By M.Freethan M-42801. FACTS AND ILLUSIONS Introduction Control valves Types of control valves...

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, mixor regulate the flow, pressure or temperatureof 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 SpecialAlloys.

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 causingcavitation, 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 actedupon 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 CONNECTIONDesigned 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 Actuator (Subassemblies)

Globe Butterfly Ball Eccentric (Construction)

Air To Close Valve

Δ P FLOW =P1-P2Δ P SHUTOFF =P1-PLOAD1. 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 = ΣFdownP1 * APort+ FSpring+ ζPacking=PLOAD* ADiaphragm+

P2 * APlug

Note: Inlet Pressure tends to push open plug

Air To Open Valve

Δ P FLOW =P1-P2Δ P SHUTOFF =P1-PLOAD1. 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 = ΣFdownP1 * APort+PLOAD* ADiaphragm + ζPacking= FSpring +P2 * APlug

Note: Inlet Pressure tends to push open plug

GLOBE VALVES

Most common, Rising stem control valvesDesign simplicity, versatility of applications, easyMaintenance & 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 VALVESEarly 1930’ used as ON-OFF block valves,

Now used as throttling valves- High performanceButterfly valves is a quarter turn rotatory motionValve that uses rotating round disk as a regulating Element.

Size< 2 in- 8 in >

Cons – deadbandPros – 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 Electronic motor Electro hydraulic

Diaphragm Piston

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.

• 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.

Reasons To use Positioners

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