Page 408 - Pipeline Pigging Technology

Pigging through Yftttlngs

The angular turn associated with the piggable wye fitting gives rise to a
phenomenon known as "nose-diving". Fig.3 shows a typical dual-module pig
passing through a wye fitting; when the front module is fully in the outlet
section and the rear module is still located in the inlet section, there is a
significant bind on the connecting joint between the two modules. This is
encountered because the rear portion of the front module tends to centre
itself coincident to the axis of the outlet, while the front portion of the rear
module tends to centre itself coincident to the axis of the inlet. This action
results in the connecting joint being pulled in different directions, and causes
the seal loads and resulting frictional drag to increase as the rear module
approaches the outlet. Although this is similar to the problem of pigging
through a mitred joint, there are several distinct differences. First, the
presence of the crotch opening has the effect of reducing some of the seal
compression, and hence the drag forces on the pig. Secondly, the rear module
can move slightly toward the centre of the wye, further reducing the frictional
drag. The net increase in frictional drag loads associated with the "nose-dive"
phenomenon is one of the main reasons for differential pigging pressures to
increase inside the fitting on multiple-module pigs. It should be noted that the
"nose-dive" phenomenon is sensitive to the magnitude of the angular turn
made by the pig and, therefore, is worst on non-symmetric wye geometries.
A considerable number of pigging tests have been conducted to evaluate
the operational performance and pigging characteristics of various types of
pigs passing through a wye fitting geometry. For the test results presented
herein, a nominal lOin x lOin x lOin symmetric fitting was used with a 30°
angle between inlets. A symmetric configuration was selected because
several dual-module pigs were to be tested. Based on geometric considera-
tions and studies with scaled models[5], it was believed that loads on the
connecting joints would be unacceptable if a non-symmetric configuration
was used.

PIG-TESTING FACILITY

A pigging facility was designed and built to test various types of pig under
a wide range of flowing conditions. The pigging facility is illustrated
schematically in Fig.4. It was decided to use compressed air to pressurize a
water tank and generate flow, rather than a conventional approach using
pumps. This was done because very high flow rates (in excess of 50,000brl/
day) could be achieved for short periods at relatively-low cost. Also, the
system could be adaptable for gas tests using air rather than water. A

389