Page 420 - Pipeline Pigging Technology

Pigging through Yfittings

All of the pigs successfully passed through the symmetric wye geometry
without problem. None of the pigs were damaged as a result of the excursion
through the wye fitting and no damage was observed to the fitting during any
of the tests. The pigs demonstrated several consistent performance features
as follows:

1. The peak pigging differential pressure in the fitting was generally less
than that encountered in the transit spool when medium to high
flow conditions occurred in the opposite side and the pig was
travelling at speeds greater than 1.35ft/sec. This "flow assist" effect
appears in all of the pigs that were tested.
2. At low pigging speeds, i.e. less than 1.35ft/sec, the peak differential
pigging pressure in the wye appears to increase when medium to
high flow occurs on the opposite side.
3. At high pigging speeds, the short, light, single-module pigs pass
through the fitting without difficulty and generally require less peak
differential pigging pressure in the wye than in the transit spool.

In addition to the test results presented in Tables 2 through 11, stall tests
were also performed on each type of pig. In the stall tests, the pigs were
positioned in the wye fitting by pigging very slowly until a by-pass condition
was observed. Flow was then increased, and in each case the pigs moved
freely through the fitting and through the outlet without incident. Several pigs
were checked for stall characteristics using gas (air). It was found that for the
three pigs tested, (TDW Redskin foam pig, S.U.N.Engineering squeegee pig
and F.H.Maloney sphere), none of them could be stalled in the fitting. This
characteristic was found to result from the wye fitting ID being slightly smaller
than the transit spool in the area just before the crotch opening. When the
pigs were slowly pigged into the wye fitting, they would stop at the restriction
until the pressure was increased sufficiently to force them past the restriction.
As soon as the pig passed the restriction, the stored energy in the transit spool
was sufficient to push the pig through the wye fitting and into the outlet. This
characteristic is of extreme importance, and suggests that wye fittings can be
made more "pig-friendly" by relieving the ID just in front of the crotch area.
Several pigging tests were also performed on a Select Industries soluble
sphere. However, since there is essentially no differential pigging pressure
required, the conventional data reduction techniques could not be used. In
the two tests performed, the soluble sphere passed through the wye fitting at
flow rates of approximately 75gpm and lOOgpm, respectively, with no
problem. In fact, during these tests, the soluble sphere actually flowed uphill
and through the fitting without falling down to the opposite side, which had
no flow and was at a lower elevation than the outlet.
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