Page 143 - 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
microns to 760 torr. This is practically speaking the rough
APvac = [F) = 0.794 in. water (for 350’) vacuum range noted above. For reference:
(350)
= (0.794/13.6) = 0.0584 in. Hg 1 torr = 1 mm mercury (mmHg)
1 in. mercury (in. Hg) = 25.4 torr
1 micron (pm 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
For piping with air in streamline flow at absolute pres- determination is to determine what is a reasonable pres-
sures in the range between 50 microns and 1 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
to broad interpretations. Method [ 181, by permission:
For this method 10 be applicable, the pressure drop is
limited to 10% of the final pressure. 1. Convert mass flow rate to volumetric flow rate, q,.
q, = M7 (359/M) (760/P,) (T/(32 + 460) (1/60),
Method [54]
cu ft/min (2-1 28)
Refer to Figure 2-44 for low pressure friction factor and
air viscosity of Figure 2-45 to correspond to Figure 2-44. where P, = pressure, torr
T = temperature, “R
W = mass flow, lbs/hr
M = molecular weight
4f Lpv 2
- pg 1 ___- (2-127)
2gD (144)’ psi
2. Calculate section by section from the process vessel to
the vacuum pump (point of lowest absolute pressure).
where = upstream static pressure, psi abs. 3. Assume a velocity, v, ft/sec consistent with Figure 2-
lPlz = downstream static pressure, psi abs. 46. Use Table 2-21 for short, direct connected con-
f = friction factor, from Figure 2-44. nections to the vacuum pump. Base the final specifi-
E = length of pipe (total equivalent), ft, incl. valves
and fittings cations for the line on pump specifications. Also the
p = average density, Ibs/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 of 100 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-
p = abs. viscosity of air, Ibs/ft-sec uum classification.

~
V ~ for other Gases and Vapors Sonic velocity, v, = (kg [1544/M] T)l/*, ft/sec.
c
~
Use v from Figure 246, and qm from Equation 2-128.
ans and Roper categorize [18] vacuum in process
systems as: 4. Determine pipe diameter,
Category Absolute Vacuum (Absolute Pressure) D = 0.146d q,/v (2- 129)
Rough vacuum 760 torr to 1 torr
Medium vacuum I to torr Round this to the nearest standard pipe size. Recal-
High vacuum to io-’ torr culate v based on actual internal diameter of the line.
Ultra hiel? vacuum IO-’ torr and below
(text continued on page 132)