21 Pipe Sizing
When it comes to sizing pipe, there are a few considerations for how to do it and why it matters:
- How big is the hole the material will go through?
- How far is the material going?
- How thick does the wall of the pipe need to be?
- What factors are working against the system (friction, possibility of erosion, etc.)?
The pipe needs to be the proper size to handle what it’s carrying (water, gas, air, drainage, etc.), but it also cannot be too small, or it could cause the volume to travel too fast. And that could cause serious issues, such as water hammer (a hydraulic shock or pressure surge), which in turn could damage the pipe system and the fixtures attached to the piping.
Days Gone By
Determining pipe sizes can be somewhat confusing, primarily because of the historical method of sizing and the fact that some current sizing still refers to those legacy systems.
Take this example:
Years ago, a half-inch pipe had an inner diameter of [latex]\tfrac{1}{2}[/latex] inches. It also had thick walls, as that was the manufacturing standard at the time.
Over the years, improved technology has enabled the walls to be thinner while retaining the same capabilities. However, to match up with existing (larger walled) pipe, the inside diameter of the new pipe had to be larger. Which means the pipe was not [latex]\tfrac{1}{2}[/latex] inch, no matter which way you measured.
When mass production of pipe began, there was a need for standardization. In 1927, the American Standards Association – which has since evolved into the American National Standards Institute (ANSI) – convened a committee to standardize the dimensions of wrought steel and wrought iron pipe and tubing. Back then, only a few wall thicknesses were used: standard weight (STD), extra strong (XS), and double extra strong (XXS), based on the iron pipe size (IPS) system of the day.
By 1939, schedule numbers were starting to come into use, but the original terms stuck and are often still used today, though XS and XXS were revised slightly to extra heavy (XH) and double extra heavy (XXH), respectively.
By the 1950s, stainless steel was coming into use more frequently, allowing the use of thinner pipes (e.g., 5S and 10S), which were based on pressure requirements. (It is worth noting that because of their thin walls, the smaller “S” sizes cannot be threaded but must be fusion welded.)
Pipe Sizing
If the legacy of sizing isn’t confusing enough, consider that pipes have been sized differently over the years depending on the specific sizing system:
-
Iron Pipe Size (IPS) — sized by reference to the inside diameters; the standard from early 19th century through just after World War II; still used in PVC manufacturing and steel gas and water piping
- Ductile Iron Pipe Size (DIPS) — similar to NPS but used for larger pipes
- Copper Tube Sizing (CTS) — in the 1920s, this was combined with the IPS standard; the inside diameter is measured in the “types” (M, L, and K for thinnest, thicker, and thic