Page 398 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

366 Applied Process Design for Chemical and Petrochemical Plants
(text r.;mlinued [rom page 363) Table 6-6
Air Inleakage into System Vacuum Pump Capacities From Steam Surface
Condensers
Few vacuum systems are completely airtight, although
some may have extremely low leakage rates. For the ideal
system the only load for the ejector is the non-condens-
ables of the process (absorbed gases, air, etc.) plus the sat- DRY AIR AT 70° F.
urated vapor pressure equivalent of the process fluid. Maximum Steam Serving Turbines Serving Engines
Practice has proven that allowance must be made for air Condensed,
leakage. Considering the air and non-condensables. For Lb. Per Hr. SCFM Lbs./Hr. SCFM Lbs./Hr.
"base" ejector capacity determine inert gases only by: Up to 25,000 ..... 3.0 13.5 6.0 27.0
25,001 to 50,000 ..... 4.0 18.0 8.0 36.0
50,001 to 100,000 ..... 5.0 22.5 10.0 45.0
100,001 to 250,000 ..... 7.5 33.7 15.0 67.4
(Pounds/hr air + non-condensables + process released air 250,001 to 500,000 ..... 10.0 45.0 . . . . ....
56.2
12.5
+ process released non-condensables)/Hr = Air in leakage, 500,001 and up ........ --------- . . . . ....
lb/hr (6-7) Rapid Evacuator Capacities, Dry Air.
cfm at 70° F., 15 in. I-lg. abs.
-------------------
Up to 75,000 .....
150
For design of a new system, it is recommended that the 7 5,000 to 250,000 ..... 300
results of the summation above be multiplied by 2 or 3 to 250,001 to 600,000 ..... 600
establish the jet system inert (noncondensables) capacity, 600,001 and up ........ 900
and add to this the non-condensed process vapors that are
released into the jet suction system. *Standards of Heat Exchange Institute, Steam Surface Condensers,
Air leakage occurs at piping connections (flanges, Third Edition, Ref. (12) by permission.
screwed fittings, valves), stuffing boxes, mechanical equip-
ment seals, etc. Whenever possible a system should be Table 6-7
tested to determine the air leakage [ 12, 14], but for new Estimated Air Leakage Into Equipment Vacuum System
designs and those situations where tests cannot be made,
the recommended values of the Heat Exchange Institute Estimated Average
are given in Table 6-6 for ejectors serving surface con- Type Fitting Air Leakage, Lbs./Hr.
densers, and are minimum safe values. A very tight system Screwed connections in sizes up to 2 inches. . . . . . . . 0.1
will show heller performance. Screwed connections in sizes above 2 inches....... 0.2
Figure 6-21 gives maximum air leakage values for com- Flanged connections in sizes up to 6 inches. . . . . . . . 0.5
mercially tight process systems which do not include any Flanged connections in sizes 6 inches to 0.8
24 inches including manholes. . . . . . . . . . . . . . . . . .
agitator equipment. For design purposes the ejector is usu- Flanged connections in sizes 24 inches to 6 feet. . . . 1.1
ally purchased to operate on a load at about twice these Flanged connections in sizes above 6 feet. . . . . . . . . . 2.0
values. For systems with agitators and ordinary shaft seals, Packed valves up to !/!" stem diameter. . . . . . . . . . 0.5
the system leakage should be increased by 5 pounds of air Packed valves above !/!" stem diameter. . . . . . . . . . . 1.0
per hour [12] per agitator. If special seals are used this Lubricated plug valves......................... 0.1
0.2
Petcocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
value may be reduced to 1 or 2. The more rotating shafts Sight glasses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.0
which must be sealed to the outside atmosphere, the more Gage glasses including gage cocks. . . . . . . . . . . . . . . . 2.0
likely will be the possibilities of increased leakage. Liquid sealed stuffing box for shaft of agitators,
pumps, etc., per inch shaft diameter........... 0.3
An alternate design for air inleakage used by some Ordinary stuffing box, per inch of diameter. . . . . . . . 1.5
manufacturers and process engineers is Figure 6-21 plus Safety valves and vacuum breakers, per inch
the summation obtained by examining the process sys- of nominal size. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.0
tem using the factors of Table 6-7. This method is con- * From D. H. Jackson, Selection and Use of Ejectors, Chem.
sidered to be conservative, however, as in general the Eng. Prog, 44, 347 (1948)
incremental cost may be very small between a unit bare-
ly large enough and one which has ample capacity to take
surges in air leakage. the basic type of plant, maintenance practices, opera-
Since the determination of air inleakage involves con- tional techniques of the production personnel, and
siderable knowledge of vacuum systems and judgment, other related items. Thus, for a tight and efficient plant,
no empirical method can be expected to yield exact and the leakage values of Figure 6-21 may sometimes be
correct values. Most manufacturers use one of the meth- reduced to 0.75 of the values read, while for a sloppy,
ods presented here, together with a factor to account for loose-run plant the values might be multiplied by 2 or 3,