Size a control valve for service on a line carrying water wi

Size a control valve for service on a line carrying water with a maximum flow rate of 200 gpm nd a minimum flow rate of 20 gpm. Assume an equal percentage valve with the pressure drop versus stem position shown in the table for this problem. The C_v for equal percentage valves of different sizes are presented in the Table 2.1

Pressure Drop versus Flow Rate for an Installed Valve for Problems 2.3.9-2.3.11 Q (gpm) AP (psi) Q (gpm) AP (psi) 220 30.6 21.5 240 30.5 20 260 30.2 20.2 40 18.7 280 29.9 60 300 17.0 80 29.5 15.0 320 28.9 100 12.6 340 28.2 120 360 9.9 140 27.4 8.6 380 26.5 160 7.4 400 25.4 180 420 6.1 24.2 200

Solution

solution:

1)valve selection has to follow by following procedure

i)select limits of flow rate

Qmin=20 gpm

Qmax=200 gpm

ii)where pressure drop and coefficent discharge for water flowing through pipeline we have following data relation as

Cv=Q(S/dP)^.5

for water S=1 for normaltemperature limit of 60 F

Cv=Q/(dP)^.5

for minimum and maximum flow rate value of Cv is given by

Cvmin=20/(30.5)^.5=3.6214

Cvmax=200/(24.2)^.5=40.65

iii)for better operation of valve minimum flow rate should not achieve below 20% flow rate and maximum flow rate occure between 60 to 80% percent valve opening.

so for valve with size of d=3 inch

has Cvmin1=5.77 for 20% opening

has Cvmax1=60.4 for 70% valve opening

as Cvmin1>Cvmin that is 5.77>3.62 and hence minimum flow rate occure accurately

as Cvmax1>Cvmax that is 60.4>40.65 and hence maximum flow rate achieved accurately

hence it gives valve sizing as d=3 inch as accurate valve sizing and it is slight oversized but it is good practice to use oversized valve than undersized valve

iv)for globe valve valve diameter should be within .5 inch tolerance to line diameter.

2)here accurate valve size is body size of d=3 inch with Cvmin20%=5.77 and Cvmax70%=60.4 and it is accurate valve selection from given data.