.026P0.6 "0).60 (6-17)
4
ment at the suction conditions required. Do not put a safe-
ty factor on tile suction condition of temperature or pres-
sure, provided the worst expected conditions of operation Estimate air inleakage for individual system specific
are specified. This requires a close examination of the leak rates, 8, from Table 6-8 [22] and from (w � 5 lb/hr)
process flow sheet range of operation. A safety factor of 2.0
is recommended for multistage steam jets with compres- 0.1 � p < 1 torr: Wa = 7t D8P0.6 4 (6-18)
sion ratio above 6:1; while a 1.5 factor is adequate for most
mechanical pumps and single-stage jets with a compres- Note that estimating maximum acceptable differs from
sion ratio of under 4: 1 [22]. The above procedure can be the design equations for Wa and W'a·
simplified for preliminary inleakage calculations [22]: Calculate total acceptable air inleakage rate, WT.

Multiply W'a X 2
(6-19)
For equipment with rotary seals allow additional 5
lb/hr for each conventional seal (packing type) and 2 A simplified alternate to the previously cited proce-
lb/hr for each mechanical seal and 0-ring. dures is suggested by Gomez [29] for calculating air
To account for air inleak a for vessels containing a liquid inleakage, but it is not presented in detail here.
level (portion of vessel submerged), the following applies.
If there is a large pinhole leak a few inches below the
liquid surface, it will behave like a leak above the liquid. Total Capacity at Ejector Suction
However, a small pinhole leak in the same location may
have zero inleakage due to capillary effects. The problem The Lota) capacity is the sum of all the expected con-
becomes complicated as the depth becomes large, and densables and non-co nde nsable flow quantities (in