Calpine Geysers? Units 18 and 20?s Stretford System Froth and Balance Tank Level Measurement

Abstract

Depending on the global location of the geothermal plant, H2S abatement may be required to meet local environmental regulations. At Calpine?s Geysers, a number of the operating power plants have Stretford H2S Abatement systems while others are equipped with burner-scrubber systems. The primary abatements systems for surface condenser vent gases are the Stretford Abatement systems. Burner-scrubber systems are in use on the direct-contact condenser units. Ten of the Geysers power plants use Stretford Abatement; the five remaining units use the burner-scrubber system. This paper focuses on the liquid level measurement technology in use in the Stretford system froth and balance tanks. These tanks were initially designed with ?bubbler? level measurement systems, when the plants were constructed by Pacific Gas & Electric (PG&E) in the 1980?s. Tank level measurement in these systems is particularly challenging because the Stretford solution and froth are two phase solutions containing high concentrations of suspended solids and have a highly variable solution density. Lower density foam and froth layers tend to form on solution surfaces causing false level indications in instruments that depend on solution height and density such as bubblers. Additional difficulties are encountered when suspended solids settle in the tank bottoms, effectively reducing the tank volume. In 2012, the bubbler systems were replaced with non-contacting radar level transmitters. Non-contacting radar level transmitters have been used on the froth and balance tanks since that time. The Stretford process is reviewed; the froth and balance tanks, which are part of the Stretford Abatement system, are detailed; bubbler technology is over-viewed; and finally, non-contacting radar as a solution in these applications is discussed.