Page 145 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Fluid Flow 129

Total line pressure drop: The majority of industrial chemical and petrochemical
plants' vacuum operations are in the range of 100
( o.2259) microns to 760 torr. This is practically speaking the rough
/'I.P vac (350) = 0.794 in. water (for 350') vacuum range noted above. For reference:
100
(0.794/13.6) = 0.0584 in. Hg l torr = l mm mercury (mmHg)
1 in. mercury (in. Hg) = 25.4 torr
l micron (urn Hg) = 0.0010 torr
Final calculated pressure = 0.6 + 0.0584 = 0.6584 in. Hg For other conversions, see Appendix.
10% of 0.658 = 0.0658 in. Hg In general, partially due to the size and cost of maintain-
Therefore the system is applicable to the basis of the ing vacuum in a piping system, the lines are not long (cer-
method, since the calculated pressure drop is less than tainly not transmissions lines), and there is a minimum of
10% of the final pressure, and w/d = 25.5, which >20. valves, fittings, and bends to keep the resistance to flow low.
The procedure recommended by Reference [18] is
Low Absolute Pressure Systems for Air [54] based on the conventional gas flow equations, with some
slight modifications. The importance in final line size
Fer piping with air in streamline flow at absolute pres- determination is Lo determine what is a reasonable pres-
sures in the range between 50 microns and l millimeter sure loss at the absolute pressure required and the corre-
of mercury, the following is a recommended method. Cal- sponding pipe size to balance these. In some cases a
culation procedures in pressure regions below atmos- trial/ error approach is necessary.
pheric are very limited and often not generally applicable Method [18], by permission:
to broad interpretations.
For this method to be applicable, the pressure drop is
limited to 10% of the final pressure. l. Convert mass flow rate to volumetric flow rate, gm.
qm = W (359/M) (760/P,) (TI (32 + 460) (1/60),
ivlethod [54] cu ft/ min (2-128)

Refer to Figure 2-44 for iow pressure friction factor and where P, = pressure, torr
air viscosity of Figure 2-45 to correspond to Figure 2-44.
T = temperature, R
0
W = mass flow, lbs/hr
4fLpv 2 . (2-127) M = molecular weight
2gD(144)' P SI
2. Calculate section by section from the process vessel to
the vacuum pump (point of lowest absolute pressure).
where P' 1 = upstream static pressure, psi abs. 3. Assume a velocity, v, ft/sec consistent with Figure 2-
P' 2 = downstream static pressure, psi abs. 46. Use Table 2-21 for short, direct connected con-
f = friction factor, from Figure 2-44.
L = length of pipe (total equivalent), ft, incl. valves nections to the vacuum pump. Base the final specifi-
and fittings cations for the line on pump specifications. Also the
p = average density, lbs/ cu ft diameter of the line should match the inlet connec-
v = average velocity, ft/ sec tion for the pump. General good practice indicates
g = acceleration due to gravity, 32.17 ft/sec-sec that velocities of100 to 200 ft/sec are used, with 300
D = inside diameter of pipe, ft to 400 ft/ sec being the upper limit for the rough vac-
µ = abs. viscosity of air, lbs/ft-sec uum classification.

Vacuum for other Gases and Vapors Sonic velocity, v5 = (kg [ 1544/ M] T) 112, ft/ sec.
Use v from Figure 2-46, and qm from Equation 2-128.
Ryans and Roper categorize [18] vacuum in process
systems as: 4. Determine pipe diameter, D, ft,

Category Absolute Vacuum (Absolute Pressure) (2-129)
Rough vacuum 760 torr to 1 torr
Medium vacuum l to 10- torr Round this to the nearest standard pipe size. Recal-
3
High vacuum 10- to l o-i torr culate v based on actual internal diameter of the line.
3
7
Ultra high vacuum 10-- torr and below
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