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Section - VI, Pipe Fabrication Issues

A: Checking of Pipe Fabrication Shop Drawings

By: James O. Pennock

There will come a time the life of almost every project when the question of checking of pipe fabrication shop spool drawings will come up. Projects that are totally field fabricated do not have shop spools and therefore the need does not exist. However for the vast amount of the projects there will be a pipe fabrication shop, shop spool drawings. Any time drawings are created there will be the potential for errors. The shop is responsible for checking their own work, but it is prudent for the engineering company to do their own check to verify that the shop is doing a proper job.

The prime objective for checking shop drawings is to eliminate (or reduce) field changes to shop fabricated spools. One person cannot be expected to check ALL of the shop drawings and still meet the construction schedule. So you must use your time wisely. You must have a plan. You should also talk to the piping foreman in the field and ask him/her, what are some of the typical errors that cause them the most problems during installation?

A) First, sort the shop drawings by line number and “cost groups” i.e.: separate by material, wall schedule and line size. The reason you need to do this is that it will allow you to focus your effort on the high cost piping. The most costly piping is the large diameter, heavy wall carbon steel, any alloy materials and any material that requires post-weld-heat-treatment (PWHT). Start with the most costly lines and their shop drawings.

B) Next, review the spool drawings for each line first for continuity. By this, I mean, do you have all the shop drawings to complete “that” specific line? If the answer is “yes” then proceed. If the answer is “no” then have someone contact the shop to find out when the missing drawings will be available. It is always possible that they forgot to draw one or to send one.

C) Next, check the drawings against the engineering drawings for configuration. By this I mean does the line turn when, where and in the right direction to match the design? If not then this will require a change in the shop (If the spool piece is not yet shipped) or a change in the field (if the spool piece has already been shipped). Shop errors can, in most cases be back-charged to the shop. However, any error can cause a delay to the schedule which adds cost to the project.

D) Next, review the material used against the piping line specification for the material that is required. If the shop used the wrong material, STOP. This line or spool will need to be refabricated.

E) Next, review the shop drawings against the design looking for places where a wrong dimension in a welded-out configuration would do the most harm. Don’t worry about fitting make-up such as flange-reducer-flange. These have a low possibility for error. Don’t worry about spools that terminate with a field weld. These have a built in opportunity to make a field correction.

F) Next, look for any place where there is a requirement for or the possibility for odd flange bolt-hole rotations. Did the shop do it correctly? On the other hand, did the shop call for and fabricate an odd bolt-hole rotation that was not called for?

G) Next, spot check 10% of the dimensions within each “cost group.” If you do not find any error trends then you may want to consider backing off additional checking. However, if you do find an error trend within a specific cost group (material, wall schedule, size, etc.) then note the name of the person who did these high-error shop drawings. The next step is to notify the shop of your concerns and request that they launch their own investigation. In the meantime notify the construction manager and based on the “cost groups” look at all of this persons work.

This should give you some ideas to consider. Please feel free to E-mail me if you have any questions.

James O. Pennock is a former Piper with more than 45 years experience covering process plant engineering, design, training, pipe fabrication and construction. He is now retired and lives in Florida, USA.

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