can be seen that a compressed flow of 5.8 feet3/minute will result in a
pressure drop per 100 feet ranging from 23.1 psi for 3/8-inch pipe to 0.55
psi for 1-1/2-inch pipe. The candidate pipe sizes seem to be 1/2 inch, with
a pressure drop of 6.90 psi, and 3/4-inch with a pressure drop of 1.62 psi.
The increase in density requires that these values be increased 2.4 times to
16.6 psi and 3.9 psi, respectively. An equivalent length of 150 feet (as an
estimate) will cause 25 psi pressure drop in the 1/2-inch pipe. Although
1/2-inch pipe might be selected, the selection of a 3/4-inch pipe size
appears more conservative and desirable.
The total length of pipe is 120 feet. Using Figure 5-2, it is determined
that the equivalent 3/4-inch pipe lengths for the three fittings are: (1) for
the globe valve -20 feet, (2) for the standard elbow - 2 feet, and (3) for
the angle valve - 11 feet. The pressure drop for the equivalent pipe length
of 153 feet is 3.9 psi (the value in Table 5-10 corrected for density) times
1.53 or about 6 psi. This is sufficiently less than the desired maximum of
25 psi that consideration could be given to tubing with an inside diameter
between 1/2-inch and 3/4-inch Schedule 40 pipe. This is particularly true
with oxygen since tubing rather than pipe might have to be used for
compatibility. The basic dimensions for commercially available tubing are
shown in Table 5-12.
(7) Approximation for Helium, High Flow. Let the maximum required
flow in the system shown in Figure 5-3 equal 2220 scfm of helium with an exit
pressure (P2) equal to 850 psia. If it is assumed that the length of pipe
and types of components are accurately known, and that a high gas velocity is
desirable, let P1 - 40% P1 = 850 psia. Therefore, P1 = 1420 psia and
the average or effective pressure = P1 + P2 = 1135 psia. At a pressure
of 1135 psia, a flow of 28.8 feet3/minute is required to supply 2220 scfm.
At 1135 psia, helium has a weight density of 0.77 pound/feet3. This weight
density is 1.3 times the weight density of air at 100 psi.
Referring to Table 5-10, it can be seen that a flow of 28.8 feet3/minute of
air causes a pressure drop in 100 feet of pipe ranging from 37.9 psi for
3/4-inch pipe to 0.022 psi for 3-1/2-inch pipe. Normally, either a 1-inch
pipe with a pressure drop of 10.8 psi or 1-1/4-inch pipe with a pressure drop
of 2.59 psi would be the candidate pipe sizes to keep the gas velocity low.
However, a high gas velocity is desired and the 3/4-inch pipe is examined
first. Because of the increased density of the helium, the 37.9 psia must be
multiplied by 1.3 to give an effective pressure drop for 100 feet of pipe of
As noted in Table 5-10, when a different schedule pipe is used than Schedule
40, the tabulated values must be corrected. The inside diameter of 3/4-inch
Schedule 40 pipe is 0.824 inch (see Table 5-11). If Schedule 160 pipe were
selected for the system the inside diameter of 3/4-inch Schedule 160 pipe
(see Table 5-11) is 0.614 inch. The ratio of the two inside diameters is
1.34. This raised to the 5th power (see note on Table 5-10 under
Calculations For Pipe Other Than Schedule 40) is 4.33. Thus, the pressure
drop per 100 feet of Schedule 160 pipe is 4.33 x 49.3 or 214 psi.
Using Figure 5-2 with an inside pipe diameter of 0.6 inch (for Schedule 160
pipe) it is found that the equivalent pipe lengths for the 3 components are