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Pigging research

Fig.3. Flow velocity vs pressure to move a sphere.
the numerous interesting observations was: "that the actual pressure required
for transporting two spheres simultaneously is 10% smaller than the sum of
the pressures for transporting a single sphere".
They also noted that the 5D bends showed no increase in the differential
pressure across the sphere, and "can be assumed as parts of the straight line".
Of particular interest, however, was the reduction of differential pressure
across a sphere, with an increase in velocity. This is shown graphically in
Fig.3, which is reproduced from their paper. Although these reductions were
2
only of the order of O.Tpsig (O.OSkg/cm ), when the velocity was doubled
(from Im/sec to 2m/sec), it may well have a significant impact on the results
of some further research which they carried out, details of which were
published early in 1979[5].
This later work was designed to study the mixing of dissimilar fluids when
separated by spheres at the interface. It produced a great deal of interesting
data concerning sphere performance in general.
It confirmed that although the frictional resistance (and hence differential
pressure) is nearly constant, it does decrease slightly with increasing velocity.
It was noted that flow forward and flow back was equal at a velocity of
about 1.3m/sec (4.3ft/sec). At lower velocities, flow back decreased, but flow
forward increased while at higher velocities, the reverse applied; the graph
showing this is reproduced in Fig.4.
They made the reasonable assumption that product flows forward due to
the frictional resistance of the sphere (i.e. the differential pressure) and flows
back due to product viscosity.
In pigging, it is generally the flow back which needs to be minimized (e.g.
for dewatering, condensate removal, etc.) so for optimum liquids' removal
using a sphere, these tests indicate that speeds should probably not exceed

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