: Effective surface area( m 2) on which the pressure acts. This area varies according to the valve type( e. g. the area of the disc of a butterfly valve, the area of the sphere of a ball valve). π www. valve-world. net Valve World August 2025 43
Fluid control
Important note:
• Torque is a vector quantity. That is, it has both magnitude and direction. The direction is found using the right-hand rule.
• The greatest torque is obtained when the force is applied perpendicular to the position vector( θ = 90 °, sin( 90 °) = 1). If the force passes through the axis of rotation or is parallel to the position vector( θ = 0 ° or θ = 180 °, sin( 0 °)= sin( 180 °) = 0), the torque is zero and there is no turning effect.
Torque calculation in valves
Torque calculation in valves means determining the rotational force required to open or close the valve. This calculation is vital for the correct selection of actuators, especially those used to automatically control valves. Choosing an actuator with the wrong torque value can lead to improper valve operation, premature failure or problems in the system.
Factors affecting torque in valves
There are many factors that affect the torque required to open or close a valve:
• Valve size( diameter): Generally, larger diameter valves require more torque because they have a larger surface area in contact with the fluid.
• Fluid pressure: As the fluid pressure inside the valve increases, the force exerted on the valve flap or ball also increases, creating a greater torque requirement to move the valve.
• Viscosity of the fluid: More viscous( denser) fluids( such as oils) can create more resistance on the moving parts of the valve, increasing the torque requirement.
• Valve type: Since the operating principles and internal structures of different valve types( ball valves, butterfly valves, gate valves, globe valves, etc.) are different, their torque requirements also vary. For example, quarter-turn valves such as butterfly valves or globe valves exhibit different torque characteristics than linear motion valves such as gate valves.
• Seal material( seal type): The material and design of the seals affect the coefficients of friction. High friction seals may require more torque to move the valve. For example, a valve with a rubber seal may require less torque than a valve with a metal seal.
• Friction: Friction between moving parts within the valve( bearings, seals, etc.) directly affects torque. Dirt, corrosion, or inadequate lubrication can increase friction.
• Temperature: High temperatures can change the properties of some materials, which can affect the torque requirement.
• Operating conditions: Environmental factors such as how often the valve opens and closes, vibration and impact can also play an indirect role in torque calculations.
Components and situational variability of valve torque
Valve torque is not a fixed value; it is generally divided into three basic components( Crane, 2018):
While only mechanical frictions are measured prior to assembly( idle / dry testing), once assembly and flow come into play, torque requirements increase due to flow forces and hydrodynamic effects( ISA, 2020).
Torque calculation in valves
Torque calculations for valves are often done using complex engineering formulas and experimental data. Specific torque values for each valve type and size are provided by valve manufacturers. These values are obtained by testing the valve under specific operating conditions. In general, the opening and closing torque of a valve consists of two main components, namely static torque and dynamic torque: 1. Static torque( starting torque): The friction force that the valve must overcome after being closed for a long time or at the first moment of movement. In particular, conditions such as seals sticking to the valve or solidification of the fluid can increase the static torque. This is usually the highest torque value required to move the valve.
2. Dynamic torque( operating torque): The torque required when the valve is in motion( opening or closing). This is usually lower than the static torque because friction decreases once motion has begun.
When selecting an actuator, the static torque value, which is the highest torque requirement of the valve, is taken as a basis. Usually, a safety factor is added to the valve torque value to select the actuator, which can usually be 25 % or more of the valve torque value. This allows for an allowance for possible unexpected friction increases, fluid viscosity changes or corrosion. Valve manufacturers usually specify“ opening-closing torque” values( in Nm) for certain diameters and pressures in their catalogues or technical data sheets. These values are used directly when selecting an actuator.
Torque Component Explanation Affecting Factors
Starting Torque
Working( Flow) Torque
Seating Torque
The torque required to initially move the valve.
The torque developed as the valve moves during flow.
The torque required to keep the valve fully closed or fully open.
Seal compression, friction forces
Flow pressure, velocity and fluid forces
Fluid pressure, sealing force
General approach for torque calculation in valves
Valve manufacturers determine the exact torque values by performing detailed tests for their specific valve designs and materials used. These values are usually presented as a series of curves or tables and are used for actuator selection. However, to roughly express the main factors that affect the torque required by a valve, we can consider the following equation:
Let’ s explain this equation with its components:
τ valve
: Total torque( Nm) required to open or close the valve.
F pressure
: The force( N) exerted by the fluid on the valve flap or ball. This force is directly related to the pressure inside the valve and the surface area of the valve flap or ball. It can generally be expressed as:
P: Fluid pressure in the valve( Pa or N / m 2).
A effective
: Effective surface area( m 2) on which the pressure acts. This area varies according to the valve type( e. g. the area of the disc of a butterfly valve, the area of the sphere of a ball valve). π www. valve-world. net Valve World August 2025 43