Control Valve

• Actuator– Pneumatic– Hydraulic– Electromechanical motor

• Valve Body– Changes clearance between plug and seat available

for flow– Thus alters flow– Design decides valve characteristics

Fig.2: AIR OPERATED DIAPHRAGM CONTROL VALVE

Fig. 3: COMPONENTS OF A CONTROL VALVE

Control Valve• Should respond accurately to change in signal• Operate over a wide range of flows• Exhibit little dead time• Exhibit little hysteresis• React to incremental changes in signal (resolution)• Respond with appropriate speed (stroking speed)• Offer acceptable mechanical/hydraulic behavior

(noise, vibrations, unbalanced forces, pressure drop, etc.)

Control Valve Basics

• Inherent Valve Characteristics– Depends on valve geometry– Independent of flow characteristics– Flow rate v lift at constant P

• Installed Valve Characteristics– Flow rate v lift at actual P– Coupled with characteristics of elements in the flow

circuit– Could be very different from installed characteristics

Inherent Valve Characteristics• The flow rate through a control valve is given by

where,q is flow ratep1 is upstream pressurep2 is downstream pressureC is flow coefficient is densitya is area available for flow

)21

( ppCaq

• Valve designs differ in the way ‘a’ is related to ‘x’, the fraction of valve stroke.

• The a-x relationship decides the q-x relationship.

• Standard q-x relationships are:– Linear– Equal percentage– Square root

Linear Characteristics

• q / qmax = x

Where, x = fraction of total valve stroke = 0, fully closed valve

= 1, fully open valveqmax = maximum flow (q at x = 1)

Equal Percentage

• d (q/qmax) / dx = k (q/qmax)

• (q/qmax) = (qmin/qmax) at x = 0

• (q/qmax) = (qmax/qmax) =1, at x = 1

• (q/qmax) = (qmin/qmax) (qmax/qmin)x

Square Root

• d (q/qmax) / dx = k / (q/qmax)

• (q/qmax) = 0 at x = 0

• (q/qmax) = 1, at x = 1

• (q/qmax) = x

Fig. 5: CONTROL VALVE CHARACTERISTICS

Choice of Valve

• Process considerations become important in selection of proper inherent valve characteristics.– Temperature Control of HX outlet temperature– Constant terminal pressure circuit– Pump circuit with constant source-sink pressures

• Valve-system combination (installed valve) should have linear characteristics or constant gain while each could separately have non-linear behavior

Fig.6: BLOCK DIAGRAM - TEMPERATURE CONTROL SCHEME

Fig.7: VALVE FLOW CIRCUIT - CONSTANT TERMINAL PRESSURES

Fig. 8: PRESSURE DROP PROFILE

Fig. 9a

Fig. 9B

Fig. 10: FLOW CIRCUIT WITH DUMP

Fig. 11: PRESSURE PROFILE

Fig. 13: NOMOGRAPH FOR SIZING CONTROL VALVES

Selection Heuristics

• Choose Valve with Linear Inherent Characteristics – For liquid level or flow control service– When P across valve is expected to be fairly

constant– When major portion of system P (i.e. across

process and valve together) is across control valve

Selection Heuristics

• Choose Valve with Equal Percentage Inherent Characteristics – For temperature control service– For pressure control service– When major portion of system P (i.e. across

process and valve together) is across control valve

Valve Heuristics

• The valve gain should be > 0.5 everywhere• Use only 10-80% of the stroke for better

control• Pressure drop in CV in steam lines to

turbines, reboilers or process vessels should be 10% of the design pressure of the steam system or 5 psi, whichever is greater.

Valve Heuristics

• If static pressure is the moving force for liquid transport from one vessel to other, allocate 10% of the lower terminal vessel pressure to CV or 50% of the dynamic pressure loss, whichever is higher

• For CV in a suction or discharge line of a centrifugal compressor, allocate 5% of suction pressure or 50% dynamic to CV.

Valve Heuristics

• In a pump circuit, pressure drop allocation to CV should be 33% of the dynamic losses in the system at rated flow or 15 psi, whichever is larger.

• Do not recommend 1 size lower if it means significantly different CV size as compared to line size.

Valve Noise

• Caused by vibrations, cavitation, high velocity pressure oscillations, unsteady flow etc.

• Typically 100 dB• Normal conversation 60 dB• Pain threshold of ear-drums : 130 dB• Convey noise specs of process to

supplier.