High Pressure Waste Heat Boilers

Exchanger Equipment Areas - Old Problems and New SolutionsHigh Pressure Waste Heat BoilersCertain 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 ExchangersIn 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. For heat exchangers where vibration damage is indicated from inspection (pulled tubes have long external wear patterns or baffle notches), replacement equipment procurement should be seriously considered to maintain high on-stream time.  New designs to replace damaged heat exchangers should examine higher (existing and future) plant capacity as a design basis and/or include means of "stiffening" the tube bundle, such as NTIW (no tubes in windows) baffles to shorten the unsupported tube span, use of thicker baffles or tighter tube/baffle clearances and consider other improvements such as changing baffle or shell type, the use of impingement plates, increasing the drop under shell side nozzles or changing tube thickness, tube pitch or tube orientation.