Nozzle orientation drawings - how to specify angles

[mgp]

This may be an odd one and I'm not sure if this the right forum but anyway...

I had a discussion with my colleagues about which way is the correct to specify angles when you specify nozzle orientations on vessel heads:

1. Should 0 degrees start at the X-axis or the Y-axis?
2. Should the angular dimension be specified clockwise or anticlockwise?

When vessels are specified, normally a datasheet is prepared with a nozzle orientation sketch. Later this will be transferred to an fabrication drawing.

Very often these are dimensioned clockwise starting at the Y-axis (12 o´clock)

At the same time all the CAD programs always use anti clockwise, starting from X axis. This is also the correct way according to any math book I have seen.

If one could agree to always use the "CAD" way, things might be less confusing, however I'm sure many think it should be the other way around.

Are there any good (historical/regional etc.) reasons why one should not use the CAD way? (I know the manway might end up at 270 degrees)

regards
Mogens

 

[prex]

My personal preference is for 0 at Y axis and anticlockwise direction.
However it is a matter of preference: the best is to follow the way shop people are accustomed to, as they might get confused.

prex

http://www.xcalcs.com

Online tools for structural design

 

[jte]

MGP-

My experience is different from prex's. Most drawings I've drawn and seen start the orientation at the Y axis and go clockwise. Don't think of this as math. If you look at this from a construction - and more to the point, a surveying perspective, it might make more sense. North is up, 90° is East, 180° is South, and 270° is West. Most of the time these coordinates are adjusted to a "plant north" since most plants are not perfectly aligned on a north-south axis.

jt

 

[dig1]

Normally we start with North up and zero and with 90 degrees being East.

Field erection crews seem to like this better, in our experience.

 

[roca]

I agree with MGP and dig1. I started work as an apprentice at a vessel fabricators back in 1974 and that was the way we taught.
From memory also in the good old days of the drawing board piping design draughting departments used the same method.
It is only since the introduction of the CAD programs where this has been changed.

 

[chicopee]

Autocad designers have the option of using either the East, North, West or South line as the reference for starting angles; they also can either assign counter clockwise or clockwise as the the positive angle direction.
Based on what you said, to simplify matters I would use the North line as my reference and clockwise as the positive direction for angles to be consistent w/ your practices.
Dont forget most autocad designers are not engineers, they are drafters and they will practice those methods that they learned during their ACAD education.

 

[prex]

Just one further thought: for a horizontal vessel if angles are quoted clockwise as viewed from one end, they will of course be counter clockwise if seen from the other end! As there is not necessarily a preferred end for viewing (unlike for vertical vessels) the direction for measuring angles is really unimportant and should suit the preferences of the shop people (if any).

prex

http://www.xcalcs.com

Online tools for structural design

 

[jamesl]

I agree with jte. All the drawings I have seen or we have created as vessel manufacture start 0 degree at Y-axis and go clockwise. There is kind of industry practice. Megysey's handbook has a section on vessel detailing(drawing).

This is not just preference issue. If the degrees go counterclockwise, shop people could interprete it as a view from the "other" end. The nozzle orientation would be messed up.

 

[roca]

Apologies
My reply should have said I agree with jte and dig1
North is 0 degrees and east 90 degrees
Every vessel drawing orientation I have seen or produced myself has been done in this way.