Flow Measurement Krohne Helps a Wastewater Plant Measure Flow Accurately in a Part-Full Pipe
Some people see the glass as half full, some as half empty. Neither approach is inherently bad, it’s just two different ways of looking at things. If a pipe is only 50 percent full, however, and you want to measure flowrate, the problem is more than just one of terminology. As one US utility company found, few instruments can accurately measure flow in a part-full pipe.
Primnath Rambissoon, instrumentation supervisor at the Back River Wastewater Treatment Plant in Baltimore, was all too familiar with this issue. “We had a process in the plant that had been shut down for quite a while, and when we restarted it, we had problems taking the measurement in that pipe,” says Rambissoon, who is also in charge of the plant’s remote pumping stations and remote media stations. “It was an application where the flowrate was very low and the pipe wasn’t completely full. Unfortunately, the flowmeter we were using was simply unable to measure flow when it was that low.”
The problem area was a section of 12-inch pipe in the plant’s sludge handling process. The pipe was oversized for the amount of material it carried, and a previous attempt to remedy the situation by installing a reducer to bring the diameter down to 8 inches had still not been able to make the existing flowmeter work.
“When I came here about three years ago, operations told me about it, but there was nothing I could do without a full pipe to read,” Rambissoon says. “When the pipe was full they could read the meter, but for much of the time they just had to guess.”
“What’s worse, since we couldn’t get a flow reading, we couldn’t operate on automatic mode in that process, which would have reduced our operational costs. We operated in manual mode for years, probably from the time the system was designed.”
Then, late in 2008, David Spitzer of Krohne was making one of his regular sales calls to Back River. He told the company that Krohne had a unique product capable of measuring flow in partially filled pipes.
Rambissoon was intrigued by this prospect and decided to investigate further. Back River already had several Krohne flowmeters that were up to 20 years old, and Rambissoon was familiar with Krohne electromagnetic flowmeters from the plant where he previously worked. Back River had also recently bought a couple of Krohne’s Optiwave 7300 C radar level meters.
As a result of Spitzer’s presentation, Back River bought the Krohne Tidalflux electromagnetic flowmeter at the end of 2008; it was delivered and installed in early 2009. The Tidalflux, which is ideally suited for use in partially filled pipelines, is installed mainly in the intake and outfall structures of sewage works and stormwater basin outlets, as well as in transfer stations.
Level as well as flow
The Tidalflux is similar to a conventional electromagnetic flowmeter in that it has two measuring electrodes. The difference is that in the Tidalflux the electrodes are located at around the 10 percent fill level, allowing the instrument to respond even when the pipe is nearly empty.
To allow it to calculate flowrate when the pipe is not full, the Tidalflux also measures the fill level in the primary head with millimeter accuracy. The large-area capacitive level sensors are embedded in the liner of the primary head, making the level measurement almost independent of flow profile and the presence of waves or debris on the surface of the liquid.
“This is kind of a unique configuration within the industry,” says Spitzer, who since this project has started his own company, Chesapeake Flow Solutions. “There are a couple other manufacturers out there that try to measure partially full pipes; they use multiple electrodes around the pipe, so in between they’re just estimating.”
As well as high measuring accuracy, the Tidalflux also boasts low hydraulic losses and low maintenance, with no field calibration required. Rambissoon says that the Tidalflux meter has not only performed just as expected, but presented few challenges during installation and set-up.
“The entire set-up and installation was extremely simple,” he says. “In fact, we didn’t even need a factory person to come out and install it; we did it ourselves in house. And our operations people love it because it makes their job so much easier.”
“But the most important benefit is the meter’s functionality and accuracy. Most of the time we wouldn’t be able to read a flow unless it was relatively high. Now we can get an accurate read at flow levels as low as 2 gallons per minute. Also, we can’t overstate the benefit of being able to run the meter in automatic mode.”
“There’s one guy there who is skeptical of just about everybody’s product,” Rambissoon adds. “After we were able to get the measurement from that pipe for the first time ever, he actually shook his head and said, ‘It works’.”
Based on the success of this project, as well as the earlier success with the Optiwave Radar Meters, Spitzer will continue to supply Krohne products to Back River through his new company.
“Since the Tidalflux was installed, I’ve sold them a couple of levels, and I also get the occasional request to quote for other flowmeters,” he says. “Primnath indicated that he has two other places in the plant where measurement of low flows is a problem, so he intends to purchase the Tidalflux for those areas as well.”
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