You will get much more pressure on a turn then on a straight pipe. On a turn you will encounter great resistance where on a butt welded length of straight pipe you will encounter very little pressure at all.

That's as simple as I can get it.

First, butt-weld fittings are NOT rated 150# and 300#.

In the USA:

Flanges are rated: class 150, class 300, class 600, etc. On drawings these are often noted using the pound sign (#) because it is faster and takes less room. (ASME B16.5 for 24" and smaller flanges. ASME B16.47 for flanges larger than 24")

Large bore (3" and larger) piping fittings, like pipe, are rated by schedule number related to the wall thickness. Example sch. 40, sch. 80 also std (standard weight), XS (extra strong), XXS (double extra strong, etc. (ASME/ANSI B16.9 - 2001 - Factory-Made Wrought Steel Buttwelding Fittings)

Small bore (2" and smaller) screwed piping fittings are rated as you noted above with a "Nominal" classification 2000#, 3000# and 6000#. These numbers themselves have nothing to do with the actual allowable pressures and temperature. ASME/ANSI B16.11 - 2001 - Forged Steel Fittings, Socket-Welding and Threaded

Small bore (2" and smaller) socket-weld piping fittings are rated as you noted above with a "Nominal" classification 3000#, 6000# and 9000#. These numbers themselves have nothing to do with the actual allowable pressures and temperature. (ASME/ANSI B16.11 - 2001 - Forged Steel Fittings, Socket-Welding and Threaded)

See (http://www.pipingdesigners.com/codes%20&%20standards/Codes%20&%20Standards%20-%201.htm) for other code applications

For 2000#, 3000#, 6000# fittings - that is actually the Water Oil Gas non-shock working pressure of the fittings at 100 deg F. That is, a 3000# socket weld fitting has a non-shock working pressure of 3000 psi at 100 F.

www.superlok.com/korean/forged.pdf

http://www.pennusa.com/pennpage/directory/catalog/nonshocktempchart.html

Pipe wall schedule numbers are a carry-over from early calculations, from ASME B36.10M:

The original intent of the Committee was to establish a system of Schedule Numbers for pipe size/wall thickness combinations which would have an approximately uniform relationship equal to 1000 times the P/S expression contained in the modified Barlow formula for pipe wall thickness as defined in the Appendix to this standard. The resulting Numbers departed so far from existing wall thicknesses in common use that the original intent could not be accomplished. The Schedule Numbers were then adopted strictly as a convenient designation system for use in ordering.

Flanges are another story - see here for a discussion: www.ebaa.com/pdf/Connections.FL-01.pdf

Hi...thank you for your response...i truly appreciate it!

Thank you...much appreciated!