Chemical Engineering October 2020 - 28

FIGURE 4. Pipe clamps and braces can provide
support in two directions, while also allowing axial
movement
FIGURE 5. Axial movement can occur via the slight
bending of the two-force guides
of a wide flange beam, the stiffness
of that anchor is questionable. Weak
axis bending and torsional instability
of the beam may reduce the effectiveness
of the anchor. If the anchor
isn't relatively rigid, then all other
stress and displacement calculations
based on that anchor location may
be called into question. In designing
an anchor, it is critical that the
anchor and the structure supporting
it are evaluated together to ensure
they interact together to achieve the
desired performance.
Guides are applied to allow axial
movement, generally due to thermal
expansion or contraction, and
to restrict lateral movement due to
wind loads, plant vibrations, or even
seismic forces. When a pipe is running
along a pipe rack with regularly
spaced support beams, the selection
and evaluation of the guides is
simple. For these situations, many
off-the-shelf guides from pipe manufacturers
exist. Selecting guides
for some other conditions can be
more challenging - for example, a
pipe running a few feet below roof
steel, or vertically a couple of feet off
28
structural steel. In these instances,
an off-the-shelf catalog guide probably
won't work without incurring
significant costs in additional structural
steel. A low-cost solution for
examples like these is to use pipe
clamps, and either angle iron braces
or threaded rod and turnbuckles to
create two-force members that provide
support in two directions, but
allow axial movement via the slight
bending of the two-force members
(Figures 4 and 5).
Slides are applied in very similar
instances as guides and are typically
used together in thermal expansion
and contraction applications.
Slides provide support in one
direction (generally perpendicular to
the pipe), most commonly supporting
a horizontal pipe from below.
Like guides on a pipe rack with
regularly spaced beams, slides can
be selected in a straightforward
manner, and there are many off-theshelf
slides available. They are typically
selected to allow two degrees
of freedom, usually lateral and axial.
But once again, in instances where
there is not a uniform framing level
to support the slides, the alternatives
must be investigated. In many
cases, a simple clevis hanger can
be used, as long as the hanger rod
FIGURE 7. If the post can tolerate bending, then a
U-bolt clamp may work as a pipe support
is long compared to the expected
displacement of the pipe.
Practical pipe supports
Taking a practical approach to pipe
supports can simplify the design of
the support system and can usually
make supports less expensive. The
following are some examples of notso-practical
supports, along with a
brief explanation as to why they are
impractical, and a better alternative
to the example.
Tall post with pipe slide, guide, or
anchor on top. A tall post is likely
going to experience a relatively large
deflection at the top, depending on
the section properties of the post,
and even with a polytetrafluoroethylene
(PTFE; Teflon) or ultrahigh-molecular-weight
(UHMW) polymer slide or
guide, there is still a friction force that
will be applied to the post (Figure 6) .
FIGURE 6. A tall post with a pipe slide, guide or
anchor at the top is likely to experience relatively
large deflection at the top
FIGURE 8. One concern surrounding tall posts with
cantilevers involves the torsion of the post
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM OCTOBER 2020
http://WWW.CHEMENGONLINE.COM
Chemical Engineering October 2020

Table of Contents for the Digital Edition of Chemical Engineering October 2020
