Section - VI,
Pipe Fabrication Issues
A: Checking of Pipe Fabrication Shop
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. Dont worry
about fitting make-up such as flange-reducer-flange. These have
a low possibility for error. Dont 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.