Control Valve Actuation

WHEN ACCURACY MATTERS

IntroductionActuation Technology


What is Pneumatics?What is Pneumatics?

• Pneumatics is an application of fluid power. Pneumatics uses air, which is compressible. Most industrial pneumatic applications use pressures of about 40 to 100 pounds per square inch (psi)

• Advantages of pneumatics– The working fluid is very light in weight so supply hoses are not

heavy. – Because the working fluid is (mostly) just air, there is usually no

need for a return line for the working fluid and leaks of the working fluid tend not to be messy.

– Because air is compressible, the equipment is less likely to be damaged by shock. The air in pneumatics absorbs excessive forces due to sudden valve closures or shift, etc.


• What is torque?– By definition “Torque” is a turning or twisting force. Usually a

force on a moment arm (lever).

• What is torque?– By definition “Torque” is a turning or twisting force. Usually a

force on a moment arm (lever).

Torque is expressed in terms of Torque is expressed in terms of inch pounds or foot pounds for the inch pounds or foot pounds for the U.S. and Newton-meters for S.I. U.S. and Newton-meters for S.I. (international).(international).

It’s magnitude can be increased It’s magnitude can be increased by increasing the force or the by increasing the force or the length of the moment arm, or both.length of the moment arm, or both.

The Turning Force - TorqueThe Turning Force - Torque

Torque can be calculated by: T = F x M.A. Where: T = Torque in in. lbs. F = force in Pounds M.A. = Moment Arm in inches (“r” on above graphic)


ForceForce• The most common device used to generate the force for

making torque by actuators is compressed air working in a linear cylinder as shown below.

• This generates the linear force which, thru the lever arm, is converted to torque


Crank ArmCrank Arm• The “Crank Arm” uses a lever attached to the valve stem

which is “pushed” by the linear cylinder

The torque output from a crank arm:

Torque at center of Stroke

T = P x A x MA

Torque at beginning and end of stroke:

T = P x A x Cos. 45°x MAWhere: T = Torque in in. lbs.

P = Operating Pressure in psigMA = Moment Arm in InchesA = Area of the piston in square inches.


Power to the LeverPower to the LeverPower to the LeverPower to the Lever

• Scotch Yoke actuators generate torque on the lever arm principle. The lever arm length is the

distance between the center of the pinion and the thrust pin bearing interface to the yoke..

Force is applied to the pinion thru air pressure on the piston and is converted to torque thru the lever arm length.


Scotch Yoke MechanismScotch Yoke MechanismTypical Torque CurveTypical Torque Curve

TO

RQ

UE

0 45 90

100% Break

50% Run

80% End

Typical Ball Valve

Scotch YokeMechanism

Rotation of Actuator


Pneumatic Linear to Rotary Scotch Yoke Piston Actuator Double Acting/Spring Return

• Advantages– Spring Return Failure

– High Torque Output

– Adjustable Limit Stops


Pneumatic Linear to Rotary Scotch Yoke Piston Actuator Single Acting

• Advantages– Spring Return Failure

– High Torque Output



Lever Arm for Rack & PinionLever Arm for Rack & PinionLever Arm for Rack & PinionLever Arm for Rack & Pinion

• Rack & Pinion actuators generate torque on the lever arm principle. – The lever arm length is the

distance between the center of the pinion and the gear tooth, as shown to the left.

– Force is applied to the rack thru air pressure on the integral piston/rack and is converted to torque thru the lever (moment) arm length; rotating the pinion.

Lever arm

L

ever arm

leng

thlen

gth

Looking down from top of

actuator


Pneumatic Linear to Rotary Rack &PinionPiston Actuator Single Acting

• Advantages– Compact Size– Spring Return Failure– High Torque– Adjustable Limit Stops


Pneumatic Linear to Rotary Rack & Pinion Piston Actuator Double Acting

• Advantages– Compact Size

– High Torque



Pneumatic Rotary Vane Actuator Double Acting

• Advantages– Compact Size

– High Torque


– Light Weight


Torque Output ComparisonTorque Output Comparison


• Advantages– Rolling Diaphragm– Constant Area– Ease of Maintenance– 1 Serviceable Softgood– Cost effective– Compact & Light Weight

Pneumatic Linear to Rotary Lever ArmDiaphragm Actuator


Manual Override

• Single Acting • ATO/FC

– Side Mounted H/W

CAMFLEX II


Pneumatic multi-spring diaphragm actuator

•Fail Closed


• Advantages– Multiple spring ranges– Field reversible– Constant diaphragm area– Reduced inventory– Cost effective– Compact

Pneumatic multi-spring diaphragm actuator

•Fail Open


Manual Override

• Single Acting

• ATC/FO – Side Mounted H/W


Pneumatic single spring diaphragm

actuator

•Fail Open


• Advantages– Simple– Low friction– Cost Effective– Simple to maintain

• Disadvantages– Diaphragm area not

constant– Not field reversible

Pneumatic single spring diaphragm actuator

•Fail Closed


Manual Override

• Single Acting • ATC/FO

– Allows local Override


• Piston or Cylinder Actuators are typically used:– where high delta-p shut offs are

required

– Long strokes are required

– Heavy Cycling

– Spring Return Plus Air Assist

Spring Return Piston Actuators


Piston Actuators• Single acting with spring

return• Double acting with or

without spring return • Leakage across piston is

prevented by a seal on the piston.

• Advantages

– Capable of High Thrust

– Stable at high DP

– Long travels are available

• Disadvantages

– Higher frictional loads than spring diaphragm actuators.


Manual Override

• Single Acting/Double Acting– Hydraulic Hand Pump

– Directional & Bi-Directional

Operate to Retract


•Required Force = (Class IV seat load) + (Packing friction) + (Process force)

Actuator Sizing

- Seat Load Force- Packing Friction- Shutoff Force


ANSI/FCI 70-2 Leakage ClassANSI/FCI 70-2 Leakage Class

• Class I no test procedure• Class II 0.50 % of rated Cv (air)• Class III 0.10 % of rated Cv (air)• Class IV 0.01 % of rated Cv (air)• Class V 0.0005 ml/min of water per

inch of orifice dia. per psi• Class VI ml/min per orifice dia. (air)



•Class IV Seat load Pressure (C1)

•C1 = (π) x (Seat Diameter) x (Seating Force)•Seating Force is determined by shutoff class

•Seating Force= 30 lbs/inch Class II, III, IV

150 lbs/inch Class V

50 lbs/inch Class VI

•C1 = (3.14) x (1.625 inch) x (30 lbs/inch)•C1 = 153.08 pounds

•Seat Diameter

Actuator Sizing


Soft Seal Metal Seal


•Required Force = (Class IV seat load) + (Packing friction) + (Process force)• Valve packing friction load (C2)

• C2 =(Friction Coefficient) (3.14) (Stem dia.) (Packing Height) (Packing load)

• Friction Coefficient is a function of the packing material• Friction Coefficient = .03 Teflon

.15 Graphite

• Packing Load is a function of inlet pressure• If inlet pressure is>1000psi Load=1000psi• If inlet pressure is <1000 psi Load=Inlet Pressure

• C2 = (0.03) x (3.14) x (0.50”) x (1.1”) x (1000Psi)

• C2 = 51.81 pounds

Packing H

eightActuator Sizing



• Process Force (C3)

• C3 =Area of Orifice = (Seat Diameter) (π/4)

• = (1.625”)2 (0.7854)

• C3 =Area of orifice = 2.07 sq. inch (2.0739)

• C3 =Process Force = (90 PSIG inlet) (2.07 sq. in)

• C3 =Process Force = 186.3 pounds

90 PSI Inlet

Actuator Sizing



•Required force = (153.08 pounds seating force) + (51.81 pounds packing friction)

+ (186.3 pounds process force)

• Required force to Close= 391.19 pounds

Actuator Sizing


Valve Required Force to OpenFlow to Open

• Required Force = (Packing Friction) – (Process Force)

• Required force =(51.81 pounds packing friction) - (186.3 pounds process force)

• Required force to Open= -134.49 pounds Based on Physics (Select Actuator to be (Packing Friction)

• Required force= 51.81 pounds


Flow to Open Direction


Flow to Close Direction


Unbalanced Trim

• Valve closure member which is under unbalanced dynamic forces induced by process media.

• Process force tends to influence trim generating additional forces in the flow direction.

• Unbalanced Trim in moderate to hi-pressure applications required larger thrust actuators.


Unbalanced Trim

• Globe Valve• Unbalanced Design• Post Guided• Lo-dB Trim • Class-IV & Class-V Shut-Off• Flow To Open


Balanced Trim

• Valve closure member which is equalized/ balance with dynamic forces induced by process media.

• Balance trim designs include Semi-Balanced and Fully-Balanced.

• Balanced Trim in moderate to hi-pressure applications required much smaller thrust actuators compared to unbalanced trim.


Balanced Trim

• Globe Valve

• Fully-Balanced Design

• Cage Guided

• Composite Polymer Balancing Seal

• Class-V Shut-Off

• Flow To Open


Semi - Balanced Trim

• Globe Valve• Semi-Balanced Design

• Dual Post Guided• Double-Port (Seat) Design• Class-III Shut-Off


Questions? Questions?

Engineering

Control Valve Actuation