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Since higher pressure boilers demand lower boiler water silica concentrations to ensure the minimum silica carry-over in the steam, constant manual sampling and analysis are necessary. The silica limit for the 2000-psi boilers at the Los Angeles Department of Water and Power Scattergood and Valley steam plants is 1.0 ppm. The limit for the main steam is 0.02 ppm silica. The normal laboratory hand method consumes about 30 min for a silica determination. However, in most instances, when a generating unit is returned to service in the early morning hours with expectation of being at full load by 8 a.m., the silica concentration usually rises above normal. This is a period when there is no laboratory coverage, and the assignment of an operating assistant to make these tests is not practical. Some means of continuous automatic sampling and analysis was sought to assist the operating personnel in returning a unit to service and raising boiler pressure and load in accordance with a curve relating boiler pressure to silica concentration. After investigating several of the various types of silica analyzers on the market, the Department of Water and Power purchased an instrument in November 1960. The instrument is mounted on a collapsible cart making it completely portable. It has been used with good results at the Valley and Scattergood steam plants. It is also expected to see service during the start-up of the units at the new Haynes plant. The test method used is a modification of ASTM Method D 1689 — 59 T using the 1-amino 2-napthol 4-sulfonic acid reduction of the yellow silico molybdic acid forming molybdenum blue. However, since the samples are much smaller than the normal hand sample, the reagent concentrations are reduced (see Fig. 1). To minimize the precipitation of ammonium molybdate in the tubing, the amount used is reduced to 10 g per liter. The amino napthol sulfonic acid solution is diluted 1 to 10 before use. Five per cent oxalic acid instead of the usual 10 per cent is used. In an effort to eliminate the use of the poisonous oxalic acid, both citric and tartaric acids have been tested. No difference in the test results was found. However, both of these acids tend to form a mold or fungus growth within a short time, and small pieces could plug the tubing in the system. The use of oxalic acid was reinstituted. The three 4-liter reagent bottles are carried on the cart, and this amount is sufficient for two days of continuous operation.
Allen, R. C.
Laboratory Technician, Los Angeles Dept. of Water and Power, Los Angeles, Calif.