What is a valve class rating?

Key Points About Valve Class Ratings:

1. Standards: Valve class ratings are defined by international standards, such as:
   • ASME B16.34 for pressure-temperature ratings of valves.
   • API standards for specific valve types in industrial applications.
2. Unit of Measure: The rating is expressed as a numerical class, such as Class 150, Class 300, Class 600, etc. These numbers represent the pressure in psi (pounds per square inch) at a reference temperature, typically around 100°F (38°C).
3. Class Groupings: There are 3 kinds of class ratings, standard, uncommon, and restricted classes which refer to how commonly used or available these pressure-temperature ratings are for valves. These classifications help engineers and purchasers understand the availability, cost, and application scope of different valve classes.
4. Pressure-Temperature Relationship: As the temperature increases, the pressure a valve can safely handle typically decreases. Manufacturers provide pressure-temperature charts for their specific products to clarify these limits for each class rating.
5. Material Dependence: The actual pressure a valve can withstand also depends on the material of construction. For instance:
   • Carbon steel valves can handle higher pressures than cast iron valves for the same class.
   • Stainless steel or other alloys may provide better performance in high-temperature or corrosive environments.

Example:

• A Class 150 valve made of carbon steel can handle up to:
  • ~285 psi at 100°F
  • ~20 psi at 800°F
• A Class 300 valve of the same material can handle up to:
  • ~740 psi at 100°F
  • ~100 psi at 800°F

Why It’s Important:

Having or providing a valve class rating helps engineers and technicians in a few ways:

• To narrow down the vast variety of valve product options and more easily select the correct valve for specific process conditions.
• They help to ensure safety and reliability in applications involving high pressures or extreme temperatures.
• And finally, having well defined class ratings helps manufacturers and end users alike to more easily comply with regulatory and industry standards.