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Pipe racks

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I have no disagreements with William, but to add to it...

Expansion joints are being regulated out of many services due to zero
emmission requirements.  Pipe loops are a good way to deal with thermal
expansion with hard pipe instead of expansion loops.  A lot of
industrial facilities make things interesting by changing/adding
processes and forcing you into some highly constrained geometries. 
Friction can conspire with cyclic thermal loads to create
"walking"...I've seen pipe shoes clear off the beam due to that.  I've
also seen pipe racks themselves bent by thermal forces.  For most
processes and pipe racks, the two big issues are static loads (including
the localized loads due to big honkin' valves) and thermal loads.  Of
course, you have to deal with seismic if you're in such a zone.

Don't forget the fluid forces, both due to valve closure/opening as well
as pumps.  That gets overlooked when *just* the structural issues are
looked at.  Valves are more of an issue with larger bore (24" and above)
piping.  Pumps can just be ornery, esp. if you don't look at the local
response tie-in upstream and downstream of the pump...but unless its a
very "bad" fluid or a critical site, you normally don't get into this
unless something is already having a problem.  

There are several pipe stress packages that do a good job of handling
all of this, to include modeling the pipe rack themselves.  CAESARII is
standard for much of the chemical industry...the Algor PipePlus package
is a bit easier to use, though.

Bart Kemper, P.E.
Baton Rouge, LA


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From: "Sherman, William" <ShermanWC(--nospam--at)cdm.com>
To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
Subject: RE: Pipe racks

The forces on the structure due to piping depend on the details of the
pipe
joints, the layout of the piping, internal pipe pressure, and on the
arrangement of the pipe supports. If the pipe joints are all restrained
(i.e. can transfer axial tension) and the supports do not restrain axial
movement, then there should be no need to resist static pipe thrust at
bends. But if there are unrestrained joints (e.g., pipe expansion
joints,
couplings, etc), then the internal pressure will cause unbalanced thrust
at
bends (or valves). (An easy way to look at pipe thrust due to internal
pressure is to draw force vectors along the axis of the pipe pointing
towards each bend. With restrained joints, the forces at each end of a
straight length of pipe will balance and will be resisted by tension in
the
pipe wall. But if the pipe tension is broken due to an unrestrained
joint,
then the force vector becomes unbalanced and must be externally
restrained.)


Pipes with restrained joints should be provided with "pipe guides" to
laterally and/or vertically support the pipe periodically against
movement
due to wind or seismic, but generally should not be axially restrained.
However, a single axial restraint near the center of a long run may be a
good idea to prevent longitudinal movement, but you should make sure the
bends can move freely to prevent restraint relative to the axial anchor.
Axial friction forces should be considered due to longitudinal thermal
movement or axial strain. Thermal movement causes very high forces if
you
attempt to restrain axial movement but only frictional forces if the
pipe is
free to expand axially. Friction coefficients for steel on steel can be
0.5
to 0.7; Teflon bearings can reduce friction to 0.04 to 0.10. 

If you have structural beams between supports, I would recommend
providing
slotted holes at one end support to allow thermal movement.

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