Page 407 - Pipeline Pigging Technology

Pipeline Pigging Technology

Fig.3. The 'nose-dive' phenomenon.

a piggable lateral connection used in the Adriatic Sea. The information
presented herein will strictly relate to forged wye fitting configurations.
In both the symmetric and non-symmetric wye configurations, the length
of the crotch opening is approximately:

Sin(a)

where (D) is the inside diameter of the wye fitting and (a) is the angle
between inlets.
The length of the crotch opening is an important design consideration for
a piggable wye fitting. This length effectively defines the distance required
between seals to prevent a by-pass condition that could stall the pig inside the
wye fitting. Generally, selection of the angle (a) involves some trade-offs. For
example, reducing the angle between inlets will decrease the magnitude of
the turn that must be negotiated by the pig, so it tends to be viewed as
improving the pigging geometry. However, the length of the pig may have to
increase to avoid a by-pass condition and, therefore, there may be no
improvement, or in fact a reduction in piggability. Moreover, the longer
crotch opening generally leads to higher stresses in the fitting, which adds to
the cost of the wye fitting. In general, for any given application, selection of
the angle between inlets should be made taking into consideration the
particular types of pigs to be used as well as the overall cost of the fitting.

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