Final Control Elements

Final Control ElementsControl Instrumentation–Lecture 13

Alpha Control Lab, Faculty of Information Technology, TUT

14.12.2016

Final ControlElements

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Final ControlElementGlobe ValvesButterflyValvesBall valveDiaphragmvalve

CharacteristicsCharacteristicsTextbookcharacteristicsControl valveproblems

ActuatorsPneumaticHydraulicElectro-mechanical

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Figure: Old Control System


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Wiring II

A lot wires you say?

Figure: Troubles


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Wiring III

Figure: New Control System


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Control Valve

The control valve, or final control element, is the last device inthe control loop. It takes a signal from the process instrumentsand acts directly to control the process fluid. Control valvesmaintain process variables such as pressure, flow, temperature,or level at their desired value, despite changes in processdynamics and load [1, 2, 3].

Final control element:damper,variable speed pump,motor relay,saturable reactor,valve.


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Globe Valves

The essential feature of a globe valve is its plug and seat assembly.The relationship between the stem position and the flow through theannulus is known as the valve’s characteristic.

InletP1

P2Outlet

Forcesoppose

Plugs

seats

In a double-ported globe valve, there will be two opposed forcevectors. This makes for a control valve that is easier to actuate.With double seated valve it is difficult to achieve-shut off in bothseats [4].


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Features

AdvantagesReduced actuator force due to balancing.Action easily changed (Direct/Reverse).High flow capacity.

DisadvantagesPoor shutoff.Only semi-balanced.


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Butterfly Valves

The maximum rotation is about 90 ◦.

inlet oulet

Its main advantage is high capacity in a small package and avery low initial cost. The modified shape and contour of thedisc are used to reduce dynamic torque and drag. This alsopermits higher pressure drops. As the disc is never hiddenbehind the shaft, good control through the 90 degrees ofoperation is possible with a linear characteristic [5].


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Features

AdvantagesLow cost and weight.High flow capacity.Low stem leakage.

DisadvantagesOversizing.


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Ball valve

Rotation of the ball varies the alignment of a cylindrical portthrough the ball with the flow passage through the body andpipework. The standard ball valve has a port diameter that isabout 80% of the pipe diameter, which makes for a compactvalve.

inlet oulet

The full-port ball valve is the only type of valve that is whollynon-invasive. However, it is more bulky and requires a morepowerful actuator [6].


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Features

AdvantagesLower cost and weight.Higher flow capacity (2-3 times that of the globe valve).Tight shutoff.Low stem leakage.

DisadvantagesOversizing.


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Diaphragm valve

The valve is shown open; closure is achieved by forcing aflexible membrane down onto the weir.

inlet oulet

weir

flexiblemembrane

compressor

Diaphragm valves are good for slurries and liquids withsuspended solids, are low cost devices, but tend to require highmaintenance, and have poor flow characteristics.


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Essential Characteristics [3]

Globe Butterfly Ball DiaphragmSize 0.5−40 cm 2− 500 cm 1.5−90 cm 0.5− 50 cm

Tmax 650 ◦C 1200 ◦C 750 ◦C 175 ◦C

Pmax 10−500 bar

5−400 bar 5−100 bar 2− 15 bar

Apps Good forextremeconditions

Large size.Good withviscosefluids.

Good forslurries butnot grittymaterials.

Good fordifficultfluids:corrosive,suspendedsolids, stickyliquids.


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Features

Globe Butterfly Ball Diaphragm+ Wide variety

of types.Nose andcavitationhandling.

Good

accuracy.

Cheaper than

globe valves if

d > 5 cm.

High capacity.

Wide range.

Moderate cost.Hight capacity.Goodrangeability.Low pressuredrop.

Low leakage.

Low cost.

Simple

construction.

Low leakage.

- Hight cost.

Moderate

rangeability.

Hight torque

for shut-off.

Nose and

cavitation.

Sensitive to

specification

errors.

High friction.High torque.Vulnerable toseal wear, deadspace in bodycavity.

Noise,

cavitation.

Limited operatingconditions.

Poor rangeability.

Maintenance

costs high due to

diaphragm wear.


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Control Valve Characteristics

Q = Cv

√P1 − P2

G, (1)

where Q - flow rate,Cv - valve sizing coefficient,P1- upstream pressure,P2 - downstream pressure,G - liquid specific gravity (water = 1).

The flow characteristics of a control valve show the rate of flowfor the range of valve operation. There are two types of flowcharacteristics for control valves:

inherentinstalled


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Textbook characteristics

Control valve trim is manufactured in a variety of different“characteristics” to provide the desired installed behavior.

1 Linear,2 Equal percentage,3 Quick-opening

0 100%

100%

linear

Quickopening

Equalpercentage

%ofposition

%offlow


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Quick opening and Linear

Quick opening

Used for full ON/ full OFF control applications.Relatively small motion of the stem results maximum possibleflow rate through the valve.

LinearValve determines the pressure drop.

Q

Qmax=

S

Smax, (2)

where Q - flow rate, S - stem position.


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Equal percentage

Equal percentageDoes not shut off flow completely in its limit of stem travel.Curve depends on the rangeability

R =Qmax

Qmin. (3)

Flow rate depends on the extend to which the valve is already open.

Q = QminRS/Smax (4)

SizingValve flow coefficient Cv .

Q = Cv

√P1 − P2

G, (5)

where G (Sg ) - liquid specific gravity.Value of Cv depends on the size of the valve.


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Globe and Ball valve trims

Different valve characteristics may be achieved by re-shapingthe valve trim.

Figure: Quick, linear and equal shapes

Figure: Quick, linear and equal shapes


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Control valve problems

1 Mechanical friction2 Flashing3 Cavitation4 Choked flow5 Noise6 Corrosive chemicals

The point of lowest pressure inside the valve (called the venacontracta pressure, or Pvc).

Pvc

P1

P2


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Bubbles attack

Figure: Flashing and errosion problems [4]

If the fluid being is a liquid, and its absolute pressure ever fallsbelow the vapor pressure of that substance, the liquid will beginto boil. This phenomenon is called flashing.

The pressure recovers to a point greater than the vaporpressure of the liquid, the vapor will re-condense back intoliquid again. This is called cavitation.


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General Info

An actuator is a device that applies the force (torque)necessary to cause a valve’s closure member to move. Fluidpower actuators:

linear,rotary.

Hydraulic or pneumatic are the linear actuators.

In rotary valves, maximum friction occurs in the closedposition, and the moment necessary to overcome it is referredto as breakout torque.

Actuators often provide a fail-safe function. In the event of aninterruption in the power source, the actuator will place thevalve in a predetermined safe position, usually either full-openor full-closed.


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Pneumatic

The most common–the spring and diaphragm design. Thecompressed air input signal fills a chamber sealed by anelastomeric diaphragm. The pressure force on the diaphragmplate causes a spring to be compressed and the actuator stemto move. This spring provides the fail-safe function andcontributes to the dynamic stiffness of the actuator.

Provides a proportional relationship between the forcegenerated by air pressure and stem position. The chiefadvantages of spring and diaphragm actuators are their highreliability, low cost, adequate dynamic response, and fail-safeaction–all of which are inherent in their simple design.


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Features

AdvantagesFail safe operation

DisadvantagesActuator force must work against spring.Temperature limitations


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Hydraulic

Typical hydraulic actuators is similar to that of double-actingpiston pneumatic types. One key advantage is the highpressure [typically 35 to 70 bar], which leads to high thrust in asmaller package. The incompressible nature of the hydraulic oilmeans these actuators have very high dynamic stiffness.

The incompressibility and small chamber size connote faststroking speed and good frequency response. Thedisadvantages include high initial cost, especially whenconsidering the hydraulic supply. Maintenance is much moredifficult than with pneumatics, especially on the hydraulicpositioner.


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Features

AdvantagesFast response to control signals.High loads.Variable stroking speed and stiffness.

DisadvantagesRequires external hydraulic supply.Not spring loaded, generally not fail-safe.


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Electromechanical

A solenoid is a device that converts an electrical signal intomechanical motion. Solenoids are used when a large, suddenforce must be applid to perform some job [7].Electric actuators are often used for on/off service.

The most common electric actuators use a typical motorthrough a gearbox.

DC Motor

Series field: large starting torque, but difficult to speedcontrol (quick opening valves).Shunt field: smaller sturting torque, good speed control(conveyor systems).Compound field: best features for both types.

AC Motor


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Features

AdvantagesHigh thrust.Easily interfaced to control system.

DisadvantagesLarge.More expensive than pneumatic.High power electrical source.Poor controlability.


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N. A. Anderson, Instrumentation for Process Measurementand Control, 3rd ed. Crc Press, 1997.

G. K. McMillan, Process/Industrial Instruments andControls Handbook, 5th ed., D. Considine, Ed.McGraw-Hill Professional, 1999.

J. Love, Process Automation Handbook: A Guide toTheory and Practice. Springer-Verlag London Limited,2007.

T. R. Kuphaldt. (2016) Lessons in industrialinstrumentation. [Accessed: September, 2016]. [Online].Available: http://www.pacontrol.com/download/Lessons-in-Industrial-Instrumentation.pdf

S. Medida. (2007) Pocket guide on industrial automationfor engineers and technicians. IDC Technologies. [Accessed:

http://www.pacontrol.com/download/Lessons-in-Industrial-Instrumentation.pdf

http://www.pacontrol.com/download/Lessons-in-Industrial-Instrumentation.pdf


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November, 2016]. [Online]. Available:http://www.pacontrol.com/download/Industrial-Automation-Pocket-Guide.pdf

W. Bolton, Instrumentation and Control Systems.Elsevier Science & Technology, 2004.

C. D. Johnson, Process Control InstrumentationTechnology, 8th ed. Pearson Education Limited, 2014.

http://www.pacontrol.com/download/Industrial-Automation-Pocket-Guide.pdf

http://www.pacontrol.com/download/Industrial-Automation-Pocket-Guide.pdf

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