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Pipe Schedules & Sizes

Pipe Schedules & Sizes

Pipe Schedules & Sizes

Does anyone know the history here for typical small pipes, such as 1/4 inch IPS to 2 inch IPS? Why was 1/2 IPS pipe made 0.840 outside diameter?, what was the logic for developing "Schedule" pipe, what was the origin of "Schedule 40 pipe, Schedule 80, etc?

History questions that have not had answers for me, although searching for many years. Thanks in advance for any info.

RE: Pipe Schedules & Sizes

Originally they defined the ID of Iron Pipe, but this was based on how pipes were manufactured when the standard was initially established but evoving manufacturing techniques as well as improved metallurgy has basically made those number arbitrary in nature since the original OD's were retained as the standard while the ID's simply ended up where they did.

Most of this is covered, including the meaning of the 'schedule' numbers, in the Wiki items below:



John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
Siemens PLM:
UG/NX Museum:

To an Engineer, the glass is twice as big as it needs to be.

RE: Pipe Schedules & Sizes

Inertia and economic (what-can-be-sold) vs (what-can-be-made-cheaply-with-what-is-in-stock) vs (what-needs-to-be-built-to-hold-higher-pressures) vs (what-the-designer-wants-to-get) pressures.

The "hole problem" started with very bad cast iron pipes and valves being built to hold very lower pressure steam and water in the first 15 to 30 psig steam boilers and locomotives in the very early 1750's and 1800's.

They could not weld anything, and only had cast iron to work with, and almost no machining abilities: lathes and tool steel and lead screws and drills and even saws were hard to come, or had to themselves be invented. Further, even if a single shop standardized, nothing was standardized even inside a city to that shop's competitors, much less across a nation.

So valve castings and pipe patterns (the wooden molds and patterns) were essential, were expensive - and were very, very valuable. The ID of each pipe was "to size" (1/2 inch, 1 inch, 1-1/2 inch, 3 inch, or whatever" and the OD became what was needed for the cast pipe (and fitting or valve) to hold the low pressure steam adequately. (And often, even those fittings broke.) the gines worked. More or less at least.

Go a few years later past the 1830's and into 1860's: steam engine locomotives regularly exceeded 60 mph (100 km/hour), and but steel was not yet available.

When the engine designers needed more pressure, they wanted piping capable of higher and higher pressures and boilers able to generate those pressures - but still had to use poor quality cast iron - and so the pipe yards increased wall thickness by reducing the pipe ID. This was because the existing fitting and valve casting shops did NOT want to change their valve molds and cut new valve and fitting patterns, so the pipe OD needed to stay the same. Pipe ID was now often - but not always - smaller than "nominal". Flow was less, but at higher pressure. Designers put up with the difference - often not really knowing what they were really getting anyway since instruments were still being invented as well.

When Bessemer and Kelly "invented" cheap enough steel at high enough quality that it could be relied on, the designers still wanted even higher pressures and temperatures. Steel has better strength and fewer flaws, so pipe walls could be thinner and still hold high pressures. But now, the pipe manufacturers did NOT want to change their rolling mills and equipment, so THEY demanded pipe OD be kept the same as it was. Therefore, pipe walls got thinner (to save money by requiring less steel!) but OD stayed the same - so pipe ID got "larger" than nominal size. Designers were happy: they got more flow at lower weight and less cost and greater reliability.

So two more things happened while keeping pipe OD the same:

Since pipe roll mills were still fixed on the pipe OD, higher pressures and superheated steam required pipe "schedules" (originally only "40" or "standard") started to get thicker and thicker walls: 80, 100, 120, 160, etc. now had pipe wall ID back smaller than the original "nominal." Lower pressure applications or lower temperature applications could use thinner walls than "standard" so 30, 20, 10 were "invented" .... Those low pressure pipes had pipe ID's bigger than "nominal".

Remember those old fitting and valve castings? They were trying to keep up with the higher pressures as well. But fitting and valve shops were outdated by the change from cast iron to cast steel - plus the higher pressures as well - and HAD to change some of their patterns and melting pots and kilns and their molds.

But many patterns based on the original cast iron pipe OD's were still useable ... until pressures rose even higher.

Thus fitting "classes" were created: originally their names came from the steam pressure rated at saturation (150 psig steam, 300 psig steam, 600, etc.) As higher-rated steam fittings were needed, the fitting manufacturers could not get the pipe mills to change their pipe OD (and thus the typical screwed thread "socket" ID and valve wall). So valve and fitting' sockets for the threads had to get thicker. the old sizes were in use and adequate, so old standards were kept - and new ones got thicker-walled.

Stainless really wasn't an issue - when it came into play as chemistry got better, stainless pipe had to adapt to the rolling mill sizes and conventions of "pipe OD is fixed, ID becomes what needs to be to sell most cheaply".

Time zones showed the advantages of standardized country codes for industrial products between 1880 and 1900, and bigger and bigger monopolies wanted cheaper products they could sell across more wide and area of the country, so "country" and "agency" or professional standards became acceptable to more and more companies - and their customers. Monopoly "busting" - such as Westinghouse and Edison and Thomson warring over light bulb sales and patents and thread sizes and electrical voltages and frequencies - also paradoxically encouraged businesses to agree with each other on things they previously fought to keep unique.

Since NOBODY (designers, companies purchasing pipes and fittings, pipe roll mills, steel casting mills, and fitting mold shops) wanted to change anything by this time, the existing conventions of "Nominal pipe" not being equal to pipe ID, OD, or fitting size were written into codes and laws and specifications - particularly for government and navy work. And, once "those" were written - well, forget it.

RE: Pipe Schedules & Sizes

Very good explanation. And then there's standard pipe, STD., Extra Standard 4X and Double Extra Standard 4XX as well as tubing. Sometimes if your checking something and there's a call out for 4" pipe - you find out later that it is a Schedule 20 or something which you don't find out until you see the actual shop drawing or order ticket.

Worst mistake I made was not paying attention to the weight of rail - which is called out lbs/yard! I think that it is the only steel material in the AISC steel manual that is called out in lbs/yard.

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