Control valves are used for gas/fluid flow control in piping or tubing. In order to improve the performance of your control valves, valve accessories have cooperated into the use of the main structure. Here are the reasons that you may need valve accessories.
1. Valve accessories help improve process control, which elevates the efficiency of the whole control systems
2. Safety issue is always what comes first in every working setting, they improve safety for the process or personnel
3. In some cases, proper installment of accessories can improve control valves performance
4. They help automatically diagnose potential issues
The structures of the equipment are simple, but you still need some basic knowledge in order to find the one that best suits your control valves. The information below provides what you should know and a complete understanding of the functions of them. Valve accessories can be useful and reliable when you do know them well.
A control valve with equipped proper accessories becomes more useful for process control. Also, the accessories provide smooth and efficient operation. That is why more and more business owners corporate these devices with their piping.
Of course, there is a variety of accessories that you can find on the market, but how to find the most suitable one for your business? Here are some popular choices that you’ll be interested in, which include pneumatic positioners, I/P transducers, volume boosters, position transmitters, limit switches, air lock relay, and handwheel.
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This type of valve accessories minimizes the effect that friction, hysteresis, and dead band impose on the valve stem. Also, it allows the control valve to change the output of the gas/fluid flow, which is also known as “signal reversal”.
There are two acting modes that pneumatic positioners can provide: direct acting mode and reverse acting mode. The former one increases the output air pressure. In reverse acting mode, it helps the pipe system decrease the output air pressure.
Pneumatic positioner also increases the speed of response and can change the flow characteristics. Common types of flow characteristics include equal percentage flow characteristic, linear flow characteristic, and quickly open flow characteristic.
I/P transducers are used for regulating gas/fluid pressure by applying electrical control signals. They are extensively used in valve actuation. The structure of I/P transducers is simple but effective in increasing the ability to control the pipe system.
In an actuator, the positioning speed of response decreases when the volume becomes large. This is because the flow control systems cannot respond or adjust the large volume within a short time. It must take more time to deal with the volume changes in the pipes with a larger flow. The use of volume boosters provides additional pneumatic thrust output capacity. This is beneficial to a valve assembly.
It provides independent valve position feedback to the flow control systems. It is particularly important for process monitoring, startup or shutdown verification, or other necessary operations by the operator in the flow control room. It can be installed and is mounted on the valve to measure the position of the stem or shaft.
Limit switches are used for creating discrete open/close input signals to the control system. They are directly mounted on the control valve yoke. Just like a position transmitter, limit switchers are useful when an operator needs to monitor the process or verify the function of startup/shutdown.
If the air supply in a trip system is lost, the application of this type of valve accessories can come to help. They are commonly used in double acting actuators with no inherent air state. Usually, the trip system has a trip point set by the operator. When the instrument air supply pressure falls below it, the controller engages to lock the control valve to protect the whole system.
Airlock relays can be seen in the applications of double-acting devices. They are responsible for providing extra reserve air capacity. When the air supply fails, the reserve capacity fills in as a protector of the system and maintains the pressure above the trip point. Then filling process stops as the system resumes its functions.
They are commonly used as adjustable stops in diaphragm actuators. According to the way they are arranged to a control valve, they can be divided as side-mounted handwheel and top-mounted handwheel. Here are the differences between the two types of a handwheel.
a. Side-mounted handwheel
In the actuator stem travel, it can be used to open/close the valve in any direction at any point. However, when it is placed to limit travel, it can only work in one direction at a time. By keeping the handwheel of the control valve in the neutral position, automatic operation is possible throughout full valve travel.
b. Top-mounted handwheel
The operation of top-mounted handwheels is commonly seen in infrequent service. They are to manually stroke the valve.
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