pipe movements where movement is necessary to
so this expansion and contraction cannot take place,
avoid overstress due to expansion or contraction of the
large forces are generated. For example, a 40-foot long
piping. Rigid supports are generally used in cold piping
straight section of 6-inch, schedule 40, carbon-moly
systems. Cold systems in this context means systems
steel pipe will elongate about 3 inches when heated
where the fluid temperature does not exceed 120F.
from ambient temperature to 454C (850F). If the pipe
One end is usually bolted or welded to a clamp on the
is confined so that expansion (or bending) cannot take
pipe and the other end bolted or welded to the ship
place, a force of 870,000 pounds is generated within the
structure.
piping material. As containment of a force of this
magnitude is difficult, the procedure normally used is
Similar alternate rigid-type designs using pipes and
to allow the expansion or contraction to take place and
angles instead of flat bars are also commonly used.
absorb the motions in expansion bends or expansion
Since the flat bar is relatively weak in bending in one
joints.
direction, it is often necessary to use supports
incorporating either angles or pipe to provide the
PRESTRESS
necessary rigidity to resist movement of the pipe that
would occur due to ship motion or shock forces.
Expansion bends and joints can be moved in both
directions from their normal position. Install them so
In hot piping systems, because of the movement of
they are stressed in one direction when piping is cold,
the piping due to thermal expansion, it is generally
and in the other direction when piping is hot. The
necessary to use resilient supports. In some locations
appropriate ship drawings indicate the amount and
where the pipe movement is small, it is sometimes
direction of this prestress or rold pullup to be applied to
possible to use rigid style supports. To balance the load
the piping and expansion bends or joints during
between rigid style supports and resilient supports, it is
installation in the cold condition. Carefully follow
necessary to make rigid types adjustable. Generally this
these instructions to prevent overstressing and possible
is accomplished by using threaded rods or turnbuckles.
failure of the expansion joints, piping, and connected
VARIABLE-SPRING PIPE
equipment.
SUPPORTS
PIPE HANGERS, SUPPORTS,
Variable-spring supports are generally used in hot
AND SWAY BRACES
piping systems to carry the weight of the piping while
Hangers and supports are used to describe the
allowing movement of the piping due to thermal
various devices used to carry the weight of piping
expansion. The variable-spring pipe support consists
systems. Although often used interchangeably and
of a heavy enclosed spring, generally attached to the
sometimes even together, the term hanger more
pipe, using an adjustable rod and pipe clamp. The
correctly refers to a device that carries the piping weight
variable springs are marked by the manufacturer with
from above, and the term support generally refers to a
hot and cold load indicator settings. The term variable
device that carries the piping weight from below. The
comes from the characteristic of the unit that the force
term support is commonly used, in a broader sense and
applied to the pipe is directly proportional to the
in this chapter, to include any device that carries the
movement of the pipe. The cold setting is calculated by
piping weight from above, from below, or even from
the designer and it facilitates adjusting the spring
the side.
support with the piping system cold. When the piping
system is at its maximum operating temperature, the
Supports are generally arranged so that they carry
load indicator should be at the hot setting. Figure 15-33
an equal share of the piping weight so pipe stresses due
indicates some applications and examples of this type
to bending effects are minimized. When heavy load
of support.
concentrations, such as vertical runs, valves, and
strainers are present, additional supports are provided
CONSTANT SUPPORTS
as close as practical to these items.
Design of supports are either of the rigid (solid)
Constant supports arc also used in hot piping
type or resilient type. Rigid types provide significant
systems and provide a constant supporting force for the
restraint in at least one direction. Resilient supports
piping throughout the expansion and contraction
include one or more elastic members, such as springs
deflection cycle of the piping system. This is done by
or rubber elements, and are selected to permit limited
the use of a heavy encased spring working with a
15-34