Control valve

7.8 Control Valves A Control Valve is a power-operated device used to modify the fluid flow rate in a process system.

There are two basic types of control valves: 1.Rotary motion valves . 2.Linear motion valves.

7.8.1Control Valve Classification

7.8.1.1Linear Motion:Types Of Closure 1.Globe valveA) Single SeatedB) Double SeatedC )Three way2. Diaphragm3. Pinch

7.8.1.1Linear Valve Features

Tortuous Flow Path Low Recovery Can Throttle Small Flow Rates Offers Variety Of Special Trim Designs Suited To High-pressure Applications Usually Flanged Or Threaded Separable Bonnet

7.8.1.2 Single Seated Globe Valve FeaturesLarge amount of force required to drive the stemTight shut offUsed in small diameter applicationsApplicationsUsed for controlling large range of process parameters hence used in oil and gas production, power generation

7.8.1.3 Double Seated Globe Valve FeaturesStem can be driven with small amount of forceCan not achieve perfect closure resulting in leakageUsed in larger diameter applicationsBody parts wear out due to erosion and cavitations which results in annoying noisesApplicationsUsed for high flow and high pressure services

7.8.1.4 Three Way Valves Features

High flow capacityValve used for mixing and diverting liquids.High rangeabilityLeakage capability consistent with normal single seated globe valveApplicationsUsed in textile industries

7.8.1.5 Diaphragm Valves Features

Consists of a saddle or seat upon which the diaphragm closes the valves.Can handle corrosive fluids.ApplicationsUsed as shut off valves in food and beverage , pharmaceutical and biotech industries

7.8.1.6 Pinch Valve Features

Tight shut off The flexible sleeve allows the valve to close drop tight around solids , solids that would typically be trapped by the seat or stuck in crevices in globe, diaphragm, butterfly, gate or ball valves. ApplicationsBest used in controlling corrosive fluids, steam and thermal fluids.

7.8.1.2Rotary Motion

Types Of Rotary valve

1. Ball2. Butterfly3. Plug

7.8.1.2Rotary Valve Features Streamlined flow path High recovery More capacity Less packing wear Can handle slurry and abrasives Flangeless Integral bonnet High range ability

7.8.1.2.1Ball ValveFeaturesTight Shut OffMinimum obstruction for the flowing media.High flow capacityCan withstand high pressure and temperatures

ApplicationsUsed for hazardous and corrosive fluids

7.8.1.2.2 Butterfly ValveFeatures

High flow capacityUnlike ball valve the disc is present in the flow of the liquid and hence a pressure drop is always induced in the flow.Tight shut off

Applications

Used in Steel , Sugar and textile industries

7.8.2 Selection criteria for controlvalve.

1.Maximum operating pressure2. Maximum flow rate3. Operating temp range4. Rate of cycling5. Response time6. Air/Fluid quality7. Mounting method8. Leakages9. Maintenance.

7.8.3Control valve characteristic

7.8.4Noise & cavitations

7.8.5 Actuators 1.In control valve we have seen that , the operation of a control valve involves positioning its movable part (the plug, ball or vane) relative to the stationary seat of the valve.

2.The purpose of the valve actuator is to accurately locate the valve plug in a position dictated by the control signal.

3.The actuator accepts a signal from the control system and, in response, moves the valve to a fully-open or fully-closed position, or a more open or a more closed position

7.8.5.1Pneumatic ActuatorsThey are basically of two types

1.Piston actuator

2.Diaphragm actuator

7.8.5.1.1Piston ActuatorThey can withstand higher input pressures.Can offer small cylinder volumes.They are generally used where the stroke of a diaphragm actuator would be too short or the thrust is too small.

7.8.5.1.2Diaphragm Actuators

7.8.6Installation Issues