Page 475 - Pipeline Pigging Technology

Pipeline Pigging Technology

Fig.4. Leakage past a sphere.
about Imph (0.5m/sec). However, with an increase in velocity, it has already
been established that there is a very small decrease in differential pressure,
and the graph shows that there is similarly a decrease in flow forward. This
supports the previously-mentioned assumption, but it does beg the question
as to whether, if a small increase in differential pressure could be induced
when operating at velocities above this theoretical optimum, it would not
only increase the flow forward, but also significantly decrease the flow back.
It must be said that in the development of a computer program called
TAPTWO during 1978[6], Kohda et at, also at Nippon Kokan KK, contra-
dicted some of the previous findings. In particular, they stated that "pressure
drop across a pig is independent from the pig velocity and a function of pig
diameter". This statement is certainly valid for the pig diameter, and may have
some validity with respect to velocity too, if the change in pressure drop is
considered in relative terms. Certainly, the pressure drop with increase in
velocity is very small, but it could be vitally important.
Some relatively-simple research, followed by some basic design engineer-
ing aimed at controlling the differential over a very small pressure range, may
well result in the ability to tailor a pig to provide optimum (and predictable)
performance for any particular pipeline, regardless of its velocity.

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