Valve Torque: Key Calculation, Factors and Practical Use

In industrial production, valves are indispensable components in piping systems, responsible for controlling the flow of fluids. Valve torque, as a key parameter in the operation of valves, plays a crucial role in ensuring the normal operation of valves and selecting appropriate actuation devices. This article will delve into the calculation methods, influencing factors, and practical significance of valve torque to help readers better understand and grasp this important concept.

Valve torque refers to the rotational force required to be applied to the valve actuation device (such as a handwheel, electric actuator, or pneumatic actuator) to open or close the valve core (e.g., gate, butterfly plate, or ball in a ball valve). It is primarily generated by the following factors.

The sealing structure of a valve requires a certain sealing specific pressure between the valve core and the seat to ensure that the valve does not leak when closed. When opening the valve, it is necessary to overcome the frictional torque generated by this sealing specific pressure. For example, in a gate valve, the sealing surfaces of the gate and the seat are tightly fitted when closed. To open the gate valve, one must overcome the friction between the gate and the seat, and the torque corresponding to this force is part of the opening torque.

In pressurized piping systems, the medium pressure acts on the valve core, generating a force that keeps the valve core closed or hinders its movement. For example, in a closed ball valve, the medium pressure in the pipeline presses the ball tightly against the seat. To open the ball valve, it is necessary to overcome not only the friction between the ball and the seat but also the resistance torque generated by the medium pressure on the ball, allowing the ball to rotate and the valve to open.

There is friction between the valve core, valve stem, packing, and other components. For example, the friction between the valve stem and the packing: during valve operation, the valve stem needs to move up and down or rotate, and the packing exerts friction on the valve stem, which also needs to be overcome by applying a certain torque.

The calculation of valve torque is essential for ensuring the normal operation of valves and selecting the appropriate actuation devices. Here are the calculation methods to help readers better understand and apply them.

The formula for calculating valve torque is as follows: First, calculate the area of the valve plate by squaring half of the valve diameter and multiplying by 3.14. Then, multiply this area by the working pressure to obtain the static pressure on the shaft. Next, multiply by the friction coefficient (for general steel, the friction coefficient is 0.1; for steel against rubber, it is 0.15), and then multiply by the shaft diameter divided by 1000 to get the valve torque in newton-meters. For electric actuators and pneumatic actuators, the safety factor is 1.5 times the calculated valve torque.

When designing a valve, the selection of the actuator is based on estimation, which is divided into three parts: the frictional torque between the sealing elements (e.g., ball and seat), the frictional torque of the packing on the valve stem, and the frictional torque of the bearing on the valve stem. Therefore, the calculation pressure is generally taken as 0.6 times the nominal pressure (approximately the working pressure), and the friction coefficient is determined based on the material. The calculated torque is then multiplied by 1.3 to 1.5 times to select the actuator.

Although the calculated value of valve torque is highly significant for reference, it should not be applied rigidly. This is because there are too many types of valve plates, seats, and packing, each with different frictional forces, as well as varying contact areas and clamping degrees. Therefore, in practice, measurement with instruments is generally used instead of calculation. Under the influence of many factors, the calculated valve torque is not as precise as the results obtained from experiments.

The magnitude of valve torque is not fixed and is influenced by a combination of various factors. Understanding these factors helps to more accurately calculate and select the appropriate valve torque, thereby ensuring the reliability and safety of valves in actual working conditions. The main factors are as follows.

Different types of valves have significantly different torque characteristics. For example, the torque of a gate valve is mainly used to overcome the friction between the gate and the seat and the medium pressure, with a relatively large and steady torque during the opening process. In contrast, the torque of a butterfly valve is relatively small at the beginning of opening and closing, but it increases rapidly when approaching the closed position due to the sealing effect between the butterfly plate and the seat.

Generally, the larger the nominal diameter of the valve, the greater the required torque. This is because larger-diameter valves have a larger core area, greater medium pressure, and higher friction on the sealing surface. For instance, a large-diameter ball valve with DN1000 requires much greater opening and closing torque than a small-diameter ball valve with DN100 under the same pressure and working conditions.

The higher the medium pressure in the pipeline, the greater the pressure difference the valve has to withstand, and the greater the resistance that needs to be overcome during valve operation, resulting in a larger torque. For example, a valve in a pipeline with a pressure rating of 10MPa has to withstand much greater pressure than a valve in a pipeline with a pressure rating of 1MPa, and its required operating torque also increases accordingly.

Different sealing types, such as soft sealing and hard sealing, have different torque requirements. Soft-sealed valves generally have better elasticity in their sealing materials, lower sealing specific pressure, and thus require less torque. In contrast, hard-sealed valves typically have metal-to-metal contact on the sealing surface, higher sealing specific pressure, and larger operating torque.

Valve torque, as a core parameter in valve operation, is significant throughout the entire process of valve design, selection, installation, operation, and maintenance. Accurate understanding and rational application of valve torque are indispensable for ensuring the safe and efficient operation of industrial systems.

Accurate calculation and understanding of valve torque are key to selecting the appropriate valve actuation devices. Only by choosing an actuation device with matching torque can the valve reliably open and close under normal working conditions, avoiding situations where insufficient torque prevents valve operation or excessive torque causes damage to the actuation device and valve components.

In the design and operation of piping systems, understanding valve torque helps ensure system safety. If valve torque is not accurately calculated, it may lead to valves not functioning properly during system operation, causing media leakage, overpressure in pipelines, and other safety accidents.

The opening torque (torque) is also an important indicator for measuring the quality of a valve product. In the piping valve standards of some advanced industrial countries, it is used as one of the assessment criteria, stipulating that the opening torque of manual valves should not exceed 360N•m. If this torque is exceeded, it is necessary to consider selecting an appropriate actuation device (such as electric, pneumatic, or hydraulic). The size of the opening torque reflects the ease of valve operation, and people often describe the quality of a valve by its easy and flexible opening.

Understanding the operating torque characteristics of different types of valves helps to better select and use them, ensuring their efficient operation in actual working conditions. Here is an analysis of the operating torque characteristics of several common valves.

When the valve opening is above 10%, the axial force of the valve, i.e., the operating torque of the valve, does not change significantly. When the valve opening is below 10%, due to the throttling of the fluid, the pressure difference before and after the gate valve increases. This pressure difference acts on the gate, requiring a larger axial force on the valve stem to move the gate, so the change in valve operating torque is relatively large within this range.

When a globe valve is opened, the valve disc reaches a certain proportion of the nominal diameter of the valve, the flow has already reached its maximum, indicating that the valve has reached the fully open position. Therefore, the fully open position of the globe valve should be determined by the travel of the valve disc. The situation when the globe valve is closed and when it is opened again after being tightly closed is similar to that of a forced-seal gate valve. Hence, the closed position of the valve should be determined by the increase in operating torque to the specified value.

The operating torque characteristic curve of a butterfly valve is high in the middle and low at both ends. This phenomenon occurs because when the butterfly valve is in the middle position, the fluid is obstructed by the butterfly plate and flows around it, creating vortices on both sides of the plate. These vortices exert a hydrodynamic torque on the plate, which tends to close the plate. As the butterfly plate opens or closes, the influence of the vortices decreases, and when the vortices disappear, the resistance on the plate also decreases, resulting in a characteristic curve that is high in the middle and low at both ends.

The operating torque characteristic curve of a ball valve is very similar to that of a butterfly valve, also due to the influence of vortices caused by the change in fluid direction within the ball. The influence of these vortices gradually decreases as the valve opens or closes. The ball valve rotates 90 degrees from fully open to fully closed, and mechanical stops are required. The open and closed positions of the ball valve should be determined by the rotation angle of the valve stem, so the ball valve is positioned by travel.

Valve torque is a key parameter in valve operation, influenced by various factors such as valve type, size, medium pressure, and sealing type. Accurate calculation and understanding of valve torque are of great significance for selecting the appropriate actuation devices, ensuring the safe operation of piping systems, and measuring valve quality. Although it can be calculated using mechanical formulas, experimental measurement often provides more accurate results in practical applications due to the presence of various complex factors. It is hoped that this article will help readers better understand and grasp the knowledge of valve torque and make more rational decisions in their actual work.