Tag Archive: pipe sizes

  1. What is Pipe Schedule?

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    Image by cwizner from Pixabay

    Pipe Schedule (SCH) is a standard that measures the nominal wall thickness of a pipe, given by the ANSI / ASME B36.10M standard for Stainless Steel Pipe dimensions, and API 5L standard for seamless and welded steel pipes.

    The pipe schedule number is non-dimensional and depends on the nominal pipe size, internal pipe working pressure, and the material used for the pipe wall.

    Nominal Pipe Sizes.
    Image Credit: The Process Piping

    Today, fourteen pipe schedule numbers are used: SCH 5, SCH 5S, SCH 10, SCH 10S, SCH 20, SCH 30, SCH 40S, SCH 60, SCH 80, SCH 80S, SCH 100, SCH 120, SCH 140, and SCH 160.

    In the past, only three pipe wall thicknesses were used, but these measurements are very vague and aren’t used today

    • Standard (STD)
    • Extra Strong (XS)
    • Double Extra Strong (XXS)

    The most used pipe schedule is SCH 40. For NPS 12 or smaller pipes or pipes with outer diameter of around 12 inches, SCH 40 is the same wall thickness of the previously used value. The SCH 80 wall thickness, on the other hand, is the same as the previously used XS wall thickness for pipe sizes up to NPS 10.

    The best way to see the relationship between pipe size, schedules and wall thicknesses is simply to refer to a conversion chart (below):

    Image Credit: Metal Supermarkets

    The following equation can be used to obtain the right pipe schedule for a given usage scenario:

    SCH = (1,000) *(P/S), where P is the internal working pressure od the pipe, while S signifies how much stress the material can be subjected to.

    Let’s have a look at an example of a pipe with internal working pressure of 450 psi and an S value of 12,000:

    (1,000) *(450/12,000) = 37.5 or roughly equivalent to a SCH 40. Another essential thing to note is the alteration of the wall thickness depending on the NPS (Nominal Pipe Size). The larger the nominal diameter of the pipe, the thicker the wall becomes, even for the same SCH number. Let’s take the same SCH 40 wall thickness as before. For an NPS 1 pipe, an SCH 40 wall thickness is 0.133-inches, while for an NPS 2 pipe, the SCH 40 wall thickness is 0.154-inches.

  2. What is “Nominal Pipe Size”?

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    You’ve probably encountered the abbreviation NPS previously on our or similar websites. It means “Nominal Pipe Size,” and it is the North American standard for measuring pipe sizes used for various pressures and temperatures. The NPS standard is widely accepted in the USA, but some things have to be observed correctly in order to find the right nominal pipe size.

    The reason is very straightforward – NPS tells you the diameter of the outer wall of the pipe, but only for sizes larger than NPS 14. For smaller pipes of NPS 1/8 and up to NPS 12, the number is not directly connected to the outer diameter of the pipe. More precisely, an NPS 12 pipe will have an outer diameter of 12.75 inches, while an NPS 14 pipe will have an outer diameter of 14 inches.

    Image Credit: The Process Piping

    It may sound confusing, but luckily most tables that have the NPS number also contain the pipe schedule and the metric DN standard, where the diameter of the outer wall is measured in millimeters. The reason why there is a disparity between smaller and larger pipe sizes is that they are two slightly different standards. When the NPS standard was first introduced, three wall thicknesses were used: Standard (STD), Extra Strong (XS), and Double Extra Strong (XXS), so the NPS values always gave the same inner diameter of the pipe. However, today multiple pipe thicknesses are used, and the NPS value became indirectly related to the inner and outer diameter of the pipe.

    Things like the inner diameter or the thickness of the wall won’t be mentioned anywhere in the NPS standard. Those details can be found in pipe schedule parameters. Please note that the American Standard for funnel assignments also uses the term NPS, but there it signifies “National Pipe Thread Straight.”