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Exchanger Equipment Areas - Old Problems and
New Solutions

High Pressure Waste Heat Boilers

Certain designs of vertical process gas waste heat
recovery boilers have had problems with corrosion
failure of tubes when the shell side fluid is boiler
feedwater at failure regions near the tubesheets.  
Accumulation of corrosion damage is slow in early life
of the equipment, but accelerates after about 10-15
years of service for such exchangers, often mounting to
multiple failures per year, each involving a temporary
repair outage.  Retubing or plugging when failures begin
to mount is time consuming and difficult and is generally
is done in the field, because the equipment is
welded-in, connecting to other equipment, such as
water-jacketed transfer lines and boiler high pressure
steam system piping.  A permanent solution that has
proven successful for more than a decade is to replace
the waste heat boiler with a new design with boiler
feedwater arranged up-flow in the tubes to eliminate
the possibility of trapping corrosive boiler dissolved
solids on the outside of tubes in the lower region of the
bundle shell side.  The cost of such retrofits can be
similar to or only modestly greater than replacement of
the exchanger in-kind.  When historic costs for outages,
repairs and lost production are weighed, such new
designs have strong economic advantages.
Process Gas Feed/Effluent Exchangers

In most Ammonia process designs, heating and cooling of synthesis gas streams is accomplished by reactor effluent being
used for heating reactor feed, recovering a substantial part of exothermic reaction heat.  Examples of such heat exchanger
equipment include Methanation and Ammonia Synthesis feed heating.  When plants are expanded in capacity, these
exchanger services often become a reliability problem because of tube leaks from failures caused by shell side induced tube
vibration, resulting in wear from contact with adjacent tubes or shell baffles.  Generally the plant contractor designs
exchangers for modest increased capacity to maximize contract profits.  Shell side gas velocities induce tube vibration and
develop into tube leaks when plant rates are pushed typically beyond 20-40% higher production through incremental
expansion projects.  It is not always clear why these exchangers initially begin to fail because the expansion projects are
generally spread out over several years.  Usually such tube failures do not show up in any consistent pattern, but instead are
spread out rather uniformly throughout the tube bundle.  Time is not so much the cause of these failures, but instead, plant
rate is the key initiator.  Plugging tubes in brief repair outages will buy time to confirm vibration as the failure mechanism and
slightly curtailing plant rate can reduce the frequency of these tube leaks.  Sophisticated heat exchanger design and rating
software can identify key parameters that indicate likelihood of vibration induced tube leaks, including tube span, crossflow
velocity, critical velocity, fluid flow-energy (Density times Velocity squared), vortex shedding frequency, Strouhal number,
frequency ratio, turbulent buffeting frequency, baffle and collision damage numbers and other related parameters.
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