Principles and Design of Pilot-operated Control Valves

 
Pilot-operated control valve is a pivotal control valve in industrial automation due to their unique structure and efficient control method. The working principle of this control valve relies on the precise cooperation between the pilot valve and the main valve. This enables the pilot-operated control valve to have precise flow control and stable operational performance under various working conditions. In this article, we will delve into the working principle of pilot-operated control valves and analyze the key points of structural design to ensure the valve's superior performance under high temperature, high pressure, and complex application conditions.

Working Principle

 
The pilot-operated control valve utilizes the cooperation of the pilot valve and the main valve to achieve efficient and precise flow control. The working principle can be detailed by explaining the valve's closed and open states.

1. Closed State

 
When the valve needs to be closed, the actuator applies force through the valve stem to close the pilot valve. The closure of the pilot valve leads to the closure of the main valve. At this point, the medium flows through the guide ring and the gap between the main valve and the sleeve into the upper chamber of the sleeve, exerting pressure on the main valve to ensure the valve's tight closure.
 
Pilot valve closure: The closure of the pilot valve is achieved by the output force of the actuator, ensuring that the main valve also closes.
Main valve sealing: The sealing of the main valve depends on the output force of the actuator and the back pressure of the medium. The medium pressure is applied to the main valve through the gap between the main valve and the sleeve, ensuring its tight closure.

2. Open State

 
When the valve needs to be opened, the pilot valve is opened first. This is because the pilot valve adopts a balanced design, allowing it to operate stably under back pressure. The actuator first opens the pilot valve through the valve stem, allowing the medium to flow through the guide ring, the gap between the main valve and the sleeve, and the channel of the pilot valve seat to the downstream of the valve.
 
Medium flow and pressure balance: Since the flow capacity of the pilot valve seat is much greater than that of the guide ring and the gap between the main valve and the sleeve, the flow of the medium can quickly establish a pressure balance between the valve front and back. This allows the main valve to open with a smaller force.
Opening of the main valve: After the pressure balance is established, the main valve will also open following the opening of the pilot valve. Due to the large flow capacity of the pilot valve, the main valve can open more easily.

3. Top-to-Bottom Flow Structure

 
Pilot-operated control valves feature a top-to-bottom flow structure design, offering several advantages:
 
Pressure Balance: When the valve is closed, the medium pressure generates back pressure through the gap between the main valve and the sleeve, which helps seal the valve seat. Higher pressure improves the sealing effect of the valve.
Ease of Opening: When the valve needs to open, the pilot valve opens first. Its high flow capacity rapidly balances the main valve, allowing it to open with less force. This ensures quick response and good sealing performance.

Key Design Considerations of Pilot-operated Control Valve

 
The design of the pilot-operated control valve must address several key technical challenges while ensuring the valve's efficient and stable operation. The following is an analysis of its main structural design points.

1. Pilot Valve Seat Passage Design

 
Flow Capacity Requirements: The passage design of the pilot valve seat must ensure that its flow capacity is significantly greater than that of the guide ring and the gap between the main valve and the sleeve. Insufficient flow capacity can result in incomplete valve opening or difficulty in adjusting at low flow rates, affecting overall performance. The passage design should be optimized based on actual operating conditions and medium characteristics to meet flow requirements.
Opening Degree Control: The opening degree of the pilot valve should be controlled to less than 10% of the total valve opening. This control helps ensure precise adjustment and prevents flow control issues or operational instability due to excessive opening. Proper design allows for high-precision flow regulation and stable control performance.

2. Spring Design

 
Stiffness requirements: The design of the spring must ensure sufficient stiffness to resist fluctuations in medium pressure. Insufficient spring stiffness may cause vibration between the pilot valve and the main valve, affecting the stability and sealing effect of the valve. The design should consider the fluctuation range of medium pressure, the working environment of the valve, and service life to choose the appropriate spring stiffness.
Load and vibration control: The spring should also be designed to effectively control the load and vibration of the valve flap, ensuring smooth and reliable operation of the valve. The selection and configuration of the spring should match the operating frequency and working pressure of the valve.

3. Guide Ring Design

 
Stability improvement: The guide ring plays a role in improving the stability of the valve assembly in the design. Its design should ensure it can effectively limit changes in the flow capacity of the gap between the main valve and the sleeve caused by temperature changes. The precision and material selection of the guide ring are crucial for the stability of the valve, ensuring that the performance of the valve is not affected by gap changes under different working conditions.
Precise manufacturing: The guide ring needs to be precisely manufactured and assembled to avoid performance issues caused by processing errors or assembly problems. The design should also consider the wear resistance and corrosion resistance of the guide ring to extend its service life.

4. Stop Pin Design

 
Control Rotation Trend: The stop pin design controls the rotation trend of the pilot and main valves, preventing rotational effects caused by vortex flow. Effective stop pin design maintains the correct movement trajectory of the valve components, enhancing operational stability and response speed.
Fixation and Positioning: The stop pin should have sufficient strength and durability for long-term stable operation. The design needs to address fixation methods and positioning accuracy to ensure reliability under high pressure and flow conditions.

5. Spring Control

 
Up and down vibration control: The spring in the pilot-operated control valve is not only responsible for controlling the up and down vibration of the valve but also plays a role in adjusting the force balance when the valve opens and closes. The design should ensure that the spring can maintain stable performance under different operating conditions to prevent sealing failure or unstable operation due to vibration.
Force balance adjustment: The design of the spring should ensure it can provide the appropriate force balance, allowing the valve to operate smoothly during the opening and closing process. The stiffness and configuration of the spring should match the working pressure, medium characteristics, and operating frequency of the valve.
 
Pilot-operated control valves excel in high-temperature, high-pressure, and large-caliber applications with their superior sealing performance and high cut-off capability. The structural design not only considers the stability and control performance of the valve but also ensures reliability and long-term service life under various working conditions through precise component design and pressure balance mechanisms. Precise structural design and functional optimization make pilot-operated control valves the ideal choice for tight cut-off and high-precision control.