Taking Turbulence in Stride

Ultrasonic level detectors overcome obstacles and improve wastewater flow studies

Taking Turbulence in Stride

This illustration shows the direction of water flow in the 8-inch-diameter pipes where they tie in to the manhole.

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Ultrasonic noncontact distance measurement technology is the most accurate and cost-effective method for measuring the water level within pipes because the ranges of the distance to be measured are very short due to the small diameters of the pipes. Most ultrasonic sensors, however, have two major problems performing the measurements in the operating environment of a sewer.

The first problem is that the sensors typically have a minimum measuring range of 4 inches or longer, and they are also usually several inches thick. Therefore, if they are installed at the top of a pipe, they can’t measure the water level if it is less than 6 or 7 inches from the top of the pipe. Since many of the pipes in a typical system are 8 inches in diameter or less, these sensors cannot obtain readings if the water level is above the lowest 10 percent of the pipe.

The second problem is that most ultrasonic sensors utilize transducers with very narrow radiation patterns that are typically around 10 degrees. This means that the sound radiates from the sensor in a very narrow conical 10-degree beam. This type of design allows the sensor to obtain longer detection ranges when the target is flat and perpendicular to the beam, but it does not work well when the reflecting surface is very uneven, which is the case with the turbulent surface of water rapidly flowing in a pipe. This uneven surface causes the reflection of the sound pulse to scatter in many directions so that the echo is outside the detection angle of a very narrow beam transducer.

Ultrasonic sensors can be properly designed to overcome these problems and provide the accurate liquid level measurements required in sewer applications. To accomplish this, the mechanical design of the sensor must be very thin so that when mounted in a pipe, the transducers will be as close to the top as possible. In addition, using two transducers allows one to transmit the ultrasonic sound pulse when it is driven by a large voltage and the other to receive the echo reflected from the surface of the water.

Most ultrasonic sensors contain only one transducer that both transmits and receives the sound pulse. Because the transducer is a resonant device, the excitation voltage pulse causes it to ring like a bell that’s been hit with a hammer. It takes time for the ringing voltage to dissipate until it’s less than the levels produced by the reflecting echo when it returns to the transducer. That’s why most ultrasonic sensors have a minimum detection range, or lockout, of 4 inches or more. If the sensor is designed with two transducers, the receiving transducer does not have a large transmit voltage placed across it when the sound pulse is being generated. Therefore, the sensor can detect the low-voltage pulse produced by the receiving transducer from the echo very quickly after the transmitting transducer emits the sound pulse.

If the transducers used in the sensors have beam angles that are approximately 20 degrees, the echo caused by the turbulent surface of rapidly flowing water will be detectable. In addition, an IP68 rating will ensure sensors can handle being submerged when the pipe is totally full during an unusually large water influx.

The IP68-rated MassaSonic FlatPack Ultrasonic Sensor is only 1 inch thick, so it can be shallowly mounted at the top of a pipe. It contains two transducers, one for transmitting and the other for receiving, which allows it to measure the distance to the water surface when it is as close as 1 inch. The transducer radiation patterns are also 20 degrees, which enables detection of the echo pulse when the reflection is scattered by the turbulent surface.

A good example

A recent, very successful water flow study conducted in the MetroWest suburbs of Boston showed how a thin ultrasonic sensor with dual broad-beam transducers, such as the MassaSonic FlatPack, can effectively provide the required level detection in the pipes of a sewer system to enable reliable and accurate flow measurements. 

This college town was about to have an increase of flows into the sanitary sewer system. The college was planning on erecting a new building on its campus and therefore petitioned the town conduct a study of the flows within the sewers so that the proper sanitary accommodations could be taken. Because the college effluent is directed into two separate systems, an impact assessment was carried out by an independent engineering firm to support construction permitting. The flow study was replicated three times between January and June 2017 to ensure the accuracy of the range of flow rates measured. 

Within the sewer shed, peak flow rates were measured at different manholes. FlatPack Sensors (Massa) were mounted at the top of pipes just before they entered the manholes.

The recorded measurements revealed that the sewer pipes’ flow levels were ranging between 14 and 22 percent of full pipe capacity. This information is being used to help the college and town quantify the actual inflow and infiltration.

The campus building project is projected to increase the sanitary sewer flows by 750 gpd, which will likely need mitigation within the sewer shed. The Massachusetts Department of Environmental Protection’s Guidelines for Performing Infiltration/Inflow Analyses and Sewer System Evaluation Surveys recommends assuming a 50 percent peak infiltration removal (measured as 50 percent of dry-weather baseflows) as a preliminary estimate of sanitary sewer flow reduction after implementation of rehabilitation measures.

During all three of these studies, the MassaSonic FlatPack Sensors were able to accurately measure the level of water in the pipes, which allowed for precise and repeatable measurements of the water flow during the entire flow study program.


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