Control valves

Diaphragm Control Valves
• Just what is a control valve? • An automated valve used to restrict flow to the process – We can manipulate the process

2 v1 P2 v2 Z1 + + + H p = Z2 + + + hl ρ 2g ρ 2g

P 1

2

Purpose of a Control Valve
The purpose of a control valve is to throttle flow. The pressure drop ∆P across the control valve required to do this is an energy expense.
Cv is proportional to cross sectional area for flow. An increase in valve capacity (Max Cv) will decrease the valve pressure drop Its relationship to stem position can be linear or nonlinear For linear trim, Cv is proportional to position. Its response to a signal is adversely affected by stick-slip and dead band.

For Liquid flow: FL = CV ∗ sqrt [∆PV / SG] Where: FL = Liquid flow (gpm)

CV = Control valve flow coefficient per characteristic curve (gpm/psi1/2) ∆PV = Pressure drop across the valve (psi) SG = Liquid specific gravity

Control Valve, Two Basic Designs:
• Sliding Stem, globe gate, needle etc. • Rotating Stem, Ball, Plug, Butterfly, Disk, etc

Sliding Stem

Rotating Stem

Sliding Stem (Globe) Control Valve
Plug stem connected to actuator shaft that moves up and down to stroke the valve
Path for escaping process vapors

Stem packing - tightened and rough surface to reduce escape of process increases friction that opposes stem movement Stem guiding helps keep plug aligned for high pressure drops

Integral flange

Flow in

Flow out

Seat ring - designs to make the sealing of the surfaces between the plug and seat tighter reduces leakage when valve is closed (tighter shutoff) but increase friction that opposes stem movement. The plug is stuck in the seat. When enough pressure builds up in the actuator, the plug breaks free, jumps, and overshoots the desired position

Severe Service Options (high pressure drop, noise, and cavitation)
Plug guiding prevents deflection Balanced plug reduces actuator size but increase leakage

Whisper trim reduces noise