12
To:
Subject:
Origins of nominal flange bore
sizes
Question:
Can you explain the origins of flange
bore sizes like 4-1/16",
7-1/16", etc. in API flanges?
Answer:
The question you raise fits together with another
historical question concerning the origin of the specific dimensions of
pipe schedules. We have never found an authentic historical account that
explains either. Because we have the same curiosity as yourself, and
since we have known and had conversations with some of the people ( now
deceased ) who worked on the earliest API Flange Specifications, we will
offer a “speculative history.”
All flanges seem to have originally derived from
pipe. Pipe nominal sizes like 2”, 3”, and 4”, when originally
standardized, had approximately 2”, 3”, and 4” inside diameters.
Flanges, as originally standardized, had their nominal sizes designated
according to the pipe with which they corresponded. As industry found
the need to contain higher pressure, standardized pipe needed thicker
walls to contain this pressure. This resulted in “schedules” designating
wall thicknesses on given outside diameters of pipe, these outside
diameters now fixed in size with the advent of standardized pipe threads. As
electric arc welding became a common procedure,
Welding Neck Flanges
became available with necks and inside diameters to match the various
“schedules” of pipe.
For oil and gas well use, flange connections
needed standardized bores so that standardized tools could pass through
these bores. In contrast to pipe, flange design allowed the necks of
flanges to increase in size to allow thicker walls to contain higher
pressure, keeping the working bore's inside diameter the same. Pipe
“nominal” size to flange “nominal” size “tradition," along with the fact
that API adopted ASA flange face and thickness dimensions for
6B
flanges, resulted in the original flange designations, e.g., 2” Series
1500, with 2-1/16” bores. Flanges with 2-1/16” bores would pass tools 2”
in outside diameter; 4-1/16” bore flanges would pass tools 4” in outside
diameter, and so on. As standardization continued and specialized
requirements presented themselves, bore sizes like 7-1/16”, 11”,
13-5/8”, and others developed. Higher pressure requirement also
contributed to the need for new flanges, and in the 1950’s, API adopted
6BX flanges for this purpose.
For years 6B flanges had their sizes designated as
nominal 2”, 3”, 4”, 6” etc. With the advent of 6BX flanges, API
designated flanges by bore and working pressure.
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