- Published on Monday, 06 May 2013 16:56
- Written by Paul Major
Contrary to what might be expected, valve design in the power industry is not necessarily driven by codes and standards. So what is it, then, that sends engineers to the drawing board?
In a recent presentation at VMA’s 2013 Technical Seminar on valve and actuator trends, Paul Major of Velan Valve Corporation answered that question while illustrating the importance of codes and standards for the smooth, safe operation of power plants. Following is a summary of the key points Major made during his presentation.
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.
Standards are developed by an organization or committee that usually involves all the affected parties, including manufacturers, contractors and users. Revised every few years, standards are used to ensure interchangeability of components such as valves, bolts, screws and nuts, flanges, fittings, piping, welding and threads.
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.
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.
A prime example for the power industry is the ASME Boiler and Pressure Vessel Code, which has rules that cover most of the components in a power plant. ASME was formed in 1880 in response to widespread steam boiler explosions in the 19th century. The first ASME code issued in 1884 was for the testing of boilers, and the first version of what was to become the ASME Boiler and Pressure Vessel Code was published in 1915. It was 114 pages long. This code is now law in the United States, Canada and more than 60 countries.
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
Code Cases (BPVC and Nuclear) – Code case 1621, which became N-62, was first introduced in 1974 in response to concerns about non-pressure retaining parts involved in a nuclear power plant including stems, seats, discs, springs and yokes. This code case gave increased allowable stresses for use in these components for certain materials.
While there are requirements for valves in various sections of the codes, they are still not the primary drivers for valve innovation.
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.
Most valve specifications for the power industry have ASME B16.34 near the top of the list. If the valve is flanged, ASME B16.5 is listed. API 598 is usually invoked for testing, and the valve is fairly well defined by these and the referenced standards. The same valve may have been supplied for decades and would meet all the requirements with very little modification and these changes often are only adding accessories.
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.
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.
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.