Leverage ASME classes to reduce valve cost by over 50%

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 high-pressure weld end valves, this can lead to unnecessary costs. This article explores a lesser-known ASME provision that allows safe, cost-effective valve re-rating using design safety margins and NDE.

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.

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.

Table 10. For ASME Standard CL1500 Valves⁽¹⁾

Figure 2: This ASME Standard CL1500 valve chart shows the maximum allowable pressure and temperature ratings of Fisher CL1500 valves made of different materials.

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.

In addition to ASME Standard Class valves, there are also ASME Special Class, ASME Intermediate Class and ASME Special Intermediate Classes available. Each of those options are discussed below.

Table 12. For ASME Special CL1500 Valves⁽¹⁾

Figure 4: While ASME Special Class valves have the same pressure rating as Standard Class valves at ambient temperatures, they have elevated pressure ratings at higher temperatures.

Table 6. NPS 4 CL1500 HP Series Valves with CL2017 Intermediate Rating

Figure 4: While ASME Special Class valves have the same pressure rating as Standard Class valves at ambient temperatures, they have elevated pressure ratings at higher temperatures.

ASME Special Class pressure ratings

In the case of weld end valves, some valve manufacturers offer ASME Special Class ratings. The ASME Standard Class ratings assume a base level of material and casting quality, which results in relatively conservative pressure and temperature ratings. However, most manufacturers can perform NDE testing on a specific valve body, and based on these test results, the ASME Code allows the manufacturer to rate the valve at higher pressures at elevated temperatures, even though the base level of material and casting quality does not change. For example, one can compare the Standard Class CL1500 chart in Figure 2 with the Special Class CL1500 chart in Figure 4. At ambient temperatures, both the Standard Class and Special Class valves have the same pressure ratings. But at higher process temperatures, the Special Class pressure ratings elevate considerably. A Standard Class CL1500 WC9 (carbon steel) valve is rated for 2245 PSI at 900 °F. The Special Class CL1500 WC9 valve is rated for 3000 PSI at that same temperature. If Special Class pressure ratings exceed the process requirements, the user can pay a relatively small extra charge for NDE testing, and they can then use a Special Class CL1500 valve instead of the much larger and more expensive Standard Class CL2500 valve. This saves significant money on the valve purchase itself, as well as reducing valve weight, space requirements and overall installation costs. Sometimes the Special Class pressure rating is still not enough to meet the process needs. In this case the user can consider an ASME Intermediate Class valve.

ASME Intermediate Class pressure ratings

Certain weld end valve body designs incorporate valve wall thickness exceeding that required by code. In these cases, some manufacturers can certify these valves to a higher-pressure class rating than Standard Classes. The specific pressure rating will depend upon the valve itself, as well as the material of construction, and the resulting pressure class will usually fall between the Standard Class ratings.

For instance, a Fisher HP series WC9 CL1500 valve could carry a CL2017 Intermediate rating. As Figure 5 shows, the pressure ratings at elevated temperatures may not be much higher than the Special Class CL1500 valve—in this case the Intermediate Class CL1500 rating at 900 °F is 3020 PSI versus 3000 PSI for the Special Class valve. However, there is a significant difference at ambient temperatures. As the chart indicates, the Intermediate Class CL2017 valve is rated for 5045 PSI at ambient temperatures versus 3750 PSI for the Standard and Special Class CL1500 valves.

An Intermediate Class valve does not require any additional NDE testing, but it is only offered in a limited range of weld end valve body styles. It does require the Standard Class pressure rating to be removed and replaced with a new pressure rating, so there is some additional cost, though less than the NDE testing and certification of a Special Class valve. Similar to the Special Class valve, an Intermediate Class alternative may present significant savings if it avoids having to move to the next pressure class rating.

ASME Intermediate Special Class pressure ratings

A third ASME approved pressure rating is the Intermediate Special Class. Essentially, this rating is a combination of the Special Class and Intermediate Class. Certain weld end bodies can be re-rated to an Intermediate Class, NDE testing can then be used to qualify for the Special Class rating as well. The net result is a valve that has pressure and temperature ratings higher than the Standard, Special and Intermediate ratings. While the cost for an Intermediate Special Class valve is higher than either the Special or Intermediate Classes, the ability to use this option can yield enormous savings, particularly for large valve body sizes.

Savings

Here are a few examples that illustrate the potential savings that can be achieved by taking advantage of the Special, Intermediate and Intermediate Special Class ratings. Note that these savings are only for the valve cost, and do not take into account additional savings realised by using these smaller and lighter valves.

• An application requires a 2” WC9 carbon steel valve at 1000 °F and 1428 PSIG.
  • A Standard Class CL2500 valve would typically be required for this situation costing about $20,000.
  • An Intermediate Class CL1694 valve would also work, costing about $14,000.
  • Therefore, an Intermediate Class valve saves about $6,000.
• An application requires a 6” WC9 carbon steel valve at 1000 °F and 1570 PSIG.
  • A Standard Class CL2500 valve would typically be required for this situation costing about $93,000.
  • An Intermediate Class CL1876 valve would also work, costing about $59,000.
  • Each Intermediate Class valve saves about $34,000.
• An application requires a 6” WC9 carbon steel valve at 1000 °F and 2030 PSIG.
  • A Standard Class CL2500 valve would be required for this situation costing about $93,000.
  • A Special Intermediate Class CL1876 valve would also work, costing about $59,000 + about $16,000 for additional NDE testing and certification.
  • Each Special Intermediate Class valve saves about $18,000.
• As previously mentioned, the savings escalate quickly with larger valves or special alloys. An application required a 16” x 12” Valve in C12A martensitic stainless steel suitable for 1085 °F and 655 PSIG.
  • Fisher had three options: quote an existing 16X12 2500# design, design a completely new 900# valve to meet the requirements, or see if our existing 600# design had any extra minimum wall thickness that would allow us to intermediate rate it, which it did.
    • The existing Standard Class CL2500 valve, cost about $510,000.
    • There was insufficient lead time to design a new CL900 valve.
    • An Intermediate Class CL688 valve would also work, costing about $217,000. Large sizes like this are typically in a valve manufacturer’s portfolio, and were in this case, but they may not be available in all sizes
  • The Intermediate Class valve saves about $293,000.

Conclusion

When specifying weld end valves at elevated pressures and temperatures, end users would be wise to consult their valve vendor about ASME approved Special Class, Intermediate Class and Special Intermediate Class options. As indicated by the examples above, there are certain situations where these alternative class ratings can safely meet the process design conditions, while saving significant cost in the initial valve purchase price, as well as generating additional cost reductions from the decreased weight and size of the valves themselves.

Figures all courtesy of Emerson

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This Technical Story is an article from our Valve World Magazine, June 2025 issue. To read other featured stories and many more articles, subscribe to our print magazine. Available in both print and digital formats. DIGITAL MAGAZINE SUBSCRIPTIONS ARE NOW FREE.

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