Abstract
Some major disadvantages of traditional drum water level gauges for high pressure boilers are considered, and features are presented of a new type of gauge—the ‘Hydrastep’—based on a series of independent electrical resistance measurements made concurrently to determine the presence of water or of steam at discrete levels in a side-arm vessel.
The thermal considerations are described which result in a density error for Hydrastep systems of only one-sixth of the error of conventional visual gauges, and evidence is presented of the validity of resistance measurements for discrimination between water and steam at temperatures up to at least 360° C. Philosophies of redundancy, as well as of the use of parallel binary information collection and transmission, are incorporated in the basic gauge to give both ‘fail-safe’ and ‘fail-operative’ characteristics. The application of these philosophies is described to systems using a number of Hydrastep gauges on one boiler, whereby enhanced security of measurement may be achieved.
Reference is made to the field trials to which a number of single Hydrastep gauges have been subjected on boilers operating at both 110 and 183 bar (1600 and 2650 lb. f/in2, 318 and 358° C respectively), and to the electronic reliability and electrode survival times achieved. Both indication and alarm facilities were provided, and the possibility is mentioned of extending the use of the equipment to provide signals for automatic control purposes, without endangering security of indication. A guide to the costs is included.
Hydrastep has been approved by H.M. Chief Inspector of Factories as a legally acceptable alternative to the visual sight glass (Factories Act, 1961: Part II Section 32). The system is protected by U.K. and Foreign patents.
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