57. Formula for Process Control Valves
• Change in the flow rate=k(change in stem
displacement)
• Q/Qmax=S/Smax
• ∆Q/Q=k ∆S
• Q/Qmin=(Qmax/Qmin)(S-Smin)/(Smax-Smin)
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58. Where
• k=constant
• Q=is the flow rate at a valve displacement S
• Qmax=the maximum flow rate at the
maximum displacement Smax
• Q/Qmax=S/Smax
• Therefore the percentage change in the
flowrate equals the percentage change in the
stem displacement.
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59. Sample Problem
• An actuator has a stem movement which at full
travel is 40 mm. It is mounted on a process
control valve with an equal percentage plug and
which has a mimimum flow rate of 0.2 m3/s and a
maximum flow rate of 4 m3/s. What will be the
flow rate when the stem movement is a) 10 mm
b) 20 mm.
• Given:
• Smax=40mm
• Smin=0 mm
• Qmin=0.2 m3/s
• Qmax=4m3
/s
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60. • Required: What will be the flow rate when the
stem movement is a) 10 mm b) 20 mm.
• Solution A: Q/Qmin=(Qmax/Qmin)(S-Smin)/(Smax-Smin)
• Q/0.2=(4/0.2) (10/40)
• Answer a) 0.423 m3/sec
• Solution B
• Q/0.2=(4/0.2) (20/40)
• Answer b) 0.89 m3/sec
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62. Control Valve
Sizing Formula
• Q=Av√ ∆P/ρ
Where
Av= valve flow coefficient
∆P = the pressure drop across the valve
ρ=density of the fluid
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63. Sample problems-Control Valve
• Determine the valve size that is required to control the flow of
water when the minimum required flow is 0.012 m3/s and the
permissible pressure drop across the valve at this flow rate is
300 kPa.
• Using the equation:
Q=Av√ ∆P/ρ
Av=0.012√1000kg/m3
/300x103
Av=69.3 x10-5
Then using Table 5.1 Flow Coefficients and Valve Sizes, the valve size is 960 mm.
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