Boiler continued shutdown

During shutdowns, water can also condense on the fireside of the boiler. Each shutdown results in a recurrence of this condensation phenomenon. The moisture can combine with acidic residue on the tubes and cause severe corrosion. If allowed to continue, tube failures can occur. The problem is most pronounced in boilers burning sulfur-laden fuels, even when such fuels are burned only occasionally. 

Preparing a boiler for inspection requires the operator to first follow a sequence of specific boiler shutdown procedures, including cooling the boiler slowly to prevent undue stress to the metal and refractory. The primary purpose of the inspection process is to ensure continued boiler safety, and therefore a thorough inspection of all appurtenances is mandatory. 

This type of work is not an exact science. Consequently, other inspection and measurement methods are employed as an aid in identifying corrosion, deposition buildup, structural integrity flaws, and other types of problems. Methods employed generally are various types of NDT, and the protocols are widely used in preventive or predictive PV maintenance programs, especially for boiler plants used in continuous process applications, where a shutdown may be several years hence. Nevertheless, there is a drawback to the use of some types of NDT programs in that lengthy downtimes may be necessary to complete the testing work. Clearly, these can be cost-prohibitive. 

The sulfuric acid plant has boiler blowdown and cooling tower blowdown waste streams, which are uncontaminated. However, accidental spills of acid can and do occur, and when they do, the spills contaminate the blowdown streams. Therefore, neutralization facilities should be supplied for the blowdown waste streams (Table 15), which involves the installation of a reliable pH or conductivity continuous-monitoring unit on the plant effluent stream. The second part of the system is a retaining area through which non-contaminated effluent normally flows. The detection and alarm system, when activated, causes a plant shutdown that allows location of the failure and initiation of necessary repairs. Such a system, therefore, provides the continuous protection of natural drainage waters, as well as the means to correct a process disruption. 

The recoveiy of heat from waste gas in a waste heat boiler involves the combined operation of two processes, however. The drawback is that the waste heat boiler has to be shut down when- ever the kiln is stopped. This must be taken into consideration in the design and operation of the boiler. On the other hand, it should be possible to continue operating the kiln in the event of a fault or shutdown of the heat recoveiy system, without any adverse effects upon the burning process in the kila In my report I want to deal more particnlarly with the recovery of heat from waste gases in waste heat boilers. And I want to point out that a waste heat boiler should be considered only if there is no other possibility of utilizing the heat. 

Corrosion fatigue failure is generally not a common failure in boilers. The failure is observed when the boilers are not used continuously or subjected to frequent startups and shutdown. Proper control of startup and closedown procedures can eliminate corrosion fatigue. 

---