Understanding Schedule and Rating

I was a layman in the world of piping when I started my career and I always used to get confused of what is a schedule and what is a rating / pressure rating which are mentioned in the piping material specification document.

If you want to know / wonder about what a piping material specification document is, please go through my previous post “Understanding Piping Specification” https://www.dhirubhai.net/pulse/understanding-piping-specification-ashok-jagarlamudi-/?trackingId=AvyY65ULTnuYRnFwX6%2BS9g%3D%3D&lipi=urn%3Ali%3Apage%3Ad_flagship3_publishing_published%3BaA%2BSH6G7QPaRpYtFuxowCQ%3D%3D

So let me try to explain it in a simple way so that a layman like me could understand at least a little bit so that he / she can take off from there.

First of all, there are some basic terminologies which we need to understand.

1.?????NPS : is an abbreviation for Nominal Pipe Size, where we generally use this term to identify the diameter or size of a pipe / fitting. Also called as “DN” which stands for Nominal Diameter as per the International and European metric system equivalent of NPS.

2.?????OD : is Outside Diameter or the measurement of the outside edges of the pipe passing through its center.

3.?????ID : is for Inside Diameter / Internal Diameter, it is generally obtained by taking outside diameter of the pipe less than twice the wall thickness (ID = OD – 2* Wall Thickness).

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Schedule, abbreviated as SCH, is a measurement that denotes wall thickness of a pipe. Remember, it has no unit such as millimeter or inch as commonly misunderstood. It is a numbering system inspired by Barlow’s formula and introduced by the ASME to simplify the identification of pipe thicknesses.

By formula, Schedule Number = 1,000 x (P/S);

Where, P = working stress or service pressure in (psi)

S = allowable stress in (psi)

According to ASME B36.10 and B36.19, standard pipe schedules are: 5, 10, 20, 30, 40, 60, 80, 100, 120, 140, 160, STD, XS, and XXS for carbon steel and wrought iron pipes; and 5S, 10S, 40S, and 80s for stainless steel pipes. The suffix “S” in the schedule number is used to differentiate B36.19 pipe (stainless steel pipe) from B36.10 pipe (carbon steel and wrought iron pipe).

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The wall-thickness designations standard (STD), extra strong (XS), and double extra-strong (XXS) have been commercially used designations for many years. The Schedule numbers were subsequently added as a convenient designation for use in ordering pipe or for procurement purposes.

As per ASME B36.10, Standard and Schedule 40 are identical up to NPS 10 (DN 250), inclusive. All larger sizes of STD have 3∕8in. (9.53 mm) wall thicknesses. XS and Schedule 80 are identical up to NPS 8 (DN 200), inclusive. All larger sizes of XS have1 ∕2in. (12.70 mm) wall thicknesses. Pipe of sizes and wall thicknesses other than those STD, XS, and XXS, and Schedule numbers which were adopted from API 5L. It was not considered practical to establish Schedule numbers or new designations for them.

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To understand this better, let us consider a piping situation where P = 500 psi and S = 10,000 psi.

Schedule Number = 1,000 x (500/10,000) = 50; this means that pipe schedule 50 is adequate for the design pressure or use schedule 60 for a conservative factor of safety.

However, there are some small details to keep in mind that Schedules 5, 5S, 10 & 10S wall thicknesses do not permit threading in accordance with ASME B1.20.1

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When it comes to the fittings, generally socket weld and threaded pipe fittings are forged products and are classified based on their pressure – temperature rating. Usually, they are used in small bore and low pressure piping up to and below 2in. sizes and sometimes may go up to 4in. also depending on the requirements.

These fittings are available in different pressure – temperature rating classes as mentioned below.

1.?????2000 class fittings available only in threaded type.

2.?????3000 & 6000 class fittings available in both threaded type as well as socket weld types.

3.?????9000 class fittings available only in socket weld type.

The correlation of fittings class with schedule / wall designation of pipe for calculation of ratings per ASME B16.11 are as follows:

Wall thicknesses for 2000, 3000 & 6000 class threaded fittings are governed by the ASME B16.11 (Forged Fittings, Socket-Welding and Threaded), wall thicknesses for 3000, 6000 & 9000 class socket welded fittings are also governed by ASME B16.11 document. Wall thicknesses mentioned in this document are of minimum values and we can also consider wall thicknesses more than these minimum requirements based on our specific requirements but not less than these minimum requirements.

The minimum requirements for Wrought Buttwelding Fittings can be found in ASME B16.9 document, for Wrought Steel Buttwelding Short Radius Elbows and Returns can be found in ASME B16.28 document, for flanges and flanged fittings can be found in ASME B16.5, ASME B16.47 document.

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For flanges you may be heard of both class / rating and schedule which may confuse you sometimes, aren’t they?

As you already know that class is a pressure – temperature rating that defines the maximum allowable working pressure of the flange. Flange classes are usually in 150#, 300#, 400#, 600#, 900#, 1500# & 2500# as per ASME B16.5 up to 24in sizes. For large diameter flanges that are from 24in size to 60in sizes are available up to 900# class.

The higher the rating of a flange the higher pressure – temperature it can withstand.

For flanges like weld neck, the neck is where the pipe going to be welded. In these cases, if the pipe is having a schedule 40, then the flange also must have the same schedule as of pipe so that there will be no miss match in the internal diameters of pipe and flange.

Sometimes we can get a doubt that can we use the same class/rating flange with a different schedule? The answer is yes, we can use, if the same class/rating flange is to be used with a different schedule pipe, then we should match the schedule of the flange also according to the pipe. If there is a weld neck flange without any schedule (which is not the case generally), then in that case we should match the schedule of the pipe itself which is going to be welded with these flanges.

It is so important to match these schedules to get correct internal diameters because if ID’s are not matching then there will be some resistance to the fluid / service which is flowing inside that pipeline at that particular joint and may result in pressure drop and the flow would no longer be in laminar steady state. In the first scenario of these type of cases that weld / joint will not get approved at all in the quality checking phase.

Thanks for the read and hopefully it might be helpful for any beginners who are seeking to understand about the Schedule and Rating.