Good engineering practice says that cold pulling is carried out after the piping erection is complete, i.e., all the welds have been done, all of the flanges have been aligned and bolted, stress relief (if necessary) has been carried out, all piping hangers and supports (which in this case use to be flexible, i.e., with springs) have been adjusted etc. etc. Only hydro test is still left.
With everything in position, you cut the specified length of pipe, pull the two resulting pipe ends together and weld them. After this, hydro test is performed.
This is how I used to do back in my days of erector engineer, in jobs under the supervision of such prestigious companies as Sargent and Lundy, The Lummus Company, Combustion Engineering and others.
Giovanni S. Crisi
Sao Paulo - Brazil
Pulling the pipe to make fitup on flanged joints puts a preload on the flanged joint that can cause leakage. If a pump is involved, it can also lead to misalignment issues with the motor coupling. Later, when the flange is unbolted it will spring out of position and make it difficult to replace the gasket and realign the joint. Cold pull that is not considered in the piping system design can also overstress the pipe when it grows due to thermal expansion and operating loads.
Marty,
in my opinion, the "cold pull" Richman is referring to (and excuse me if I'm telling something you already know) is a technique used on piping that will operate at high temperatures, live steam for example.
The piping engineer calculates how many millimeters (or inches) the piping will expand due to the high temperature it will work at. Then, in a suitable straight run of the piping, also determined by the engineer, it is cut a piece of pipe measuring half the calculated expansion. This will cater for half of the stress imposed by the expansion. The other half will be taken up by the piping flexibility, to which the engineer should devote careful attention.
Back in my days of erector engineer, constructing thermal power plants and oil refineries, I've done plenty of "cold pulls".
On the other hand, Marty, you're referring to a practice that should be by all means avoided, which is to pull the piping (not rarely using chain blocks or other means) to make the holes of two flanges coincide and be able to put the bolts into them. This will lead to the problems you've listed on your posting.
Giovanni S. Crisi
By Nanjing
Date 11-27-2009 15:01
You always pipe away from a pump so there is the minimum stress on reciprocating equipment and its associated pipework otherwise you may run into fatigue problems.
