Page 228 - Chemical engineering design

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CHEMICAL ENGINEERING
There will also be a pressure drop due to the valves used to isolate equipment and
control the fluid flow. The pressure drop due to these miscellaneous losses can be estimated
using either of two methods:
1. As the number of velocity heads, K, lost at each fitting or valve.
2
2
2
A velocity head is u /2g, metres of the fluid, equivalent to u /2 ,N/m .The
total number of velocity heads lost due to all the fittings and valves is added to the
pressure drop due to pipe friction.
2. As a length of pipe that would cause the same pressure loss as the fitting or valve.
As this will be a function of the pipe diameter, it is expressed as the number of
equivalent pipe diameters. The length of pipe to add to the actual pipe length is
found by multiplying the total number of equivalent pipe diameters by the diameter
of the pipe being used.
The number of velocity heads lost, or equivalent pipe diameter, is a characteristic of the
particular fitting or type of valve used. Values can be found in handbooks and manufac-
turers’ literature. The values for a selected number of fittings and valves are given in
Table 5.3.
The two methods used to estimate the miscellaneous losses are illustrated in
Example 5.1.
Pipe fittings are discussed in section 5.5.3, see also Perry et al. (1997). Valve types and
applications are discussed in section 5.3.

Table 5.3. Pressure loss in pipe fittings and valves (for turbulent flow)
Fitting or valve K, number of number of equivalent
velocity heads pipe diameters
45 ° standard elbow 0.35 15
45 ° long radius elbow 0.2 10
90 ° standard radius elbow 0.6 0.8 30 40
90 ° standard long elbow 0.45 23
90 ° square elbow 1.5 75
Tee-entry from leg 1.2 60
Tee-entry into leg 1.8 90
Union and coupling 0.04 2
Sharp reduction (tank outlet) 0.5 25
Sudden expansion (tank inlet) 1.0 50
Gate valve
fully open 0.15 7.5
1/4 open 16 800
1/2 open 4 200
3/4 open 1 40
Globe valve, bevel seat-
fully open 6 300
1/2 open 8.5 450
Plug valve - open 0.4 18

Example 5.1
A pipeline connecting two tanks contains four standard elbows, a plug valve that is fully
open and a gate valve that is half open. The line is commercial steel pipe, 25 mm internal
diameter, length 120 m.