A fast-growing city in the arid desert regions of the Western U.S. recently addressed the need to expand its municipal service for new residents and subdivisions. In recent years, the city's residential demand for wastewater treatment tripled from less than 1 million gallons to more than 1.5 million gallons per day. In planning for the wastewater treatment plant's expanded capabilities, the city's water engineers identified minimizing aeration basin compressed air energy costs as a goal.
The engineers at the city plant needed to place air flow meters in a rugged area of the facility with an underground vault, which required remote access. The installation site was further challenged with straight pipe run limitations, hazardous gases present, a wet/dirty environment, and fluctuating wastewater levels.
The flow meters needed to be installed in a 24-inch line for blower air flow into the aeration basins. Accurate measurement of the air flow was necessary for the control system to maintain the correct level of dissolved oxygen in the aeration basins and proper treatment of the wastewater. The meters also needed to provide accuracy over a wide flow range because the facility requires flow rates to increase from 0.5 to 3.0 MGD with future growth.
After reviewing various flow meter technologies, the city's plant engineers selected the ST98 Flow Meter from Fluid Components International (FCI) because of its accurate performance over a wide flow range, ease of installation and low maintenance requirements. The flow meter is designed with a thermal dispersion mass flow sensing element that provides excellent measurement accuracy and reliability in harsh environments.
To compensate for flow disturbances resulting from limited piping upstream, Vortab Flow Conditioners (Model VIP) were installed to ensure flow meter accuracy. The flow conditioners remove both swirl and distorted velocity flow profiles to support accurate flow measurement by the meter at all velocities under these challenging conditions.
The ST98 flow meter also includes an integral two-way HART protocol interface for direct communication with the plant's distributed control system. The HART interface allowed the plant engineers to receive multiple process variables and configure the flow meter remotely from the safety of the control room.
HART's field-proven communications protocol provides reliable two-way communication as part of the existing 4-20 mA wiring. Using integral two-way HART communications, the city's process engineers have simplified control and access to important flow meter flow data including diagnostics, calibration and configuration information.
The highly stable, constant power flow sensor design allows the meter to be used in applications where upset flow conditions - such as sudden changes in flow, temperature or moisture - are present. With its highly-reliable thermal mass sensing element, the flow meter delivers precision mass flow rate, totalized flow and temperature measurement to the city's engineers. It is ideal for air/gas flow measurement in wastewater treatment applications and offers high accuracy to ±1% of reading, 0.5% of full scale. Repeatability is ±0.5% of reading.
In wastewater treatment facilities and industrial plants, the cost of air flow can easily be one of the facility's largest energy expenses. The cost of energy to produce compressed air continues to rise along with fuel costs. Optimizing the aeration process by measuring and controlling the aeration system's air flow with an accurate, reliable flow meter reduces energy costs and overall plant operational costs. In addition, the flow conditioner can reduce piping costs, and the low pressure drop characteristics minimize compressor/pump power requirements.
As this city demonstrated, outfitting wastewater treatment aeration systems with a mass flow meter will result in improved process effectiveness and reduced energy consumption. Looking carefully at measuring accuracy and range needs, installation conditions and maintenance requirements will assist in selecting the most cost-effective flow metering solution.
Fluid Components International is an exhibitor at the WEFTEC.13 event and can be found at Booth No. 3944.
