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Structural Integrity

During the 1950s and ‘60s, several deaerator manufacturers fabricated units with medium carbon steel, keeping vessel metal thickness below ½ in. and eliminating the need to stress-relieve the welds. Numerous units developed weld cracks because of cyclic thermal and circumferential stresses, and several experienced catastrophic failure. Since then, many units have undergone weld repair, stress relieving, and recertification. Most insurance carriers now require a thorough nondestructive inspection of all welds every five years. This condition is somewhat different from vessel internal corrosion, which is caused by ineffective water treatment or deaerator performance.

When properly installed and controlled, deaerators are designed to provide continuous boiler operation, incurring significant load changes without operator involvement. Variability in pressure, temperature, or water level in a deaerator can cause water hammer, auxiliary-equipment malfunction, and boiler operational anomalies. Therefore, deaerators generally are well-instrumented. As such, the instrumentation data available not only can provide system reliability, but critical information for a boiler-system critique.

A deaerator chemical balance can determine the percent of condensate return from the process systems required for a boiler-system technical analysis. Generally, commercial and industrial plants should achieve a minimum of 75- to 80-percent condensate return unless steam or condensate is used for direct injection in the process (Figure 3).

A deaerator energy balance then can be developed to determine deaerator steam usage and other energy data needed for a critique of the entire steam system (Figure 4). After establishing condensate return and deaerator steam usage per thousand pounds of steam produced, an overall energy balance of the boiler can be developed. Knowing energy values, such as fuel energy per unit of steam production, pound of fuel per pound of steam, or British thermal units of fuel per British thermal units of steam, adds to a critique of overall steam-plant operation (Figure 5).