A town located in the metro-west suburbs of Boston was petitioned by a local college to improve the flow-rate into the sanitary sewer. Additional flow capacity was expected to be needed. The source of the increase in flow was the result of a new building on the campus. With effluent from the college directed into two separate systems an independent engineering firm was tasked with carrying out an impact assessment to support construction permitting, and to ensure that the flow rates were accurate.
The study was replicated three times between January and June 2017. The analysis of the sewer system and infiltration/inflow approaches was developed from the resulting study.
Measurment of Water Flow in Complex Sewer Systems
Measuring water flow within a complex sewer system can be difficult, and to make it reliable it is often necessary to accurately measure the level of the water as it flows in and out of the numerous manholes through the pipes within the system.
The most accurate and economical of the water-level measurement methods is ultrasonic non-contact distance measurement technology as, due to the small diameters of the pipes, the ranges of the distance to be measured are very short. However, there are two significant problems that most ultrasonic sensors encounter when performing the measurements in the operating environment of a sewer.
Limitations of Ultrasonic Sensors
The first problem is that the minimum measuring range of sensors is typically 4 inches or longer, and the sensors themselves can be several inches thick. This means that if they are installed at the top of a pipe, and the water level is closer than 6 or 7 inches from the top of the pipe, it is impossible for them to measure the water level. As many of the pipes in a typical system are 8 inches or less in diameter, these sensors won't be able to make a reading if the water level is higher than the lowest 10% of the pipe.
The second problem is that the majority of ultrasonic sensors use transducers that have very narrow radiation patterns, typically around 10°, meaning the sound radiates from the sensor in a very narrow conical 10° beam. While this design allows the sensor to obtain longer detection ranges when the target is flat and perpendicular to the beam, it does not function well with an uneven reflecting surface, like those seen when water flows rapidly from a pipe. This uneven surface scatters the reflection of the sound pulse in many directions, so that the echo is outside the detection angle of a very narrow beam transducer.
Thin Ultrasonic Sensor Provides Accurate Liquid Level Measurements
Ultrasonic sensors can be properly designed to overcome these problems and provide the accurate liquid level measurements required in sewer applications. For this to be accomplished, the mechanical design of the sensor must be extremely thin so that the transducers are as close to the top of the pipe as possible.
Furthermore, there should be two transducers in the sensor, one for transmitting the ultrasonic sound pulse when it is driven by a large voltage, and the other to pick-up the echo reflected back from the surface of the water. Most ultrasonic sensors contain only one transducer that both transmits and receives the sound pulse. This can be a problem as the transducer is a resonant device, and the excitation voltage pulse causes it to ring like a bell that has been hit with a hammer.
This ringing voltage slowly reduces over time until it is below the levels produced by the reflecting echo when it returns to the transducer. This is the reason that most ultrasonic sensors have a minimum detection range, or lockout, of 4 inches or more. This isn't a problem for a sensor designed with two transducers, as the receiving transducer does not have a large transmit voltage place across it when the sound pulse is being generated.
This allows the sensor to detect the low voltage pulse produced by the receiving transducer from the echo much more quickly after the transmitting transducer emitted the sound pulse.
Another necessary feature of transducers used in the sensors are broader radiating patterns. If the beam angles of the transducers are approximately 20°, the echo caused by the turbulent surface of the rapidly flowing water will be detectable. In addition, the sensors should have an IP68 rating, so that in the scenario of a completely full pipe from a large water influx, they can withstand submersion.
MassaSonic® FlatPack® Ultrasonic Sensor
One ultrasonic level detection that contains these design modifications for operation in a pipe is the MassaSonic® FlatPack® Ultrasonic Sensor. Shown in Figure 1, it is only 1 inch thick, allowing for simple shallow mounting at the top of a water pipe.
There are two transducers, one for transmitting and the other for receiving, allowing it to measure the distance to the water surface at distances as close as 1 inch. It also emits the transducer radiation in patterns of 20°, so the echo pulse can be detected from the reflection of the scattering by the turbulent surface of the water flowing in the pipe. The sensor is rated IP68.
Figure 1. Photograph of a MassaSonic® FlatPack® Sensor Showing Its Shallow 1 inch Thickness and Dual Broadbeam Transducer Design.
The college permitting driven flow study conducted in Metro West Boston, assessed how a thin ultrasonic sensor with dual broad beam transducer, such as the MassaSonic FlatPack, provides the required level detection in the pipes of a sewer system for reliable and accurate flow measurements. Within the sewershed, typically 8” pipe, peak flow rates were measured at SMH 948 and ranged between 14% to 22% of full pipe capacity.
These values were cross-referenced with a pre-existing study conducted in 2013 that found the location to be at 18% of capacity, and were found to agree with those findings. Peak flows recorded at SMH 827 were between 10% and 12% with the estimated existing flow rate to be 15%. Figures 2 and 3 show photographs from one SMH set-up where data was collected.
Figure 2. Photograph Looking Down into One of the Manholes In the Metro West Flow Study.
Figure 3. Photograph From the Inside One of the Manholes of the Sewer System in the Metro West Flow Study Looking Into Pipe A, Showing One of the MassaSonic FlatPack Level Sensors Mounted at the Top of the 8 inch Diameter Pipe A.
In summary, three separate flow studies were carried out between January and June of 2017 that were compiled into a program of results. The study is being used to assist the college in quantifying inflow and infiltration sources into the campus system. The campus building project was estimated to increase sanitary sewer flows by 750 gpd which will likely need to be mitigated within the sewershed.
The MassDEP’s Guidelines for Performing Infiltration/Inflow Analyses and Sewer System Evaluation Surveys recommends assuming a 50% peak infiltration removal (measured as 50% of Dry Weather Baseflows) as a preliminary estimate of sanitary sewer flow reduction after implementation of rehabilitation measures.
This information has been sourced, reviewed and adapted from materials provided by Massa Products Corp.
For more information on this source, please visit Massa Products Corp.