Dimensioning Pipe?

I recently completed drawings for a item that included some steel pipe
for a pneumatic line. [About 30' of pipe, with about 25 fittings] I
obtained dimensions for each of the fittings we needed, and
dimensioned the length of pipe by indicating the distance between
fittings. I ignored thread engagement, thinking that this is a
variable I couldn't control. I was also thinking that the installation
must, by nature, be custom fitted by the guy installing the pipe. One
experienced person said that thread can be pretty variable, and the
length of thread engagement will vary depending on how deep the maker
cut the threads, and how worn the tool was.

At some point in the past, I saw a piping drawing that gave
distances to intersections of pipe, ignoring the fittings. But, this
doesn't seem any better than the method I used, except that it doesn't
require me to know the real size of the fittings.

The item I drew is being built now, with the assumption that my
dimensions are only a general guide, so each pipe is being cut to fit
after prior pieces are already installed. I am evaluating if there
might be a better way, especially in situations where I have a number
of fittings close together and want to use standard size nipples.
Perhaps I should assume a certain depth of thread engagement and give
the total length of the pipe (including the thread)?

Any suggestions or industry standards?

Joe Dunfee

 

Machinery's Handbook will give thread engagement distances which I recommend
you use. Someone posted this link earlier this year in Eng Tips.
http://bellsouthpwp.net/r/s/rsnmar/METhreads.zip
I haven't looked at it someone responded favorably.
Here is another chart.
http://unifiedalloys.com/products/pdf_directory/main_collection/pipe_threads.pdf

 

Bruce, thank you for that very useful information.(the charts and
program providing very detailed sizes for most aspects of pipe)
However, I know the thread engagement is still a variable parameter.
I am curious how it is done among those who really know what they are
doing. Most of the piping is done on our object, and it is not
entirely straight.

Joe Dunfee

 

One thought would be to get a sampling of the sizes you are using and do actual assembly of nipples into fittings to measure the thread engagement. I think that threaded pipe joints are somewhat like sheetmetal, what works for one shop doesn't necessarily work for another.
You will have to decide on the level of precision needed. What are the position tolerances that are viable in your application?
I have done considerable piping design using 1/2" increments and when we wanted to be really precise, we went to 1/4" increments!
One thing to keep in mind is that no plumber/assembler has used a torque wrench to assemble the system to my knowledge. You will always be at the mercy of the guy on the end of the pipe wrench. In my case, that was me, more often than I liked!