ANSI B36.10/19 is the most recognized sizing specification for steel pipe in the US. Under this specification, you can find the dimensional tolerances for common pipe sizes, including the outside diameters, wall thickness and inside diameter. From these dimensions and the material type a qualified engineer can calculate burst pressures, flow rates, weights and many other important details for selecting pipe.
Prior to the ANSI specification, three long standing traditional designations were used to describe a pipes wall thickness, Standard, Extra Strong (or Extra Heavy) and Double Extra Strong. Common abbreviations are STD, XH, XS, EH, XXH and XXS.
Since all pipes must fit the same fittings, the outside diameters remain the same. Changing the wall thickness changes the inside diameter.
Extra Heavy (XH) is used to describe the minimum wall thicknesses for pipe and some pipe fittings. For sizes up to 8", XH is the same wall thickness as schedule 80. Although the "XH" description is not the most accurate, it is still very commonly used and implies the same as schedule 80. Confusion can occur when it used to specify pipe 10" and larger.
Even more confusion occurs when this traditional designation is used to describe pipe fittings. Because it is only applicable to butt weld fittings and flanges.
On a near daily basis we are asked to supply "standard" and "extra heavy" (or schedule 40 & 80) threaded or socket weld fittings. For standard fittings, it is pretty much assumed that our client wants class 150 fittings. But an XH or schedule 80 fitting can be class 250, 300, 2000 or 3000 lb.
All threaded and socket weld fittings (except some plastic and merchant steel) are classified and should be specified by a pressure class.
Most sizes of standard/schedule 40 pipe are pressure tested to 700 psi. But if you're running 300 psi, you would not want to use a "STD" or class 150 fitting. Nor would you need to use the added weight, flow restriction and cost of XH pipe. Hence the need for a pressure class fitting. Specifying your fittings with a pressure class will ease your procurement and help insure you receive the right product for your application.
If you have any questions regarding the specifications on pipe, valves, fittings or hangers, give us call. We are happy help.