This
type of expansion joint is really a combination
of several types. Its purpose is to retain
and balance the pressure thrust so that
main anchoring of the pipe or adjacent equipment
is not required, and forces and movements
on attachment flanges of delicate equipment,
such as turbines, are kept to acceptably
low levels.
Briefly, the deflections to be accepted
are handled by the proper type of expansion
joint, which normally, and as shown in the
above sketch, is a tied universal type to
accept lateral movements. However, the pressure
balanced elbow is usually required because
axial deflections are also present. In order
to accept these movements, a bellows is
added beyond the elbow with the same cross-sectional
area as the ones in the universal section.
This balancing bellows is connected by the
tie rods to the pipe beyond the universal
section; in this way the pressure thrust
is contained as tension in the tie rods.
The section of the expansion joint between
the tie rods, which includes the elbow,
is now free to move axially, with the only
resistance being a function of the spring
rates of the bellows. Because of their arrangement,
however, the spring rate of the entire expansion
joint is the sum of the spring rates of
the balancing and the universal bellows.
This is a constant volume system, in that
when the universal end compresses, the balancing
end extends the same amount.
All of the lateral deflection is absorbed
by the universal end, and there is no lateral
deflection imposed on the balancing end.
Therefore, the balancing bellows is almost
always a single bellows type.
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