Ensuring Reliable Valve Operation: The Role of Torque Safety Factors in Actuator Sizing

Pneumatic rack and pinion actuators are pivotal in automating industrial valves, enabling precise control over fluid flow in various processes. When engineers select these actuators for specific valve applications, they must ensure they can deliver sufficient torque to operate the valve under all expected conditions. This requirement leads to incorporating a torque safety factor in the sizing calculations. Understanding why this safety factor is essential involves examining the variables that affect valve torque requirements and the operational reliability of the actuator-valve system.

The Critical Role of Proper Actuator Sizing

Actuator sizing is not merely matching nominal torque values between the actuator and the valve. Valves often experience conditions that can significantly increase the torque required to open or close them. If the actuator cannot provide the necessary torque under these conditions, the valve may fail to operate as intended, leading to process interruptions, safety hazards, or equipment damage. Therefore, engineers must account for various factors influencing torque requirements to ensure reliable and safe valve operation.

Factors Influencing Valve Torque Requirements

Several factors can cause the torque required to operate a valve to exceed the nominal values provided by the manufacturer:

• Friction and Wear: Over time, valves can experience increased friction due to wear, corrosion, or the accumulation of deposits on sealing surfaces. This friction directly translates to higher torque requirements. For instance, sediment buildup in a butterfly valve can make it harder to turn, necessitating more torque from the actuator.
• Temperature Variations: Temperature changes can affect the materials of both the valve and the actuator. Thermal expansion or contraction can alter clearances and increase friction. Additionally, temperature changes can affect the viscosity of the process fluid, making it harder to move the valve.
• Pressure Fluctuations: The differential pressure across a valve influences the torque needed to operate it. Unexpected increases in upstream or downstream pressure can create additional forces that the actuator must overcome. For example, in a ball valve, higher pressure can increase the seating force, thus requiring more torque to unseat the ball.
• Valve Seat Aging: The degradation of valve seats over time can lead to increased torque requirements. Aging seats may harden, lose elasticity, or become damaged, making the valve harder to operate.
• Unanticipated Loads: External factors such as pipeline stresses, misalignment, or mechanical interference can impose additional loads on the valve stem, increasing the torque needed for operation.
• Supply Pressure Variations: Pneumatic actuators rely on a supply of compressed air at a specific pressure. Fluctuations in this supply pressure can reduce the actuator's output torque. For instance, if the air supply drops below the design pressure, the actuator may not produce enough torque to operate the valve.

The Justification for a Torque Safety Factor

Given the many variables that can increase the torque required to operate a valve, engineers incorporate a torque safety factor into the actuator sizing process. This safety factor is a multiplier, usually expressed as a percentage, applied to the nominal valve torque to ensure the actuator can handle the maximum expected torque under all conditions.

The torque safety factor accounts for the following:

• Operational Uncertainties: It compensates for unknown or unpredictable factors that may affect valve torque requirements during the valve's service life.
• Manufacturing Variances: This section considers tolerances and variations in the manufacturing of valves and actuators, which can affect performance.
• Maintenance Practices: It allows for less-than-ideal maintenance conditions, such as valves not being serviced as regularly as required.
• Extreme Conditions: It ensures that the actuator can operate the valve during extreme conditions, such as emergency shutdowns or start-ups, where conditions may not be typical.

By applying a torque safety factor, engineers build a margin of safety that enhances the reliability and longevity of the valve-actuator assembly. This practice helps prevent operational failures, reduces downtime, and maintains safety standards within the industrial facility.

Incorporating a torque safety factor when sizing pneumatic rack and pinion actuators for industrial valves is a critical engineering practice. It accounts for the myriad factors that can increase torque requirements beyond nominal values and ensures that actuators can reliably operate valves under all anticipated conditions. By doing so, engineers safeguard against operational failures, enhance system reliability, and uphold safety in industrial processes.

UNITORQ
2150 Boggs Road
Suite 410
Duluth, GA 30096
https://unitorq.com
Phone: 770-446-7074
Fax: 770-447-1825

Understanding Operating Torque and Seating Torque

A butterfly valve is a type of quarter-turn valve that regulates fluid flow. It comprises a rotating disc in the middle of a pipe and an actuator to turn it parallel or perpendicular to the flow. The disc is attached to a stem operated by the actuator.

The two key aspects that play a crucial role in the operation and functionality of these valves are operating torque and seating torque:

• Operating Torque: This is the torque required to rotate the butterfly valve disc from the fully open position to the fully closed position (or vice versa). This torque is directly related to the fluid pressure and friction caused by the moving parts of the valve. It generally stays relatively constant throughout the open/close cycle, although it can fluctuate slightly depending on the valve's position and the fluid's flow rate and pressure.
• Seating Torque: Seating torque, also known as closing or breakout torque, is the additional torque needed to fully seat or unseat the valve (i.e., to move the disc from nearly fully closed to fully closed or from fully closed to slightly open). The seating torque is usually higher than the operating torque because it has to overcome additional forces like seal friction or pressure drop across the valve when it's nearly closed.

It's essential to distinguish between these two torque types when sizing a valve actuator for a few reasons:

• Avoiding Undersizing: An undersized actuator may not have sufficient torque to open or close the valve fully, mainly considering the valve's seating torque, which can cause a valve to be stuck in an undesirable position, potentially impacting process control and safety.
• Avoiding Oversizing: Conversely, if an actuator is too large, it could produce excessive torque that may damage the valve's seals or other components.
• Safety and Reliability: Actuators are typically sized with a safety factor to ensure reliable operation under various conditions (changes in pressure, temperature, etc.). Knowing the operating and seating torque allows for more precise sizing, contributing to the valve's overall safety, reliability, and lifespan.
• Efficiency and Cost: Correctly sizing an actuator contributes to energy efficiency, as an oversized actuator uses more energy than required. It also avoids unnecessary costs associated with purchasing and maintaining a larger actuator than needed.
• Adaptability: Having precise knowledge of operating and seating torque enables the actuator to adapt to possible changes in the process conditions (such as pressure, flow rate, or media composition), which may increase the torque requirements.

Understanding the operating and seating torque is essential for the optimal selection, operation, and maintenance of butterfly valves and their actuators.

UniTorq Actuators and Controls

2150 Boggs Road, Suite 410

Duluth, GA 30096

https://unitorq.com

Phone: (770) 446-7074

Fax: (770) 447-1825