Class vs Pounds The Rating of fittings
What is the pressure rating of a Class 125 fitting? What is the pressure rating of a Class 250 fitting? The answer to both of those questions is the same. It depends!
One thing that’s been a problem over the years in dealing with fittings is the nomenclature used. For years we called our cast iron fittings 175 pound fittings. And then when we needed higher pressures we called for 300 pound fittings. I’ve had plenty of discussions with engineers over what fittings to use due to a general misunderstanding of how fittings are rated. I learned to start by trying to educate people about this subject.
The first thing to learn is that in 1973 the designations were changed from “pound” to “Class”. The 125 pound fittings became the Class 125, etc.
The second thing is that the pressure rating of a fitting depends on the temperature of the materials it is to be used with. Generally the class of a fitting is the maximum pressure at highest temperature. Usually this means steam. Thus a Class 125 cast iron fitting is rated at 125 PSI at 350°F (steam) and 175 PSI at -20°F to 150°F which is the range where most fire sprinkler water falls. Thus 175 PSI is that number we have grown accustomed to even if we didn’t know where it came from.
A Class 250 cast iron fitting is rated at 250 PSI at 400°F (steam) and 400 PSI at -20°F to 150°F.
And it gets a little more complicated for malleable iron fittings.
A Class 150 malleable iron fitting is rated at 150 PSI at 150°F (steam) and 300 PSI at -20°F to 150°F So when an AHJ states that he wants 300 pound fittings don’t let him reject it because it says Class 150 on the sumittal sheet. Make sure you include the temperature/pressure data.
A Class 300 malleable iron fitting has different pressure ratings for the size of the fitting as well. See the data chart below for a complete breakdown. Generally, the largest size had a maximum pressure of 300 PSI at 550°F (steam) and 1000 PSI at -20°F to 150°F This is why some still call them 1000 pound fittings.
|Pressure-Temperature ratings for cast iron threaded fittings(psig)|
|Pipe Size (inches)|
|-20 up to 150||200||150||150||500||300|
|Pressure-Temperature ratings for malleable iron threaded fittings (psig)|
|¼ - 1||1¼ - 2||2½ - 3|
|-20 to 150||-29 to 66||300||2000||1500||1000|