Driving Valve Design in the Power Industry

Standards

These are sets of rules used by the designer, manufacturer and end user to define a product. Originally they were needed because components made and used in one part of the country often did not match those used in other parts of the country. These standards, which now are largely taken for granted, are used by agreement, voluntarily, and help ensure adherence to code and regulatory requirements for construction and design, and only products that fully conform can be identified as such.

Organizations developing standards include ASME, API, ISO, NACE and MSS and the standards cover everything from types of valves required for specific processes to materials to wall thicknesses, pressure/temperature ratings and fire testing requirements.

While important to ensure compatibility and quality, because these standards tend to be stable for periods of time, they do not drive valve innovation.

Codes

A standard becomes a code when it has been adopted by one or more governmental bodies. They are enforceable by law and provide minimum requirements to ensure the safety of the public; they in turn may invoke other codes and standards.

The ASME Boiler and Pressure Vessel Code contains rules for the material, design, examination, fabrication, inspection and testing of boilers and pressure vessels. Of the 12 sections, the following are applicable for valve design and construction:

Section II – Materials (divided into four parts)

Part A for ferrous materials and Part B for non-ferrous materials, both of which outline the physical and chemical properties for all materials approved for use as pressure containing components.
Part C covers welding rods, electrodes and filler metals.
Part D goes into more details about the physical properties of the materials such as allowable stresses, stress intensities, yield strength, tensile strength, modulus of elasticity and thermal coefficients of expansion.

Section III – Nuclear Facilities Construction

This section has five divisions, 17 sub-sections and an appendix covering a variety of areas related to nuclear power plants. In 1968, the draft ASME Code for Pumps and Valves for Nuclear Power was created. It included pressure/temperature ratings and wall thicknesses and became part of the 1971 version of BPVC Section III.

Section V – Non-destructive examination

Section VIII – Rules for construction of pressure vessels

Section IX – Welding and brazing qualifications

While there are requirements for valves in various sections of the codes, they are still not the primary drivers for valve innovation.

Specifications

Specifications are generated by the end user and describe what the end user requires. They are passed through the procurement chain until they get to the valve manufacturer. Various sections describe the different requirements. In theory, specifications should describe exactly what the end user requires, but in practice many details often need to be finalized before the design and engineering can be completed, especially for nuclear valves.

These specifications will have references to codes and standards, but there will be specific requirements based on the end-user’s needs. The specifications should include the size, class, type of valve, construction materials, flow conditions, testing requirements, quality assurance requirements, painting/coating requirements and even packaging, crating and shipping requirements.

How Valve Design is Affected: A Nuclear Case

For North American valve manufacturers, most new requirements that require core design changes come from the nuclear industry, specifically driven through the Nuclear Regulatory Commission (NRC). NRC requirements are sent to the nuclear plant owners and operators who must comply with the regulations to keep their licenses.

Operating experience at nuclear power plants in the 1980s and 1990s revealed weaknesses in many activities, in particular, those associated with MOV (motor-operated valve) performance. Consequently, new requirements were developed after testing was undertaken.

Testing

Multi-utility-sponsored projects, individual U.S. nuclear utilities and valve manufacturers tested many valves, under a range of operating conditions, including blow-down. These tests showed potential common-cause failure mechanisms as a result of which multiple safety-related MOVs could become incapable of performing their safety functions under design basis conditions.

These tests resulted in new requirements, including the requirement that nuclear licensees develop and maintain a performance-based test and analysis program that demonstrates MOV design basis function capability over the life of the plant. Included in these programs are design basis reviews, valve and actuator sizing, switch setting criteria and periodic diagnostic performance testing and performance trending.

These new requirements have led to end users developing new specifications, which have in turn drive changes in valve design for nuclear plants, changes that then flow down to other industries.

Conclusions

While codes and standards are important for safety and ensuring compatibility, they do not have a large influence in new valve design innovation. Customer specifications are the drivers of change.

Paul Major is nuclear design manager, Velan Valve Corporation. (www.Velan.com). Reach him at paul.major@velan.com