INTRODUCTION
CONTROL VALVE
FLOW CHARACTERISTIC
ACTUATOR
ACCESSORIES
Introduction
For most of the process control system, the final control element is a control valve and these valves are typically driven by motors which are commonly called Actuators. There is almost unlimited number of hardware variations encountered in process control valves and Actuators.The control valve plays a very important part in the automatic control of modern plants, which depends on the correct distribution and control of flowing liquids or gases.
WHAT IS CONTROL VALVE?
It is the Final Control element most commonly used in processIndustries to regulate the material flow in a process. The Control Valve manipulates the flowing fluid to compensate the load disturbance and
keep the regulated process variable like pressure, flow temperature,
level, etc. As close as possible to the desired set point.
Flow Characteristic:
Quick Opening:
The valve sensitivity (change in flow for a given change in valve position) decreases with increasing flow.Linear Type:
The valve sensitivity is more or less constant throughout the flow range. Control valve plug has linear flow characteristics when the flow rate through is proportional to the lift.Equal Percentage:
Control valve plug has equal percentage characteristics if at any plug position, the same percentage of change in flow takes place for the same amount of plug movement.Control Valve Construction
Valve Body
The main pressure boundary of the valve that also provides the pipe connecting ends, the fluid flow passageway, and supports the seating surfaces and the valve closure member.Actuator
A pneumatic, hydraulic, or electrically powered device that supplies force and motion to open or close a valve.Bonnet
The portion of the valve that contains the packing box and stem seal and can guide the stem. It provides the attachment of the actuator to the valve body.Yoke
The structure that rigidly connects the actuator power unit to the valve .Types of Control Valve:
1. Linear Motion
Globe Valves Single Ported
• Single Ported Globe
• Angle
• Split
• Needle
Double Ported
Three Way
2. Rotary Motion
Ball Valve
Butterfly Valve
Control Valve Packing
Gland Packing
The part of the bonnet assembly used to seal against leakage around the closure member stem.
Types
• Teflon.
• Glass loaded Teflon.
• Carbon loaded Teflon.
• Graphite.
• Asbestos with lubricants
like graphite.
Selection criteria
• Temperature.
• Service.
• Pressure.
Actuator:
A fluid-powered or electrically powered device that supplies force and motion to a Valve Closure Member.
Types of Actuator
Diaphragm Actuator Piston Actuator
Electrical Actuator
DIAPHRAGM ACTUATOR:
Is a fluid (usually pneumatic) pressure-operated, spring-opposed diaphragm assembly, which positions the valve stem in response to an input signal.
Types Of Diaphragm Actuator:
AIR-TO-OPEN: An increase in air pressure to the Actuator is required to cause the valve to open. This is another way of saying the valve is Fail Closed or Normally Closed. AIR-TO-CLOSE: An increase in air pressure to the Actuator is required to cause the valve to close. This is another way of saying the valve is Fail Open or Normally Open.
The Advantages and Disadvantages of Diaphragm Actuator:
Advantages:
• Lowest Cost.• Can throttle with out Positioner.
• Simplicity.
• Inherent fail safe action.
• Low supply pressure required.
• Adjustability.
• Easily maintained
Disadvantages:
• Limited output capability.• Large size and weight.
PISTON ACTUATOR
A fluid-powered, normally pneumatic device in which the fluid acts upon a movable cylindrical member, the piston, to provide linear motion to the actuator stem. These units are spring or air opposed and operates at higher supply pressures than a Spring Return Actuator.
ELECTRIC ACTUATOR
Also known as an Electro-Mechanical Actuator uses an electrically operated motor-driven gear train or screw to position the actuator stem. The actuator may respond to either a digital or analog electrical signal.
Advantages:
• Compactness.• Very high stiffness.
• High output capability.
Disadvantages:
• High cost.• Lack of fail safe action.
• Limited duty cycle
Accessory
A device that is mounted on the actuator to complement the actuator's function and make it a complete operating unit. Examples include positioners, supply pressure regulators, solenoids, and limit switches.Control Valve Accessories:
Hand Wheel
Air filter Regulator
Positioner
Booster
Limit Switch
Solenoid Valve
Current to Pneumatic (I/P) Converter
Air Lock Relay
Hand Wheel
Top-mounted hand wheel for a Reverse-acting diaphragm Actuator. This unit can be used as an adjustable travel stop to limit travel in the downward direction or to manually close push-down-to-open valves.Supply Pressure Regulator
Supply pressure regulators commonly called air sets, reduce plant air supply to valve positioners and other control equipment. Common reduced-air-supply pressures are 20, 35 and 60 psig. The regulator mounts integrally to the positioner, or nipple-mounts or bolts to the actuator.Positioner
A position controller that is mechanically connected to a moving part of a final control element or its actuator and that automatically adjusts its output to the actuator to maintain a desired position in proportion to the input signal. It ensures proper positioning of the valve and provide fast response. The Positioner uses an auxiliary air supply and feedback cam to position the valve plug in accordance with the air signal from the controller.Volume Booster
A stand-alone relay is often referred to as a volume booster or simply booster because it boosts, or amplifies, the volume of air supplied to the actuator.Features:
• Quick response.• Greater stability.
• Low air consumption.
• Higher accuracy.
