Why is Control Valve Actuator Bench Set Important ?

In the technical state of process control engineering, control valves are critical in regulating fluid flow, pressure, and other factors. Among the many technical criteria for control valves, the actuator bench set is sometimes ignored or misunderstanding. However, it is crucial to ensuring optimal actuator performance and valve operation. Understanding the bench set is critical to preventing actuator misalignment, inefficient valve performance, and other process problems.

In this post, we’ll explain what the control valve actuator bench set is, why it’s important, and how it influences system performance.

What is an Actuator Bench Set ?

The actuator bench set is an important calibration parameter in control valve actuators, particularly spring-and-diaphragm actuators. It is the particular pneumatic pressure range provided to the actuator to move the valve stem through its whole stroke, from fully closed to fully open. This calibration ensures that the actuator functions as intended under a variety of operating circumstances, and it is often performed during the assembly or testing phase, prior to installation in a pipeline or system.

The bench set pressure range accounts for various factors that can influence valve performance, such as:

• Static unbalance: The force caused by process pressure acting on the valve plug or disc.
• Seat loading: The force required to maintain a tight shut-off between the valve plug and seat.
• Packing friction: The resistance caused by the valve stem passing through the packing material.

The bench set is often specified as a range, such as 3-15 PSI, 6-30 PSI, or other unique numbers based on the process needs. For example, a bench set of 6-18 PSI indicates that the actuator starts moving at 6 PSI and finishes its entire stroke at 18 PSI. These settings ensure that the actuator spring is preloaded to resist any forces occurring on the valve stem, allowing for appropriate shut-off and consistent functioning across the stroke range.

The bench set prepares the actuator spring, ensuring the valve remains closed with proper seating force.

• When appropriate seating force occurs, the valve remains closed.
• It can overcome conflicting process forces during the starting phase and retain control during operation.
• Full valve travel is obtained within the operational pressure range, avoiding problems like as undertravel and overtravel.

The actuator bench set must be properly calibrated in order to improve valve performance, maintain process efficiency, and assure the control system’s safety and reliability.

How Actuator Bench Set Working ?

To understand the bench set, we must first recognize how control valve actuators operate. 

Most pneumatic actuators use Hooke’s Law, which relates the force applied to a spring to its displacement:

F = kx

Where:

• F = Force applied to the spring
• k = Spring constant
• x = Displacement of the spring

In pneumatic actuators, the relationship between the applied pressure (P) and the resultant force (F) can be expressed as:

F = P × A

Where:

• P = Actuator air pressure
• A = Effective diaphragm or piston area

By substituting this into Hooke’s Law, we get:

x = (P × A) ÷ k

This formula describes how the actuator’s spring compresses in response to applied air pressure. The bench set calibration ensures that the compression matches the necessary valve stem movement for proper valve operation.

Why is the Bench Set Important in Control Valve?

Ensures Proper Valve Shut-Off

The bench set measures the seating force of the valve plug against its seat when the valve is fully closed. This seating force is essential for ensuring a tight shut-off, particularly in applications where even small leakage may risk safety or process integrity.

Prevents Overtravel or Binding

An improperly adjusted bench set can lead to:

• Undertravel: The valve may never fully close, causing leakage.
• Overtravel: The valve may not fully open, restricting flow capacity.

Handles Process Pressure Forces

Process pressure acting on the valve plug or ball creates unbalanced forces that the actuator must overcome. A correctly calibrated bench set preloads the spring to counteract these forces, ensuring smooth operation under all conditions.

Improves Process Control Accuracy

A properly set bench range ensures precise valve movement in response to controller signals, maintaining the desired process control parameters.

Key Adjustments in Control Valve Bench Set

Calibrating the bench set of a control valve requires carrying out exact adjustments to ensure that the valve performs effectively and reliably within its intended pressure range.

The following adjustments are necessary to achieving accurate calibration:

Stem Connector Adjustment

The stem connector serves as the mechanical link between the actuator stem and the valve stem. Correct adjustment is crucial to allow full valve travel without binding or misalignment, which could compromise valve performance.

Issues with improper stem length adjustment:

• If the stem is set too long, the actuator diaphragm may bind at the upper end of its travel, limiting the valve’s ability to fully open.
• A stem that is too short can cause the diaphragm to bind at the lower end, preventing the valve from achieving a tight shut-off.

The stem connector needs to be accurately adjusted during calibration in order to prevent these problems. In order to ensure correct alignment and reduce wear on the valve components, the actuator should operate freely and smoothly across its whole range.

Spring Adjuster Calibration

An essential part of determining the bench set pressure is the spring adjuster.   This adjustment ensures that the actuator spring balances the opposing forces exerted on the valve stem, allowing the valve to function within the defined pneumatic signal range. Proper calibration is dependent on the type of actuator and valve action:

• In an air-to-open valve, the bench set pressure ensures the plug lifts off the seat at the lower end of the range (e.g., 3 PSI for a 3-15 PSI range).
• In an air-to-close valve, the spring loading ensures the plug fully seats at the lower end of the range.

For Air-to-Open Valve (Fail-Close)

• The spring adjuster is set so that the valve plug begins to lift off the seat at the lower end of the bench set range (e.g., 3 PSI in a 3-15 PSI system).
• As the pneumatic signal increases, the actuator overcomes the spring force to fully open the valve at the upper range (e.g., 15 PSI).

For Air-to-Close Valve (Fail-Open)

• The spring is adjusted to ensure the plug fully seats at the lower end of the bench set range.
• The pneumatic signal then compresses the spring as the pressure increases, allowing the valve to fully close at the specified upper limit.

Refer the below link to know the Fail Open Vs Fail Close valves

Common Bench Set Configurations

Air-to-Open Actuator Bench Set Example

Using a 7–15 PSI bench set with a 3–15 PSI air-to-open actuator:

• The actuator stem begins to move at 7 PSI, not 3 PSI.
• Full travel occurs at 15 PSI.
• The preload accounts for process pressure and ensures sufficient seating force when the valve is closed.

Air-to-Close Actuator Bench Set Example

For a 3-15 PSI air-to-close actuator with a bench set of 3-12 PSI:

• As soon as the actuator stem reaches a pressure of 3 psi, it starts to move.
• Full travel is achieved at 12 PSI.
• The preload ensures the valve can resist process forces when fully open.

During calibration, it’s essential to:

• Verify the valve’s stroke using a travel indicator.
• Ensure there is no excessive play or binding in the stem connector.
• Check the actuator’s response to varying signal pressures to confirm full travel occurs within the intended range.

Refer the below link for Control valve actuator bench set Adjustment

Impact of Incorrect Bench Set

Valve Leakage

A bench set that does not generate sufficient seating force can lead to valve leakage even when the valve is fully closed. This issue compromises process integrity and operational safety:

• Leakage can disrupt controlled process variables such as flow rate or pressure, leading to inefficiencies in production.
• In applications involving hazardous media, leakage may pose serious safety risks, including environmental contamination or equipment damage.

For instance, in air-to-close valves, an improper bench set might prevent the plug from fully sealing against the seat, allowing fluid to bypass the valve.

Click here for Control Valve Leakage Testing, Types, and Calculation Standards

Reduced Flow Capacity

An wrong bench set may delay the actuator from performing its complete travel, restricting the valve’s capacity to fully open or close. This limitation limits the valve’s flow capacity, which directly affects the process.

• A partially open valve can cause bottlenecks in the system, reducing overall production output.
• Flow modulation becomes less precise, making it challenging to maintain process stability.

For example, in air-to-open valves, insufficient bench set pressure may result in the valve failing to open completely, restricting maximum flow rates.

Mechanical Damage

Improper stem adjustment or inadequate preload on the actuator spring can cause mechanical stress or binding, leading to excessive wear and tear on valve components:

• Incorrect alignment of the actuator and valve stems may result in the diaphragm binding at the extremes of its travel range, causing unnecessary strain on the stem and actuator.
• An incorrectly calibrated spring may wear out faster due to repeated over-compression or insufficient preload.
• Excessive stress on the diaphragm can result in cracks, leaks, or rupture, reducing actuator reliability.

Refer the below link for the Control valve Bench setting and Calibration Procedure

Bench Set vs. Air Operating Range

The bench set and air operating range are often mistaken as interchangeable, but they serve distinct roles in the operation of control valves. Below is a comparative table to clarify their differences:

Click here for What is bench-set in control valve?

Bench Set in Rotary Valve Actuators

The bench set concept applies to rotary valve actuators, such as ball or butterfly valves, by adjusting the spring preload to counteract process forces like unbalanced torque from fluid pressure.

• Spring Preload is ensures sufficient force for proper valve seating and operation across the rotation range (typically 90°).
• Terminology is often referred to as initial compression or spring preloading by manufacturers.
• Purpose is to prevents leakage and ensures smooth operation by matching spring force to the actuator’s operating signal range.

Click here for Control Valves in Process Industries: A Collection of In-Depth Articles