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
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 isproportional 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 valveflow 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 (Globe) Control Valve
Plug stem connected to actuator shaft that moves up and down to strokethe 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
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 noisebut watch out for occlusion of narrow slots and tiny holes from steam and process residue!
Hydro Thermal – Steam Mixer
Rotary Eccentric Plug Valve Design
Rotary actuator eliminates linkage needed to translate up and down shaft movement to rotary motion of plug stem
Tight plug stem and actuator shaft connection reduces dead band Lowest cost for 1”- 6” size valves but it has a verylimited choice of materials for corrosive service
Digital valve positioner measures and controls actuator shaft rotation
Rotary Eccentric Plug Valve Cross Section
Integral flange reduces potential for leaks and the installation and maintenance cost Stepped Seat Ring (less friction near the seat)
Contoured eccentric plug improves plot of flow versus stem position near the seat
Optionalattenuator disc to reduce noise
Flow Direction from cooking tube to liquefaction
V-Ball with a diaphragm actuator
Dead band (lost motion) V-ball < 0.5% Other 2 - 8%
Tight ball, stem, and shaft connections reduces dead band
Diaphragm actuator - an increase in air pressure to top presses on flexible diaphragm that causes shaft to move down. The minimum pressure to open the valve is setby force of opposing spring. For very large actuators volumes, the time to stroke full scale can become large Tight linkage design to translate up-down actuator shaft movement to rotary ball motion Unfortunately most positioners measure actuator instead of ball or disc position
V provides better flow characteristic and shear of pulp
Short shaft reduces twist (stick-slip)
Butterfly Valve(with Rotary Piston Operator)
Butterfly valves are much much less expensive than globe valves for large sizes (>6”) since there is much less metal and machining
These are often called high performance valves because of high temperature and low leakage ratings but they are low performance so far as stick-slip particularly near the closed position because of very high seat/seal friction...
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