By
M.Freethan
M-42801
FACTS AND ILLUSIONS
•Introduction
•Control valves
•Types of control valves
•Actuators
•Positioners
•Case study on handling control valves
smartly
What are VALVES?
Mechanical devices designed to direct, stop, mix
or regulate the flow, pressure or temperature
of a process fluid.
Kinds of VALVES:
On-off valves, One way valves & Throttling valves.
Types of VALVES:
Gate, Plug, Ball, Butterfly, Check, Pressure-relief
& Globe valves.
Globe valve
Gate valve
Ball valve
Plug valve
What are VALVES made of ?
Steel, Iron, Plastic, Brass or any other Special
Alloys.
How to select VALVES ?
VALVE Coefficient (Cv):
Measurement commonly applied to valves is
the valve coeff or the flow coeff.
If Cv is not calculated, the valve experiences
diminished performance.
Too small Cv :
•Valve under sized
•Starving for fluid
•Buildup of upstream pressure
•Higher backstream pressure damaging equipments
•Cavitation & Flashing
Too large Cv :
•Large oversized valve is selected
•Cost, size & weight increases
•Higher pressure drops and faster velocity causing
cavitation, flashing & corrosion
•Bath tub stopper effect
Cv is defined as number of US gallons per
minute of water at 60°F that flows
through valve with a pressure drop of
1 Psi
How do we compare VALVES ?
Rangeability:
Ratio of maximum to minimum flow that can be acte
upon by the VALVE after receiving the signal from
the controller.
Factors affecting rangeability :
•Valve’s geometry (plug and seat of a gate valve)
•Seat leakage (valve instability)
•Actuator (more stiffness at near closure)
Sensitivity :
Change in output / Change in input
SHUT OFF
Classification:
ANSI Class I – Open classification-tests not
Required, Allows specified agreement over
required leakage
ANSI Class II – Shutoff is 0.5% of the rated valve
Capacity
ANSI Class III – 0.1% rated valve capacity used
at places where higher cutoff is required
ANSI Class IV – Industry standard for single
seated Valve with metal-metal seating surface,
0.01% of rated valve capacity
ANSI Class V & VI - Throttling valves where
Shutoff is primary
ANSI Class V – Shutoff focus is defined as
0.0005cm²/min per inch of orifice dia/psi
Differential used when control valve has to be
closed for a long time
ANSI Class VI – bubble tight shutoff valve, metal
to elastometer soft seating surfaces-independent
of pressure differentials
BODY END CONNECTION
Designed to match piping connection,
• > 2 inches use thread connection
• < 2 inches > flanged
WELDED END CONNECTION - Delivers zero
leakage
•Stockweld – HP,HT fluids in < 2in
•Buttweld – HT, HP
CONTROL VALVE
A throttling valve designed with an
actuating system to work within control
loops.
Control valves are throttling valves but all
throttling Valves are not control valves
Control valves
Body assembly
Globe
Butterfly
Actuator (Subassemblies)
Ball
Eccentric
(Construction)
Air To Close Valve
Δ P FLOW
=P1-P2
Δ P SHUTOFF =P1-PLOAD
1. USUALLY Δ P SHUTOFF > Δ P
FLOW
2. Methods to increase Δ P SHUTOFF
1. Increase PLOAD
2. Increase ADiaphragm
3. Reduce FSpring
4. Reduce ζ Packing
ΣFup = ΣFdown
P1 * APort+ FSpring+
ζPacking=PLOAD* ADiaphragm+
P2 * APlug
Note: Inlet Pressure tends to push
open plug
Air To Open Valve
Δ P FLOW
=P1-P2
Δ P SHUTOFF =P1-PLOAD
1. USUALLY Δ P SHUTOFF > Δ P
FLOW
2. Methods to increase Δ P SHUTOFF
1. Decrease PLOAD
2. Increase ADiaphragm
3. Increase FSpring
4. Reduce ζ Packing
ΣFup = ΣFdown
P1 * APort+PLOAD* ADiaphragm +
ζPacking= FSpring +P2 * APlug
Note: Inlet Pressure tends to push
open plug
GLOBE VALVES
Most common, Rising stem control valves
Design simplicity, versatility of applications, easy
Maintenance & ability to handle a wide temperature
and pressure range
Size < 0.5-42 inches >
Globe valves:
•Most widely used valve in
Ammonia plant
•Has a cage to regulate flow
BUTTERFLY VALVES
Early 1930’ used as ON-OFF block valves,
Now used as throttling valves- High performance
Butterfly valves is a quarter turn rotatory motion
Valve that uses rotating round disk as a regulating
Element.
Size< 2 in- 8 in >
Cons – deadband
Pros – smaller & lighter in weight
Butterfly valves:
•Used widely in water
treatment plant since
Large pipes are used
BALL VALVES
Similar to butterfly control valves, Accurate
Control possible, Improved sealing & highly accurate
Matching of balls have provided tight shutoff.
Used in slurries or pulp applications. Can be installed
vertically in pipe lines.
Ball valves:
•Used for slurry
applications
ECCENTRIC VALVES
Growing in demand , combines the positive
Aspects of Ball, Globe, Butterfly, Rotary valves using
An offset plug to swing into a seat to close the valve.
Pressure drop < 100 psi >
Avoids water hammering effects
ACTUATORS
A device mounted on a valve that in response to a
Signal, automatically moves the valve to the required
position using an outside power source. The addition
Of an actuator to a throttling valve is called a
CONTROL VALVE
ACTUATORS
Pneumatic
Diaphragm
Electronic motor
Piston
Electro hydraulic
PNEUMATIC ACTUATORS
Air is relatively inexpensive, 90% of the
Industries employ these actuators.
HYDRAULIC ACTUATORS
Exceptional stiffness & high thrust’ are
required, Fast stroking speeds.
Hydraulic fluid above and below a piston to position
the valve.
LINEAR ACTUATOR
ROTARY ACTUATOR
ELECTROHYDRAULIC ACTUATOR
Self contained hydraulic system, electrical
Signal feeds to an internal pumps, which uses
hydraulic fluid from a reservoir to feed hydraulic
fluid above or below the piston.
Pros – Exceptionally stiff because of the
incompressibility of liquids.
Cons – Expensive and Bulky
POSITIONERS
A device attached to an actuator that receives
An electronic or pneumatic signals from the controller
And compares this signal to the actuator’s position.
3-way positioners
Send and exhaust air to only one side of a
Single acting actuator that is opposed by a range
Spring.
4-way positioners
Send and exhaust air to both sides of an
actuator which is required for double acting
actuators.
Reasons To use Positioners
• Increase control system resolution
i.e)fine resolution.
• Allow use of characteristic cams.
• Minimize packing friction effects.
• Allow Split Ranging.
• Overcome seating friction in rotary valves.
• Facilitate operation when the higher
number in the bench-set range is greater
than 1ksc
Reasons To use Positioners
• Permit use of piston actuators.
• Allow distance between controller and
control valve using the advantage of
4-20ma signal.
CASE STUDY
Implementing FUZZY concepts in controllers
driving the control valves
References
•Valves handbook
•Fuzzy logic by Lofti.A.Zadeh
•Valves presentation by
Mr.Charles Dhanraj
QUESTIONS