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 deflections 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 prohibit 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.


ISO 5211 (2017) – Industrial Valves–Part-Turn Actuator Attachments

ISO 12490 (2011) – Petroleum and Natural Gas Industries–Mechanical Integrity and Sizing of Actuators and Mounting Kits for Pipeline Valves

MSS SP-101 (2014) – Part-Turn Valve Actuator Attachment

API 6DX (2020) – Standard for Actuators and Mounting Kits for Valves

DIN EN 15081 (2008) – Industrial Valves–Mounting Kits for Part-Turn Valve Actuator Attachment

ISO 5115 (2023) – Industrial Valves — Part-Turn Valve Actuation


ISA 96.09.01_Final Draft – Quarter-Turn Actuators and Valves – Technical Specification – Mounting


ISO/FDIS 5640 – Industrial Valves — Mounting Kits for Part-Turn Valve Actuator Attachment


API 6DX – 2020

“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.”

ISO 5115 – 2023

“The intermediate support design shall be sufficiently strong by design and material selection to ensure no visibly discernible movement of the actuator from twisting or warping of the intermediate support during operation.”


ISO 12490 – 2011

“The mounting kit design and manufacturing tolerance shall ensure the following: Parallelism of the intermediate support mounting faces.”

API 6DX – 2020

“The mounting kit design shall ensure the following: Parallelism of the mounting spool mounting faces.”


ISO 12490 – 2011

“The mounting kit design and manufacturing tolerance shall ensure the following: Concentricity of the PCD of the bolting of the intermediate support and Alignment of the PCD, valve stem, coupling and the actuator drive.”

API 6DX – 2020

“The mounting kit design shall ensure the following: Concentricity of the bolting of the mounting spool and Alignment of the valve stem, coupling, and actuator drive.”


ISO 5115: 8.4 Fasteners

“Fasteners shall be manufactured to a specification with defined minimum mechanical property requirements.”

API 6DX – 2020: 5.6.3 Bolting of Mechanically Loaded Parts

“For a valve without an anti-rotation feature (e.g. dowel pin, or fitted bolts), the design shall  include a calculation of bolt tension to generate a clamping force (e.g. between the valve and actuator, between the valve and mounting kit, and between the actuator and mounting kit that is sufficient to prevent slippage of bolted connections”

ISA 96.09.01_Final Draft: 8.1 Slip-Critical Bolted Joint Calculations

“All Bolted connections within an assembly shall be slip-critical joints. Slip-critical joints rely on friction forces between the two connected surfaces rather than bolt shear. This is achieved by calculating the necessary tension to keep all bolts from slipping…”


ISO 5115: 8.2 Intermediate Support

“Intermediate support shall be designed to be capable of installation onto and removal from the valve without loosening or removal of pressure boundary bolting such as bonnet bolts, flange bolts, packing gland bolts, etc.”

Lockout Capabilities:

Another safety feature of mounting kits include the ability to mechanically lock the valve assembly in the safe position

Other Design Considerations:

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

About the Author

Chris Fabbri, a licensed Professional Engineer in Michigan, graduated with a Bachelor of Science in Mechanical Engineering (BSME) from Michigan Tech University in 2003, and worked in the following industries prior to starting his career at VanAire: precast concrete, automotive components, and polyurethane components.

In addition to his role as Product Engineer at VanAire and overseeing their Engineering and Design teams, Chris currently participates with the technical committees of several international organizations in the Valve Industry. e.g., MSS (Manufacturers Standardization Society), ISA (International Society of Automation), ISO (International Organization for Standardization), and API (American Petroleum Institute).