Page 472 - Pipeline Pigging Technology

Pigging research

Manufacturers of intelligent pigs have determined, and specify, the speeds
at which their pigs must be run to obtain optimum performance. These range
between about 1 to lOmph (0.5 to 4m/sec), although many of the geometry
pigs can perform at much higher velocities.
Conventional pigs, however, must be run at the velocity at which the
pipeline is operating. The speeds usually recommended for routine, conven-
tional, on-stream pigging are 2 to lOmph (1 to 5m/sec) for liquid lines and 5
to 15mph (2 to 7m/sec) in gas lines; these figures may differ if the pig is run
during construction or commissioning.
The two questions which immediately arise are firstly, is it conceivable that
optimum performance can be obtained at all speeds within such a wide
range? and secondly, where did these figures come from - on what are they
based?

Virtually all of the published research work carried out to date appears to
be in connection with the use of spheres.
Spheres have some obvious advantage from the researcher's point of view.
They are perfectly symmetrical, they have only one sealing surface, and
because they are inflated, their diameter can be altered. This eliminates at
least some of the variables.
Some of the earliest work was carried out in 1959 by Barrett of the Shell Oil
Co, Indianapolis[l], to reduce interface mixing in its 14-in, 250-mile Wood
River to Chicago product line. This was soon after the introduction of what
were then known as "expandable spheroids".
Although Barrett's paper is mainly concerned with reducing interface
mixing and does not specifically address the effects of velocity, there are a
number of aspects which are of general importance. One of these concerns
the effect of the sphere/pig diameter ratio on sealing efficiency; Fig. 2 is a
reproduction of the graph published at the time.
Barrett's tests were carried out in a 1-mile long 13.375-in ID meter prover,
using spheres made from a relatively-soft (45-50 Shore "A") neoprene. Later
it was found that both neoprene and nitrile rubber had a tendency to absorb
hydrocarbons and "blister" and this, together with significant improvements
in their mechanical properties, has led to the almost exclusive use of
polyurethanes today.
Among the many interesting facts observed during his research was the
relative volumes of fluid leaking past the spheres - in both directions - with
different sphere/pipeline diameter ratios. This is referred to as "flow forward"
and "flow back". He states:

"Indications are that the 'flow back' across a spheroid inflated to the
optimum diameter of about 1% larger than the pipe ID is in the order

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