Floating Ring Design
Improved Floating Ring Design Results in Reduced Stress and Lower Cost
Hardware designed to restrain pressure thrust loads on hot
shell expansion joints; generally involves a ring to distribute
loads. Gusseted lugs seldom work as pressure thrust restraints
on hot wall pipes because the resulting local stress
in the pipe is excessive. However, fi xed rings can cause
severe stress in hot wall pipes because the rings never operate
at the same temperature as the pipe itself. At media
temperatures over 1000 F (the beginning of the creep range
for most pipe materials), the stress caused by fi xed rings will
cause cracks in the pipe over time.
Senior Flexonics Pathway has been building fl oating ring
type pressure thrust restraint structures for many years by
fi xing the fl oating ring in place on the hot wall pipe with shear
pads. These rings are held in place with pads that transfer
the pressure thrust loads into the pipe shell. However, the
number of pads per load point is practically limited to two and
there is a practical limit to the size of the pads. Occasionally,
the local stress in the hot wall pipe around the pads exceeds
allowable limits even though a fl oating ring is used.
To solve this problem, Senior Flexonics Pathway engineers
developed a new fl oating ring concept in which
the primary load is transferred to a location where the
temperature is lower and the allowable stresses are
higher. In this improved fl oating ring design (featured
on the front) the pressure thrust load is transferred
through a shear ring circumferentially welded to the
expanded pipe weld end that is typically used on hot
wall expansion joints. The fl oating ring is centered by
the use of at least four slotted connectors that hold the
floating ring structure against the shear ring while allowing
for independent radial motion of the shear ring
and fl oating ring.
Finite element analysis indicates that this construction
results in signifi cantly lower stresses than the conventional
floating ring design. A rigorous thermal analysis of
the improved design also indicates that temperature of
the floating ring is low enough to allow the use of lower
cost carbon steel or low alloy steel for the fl oating ring.
With the conventional box ring design mounted directly
on the pipe, the ring material must be the same material
as the pipe, usually T304-H stainless steel.