Page 417 - Pipeline Pigging Technology

Pipeline Pigging Technology

the peak differential pressure occurs at 29.7secs, and is the difference
between the inlet side ("B" side) pressure of 43psi and the outlet pressure of
6psi, i.e. 37psi.
It should be noted that measurement errors are possible in several parts of
the above procedure. First, there are small differences between pressure
transducer readings. For example, comparison of the pressure readings at the
start and end of the test shown in Figs 7 and 8 demonstrates as much as 3psi
difference in readings at different locations. Measurement of transit spool
average velocity (and average flow rate) also has errors associated with
judging whether the front end or rear end of the pig is at the transducer
location. Hence, the location error could be in the order of magnitude of the
pig length. In some tests this is significant, since several of the pigs were over
4ft long (more than 20% of the separation distance between transit spool
transducers).
Although the quantitative results will clearly have some associated error,
it should be understood that the most important observation and, in fact, the
main objective for most tests, was to verify that the pig or pig train would
successfully pass through the wye without damaging the pig or the fitting. The
flowing conditions, average pig velocity and differential pigging pressure
serve primarily to describe the pigging conditions. It is generally believed that
the pressure measurements are within ±4psi throughout all tests. The error
in average velocity (and average flow rate) in the transit spool will be greatest
on the long pigs (TDW dual-diameter scraper pig, VetcoLog intelligent pig,
and the pig trains involving the TDW dual-diameter scraper pig), and could be
as high as 25%.

RESULTS

The results of the pigging tests are summarized in Table 1 (page 404). The
various pigs are ranked by the differential pigging pressure from lowest to
highest. The small, light, single-module pigs such as the foam pigs and spheres
demonstrated the least pigging differential pressure required. The larger,
dual-module pigs such as the Knapp Polly Pig dual-diameter (14x10) gauging
pig, TDW dual-diameter (14 x 10) scraper pig and VetcoLog intelligent pig,
required significantly higher differential pigging pressures.
It can be seen that a considerable range exists in the differential pigging
pressures recorded for any particular type of pig. There are several important
factors that account for these variations. First, the results presented are an
accumulation of data from three different test programs performed as part of

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