Page 412 - Pipeline Pigging Technology

Pigging through Y fittings

side of the transit spool on the opposite side, i.e. the side opposite to the
transit spool containing the pig or pig train.
The three 2-pin recorders were attached to a synchronizing device which
marked each chart, so that events could be measured relative to some initial
time. The pressure transducer measurements were found to be excellent pig
signallers in addition to being used to measure the differential pigging
pressures. This signalling feature allowed location of the pigs which, coupled
with the relative time and knowledge of the geometry, allowed direct
computation of average pig velocity between known positions.
The instrumentation was modified somewhat for air-driven pigging tests.
Since the flow meters were not usable for air tests, four pressure transducers
were used in the transit spool used to pass the pig (two each on two 2-pin
recorders). The additional pressure readings in the transit spool allowed
calculation of a velocity profile rather than an average velocity, which is useful
due to the greater difficulty in conducting pigging tests with gas. The other
two pressure transducers were used to record the pressure on the outlet side
of the wye and the air tank pressure.
The tank pressure vs time curve was used to approximate the air flow rate
out of the tanks. This was done by determining the rate of change of tank
pressure with time. The flow rate is then calculated as:

Row rate = V xdp
14.7 dt

where V is the volume of the two tanks (air and water) and the rate of
change of pressure with respect to time was determined using finite differ-
ence techniques with the data from the tank pressure vs time chart. It should
be noted that the flow rate is not particularly useful in characterizing pig
performance under conditions of compressible flow. Generally, the velocity
and differential pigging pressure are more useful parameters and better
characterize pig performance.

TEST PROCEDURES

For water-driven pigging tests, the following basic procedures were
followed:

1. The air tank was pre-charged to the desired pressure (charging
pressure varied between 50-125psi depending on the flow rate).

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