Page 255 - Civil Engineering Project Management, Fourth Edition

Earthworks and pipelines
235
The crane handling a heavy pipe must not slacken off until the pipelaying fore-
man is certain it is safe to do so, and no man would be put in danger if the pipe
should move. Timber props, packings and wedges in adequate numbers should
be available to secure the pipe before it is finally moved into position for jointing
by slow jacking or barring to get it into position, the wedges being continuously
adjusted to keep the pipe from moving unexpectedly.
Where large diameter pipes and fittings, such as bends, have to be fitted
together there is often difficulty in getting them set so that their joints match
accurately, especially when a bend must be fixed at an angle to the horizontal.
Before lowering such a bend into position it is worthwhile measuring it to find
and mark, on the outside of the pipe at both ends, the diameters on the true axis
of the bend and at 90 degrees to it. These should be accurately marked with a
chiselled or indelible pencil line on white paint, not marked with chalk which
will rub off. It is quite difficult to locate the axis of a bend accurately, even when
the bend is above ground; and can be frustratingly difficult when the bend is
laid in some trench or basement. It is also good practice to put a mark round the
spigot end of a pipe showing how far it must be inserted into a socket. The rela-
tionship of this line to the socket face indicates whether the alignment is satis-
factory. Spigot and socket pipes need to be lined up within 1 degree to achieve
a good joint; pipe flanges have to be lined up exactly parallel, with the bolt holes
exactly matching, and as close as possible after insertion of the necessary joint
ring before final drawing together of the flanges by progressive tightening of
the bolts. To set a 1.2m diameter 45 degree bend accurately to join a horizontal
pipe at one end and an inclined pipe at the other may take a gang of four men
and a crane driver two or three hours. If things do not go well it may take much
longer.