Pipe Schedule

  • #1

Main Question or Discussion Point

Hi,

I am hoping that someone could enlighten me about pipe schedules.

I've searched online but there have been conflicting definitions for schedules, where some people refer to it as pressure and some referring to it as thickness.

I need a pipe to be able to withstand 70bar of pressure, which schedule should I use?

and what units are the schedules?
 

Answers and Replies

  • #2
Q_Goest
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Sorry to say, there is no correlation between pipe schedule and anything such as pressure or wall thickness. Looking at the history, it appears the pipe schedule wall thicknesses were simply selected on historical values and nothing more.
In March 1927, the American Standards Association authorized a committee to standardize the dimensions of wrought steel and wrought iron pipe and tubing. At that time only a small selection of wall thicknesses were in use: standard weight (STD), extra-strong (XS), and double extra-strong (XXS), based on the iron pipe size (IPS) system of the day. However these three sizes did not fit all applications. The committee surveyed the industry and created a system of schedule numbers that designated wall thicknesses based on smaller steps between sizes,[3] although IPS and NPS numbers remain equivalent.

The original intent was that each schedule would relate to a given pressure rating, however the numbers deviated so far from wall thicknesses in common use that this original intent could not be accomplished.
If you want to select a pipe for an application it requires using the applicable piping code. In the US, it will be one of the ASME B31.x piping codes. (ex: ASME B31.3 for Process Piping)

The codes have specific equations to use, generally more than 1. They also contain a section on materials which provides "stress allowables" for each type of material used. That stress allowable is a function not just of the chemistry and strength of the pipe but also on the operating temperature. In addition, things such as corrosion and externally applied stresses need to be taken into consideration.

Even if this is for a college project, if this is actually to be built you really should ensure your design meets the applicable piping codes for your country.
 

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