The Underappreciated Mounting Kit
All too often, the mounting kit is considered trivial, an afterthought deemed a commodity within all the other specified control components that comprise a complete automated valve package.
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As a result, industry is taking formal steps to ensure safety, quality, consistency and increased uptime by considering the complete valve automation assembly, including its mounting. In 2011, the International Organization for Standardization (ISO) released the 12490 standard, Petroleum and Natural Gas Industries Mechanical Integrity and Sizing of Actuators and Mounting Kits for Pipeline Valves. The American Petroleum Institute (API) then released API-6DX: Standard for Actuator Sizing and Mounting Kits for Pipeline Valves in 2012. API 6DX and ISO 12490 are identical and together are referred to as American National Standards Institute (ANSI)/API 6DX-201/ISO 12490-2011.
These two standards contain dedicated sections geared specifically to the mechanical integrity of a mounting kit. Although they are technically written for pipeline valves, many other markets are adopting their requirements, and many valve and actuator manufacturers are using them as guidelines.
This article creates awareness of these standards and encourages readers to think twice about the mounting kit they are using in the field: Can they guarantee the end user that the mounting kit is engineered to withstand all the forces that can be applied without triggering premature valve failure?
FACTORS TO CONSIDER
The forces applied by the actuator drive the factors behind ensuring the mechanical integrity of the mounting kit. Section 7.6.1 of these standards states that the mounting kit should be designed to transfer all of the loads from the actuator to the valve, and to react to them—including loads of 1.1 or more times the maximum torque or thrust output, taking into account the following:
- For pneumatic/hydraulic actuators: The torque/thrust generated at maximum operating pressure or as limited by the relief valve or other pressure-limiting device
- For spring return actuators: The torque/thrust generated by maximum compressed spring force
- For electric actuators: The torque/thrust at stall condition or 100% torque/thrust switch setting
These standards also require that, when calculating the integrity of the mounting kit, sufficient factors of safety exist at 1.1 times the maximum torque/thrust. Section 7.6.1 specifically talks about tensile, shear, torsion and bearing stresses and, where applicable, welding strength.
It requires that tensile stresses in mounting kit components do not exceed 67% of specified minimum yield strength (SMYS) when delivering 1.1 or more times the maximum torque/thrust output. Shear, torsion and bearing stresses should not exceed the limits specified in American Society of Mechanical Engineers (ASME) Boiler & Pressure Vessel Code (BPVC) Section VIII:2004, Division 2, Part AD-132, except when design stress intensity values (Sm) is 67% of SMYS. Also, for all mounting kits, attention must be paid to deflection and strain. The standards say adherence to these allowable stress limits alone might not result in a functionally acceptable design for the actuated valve assembly. (7.6.2)
The standards also require that fillet welds are designed with a strength efficiency factor of 0.75 or better.
Although the actuator is a key factor in a mounting kit design, valve details cannot be ignored. Section 8.1.1 states that the purchaser of an actuator should ensure that the maximum allowable stem torque and maximum torque/thrust of actuator and valve are compared to ensure compatibility of the design. It is imperative this information is exchanged by the necessary entities during the quoting and specification stages. Other valve criteria includes: valve torque and/or thrust data, safety factor, and operating time and valve dimensions
ANSI/API 6DX-201/ISO 12490-2011 addresses other mounting kit details such as welding and fasteners by noting:
- For welding, repair welding, and structural welds (including mounting kit): They should be performed in accordance with procedures qualified to ANSI/American Welding Society (AWS) D1.1/D1.1M or an equivalent standard. Welders and welding operators are to be qualified in accordance with ANSI/AWS D1.1/D1.1M or an equivalent standard. (11.2)
- Also acceptable for structural welding are welding procedure specification and procedure qualification records, in accordance with procedures qualified to ISO 15607, ISO 15609 (all parts), ISO 15614-1 or ASME BPVC Section IX, as well as welders and welding operators qualified in accordance with ISO 9606-1, ASME BPVC Section IX or EN 287-1. (11.2)
- Visual inspection of welds should be in accordance with ANSI/AWS D1.1/D.1.1M or an equivalent standard. Parts should be subjected to a visual inspection in accordance with the manufacturer’s quality procedures. (12.1.3)
- For the fasteners, bolting in mounting kits should not be subjected to shear forces, unless specific exception is agreed upon. (7.6.2)
Bolted joint slip calculations are the best way to ensure the clamping force generates enough friction force to withstand the shear force generated by the maximum torque of the actuator. Adequate friction force will prevent the bolts from going into a shear loading condition.
ISO and API also provide general requirements and guidelines for mounting kits, such as:
The mounting kit design and manufacturing tolerance should ensure:
- Parallelism of the intermediate support mounting faces
- Concentricity of the pitch circle diameter (PCD) for bolting for intermediate support
- Alignment of the PCD, valve stem, coupling and the actuator drive. (7.6.2)
The mounting kit design should also consider:
- Installed orientation of the valve and actuator (Note here that valves installed with horizontal stems can require additional support, e.g., spigots, to ensure accurate alignment of valve and actuator during removal and refitting in field service.)
- External loading from environmental effects (e.g., wind, snow, seismic activity)
- Blast loading, if specified
- Frequency of cycling and speed of operation. (7.6.3)
In conclusion, every component in the valve assembly must be able to withstand the demands of the application. Users of these kits need to ensure they are using hardware engineered for valve automation so they don’t put themselves in a position of trying to explain to customers that valve problems, plant downtime or other consequences resulted from an inadequate mounting kit design.
This article highlights only a portion of the requirements found in ANSI/API 6DX-201/ISO 12490-2011. To obtain the full version of these standards, please purchase or download them online. VM
CITED REFERENCES AND KEY SECTIONS:
ANSI/API 6DX-201/ISO 12490-2011: Mechanical integrity and sizing of actuators and mounting kits for pipeline valves
Section 7: Design
Section 8: Sizing
Section 9: Instrumentation/Regulation
Section 10: Materials
Section 11: Welding
Section 12: Quality Control
Section 13: Testing
Section 14: Surface Protection
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