Page 70 - Pipeline Pigging Technology

Pipeline design for pigging

Pig-passage indicators

Currently, pig-passage indicators of mechanical design have the longest
track record. They are often regarded as unreliable, although any shortfall in
performance is usually due to the lack of preventive maintenance.
Pig-passage indicators must be:

bi-directional;
flush with the internal pipe wall; and
retractable and replaceable under pressure.

Furthermore, pig-passage indicators can be equipped with a micro-switch
for remote signalling. Such applications usually have an automatic re-set
mode, while mechanical passage indicators are manually re-set.

Bends

Bends for pigging should be of the following minimum radii:

4-in 20D
6 and 8-in 10D
10-in and above 5D

Besides the minimum radius, the out-of-roundness should also be limited
to 5%.
Special attention should be paid to the internal diameter, as these bends
are usually hot-drawn from heavy pipe wall material.
The location of the bends should always allow a straight section of at least
three times nominal diameter up- and down-stream. In particular, 30° or 45°
offset bends should have a minimum straight length between them of 6ft for
pipe diameters to 24in, and 3D for diameters of 24in and above.

Valves

Valves should be specified for pigging purposes with the following
requirements:

full-bore with specified minimum internal diameter;
guaranteed 100% opening;

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