Pound, #, lb. pressure ratings on fittings and valves?

One thing that’s been a problem over the years in dealing with fittings and valves is the terminology used when talking about pressure ratings. The pound terminology is based on PSI (Pounds per square inch) a measurement of pressure in the Imperial system of measurement.

FITTINGS

For many years industry referred to bronze fittings as 125-pound 250-pound (125# -250#) and malleable iron fittings as 150 pound & 300-pound (150# - 300#) fittings. In 1973 the designations were changed from “pound” to “Class”, unfortunately many still use the outdated pound (#) terminology to this day which causes confusion. It is important to beware that the pressure rating of a fitting depends on the temperature of the materials it is to be used with, the class of a fitting is the maximum pressure at highest temperature. It is important to refer to the pressure / temperature charts to verify the effect that increase temperature has on the allowable working pressure.

VALVES:

The pound terminology was also used for valves, for example a valve rated for 600 PSI was referred to as a 600# valve. However, the pressure rating for valves has been WOG (Water - Oil – Gas) which was is molded on the valve body as 600 WOG, the older, generic pressure rating call-out.

In this WOG designation, “Gas” refers to any non-flammable, compressible fluid in a gaseous state. It does not imply a product’s suitability or third-party approval for use with flammable gases. Similarly, in this designation, “Oil” does not imply a product’s third-party approval for use with any petroleum products.

For these reasons the WOG rating has not been a good description and there is movement within the industry to the more descriptive CWP (Cold Working Pressure). Regardless of what it is called, it remains a limitation of the pressure valve only. The valve type, material and trim are different for each application. The pressure/temperature limitations of the pressure containing vessel are a few of the several variables that must be considered.

The WOG call out is still used, however it is being replaced by CWP (Cold Working Pressure) terminology. This designates the maximum non-shock pressure a product can operate at. “Cold” is generally defined as a range between -20°F to 100°F. Again, it is important to refer to the pressure / temperature charts to verify the effect that increase temperature has on the allowable working pressure. You will see valves with the 400 CWP, 600 CWP marking becoming more and more common as manufacturers make the change to the better Cold Working Pressure designation.