Class 150 and Class 300 are pressure classes for threaded malleable iron fittings under ANSI/ASME B16.3. They are not two grades of quality and not two levels of care - they are two wall thicknesses, and the right one is decided by the pressure and the temperature the line actually runs at, not by how important the project feels.
The class is a reference rating, not the working pressure. A Class 150 malleable iron fitting is rated to 150 psi of saturated steam, and a substantially higher figure for cold non-shock water, oil or gas - the two numbers differ because steam brings temperature with it. Class 300 is the heavier wall and carries roughly double the rating at the same temperature.
The practical consequence is that quoting a class alone says nothing until the service is named. "Class 150" on a cold water line and "Class 150" on a 180C steam line are the same fitting doing very different jobs, and only one of them is near its limit.
Malleable iron loses allowable stress as it gets hotter, so the permitted working pressure falls as the line temperature rises. The published rating tables in ANSI B16.3 step down with temperature, and the number that matters is the one at your operating temperature, not the headline class.
This is where most Class 150 failures start - not at a pressure the fitting was never meant to hold, but at a pressure it could have held cold, on a line running hot. If the temperature is above ambient, read the rating at that temperature before choosing.
Steam and condensate above low pressure; high-pressure compressed air; hydraulic and oil lines; and anything subject to pressure surge, water hammer or frequent cycling, where the peak matters more than the steady reading. Refinery, petrochemical and utility work written by American engineering contractors commonly specifies Class 300 for exactly these reasons.
The heavier wall also gives more thread engagement and more metal to take a wrench, which matters on lines that will be broken and remade for maintenance over a long service life.
Building services water, plumbing, irrigation, low pressure compressed air and most gas distribution. Specifying Class 300 for these is not a safety upgrade, it is weight and cost that the design does not use - and on a large tender it can move a bid without changing what the system does.
Fire protection follows its own logic: the governing requirement there is the UL Listing and FM Approval of the figure, and the specification names those rather than a pressure class.
TUPY produces NPT fittings to ANSI B16.3 in Class 150 and Class 300, from the same foundry and the same material standard as the BSP range to EN 10242 and ISO 49. A site that needs NPT Class 300 in the plant and BSP for the buildings can be supplied from one order, with mill test certificates covering both.
Send the specification with the enquiry. Thread form, class and finish are confirmed against it before anything is quoted, which is cheaper than discovering the mismatch at the submittal stage.
Roughly double the pressure rating at the same temperature, from a heavier wall - not a different material or a better grade. Both are malleable iron to the same material standard.
The allowable working pressure does. Malleable iron loses allowable stress as temperature rises, so read the rating at the operating temperature rather than the headline class figure.
It depends on the steam pressure and temperature. Low pressure steam is commonly Class 150; higher pressure and condensate service is where Class 300 is usually specified. Send the operating conditions and it will be checked against the rating table.
They will thread together, but the line is only as strong as its lowest-rated component. Mixing classes means the Class 150 fitting sets the limit for the whole run.
Yes - NPT to ANSI B16.3 in Class 150 and Class 300, and BSP to EN 10242 and ISO 49, from the same manufacturer.
Send the figures, sizes and quantities and it will be quoted from stock in Dubai or direct from Brazil, whichever lands cheaper.
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