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Measuring Flow in Gravity Sewers

There is nothing more difficult than measuring flow in gravity sewers. WRUA bought 34 Isco Area Velocity Meters – Quality Equipment & Post-Sales Service Sealed the Deal. 

By Mary Therese Gookin

West Rankin Utility Authority (WRUA) provides wastewater treatment for seven neighboring members serving a population of 100,000.  Bruce Stephens, Executive Director of WRUA, a flow measurement expert says, “I tell my members, nothing is more difficult  than measuring flow in gravity sewers – it is the worst flow measurement application known to man and my entire system is one big gravity system. I used to measure flow in the crude oil industry – that was difficult – but nothing compares to measuring flow in a gravity sewer system.”

Back in 2007, WRUA decided to upgrade to more reliable and accurate flow metering technology; in 2009 the upgrade project was installed. Prior to this, they based their bills on drinking water sales rather than actual wastewater flows.  What they were using was inconsistent.  Further, if a city wasn’t doing a good job on maintaining sewers and had problems with I&I (Infiltration and inflow), billing based on water sales would be sorely inaccurate.

After evaluating competitive equipment, he chose Isco’s 2150 Area Velocity Flow Meters.  Isco not only had the flexibility to provide MODBUS communications which enabled him to digitally communicate to his existing SCADA system, but the competition could not provide local, post-sales integration support of the Isco meters in their SCADA system like C.C. Lynch could.  Our ability to provide this post-sales integration service sealed the deal for Bruce.  Bruce said, “With C.C. Lynch reps, I get more than a salesperson who directs me to the factory to get my questions answered…I get guys who get right down there with me and help me configure, install, integrate and fix things themselves.”

West Rankin started out with 32 flow meters and now has a total of 34.  As member cities add new developments new entry points into West Rankin’s interceptor are added and flow meters are installed.

WRUA members insist on accurate and reliable measurements.  Bruce feels confident about trusting the reliability and accuracy of his Isco meters.  Monthly, he shares raw and adjusted data with the Board and if they have any questions about its accuracy they can visit his office to see the data and analysis for themselves.  Bruce says, “We can put all the cards on table for anyone asking.” 

Every morning the first thing they do is check their SCADA to look for issues.  Historically they can pretty well predict where the issues are going to be. Where the slope is not good and you have low velocity, sediment and debris collect.  At sites where you start getting velocities below .5 ft. per sec you will have problems. You need 1 ft. per sec or more.   By using MODBUS, not only can they read velocity and depth of flow from their SCADA at each individual site in real-time, but the analytical capabilities that come with Isco’s 2150 Area Velocity Flow Meters also allows them to get multiple QA/QC signals from their sensors – signal strength and spectrum- which this, along with historical analytical data, helps them to assess from the office if a sensor has failed and in many cases assess if there is just debris on the sensor.  This level of remote troubleshooting enables the technician to bring a new sensor to the site if needed.  Since some of their sensors are over 30 ft. deep in the ground, being able to monitor instrument signals before they get to a site saves the tech from having to go down a 30 ft. confined entry space twice.  This saves travel time, decreases travel risks and decreases confined space entry risks which equates to improved employee safety, time and money savings.

Bruce is big on system maintenance as well insisting “there is no meter you can put out there and forget it – you have to continually watch and monitor all your meters to get good flow measurements.” So in addition to his daily morning SCADA monitoring, their meters are field validated every other month with an actual handheld velocity meter, debris is cleaned off and if there is a bad sensor, it is replaced.  C.C. Lynch trained the techs who do this for them.

Bruce said, “I enjoy working with people like the C.C. Lynch folks who know their products and who can provide local sales AND service – not just sales like some reps.  When I need help, I don’t need a salesman offering to take me to lunch…. I need a rep who will lay their hands on it and help me fix it.  Further, C.C.Lynch has found ways to help me save money – I know they don’t try to soak me.  They are excellent to work with.”

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