waste heat boilers (WHB) generate steam while cooling a process fluid. Depending on process and steam system considerations, varied boiler designs and pressures are utilized. Some designs are susceptible to waterside corrosion problems and tube failures may be prevalent. Examples are used to illustrate various failure mechanisms involving different WHB designs.
INTRODUCTION
There are many different types of waste heat boilers. All are designed to produce steam while cooling a process fluid. An important function of the WHB is to improve process economics by reclaiming waste heat. Boiler designs include firetube or watertube, with natural (thermal) or forced circulation on the waterside. The tube bundle geometry can be vertical, horizontal, or U-tube depending on plant requirements. One design utilizes a bayonet and scabbard tube configuration. WHB pressure typically ranges from 25 psig to 1800 psig (0.2 to 12.4 MPa), although higher pressure units are sometimes used. The boiler makeup may be demineralized water, reverse osmosis permeate, or zeolite softened water. The tubing may be fabricated from mild steel, low-alloy steel, or stainless steel (austenitic, duplex, or ferritic stainless steel), depending on BFW quality, heat flux and the process environment.
Waste Heat Boiler failures have been documented in the literature1-7. Problems can be related to design, operation, water treatment, or a combination of factors. The boiler design may be based primarily on process unit considerations, while making compromises on the water side. Plant operators may not be dedicated to boiler operation, and inadequate operator attention and/or training may be an issue. Some high pressure WHB designs leave very little tolerance for deposition or deviations in boiler water quality. To avoid internal corrosion, especially with higher pressure WHB, the water chemistry must remain within strictly controlled guidelines at all times. Even short-term contamination of the boiler feedwater may lead to corrosion-induced failures within weeks or months, particularly if a boiler system is dirty.
Some WHB only recover waste heat, for example from a flue gas convection pass. In such a case, an unscheduled outage caused by a tube leak may not be a critical problem. Other WHB are integrated into the process and are an absolute necessity to the process operation, where failure of the WHB would simultaneously require shutdown and loss of production. In this situation, unit reliability is essential from a plant operations standpoint and total costs associated with a single failure can exceed the total cost of the boiler2. This paper will review waterside mechanisms that can lead to boiler tube failure. Examples will be used to illustrate failures involving different WHB designs.