The auxiliary boiler feedwater is exposed to the same contaminants as the propulsion boiler feedwater. Auxiliary boilers are generally used for hotel service loads, and shore water used for feedwater is usually the prime source of contamination. The shore water may contaminate the feedwater system by leakage through malfunctioning galley mixing valves, laundry equipment, and

Figure 7-2.Troubleshooting guideauxiliary boilers.

hot water heaters. Shore water is usually hard water which contains high concentrations of dissolved solids and silica. Although it can have either high or low pH, in a boiler, shore water usually causes high pH. High concentrations of dissolved solids lead to boiler water carryover with the steam. Silica may be deposited on the boiler watersides and in the steam system as it vaporizes. Water hardness leads to excessive usage of boiler water treatment chemicals which causes corrosion, scale, and sludge buildup. Excessively high pH causes caustic embrittlement and subsequent erosion of boiler metal parts.

Firetube and watertube auxiliary boilers are natural circulation boilers. The water treatment for natural circulation auxiliary boilers is maintained in the same manner as the propulsion boiler water. Section 21 of Naval Ships Technical Manual, chapter 220, volume 2, describes this water treatment. The control parameters for auxiliary boiler water are alkalinity, phosphate, and chloride. In auxiliary boilers, the alkalinity of the auxiliary boiler water is measured instead of the pH because its higher alkalinity level can be more easily measured by the alkalinity test than by the pH meter test. The alkalinity range is equivalent to a pH range of 11.0 to 11.3. The auxiliary boiler limits are given in table 7-1.

The same treatment chemicals, trisodium phosphate dodecahydrate (TSP) and disodium phosphate anhydrous (DSP), are used for auxiliary boiler water treatment except that a higher level must be maintained due to the lower operating pressures. The TSP provides alkalinity and phosphate. The DSP provides additional

Table 7-1.Auxiliary Boiler Water Limits For Firetube and

phosphate without significantly affecting the alkalinity.