Control Valve Accessories - Principle, Purpose, Advantages

Control Valve Accessories - Principle, Purpose, Advantages

Control valves regulate fluid flow within process systems.

For further details, please visit our website.

According to the International Society of Automation, a control valve functions as a power-operated device designed to control fluid flow rates in process control systems.

Control Valve Accessories

The following are the most common accessories used with control valves:

1. Pneumatic Positioners
2. I/P Transducers
3. Volume Boosters
4. Position Transmitters
5. Limit Switches
6. Solenoid Valves
7. Air Lock relay
8. Handwheel

Equipping control valves with appropriate accessories enhances their utility in process control and ensures smooth and efficient operation. Pneumatic control valve accessories are vital for optimal performance in a process control system.

Purpose of Control Valve Accessories

Accessories are added to control valves for the following reasons:

• Improve process control
• Enhance safety for the process and personnel
• Boost valve performance and response speed
• Monitor and verify valve responsiveness
• Diagnose potential valve issues

Advantages of Valve Positioner

Valve Positioner benefits include:

1. Minimizing friction, hysteresis, and dead band effects on the valve stem.
2. Enabling signal range change.
3. Allowing control valve signal reversal: A positioner operates in either direct or reverse acting mode. In direct acting mode, an increase in control signal pressure raises positioner output air pressure. In reverse mode, an increase in signal pressure lowers positioner output air pressure.
4. Increasing actuator response speed.
5. Allowing changes in valve flow characteristics.
6. Supporting split range operation. In a split range control loop, one controller output drives two control valves.

I/P Transducer

An I/P transducer converts a 4 to 20 mA current input to a proportional 3 to 15 psi pressure output.

In some applications where positioning accuracy by a positioner isn't required, an electro-pneumatic (I/P) transducer can suffice.

Read: I/P Converter Principle

Volume Boosters

For large actuator volumes, control valve response speed decreases.

Volume boosters provide additional pneumatic thrust output capacity to a valve assembly.

Single-acting actuators typically use one volume booster, while double-acting actuators require at least two.

Read: Volume Booster Principle

Position Transmitter

Position transmitters provide independent valve position feedback to the control system, essential for process monitoring, troubleshooting, or startup/shutdown verification.

The position transmitter is mounted to the valve yoke to measure the position of the valve stem or shaft.

Read: Positioner Principle

Solenoid Valve

In certain applications, solenoid valves trap air in the actuator to lock the valve in its current position or vent air to allow the valve to move to its no air position.

Read: 2 way Solenoid Valve

Limit Switches

Limit switches provide discrete open or close signals to the control system when the valve reaches specific positions within its travel range.

Typically, they are mounted on the control valve and yoke, used for process monitoring, troubleshooting, or startup/shutdown verification.

For further information, please visit Xingyu.

Trip Systems – Airlock Relay

Trip systems like air lock relays are utilized in scenarios needing specific actuator action if air supply pressure is lost. These systems work with double-acting actuators lacking an inherent fail state or with single or double-acting actuators for pneumatic lock-up.

When the air supply pressure drops below a trip point, the trip valve causes the actuator to lock the control valve in its last position. For double-acting applications, a reserve air volume tank ensures the valve can operate until the air supply is restored.

Read: Air Lock Relay Principle

Handwheel

Handwheels for diaphragm actuators serve as adjustable travel stops and manual emergency positioning tools. They may be:

1. Side-mounted: Can open or close the valve in any direction at any point in the actuator stem travel.
2. Top-mounted: Used for infrequent manual operation to manually stroke the valve.

Selection, Location, Advantages and Disadvantages of Valves

Valve selection and setting position

1. Selection principle of valves used in water supply pipelines:

• For diameters ≤ 50mm, use globe valves; for diameters > 50mm, use gate or butterfly valves.
• Use regulating and globe valves to adjust flow and pressure.
• Use gate valves for sections requiring low flow resistance.
• For bidirectional flow, use gate or butterfly valves.
• Use butterfly and ball valves where installation space is limited.
• Use globe valves for frequently opened/closed sections.
• Use multifunctional valves on large caliber water pump outlets.

2. Position of valves in the water supply pipeline:

• Residential district pipeline.
• Nodes of outdoor annular pipe network.
• Start of branch pipe or household pipe.
• Household pipes, water meters, branch risers.
• Branch pipes of loop networks, connecting pipes.
• Indoor pipelines to households and public toilets.
• Pump outlet and suction pipes.
• Water tank inlet, outlet, drainage pipes.
• Equipment filling pipes (heaters, cooling towers).
• Water distribution pipes for sanitary appliances.
• Before some accessories (automatic exhaust valves, pressure relief valves).
• Lowest parts of the network with drain valves.

3. Selection of check valves:

• For low pressure, use swing, ball, or shuttle type check valves.
• For strict sealing, select check valves with closing springs.
• To minimize water hammer, use quick closing or dashpot check valves.
• Ensure check valve discs close via gravity or spring force.

Check valves are set:

• Service pipes.
• Inlet pipes of sealed water heaters/water installations.
• Water pump outlet pipes.
• Combined inlet and outlet pipes on tanks, water towers.

4. Exhaust device positions for water supply pipes:

• Install automatic exhaust valves at ends and highest points.
• At peaks of undulating sections with air accumulation.
• Pneumatic supply devices with automatic air tanks.

Advantages and disadvantages of various valves:

1. Gate Valve:

Advantages:

• Low fluid resistance.
• Requires smaller torque for operation.
• Allows bidirectional medium flow.
• Minimal erosion when fully open.
• Simple body structure and manufacturing process.
• Short structural length.

Disadvantages:

• Larger dimensions and installation space needed.
• High friction loss during operation.
• Difficult processing and maintenance.
• Long opening and closing times.

2. Butterfly Valve:

Advantages:

• Simple, compact design with material savings.
• Fast operation with minimal flow resistance.
• Suitable for media with suspended solids.

Disadvantages:

• Limited flow adjustment range.
• Not ideal for high-temperature or high-pressure systems.
• Poorer sealing performance compared to other valve types.

3. Ball Valve:

Advantages:

• Minimal flow resistance.
• Reliable use with corrosive media.
• Complete seal across pressure and temperature ranges.
• Fast operation with minimal impact.
• Reliable sealing on both sides.
• Medium isolation when fully open/closed.
• Compact and lightweight, ideal for low-temperature systems.
• Symmetrical design withstands pipeline stress.
• Can operate underground with long service life.

Disadvantages:

• Seal performance affected by temperature and material limitations.
• Poor regulating performance compared to globe valves.

4. Globe Valve:

Advantages:

• Less friction during operation than gate valves.
• Short opening height.
• Easy manufacturing and maintenance with single sealing surfaces.
• High temperature resistance.

Disadvantages:

• Higher flow resistance due to direction changes.
• Slower opening speed.

5. Plug Valve:

A simple rotary valve similar to a ball valve, mainly used in the petrochemical industry.

6. Safety Valve:

Automatically opens under excessive pressure to prevent equipment damage; closes when pressure normalizes.

7. Steam Trap Valve:

Ensures efficiency by removing condensed water, preventing steam leakage, and expelling non-condensable gases.

8. Pressure Reducing Valve:

Reduces inlet pressure to a desired outlet level and maintains stability.

9. Check Valve:

Automatically prevents backflow, protects pumps and container mediums, and may be swing-type or lift-type.

Additional resources:
Exploring RBV Series Pneumatic Ball Valve Flange