Page 416 - Pipeline Pigging Technology

Pigging through Yfittings

ues to drop until 27.3secs, indicating that the front portion of the pig has just
reached the downstream pressure transducer location. It should be noted
that after the back end of the pig passes the downstream pressure transducer,
both pressure readings in Fig.7 should be identical. Using dividers, one can
compare the two pressure charts starting from the right end and moving
leftward until a difference in readings is observed. This occurs at 28.8secs.
Therefore, the time required to travel down the transit spool is 12.6secs (28.8
minus 16.2). The average velocity is then calculated by dividing the distance
between transducers (18.75ft) by the travel time. Hence, the average velocity
is 1.49ft/sec. The average flow rate can then be calculated by multiplying the
average velocity by the flow area inside the pipe.
It should be noted that the error in the above technique is generally related
to the length of the pig, since it is often difficult to determine if the entire pig
is past the pressure transducer or just a portion of the pig. This is particularly
evident for long pigs with multiple seals.
Referring again to Fig.7, it is seen that the peak differential pigging pressure
occurs at 27.3secs when the upstream pressure is 4lpsi and the downstream
pressure is 8psi. Hence, the peak differential pigging pressure is 33psi while
the pig is in the transit spool.
Reviewing the flow rates in Fig.6 shows that the flow rate in both sides
increases after the exhaust valve opens. The flow rate on the side with the pig
(the "A" side) reaches a peak at approximately 470gpm and then gradually
decreases as the pig travels down the transit spool. At the point where the pig
reaches the wye fitting, the flow rate on the "A" side has dropped to
approximately 240gpm. Over the same period the flow rate in the opposite
side (the "B" side) remains reasonably steady between 370 and 400gpm. As
the front of the pig enters the fitting (just after 27.3secs), the flow rate in the
"A" side increases to approximately 360gpm. This behaviour is typical for
most types of pigs, and is attributable to filling (pressurizing) the opposite side
transit spool ("B" side), increased flow by-pass and, in many instances, an
increase in pig velocity while inside the fitting.
Referring to Fig.8, it can be seen that the "B" side down-stream pressure
falls after the exhaust valve is opened, and continues to drop until 29.7secs.
The pressure spike at 29.7secs indicates the front of the pig has entered the
wye. After the pig is completely past the pressure transducer on the down-
stream side of the wye, the two readings in Fig.8 should be identical. Hence,
the charts can again be compared, starting with the right hand side and
working to the left to locate where the curves start to differ. In this case, it is
found that the curves differ at times prior to 31.2secs. Therefore, at 31.2secs,
the pig is completely in the outlet. Also, by inspection, the peak differential
pigging pressure while the pig is in the fitting can be determined. In this case,

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