STANDARDS
Many valve users rely on standard American Society of Mechanical Engineers( ASME) B16.34 pressure classes, often selecting valves based on material, pressure and temperature. However, for highpressure weld end valves, this can lead to unnecessary costs. This article explores a lesserknown ASME provision that allows safe, cost-effective valve re-rating using design safety margins and NDE.
Leverage ASME classes to reduce valve cost by over 50 %
Mark Nord, Emerson
Most users who specify valves are quite familiar with the standard pressure classes defined by ASME B16.34. In many cases, a standard valve pressure class is chosen based on the valve material and the maximum process pressure and temperature, and the valve is simply ordered. While this works well for lower pressure classes( CL600 and less) and most flanged valves, this method can result in far higher costs than are necessary when applied to weld end valves in higher pressure and temperature applications. This article discusses a less common section of the ASME B16.34 code that allows valve manufacturers, and purchasers, to take advantage of the large safety factors inherently built into some valve designs, along with non-destructive examination( NDE) testing to re-rate a standard valve to a higher pressure and temperature rating. In cases where process conditions exceed those offered by standard valve classes, the resulting reduction in cost, size and weight can be significant.
ASME Standard Class pressure ratings
Engineers who routinely select valves typically have a solid understanding of ASME standard pressure classes( Figure 1). As the chart shows, a carbon steel valve is capable of handling pressures significantly above the pressure class rating at ambient temperatures. However, as process temperatures rise, the maximum pressure rating falls. In the case of carbon steel, the maximum pressure drops precipitously above 800 ° F.
Using the published standard class pressure / temperature charts, such as shown in Figure 2, end users can quickly compare their process requirements to the maximum process pressure and temperature ratings, and then select an ASME Standard Class valve that will meet those specifications. For instance, this chart indicates that a CL1500 WC9( carbon steel) valve is rated for 3750 PSI at ambient temperature and 2245 PSI at 900 ° F. If the maximum process pressure was above 2245 PSI at 900 ° F, a higher-pressure class valve would be required. The difference in cost and weight between the same size valves in the lower pressure classes is minor. But as one jumps from CL900 to CL1500, and even to CL2500, the difference in valve cost, size and weight becomes very significant. For example, Figure 3 compares a 6” CL1500 valve to a 6” CL2500 valve. The valve cost and weight are nearly double, and the line take-out dimension stretches from 33” to 47”. In addition to the cost of the valve itself, the additional cost in pipe supports and the space required to fit the larger valve into the process piping adds to the overall cost escalation. This price difference is further magnified with larger valve sizes or non-standard alloys. Flanged valves generally fall under the ASME Standard Class ratings, so if a particular pressure class does not meet the design conditions, the user has little choice but to utilise a different material, if acceptable, or move up to the next pressure class. Fortunately, there are other ASME sanctioned alternatives when dealing with weld end valves.
About the author Mark Nord is the control valve solution architect for Emerson’ s Flow Controls Products in Marshalltown, Iowa. He is responsible for solving the most challenging control valve applications using his 35 years of experience in plant operation, control valve and steam conditioning applications. Nord holds a BSME degree from the University of North Dakota.
Figure 1: This chart shows the allowable pressure and temperature ratings of several standard pressure classes for carbon steel valves. As the temperature rises, the allowable pressure falls. For example, carbon steel tends to lose strength quickly above 800 ° F.
28 Valve World June 2025 www. valve-world. net