Cold Working Pressure (CWP) Rating.

Valves are also rated by the CWP rating. This rating represents the maximum allowable working pressure at the ambient temperature to which the valve may be subjected in normal service. Sometimes it is referred to as cold rating. The pressure rating of a valve at 20 to 100⁰F (29 to 38⁰C), as listed in ASME B16.34 and other valve standards, is considered cold working pressure (CWP) or cold rating. CWP is also designated as the water-oilgas (WOG) rating. Valves marked with CWP or WOG rating are primarily intended for applications in which the flow medium is maintained at ambient temperature, such as, but not limited to, water, oil, and gas distribution and transmission systems.

NFPA Rating.

Valves to be used in fire protection systems in the United States are required to be rated for fire service. These valves are rated for 175 and 250 psi (1210 and 1725 kPa), and they are designated as UL (Underwriters Laboratories) listed and FM (Factory Mutual) approved. A valve rated for 175 psi (1210 kPa) for fire protection service may also be rated for higher pressure marked as CWP or Water Working Pressure (WWP) for general service. As such, a fire-rated valve may be used at pressures higher than fire service rating in applications termed as general service.

General Service Rating.

The general service rating may be considered to be the equivalent of the CWP rating. Some valve manufacturers assign two ratings, fire rating and general service rating, to a valve. A UL-listed and FM-approved valve rated for 175 psig (1210 kPa) may have a general service rating of 400 psig (2760 kPa) at ambient temperature. It means that this valve can be used in a fire protection system having a maximum allowable working pressure of 175 psi (1210 kPa) or less, whereas it can be used in other services termed as general services and be subjected to a maximum allowable working pressure of 400 psig (2760 kPa). One must classify the service in accordance with the code of construction, as required. If the service does not fall within the jurisdiction of a code, then prudent engineering judgment may be followed in selecting the valve.

Steam Working Pressure (SWP) Rating.

The SWP rating of a valve is intended to define the maximum working pressure corresponding to the steam temperature. A valve assigned a SWP rating must not be used in steam service at pressures and temperatures exceeding the rating.

Dual or Multiple Ratings.

A valve may be assigned one or more ratings by the valve manufacturer. Ratings assigned must be marked on the valve. The marking on the valve must be in accordance with the applicable valve standard or standards. A valve may comply with one or more valve standards. For example, a Class 600, NPS 4 (DN 100), butt-welding end, steel gate valve complying with ASME B16.34 may be marked as Class 800 valve in accordance with API Standard 603, provided the valve complies with the design and construction requirements of both ASME B16.34 and API 603. Some valve manufacturers may have proprietary design valves that are rated for specialty applications.

Dual- or multiple-rated valves may be used within the pressure-temperature rating(s) conforming to the valve standard referenced in the code of jurisdiction under which the system is designed and constructed. Dual- or multiple-rated valves have increased utility and broad market. The manufacturer benefits from reduced design and production costs.