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Horizontal vessels-Friction forces

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We do a lot of heat exchangers and other horizontal drums for refinery and
power work where thermal expansion is a major concern.  It appears that
your propane tank will see only ambient temperature changes but none the
less should be considered.  Our horizontal tanks are slotted on both ends
because the tanks are purchased early in the job and the "fixed" end is not
determined until our pipe stress group runs their numbers.  Once the fixed
end is determined our standard drawings call out for a 1/4 inch plate
washer to be field welded to the saddle after installation.  At the sliding
end we double nut and leave 1/16 inch clear from the bottom bolt to the
saddle.  Most of our designs use a galvanized slide plate at the other end
and Teflon plates and pads for only very heavy vessels.

Your co-workers are correct at the sliding end, if the slotted holes are
long enough and all is installed correctly the sliding end anchor bolts
should not see any load from thermal expansion.  In fact, one of our
clients (Shell) do not use any anchor bolts on the sliding end but instead
weld bars on the outer ends of the slide plate to act as guides.  However,
the fixed end bolts should be designed for the full friction load.  Thermal
loads are not necessarily instantaneous and will build up forces until the
sliding end actual moves and after that it theoretically remains at that

Your bolts at both ends will definitely need to be designed for transverse
seismic and note that UBC section 1632.1 does not allow you to deduct the
friction resistance from the seismic force.

A 500 kip vessel warrant's the use of Teflon slide plates.  Note that
Fluorocarbon changed it name to Furon and now has sold off their slide
plate division.  It is now an independent company named Seismic Energy


Thomas Hunt, S.E.
Duke/Fluor Daniel

----- Forwarded by Tom Hunt/DFD on 03/02/01 12:18 PM -----
                    Rajendran            To:     seaint(--nospam--at)                                                 
                    <rakamaka@yah        cc:                                                                       
          >              Subject:     Horizontal vessels-Friction forces                           
                    10:56 AM                                                                                       
                    respond to                                                                                     

As far as I am aware of, horizontal vessel, supported
on two saddles, has slotted holes on one of the
saddles (sliding end) and standard holes on the other
saddle (fixed end). Horizontal force of friction due
to thermal expansion/contraction of the vessel is,
then, equal to the product of coefficient of friction
at the sliding interfaces and the vertical reaction at
the sliding end. Consequently, one of the loading for
which the anchor bolt on the "fixed" end should be
designed for is the friction force. Because this
loading is transient, one may use a higher allowable
stress or use a reduced load factor. This is what I
have learned and practiced for about 10 years.

I have changed jobs and the lead engineers (mechanical
and civil) in this company tell me that the above
noted friction force need never be considered in the
design of anchor bolt. Their reason is that the vessel
moves on the sliding end (and, probably on the fixed
end also because the hole diameter on the fixed end is
larger than bolt diameter)and, any friction force that
may develop is so quick and instantaneous that the
bolt will not experience it. The tank in question is a
propane storage tank with an operating weight of 500
kips. Based on my experience I figured that the
friction force will be very large and that in order to
reduce it, I suggested using Teflon sliding surfaces,
such as Fluorogold product. The engineers here say
that Teflon sliding pads do not work well and that the
pads easily come off the equipment. They also told me
that it is common to have both saddles with slotted
holes and that they have never heard of using friction
reducing sliding surfaces on horizontal vessels! Each
of them has more than 20 years with reputed and large
consulting engineering companies, whose design
standards echo what I have been practising.

Rick Drake and Christopher, in particular, (because I
believe that both of you have refinery and pipeline
experience), am I overly conservative in my design
approach and is my design criteria in error?

P. Rajendran