How Mounting Kits Impact Valve Safety and Reliability

A chain is only as strong as its weakest link. What is the weakest link in your chain? It might be the mounting kit.

By Chris Fabbri, PE Product Engineer – Vanaire, Inc.

For years, many have overlooked the importance of the mounting bracket and often called it “just a piece of bent metal” or “just a piece of square tubing.” Vanaire and other industry leaders understand that without the solid structure of a robust mounting bracket, the entire valve assembly is at risk for not functioning properly.

The design of the mounting bracket should consider not only the stresses and strains developed from the applied torque, but also deflection (angular twist or bending). Excessive de-flections could prevent the valve and actuator from functioning properly. “Deflections of the mounting kit shall not prevent the valve closure member from reaching the fully closed or fully open position or restrict actuator functionality” (API 6DX, 2020). Except for special circumstances (e.g., extended mounting heights with horizontal stem orientations, small bolting patterns, and so on), mounting kits should be able to be installed in any orientation without additional support.

As industries continue to implement global standards to increase safety, consistency, and reliability, elements such as mounting kits that may have once been considered commodity-type items are now moving to the forefront. ISO, API, MSS, ISA, WIB, and other international standard organizations have all identified the mounting kit of an automated valve assembly to be a critical component in the chain of elements and are creating new standards and increasing the technical requirements of current standards.

Inadequate Mounting Kit Implications

When a mounting kit is improperly engineered it can be responsible for failures within the automated valve assembly and even result in catastrophic failure. The most common failure types include:

• Sideloading the valve stem causing leakage
• Premature valve or actuator failure
• Inaccurate process measurements due to hysteresis
• Mounting kit component failures (bracket, coupling, fasteners, locking mechanism)
• Imperative Features of a Mounting Kit

A properly engineered mounting kit has multiple considerations that it needs to adhere to in order to function properly in the field, these include but are not limited to:

• Stiffness
• Parallelism
• Concentricity
• Fasteners

Imperative Feature: Stiffness

The bracket needs to be thick enough to prevent excessive levels of stress and deflection when transmitting the actuator torque and other loads. Excessive deflection in the mounting bracket, especially with a horizontally oriented valve stem, can result in the valve stem experiencing sideloading. Sideloading on the valve stem can facilitate premature packing failure and cause leaking from the valve stem.

Imperative Feature: Parallelism

In order to ensure that the valve stem and actuator drive are axially aligned, the mounting faces must be parallel. If they are not, the result could be an angular misalignment of the valve and actuator drives, which can cause sideloading and result in valve stem leakage.

Imperative Feature: Concentricity

In order to ensure that the valve stem and actuator drive are axially aligned, the bolting patterns on the valve and actuator side of the mounting bracket must be concentric. If they are not, the result could be a parallel misalignment of the valve and actuator drives, which can cause sideloading – leading to valve stem leakage.

Imperative Feature: Coupling Drives

The valve and actuator drives on the coupling (square, DD, keyed, and so on) must be axially aligned. Drives that are not axially aligned can introduce sideloading on the valve stem. The drives of the coupling must also be designed and manufactured with tolerances that allow them to be installed, but tight enough to minimize hysteresis. Excessive clearances in the drive connections can lead to inaccurate process measurements.

Imperative Feature: Fasteners

Fasteners are critical parts of the mounting kit and are components that are trusted to unfailingly connect the gear/actuator to the valve. The typical bolted joint for an actuated valve assembly may be referred to as a friction-type or slip-critical joint. This bolted joint relies on the frictional force between joint members generated by the fastener clamping force (fastener preload) to resist shear forces (e.g., actuator torque and weight). If enough clamping force is lost and the bolted joint begins to slip, multiple issues can arise. Misalignment of the drives prohibits the sideloading valve from being fully open and/or closed, and hysteresis due to unexpected movement are possible issues that could occur. For these reasons, fasteners in a mounting kit need to be manufactured to a standard that specifies minimum mechanical property requirements, so that a proper preload can be determined.

Other Design Considerations 

• Welding (if applicable)
o Vertical stem
o Horizontal stem
• Valve orientation
• Bracket height
• Material selection
• Surface protection
• Number of cycles
• Cycle speed
• Temperature
o Process
o Ambient

Final Words

When checking off the “mounting kit” box on the to-do list, users should contemplate the severe implications of choosing a mounting kit that is not designed with the imperative features in mind. The repercussions may not be immediately apparent to the client, but there will be longevity of the project in the years ahead and one less problem to solve in the future.

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