Air Flow Instrumentation Criteria for Wastewater Aeration Systems

Air compressor operation is one of the largest energy expenses in wastewater facilities that use the activated sludge process...

Apr 1st, 2006
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Air compressor operation is one of the largest energy expenses in wastewater facilities that use the activated sludge process, and the cost of energy to produce compressed air for aeration systems continues to rise along with fuel costs. Optimizing the aeration process by measuring and controlling the system’s air flow with a suitable flow meter can help reduce those costs.


The Fluid Components International ST50 flow meter
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In evaluating and selecting flow meters for the aeration process or any other air or gas flow measurement application, there are five important factors to consider:

• Flow Sensor Technology
• Range and Accuracy
• Operating Environment
• Ease of Installation
• Maintenance and Life

Flow Sensor Technology

Three flow sensor technologies have typically been in use in aeration air flow monitoring applications in wastewater treatment plants: differential pressure (orifice plates); vortex shedding technologies; and thermal dispersion (mass flow).

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While, orifice plates have been in use for many decades in water treatment plants and vortex shedding is recognized for its high accuracy, thermal dispersion flow measurement now has the largest installed base for this application. Thermal dispersion has grown in popularity because it offers direct mass flow measurement, offers a wider turndown ratio, has no holes or moving parts to foul or clog, is an insertion style meter that installs in a single tap and is cost effective for the pipe sizes commonly found in aeration distribution systems.

Accuracy, Flow Range

At a typical large municipal wastewater treatment plant, one of the more common specifications for an aeration flow meter is to measure over a flow range from 1.5 to 150 SFPS with an accuracy of +2% of reading, +0.5% of full scale, with a repeatability of +0.5% of reading. Most aeration systems will operate with excellent efficiency at this level of accuracy. Flow meter manufacturers can provide products for higher accuracy specifications, but these products typically include extra features and functions that are unused in aeration applications and they carry a price premium.

When selecting a flow meter, it is critically important to look at the meter’s repeatability specification, which tells the user how reliably the device will maintain its specified accuracy level.

Operating Environment

The amount of air required to maintain the treatment process varies throughout the day and is dependent on environmental and climate conditions. Flow meters for such applications must be able to tolerate drops in pressure from 0.8 to 17.6 psig, which means the flow meter must be have a wide turn-down range. Temperatures also can vary widely, from -68 to 150°F. A wastewater treatment plant can be a dirty environment, which can be a maintenance issue with devices with holes that may plug or foul.

When it comes to installation, some flow meters are more straightforward than others. Be sure to ask if the flow meter can be inserted directly into the process pipe or if it requires an inline configuration that will require installers to cut and splice pipes in multiple places.

To accurately measure flow, meters require some length of unobstructed pipe straight-run upstream and downstream from the meter to achieve their specified accuracy. If your plant is short on real estate or if valves or elbows have to be placed near the flow meter, a flow conditioner will reduce the straight-run needed to ensure the flow meter is measuring accurately.

Tabbed type flow conditioners, such as those provided by the Vortab Company, have proved successful in these applications. Other flow conditioning technology choices including tube bundles, honeycombs, and perforated plates, may also be considered depending upon the specifics of the application and obstructions.

Maintenance, Life

Be sure to ask about the maintenance requirements for your flow meter. Some flow meters need more frequent recalibration and/or cleaning which can be time-consuming or, worse, require you to remove the meter from service. For wastewater aeration applications, the ideal flow meter will have no moving parts to wear out and no routine cleaning requirements. When calculating the cost of a new flow meter, be sure to look beyond the purchase price to determine what it will cost to maintain and how long it will provide service.

Energy Savings

The cost of compressed air to support wastewater treatment operations is dependent on a number of major variables. These include the plant’s physical climate, the layout, the volume of waste, the equipment in use (including the diffuser, compressor and control system), the piping configuration, the flow instrumentation and the energy supplier.

In the perfect world, all of these variable factors would work together to promote the optimum micro-organism growth needed to treat the water in the shortest amount of time. While perfection is beyond most of us, improvement is possible and valuable. If your energy costs seem high, be sure to consider all the variables, including the type of flow meters being used, where they are placed in the pipeline and their calibration for your application.

Selecting the wrong type of flow meter or improper calibration or improper installation can all result in less than optimum compressor efficiency and higher energy costs. To determine the potential savings, consider the amount of compressed air consumed daily and then look at what a small percentage improvement in compressor efficiency is worth. Then ask your flow meter supplier to help you review the performance of the instrument in the actual application.

Conclusions

Outfitting wastewater treatment aeration systems with the proper flow meter will result in improved process effectiveness and reduced energy consumption. Looking carefully at measuring accuracy and range needs, installation conditions and complexity, and maintenance requirements will result in selecting the most cost effective flow metering solution.

Over the past three years a number of flow meters from multiple manufacturers have been developed to better meet the needs of air flow measurement and other gases such as digester gases, applications found throughout wastewater treatment facilities. For example, Fluid Components International has designed a range of new and enhanced flow meters within the past 12 months that better meet the need for aeration flow measurement and other applications within wastewater treatment.

Its newest flow meter, the ST50, is designed specifically and optimized for aeration and blower air applications in wastewater treatment facilities. The ST50 incorporates thermal mass flow sensor technology for a no-moving parts insertion style element with optional wireless IR communication. Its transmitter electronics include dual analog outputs and optional digital readout, all housed in a rugged metal enclosure.

About the Author:

Steven Craig, Product Engineer Water Industry, Fluid Components International, graduated from Iowa State University with a Bachelor of Science degree in Industrial Technology. He has been with Fluid Components International for more than 10 years in engineering. He may be reached at stevec@fluidcomponents.com.

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