How to choose the actuator for pressure regulating valve (pressure control valve)

Any pressure regulating valve actuator is a device that uses energy to drive the valve. This device may be a manually operated gear set used to open and close valves, or an intelligent electronic component with complex control and measurement devices that can achieve continuous adjustment of valves. With the development of microelectronics technology, actuators have become more complex. Early actuators were simply motor gear transmission devices with position sensing switches. Nowadays, actuators have more advanced functions. They can not only open or close valves, but also detect the working status of valves and actuators to provide various data for predictive maintenance.

The most extensive definition of an actuator is a driving device that can provide linear or rotational motion, utilizing a certain driving energy source and working under the action of a certain control signal.

The actuator uses liquid, gas, electricity, or other energy sources and converts them into driving force through motors, cylinders, or other devices. The basic actuator is used to drive the valve to the fully open or fully closed position. The actuator of the pressure control valve can accurately move the valve to any position. Although most actuators are used for opening and closing valves, the design of today's actuators goes far beyond simple switching functions. They include position sensing devices, torque sensing devices, electrode protection devices, logic control devices, digital communication modules, and PID control modules, all of which are installed in a compact housing.

As more and more factories adopt automation control, manual operations are replaced by mechanical or automated equipment. People demand that the actuator can play an interface role between the control system and the mechanical movement of the valve, and also require the actuator to enhance work safety performance and environmental protection performance. In some hazardous situations, automated actuator devices can reduce personnel injuries. Some special valves require emergency opening or closing in special circumstances, and the regulating valve actuator can prevent further spread of danger while minimizing factory losses. For some high-pressure large-diameter valves, the required output torque of the actuator is very large. In this case, the required actuator must improve mechanical efficiency and use a high output motor to operate the large-diameter valve smoothly.

Valves and Automation

In order to successfully achieve process automation, the most important thing is to ensure that the valve itself can meet the special requirements of the process and the medium inside the pipeline. The production process and process medium usually determine the type of valve, the type of valve core, as well as the structure and materials of the valve internals and valves.

After selecting the valve, the next step is to consider the requirements of automation, that is, the selection of the actuator. The actuator can be considered simply based on two basic types of valve operation.

1. Rotary valve (single turn valve)

This type of valve includes: plug valves, ball valves, butterfly valves, and dampers or baffles. This type of valve requires an actuator that can rotate 90 degrees with the required torque

2. Multi turn valve

This type of valve can be a non rotating lifting stem or a rotating non lifting stem, or they require multiple rotations to drive the valve to the open or closed position. This type of valve includes: straight through valve (globe valve), gate valve, knife gate valve, etc. As an option, pneumatic or hydraulic cylinders or membrane actuators with linear output are also used to drive the aforementioned valves.

There are currently four types of actuators that can use different driving energy sources and operate various types of valves.

1. Electric multi turn actuator

Electric driven multi turn actuators are one of the most commonly used and reliable types of actuators. Using a single-phase or three-phase motor to drive gears or worm gears, and finally driving the valve stem nut, the valve stem nut causes the valve stem to move and open or close the valve.

Multi turn electric actuators can quickly drive large-sized valves. In order to protect the valve from damage, the limit switch installed at the end of the valve stroke will cut off the motor power supply. At the same time, when the safety torque is exceeded, the torque sensing device will also cut off the motor power supply. The position switch is used to indicate the on/off status of the valve. The handwheel mechanism installed with the clutch device can manually operate the valve in case of power failure.

The main advantage of this type of actuator is that all components are installed in one housing, which integrates all basic and advanced functions within this waterproof, dustproof, and explosion-proof shell. The main disadvantage is that in the event of a power failure, the valve can only remain in its original position, and only with the use of a backup power system can the valve achieve a fail safe position (fail open or fail closed)

2. Electric single rotary actuator

This type of actuator is similar to an electric multi turn actuator, with the main difference being that the final output of the actuator is a motion of 1/4 turn to 90 degrees. The new generation of electric single turn actuators combines the complex functions of most multi turn actuators, such as parameter setting and diagnostic functions using a non intrusive user-friendly interface.

The single turn actuator has a compact structure and can be installed on small-sized valves, with a typical output torque of up to 800 kilograms per meter. Additionally, due to the small required power supply, they can be equipped with batteries to achieve safe operation in case of failure.

3. Fluid driven multi turn or linear output actuators

This type of actuator is often used to operate straight through valves (globe valves) and gate valves, which use pneumatic or hydraulic operation methods. The structure is simple, the work is reliable, and it is easy to implement a fault safe operation mode.

Usually, people use electric multi turn actuators to drive gate valves and globe valves, and only consider using hydraulic or pneumatic actuators when there is no power supply.

4. Fluid driven single rotary actuator

Pneumatic and hydraulic single turn actuators are very versatile, they do not require power and have a simple structure with reliable performance. They have a wide range of applications. Usually, the output ranges from a few kilograms of meters to tens of thousands of kilograms of meters. They use cylinders and transmission devices to convert linear motion into right angle output. The transmission devices usually include forks, gears and racks, and levers. Gears and racks output the same torque throughout the entire stroke range, making them highly suitable for small-sized valves. Forks have high efficiency and high torque output at the beginning of the stroke, making them ideal for large-diameter valves. Pneumatic actuators are generally equipped with accessories such as solenoid valves, positioners, or position switches to control and monitor valves.

This type of actuator is easy to implement a fail safe operating mode

Elements for selecting executing agencies

When selecting a suitable valve actuator type and specification, the following factors must be considered:

1. Drive energy

The most commonly used driving energy source is a power source or a fluid source. If a power source is chosen as the driving energy source, a three-phase power source is generally used for large-sized valves, while a single-phase power source can be used for small-sized valves. Generally, electric actuators can have multiple types of power sources to choose from. Sometimes DC power supply is also optional, and in this case, power failure safe operation can be achieved by installing batteries. There are many types of fluid sources. Firstly, they can be different media such as compressed air, nitrogen, natural gas, hydraulic fluids, etc. Secondly, they can have various pressures. Thirdly, the actuator has various sizes to provide output force and torque.

2. Valve type

When choosing a valve actuator, it is necessary to know the type of valve in order to select the correct actuator type. Some valves require multi turn drive, some require single turn drive, and some require reciprocating drive, which affect the selection of actuator types. Usually, multi turn pneumatic actuators are more expensive than electric multi turn actuators, but reciprocating linear output pneumatic actuators are cheaper than electric multi turn actuators.

3. Torque magnitude

For valves with a 90 degree rotation, such as ball valves, disc valves, and plug valves, it is best to obtain the corresponding valve torque from the valve manufacturer in Wozhong. Most valve manufacturers test the operating torque required by the valve at rated pressure and provide this torque to customers. For multi turn valves, the situation is different. These valves can be divided into: reciprocating (lift) motion - valve stem non rotation, reciprocating motion - valve stem rotation, non reciprocating - valve stem rotation. The diameter of the valve stem must be measured, and the size of the valve stem connection thread determines the actuator specification.

4. Selection of execution mechanism

Once the type of actuator and the required driving torque for the valve are determined, the data sheet or selection software provided by the actuator manufacturer can be used for selection. Sometimes the speed and frequency of valve operation also need to be considered.

The fluid driven actuator can adjust the stroke speed, but the electric actuator with three-phase power supply only has a fixed stroke time.

Some small-sized DC electric single turn actuators can adjust their travel speed.

switch control

The biggest advantage of automatic control valves is that they can be operated remotely, which means that operators can sit in the control room to control the production process without the need to physically operate the valve on site. People only need to lay some pipelines to connect the control room and the actuator, and the driving energy directly excites the electric or pneumatic actuator through the pipeline, usually using a 4-20mA signal to feedback the position of the valve.

continuous control

If the actuator is required to control parameters such as liquid level, flow rate, or pressure in the process system, which requires frequent action of the actuator, a 4-20mA signal can be used as the control signal, but this signal may change as frequently as the process. If a very high-frequency actuator is required, only select a special adjustable actuator that can start and stop frequently. When multiple actuators are required in a process, digital communication systems can be used to connect them together, which can greatly reduce installation costs. Digital communication circuits can quickly and efficiently transmit instructions and collect information. There are currently multiple communication methods available, such as FOUNDATION FILDBU, SPROFIBUS, DEVICENET, HART, and PAKSCAN designed specifically for valve actuators. Digital communication systems can not only reduce investment costs, but also collect a large amount of valve information, which is very valuable for predictive maintenance programs of valves.

predictive maintenance

Operators can use the built-in data storage to record the data measured by the torque sensing device during each action of the valve. This data can be used to monitor the operation status of the valve, prompt whether the valve needs maintenance, and diagnose the valve.

The following data can be diagnosed for valves:

1. Friction force of valve sealing or packing

2. Friction torque of valve stem and valve bearing

3. Friction force of valve seat

4. Friction during valve operation

5. The dynamic force acting on the valve core

6. Friction force of valve stem thread

7. Valve stem position

Most of the above data exists for all types of valves, but the emphasis is different. For example, for butterfly valves, the frictional force during valve operation can be ignored, but for plug valves, this force value is very large. Different valves have different torque operating curves. For example, for wedge valves, the opening and closing torques are very large, while during other strokes, only the friction force of the packing and thread is present. When closing, the hydrostatic pressure acts on the gate plate, increasing the friction force of the valve seat. Ultimately, the wedge effect causes the torque to rapidly increase until it is fully closed. So, based on the changes in the torque curve, it is possible to predict the faults that will occur, which can provide valuable information for predictive maintenance.