Category: Pipe

Robert Briggs was the superintendent of the Pascal Iron Works in 1862.  Here he wrote a set of pipe specifications for iron pipe and distributed them to all the other local mills.

In 1862 each pipe mill still made its own pipe and fittings to its own sixes and specifications.  Briggs had seen a need to standardize the sizing.  The pipe standards he created became known as the "Briggs Standards."  And later became the American Standards, which are the same that are used today.

The current ASTM A53, A106, A312, A333, A335 and many others are based on the Briggs Standard for pipe sizes 1/2" thru 4".  Sizes 5" thru 12" are closer to the actual inside diameter and sizes 14"thru 48" are actual outside diameters.

Sizes 1/2" thru 4" came from the dies used in the Pascal Iron Works.  Briggs wrote the standards to suit thier dies and all other mills had to make the adjustment.

Pipe sizes 1/2" thur 4" do not have a single dimension the equals its call out.  There is no 1/2" dimension on a 1/2" pipe, nor a 3/4" dimension on 3/4" and so on.  Hence the name "nominal" pipe came about, meaning "cloe to" or "somewhere in the proximity of" the actual dimension.

All pipe schedules define a minimum wall thickness. Schedule 40 is the most recognized and can be traced back to 1862 from the Pascal Iron Works. The original wall thicknesses included pipe sizes up to 4". Since then everything else appears to be patchwork additions.

12" schedule 40 pipe is .406" wall or a .375" wall, depending on who you ask. Actually, 12" schedule 40s is .375" and 12" true schedule 40 is .406". But 12" standard wall is .375". Confused yet?

So are many of our callers and even some of our suppliers. If you ask a steel pipe mill for 12” schedule 40, you will get a .406” wall. Ask a stainless steel pipe mill and you will get .375” wall. Ask a PVC pipe mill and you will get .406” wall.

Hayward pipe has published a Metal Pipe ID-OD Chart. The chart clarifies the differences and duplication's between pipe schedules. This chart includes only pipes commonly defined by pipe schedule.

Since the wall thickness influences the thermal attributes, weight, cost, ID and pressure/temperature limits it is always prudent to specify the actual wall thickness, not just the schedule, when ordering pipe, fittings or valves.