Page 177 - Pipeline Rules of Thumb Handbook

164 Pipeline Rules of Thumb Handbook

Sizing control valves for thruput

These useful charts give data essential to calculation of pressure drops through regulating stations
W. G. Birkhead, Gas Measurement Engineer, United Gas Pipeline Co., Shreveport, La.

Pressure losses through piping fittings, once given little This equation may be rearranged so that C v in terms of our
consideration in the sizing of control valves, have become K factor may be expressed as:
important as more efficient control valves have been devel- 2
.
oped. With the development of high capacity valves and the 29 9 d ()
C v = (4)
desire for a more exacting flow formula and efficiency, K
designers and users look to consideration of minor losses
through elbows, tees, swages or reduced ported valves. The where d = inside diameter of the pipe or fitting, in.
capacity of control valves and regulators was once calculated
using the basic hydraulic head equation with an assumed The head loss or pressure drop of a fluid flowing through
efficiency. This is no longer the case. various pipes, valves, or fittings in series can be stated:
The head loss in any closed filled conduit has been
accepted as proportional to its length, diameter and fluid ( DP) Total = ( DP) + ( DP) + ( DP) + —— (5)
3
2
1
velocity head, and can be calculated as
Substituting Equation 3a in Equation 5:
L V 2
H L = f (1) 1 1 1 1
D g = + + + —— (6)
2
( C v ) 2 Total ( C v ) 1 2 ( C v ) 2 2 ( C v ) 3 2
where H L = Head loss of flowing fluid (ft of fluid)
f = Friction factor This equation is unwieldy if several fittings are used. By
L = Length of pipe, ft equating Equation 4, substituting in Equation 6 and dividing
2 2
D = Diameter of pipe, ft through by [29.9(d) ] :
V = Average fluid velocity, ft/sec
g = Acceleration of gravity, ft/sec/sec K Total = K + K + K + —— = S K (7)
2
3
1
Substituting in Equation 4 for K T an over-all C v can be
Sometimes L/D is referred to as the “equivalent length in
derived:
diameters.” Valves, fittings, etc., have often been expressed in
equivalent diameters. If the friction factor f is combined with 29 9 d ()
2
.
L/D, a constant K is derived for any pipe, valve or fitting. If C v =
S K
K = f(L/D), then
2
Ê V ˆ
H L = K (2) K factors for various valves and fittings
Ë 2 g ¯
For full open valves, pipes or fittings, the K factor is
The term K is called the “head loss factor” or “pressure drop approximately equal to f(L/D) or Equation 1.
factor.”
Most control valves are rated with a capacity term called where L = Equivalent length of valve or pipe or fitting, in.
C v . C v is the number of gallons of water per minute that will (or ft)
2
flow through the valve with a 1lb/in. pressure drop across the D = Diameter of the valve, in. (or ft)
valve. f = Friction factor
C v = Q D P (3) Some of the recognized K factors for various welding fit-
tings are shown in Table 1.
Q = Quantity of water in gpm Other fitting K factors can be obtained from any fluid
DP = Pressure drop in psi mechanics, hydraulics or piping handbook.
Low, average and high values of K given in Table 1 point
Rearranging Equation 3: out caution necessary in selecting specific types of fittings.
Pressure losses in valves and fittings can be more accurately
2
2
DP = Q ( C v ) (3a) calculated by consulting the references.