Level measurement can be tricky, especially when the fluid being measured has changing density and specific gravity. Emerson’s Cathy Ershen alerted me to a paper she, Emerson’s Pat Carroll and Calpine’s Anthony Derenia had published at the Geothermal Library.

The paper, Calpine Geysers’ Units 18 and 20’s Stretford System Froth and Balance Tank Level Measurement (access behind paywall), describes the challenge and solution around the process to reduce hydrogen sulfide (H₂S) to meet local environmental regulations.

Here is their 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.

A Wikipedia entry describes the Stretford process:

The Stretford process was developed during the late 1940s to remove hydrogen sulfide (H₂S) from town gas. It was the first liquid phase, oxidation process for converting H₂S into sulfur to gain widespread commercial acceptance.^[1]

Here is a description of the role of the froth and balance tank in H2S reduction:

The process in the froth and balance tanks consist of: foam; liquid; and liquid with solids near the bottom of the tank. Both tanks are agitated, open to the atmosphere and have a temperature of close to 100 °F. The purpose of the froth tank is to agitate the tank contents to a uniform consistency for the filter belt, the next unit downstream of it. Once filtered, if the sulfur meets product quality control limits, the sulfur can be sold as an agricultural soil amendment. The balance tank is used to feed the circulating pumps and ensure there is sufficient volume for them to draw from. The balance tank has a small evaporative cooler mounted directly over it, and, the liquid in the process has the color of root beer with some foam on top. Both tanks are agitated continuously.

The authors describe the bubbler system originally installed on the froth and balance tanks:

Just think for a moment of using a straw to drink liquid from a glass. Consider the glass as a tank and the straw, the bubbler tubing. As you blow through the straw you are sending “compressed air” through the liquid in the glass. The higher the level in the glass, the harder one has to blow.

Replacing this method of level measurement with non-contacting radar level transmitters:

…works independent of changing density or specific gravity. Thus, the solids in these tanks are not an issue, making radar a good fit for these applications.

Before using the non-contacting radar transmitters on the froth and balance tanks, they were proven for their reliable measurements on:

…condensers, cooling tower basins, and displacer-replacement applications.

Some physical installation challenges were addressed as part of the level measurement upgrade [hyperlink added]:

The balance tank has the cooling tower located directly above it and both the froth and balance tanks have agitators. Using a tool within the radar transmitters’ software called circular polarization allowed the radar units to be mounted within the constraints presented. Circular polarization mitigates the need to mount the radar unit at a specified distance from the tank wall; circular polarization has the capability to rule out obstacles, such as agitators and baffles that may produce echoes, which for some radar units, may register falsely as the surface level.

The authors conclude:

To date, the units have provided reliable tank level readings in tanks that have challenging processes and internal tank obstacles.

You can connect and interact with Cathy, Pat and other level measurement experts in the Level track of the Emerson Exchange 365 community.