Flow monitoring is a complex business, and when it comes to flow meters, there is no one-size-fits-all option. Choosing the right sensor for the job takes an understanding of every challenge for the specific deployment so that operators invest properly according to the exact need.
Open channel pipes carry water in countless industrial and municipal applications, with depths that can range from a trickle to a deep surge. Water quality can display a wide range of pH and solids content and might contain heavy debris or none at all. Pipes can be easy or very difficult to access. There are many more parameters and countless combinations of points along each of them.
A few complex examples illustrate how even for the most extreme applications, there is a flow meter for the job.
Large Deployment of Flow Meters for I&I Studies
Municipal wastewater facilities often conduct inflow and infiltration (I&I) studies to fully understand sewer flow, especially during rainy seasons. In these studies, flow monitoring companies deploy dozens of flow meters across a system for a limited period of time. For larger systems, more than 100 flow meters may be needed to accurately assess flow and identify I&I hot spots.
When a large number of flow meters is necessary, flow monitoring providers typically prefer submersible flow meters that are installed in the bottom of a pipe. These devices measure water velocity using ultrasonic technology and water level using a pressure level sensor. Flow is calculated inside the sensor, with data ready to output in real time. Flow providers are generally comfortable with this technology and adept at deploying a small army of them at once. One challenge, however, has been the life of the pressure level sensor, which is typically the first part of the device to wear out. When this happens, the entire device has to be replaced, even when only a small part of it needs replacing.
I&I studies are usually limited to a few months, which gives flow companies ample opportunities to proactively replace pressure sensors, helping to save money and avoid device failure during a deployment.
Wide Diameter Urban Sewer Pipe
While municipal sewer pipes typically fall into the 12-inch diameter range, interceptor pipes for large urban districts can reach up to 10 meters (33 feet) wide. In these scenarios, the volume and velocity of water can reach staggering levels, especially during a rain event. A submersible flow meter is a poor fit for this job, simply due to the risk and difficulty of installing and maintaining flow meters under deep, swiftly moving water with highly complex contents.
The ideal flow meter in this situation is a noncontact area/velocity flow meter that calculates flow from above the water surface with high accuracy. The device works by first sending radar pulses to detect the speed of the water’s surface by “reading” ripples and waves, then measuring depth using a level sensor. Using the area-velocity method (speed x cross-sectional area), the device calculates flow, with all processing taking place inside the sensor and without external equipment. Flow data is sent directly to a supervisory control and data acquisition (SCADA) system or to other monitoring systems.
As this kind of sensor is installed above the surface of the water, installation and maintenance are accomplished without entering the water. When conditions may be especially hazardous for workers, this is the sensor for the job.
Highly Caustic Industrial Sludge
Many industrial sludges are highly caustic. This is especially true for copper mining sludge, a thick, fast-moving, very low pH sludge that contains elevated levels of heavy metals and total dissolved solids. This sludge will aggressively break down metal and concrete, and calculating its flow requires a highly specialized flow meter that can survive these conditions. Standard flow meters will dissolve to the point of inoperability in a matter of weeks.
When measuring flow that presents a threat to both people and assets, a robust noncontact flow meter with little maintenance and remote data collection is the best option.
The Growing Threat of Sewer Overflows
With increasing regulations and more extreme rainfall, municipal wastewater systems are grappling with increasing overflows and the resulting penalties for such events. In addition to I&I studies that can help systems identify large breaches in their systems as a proactive strategy against overflows, flow meters can also be used reactively to help manage water flow during heavy rain events by diverting discharge to catch basins or otherwise responding appropriately.
This scenario is challenging, because pipes can go from dry to raging in a matter of minutes, and utilities need flow meters that are adaptable enough to respond to quickly changing conditions and deliver real-time calculations. Flow meters for this type of application also need to be able to analyze water with extreme turbulence, which is another characteristic of water in a combined sewer during a heavy rain event.
While both the noncontact and submersible flow meters can potentially work in these conditions, there is a vulnerability of the submersible that operators should be aware of. If the flow carries an inordinate amount of sediment, such as through stormwater runoff with road sand and silt, a submersible flow meter can be buried. In this case, the doppler effect will be inhibited, which will hinder the device’s ability to calculate flow. For municipalities where overflows have become a significant and frequent threat, investing in noncontact flow meters can be worth the cost, especially when factoring in the reduced maintenance and upkeep demands they deliver.
Combined sewers are typically designed so they can never completely fill up. Therefore, the above-water, noncontact flow meter—which depends on an air gap to calculate flow—will always be a good solution. However, there are rare instances in which an effluent pipe can completely fill, and in these cases, operators may decide to install both kinds of sensors.
As an example, a wastewater treatment plant in the western United States discharges into a wetland and uses flow meters to monitor flow from that wetland to the river. This is done through pipes that completely fill for a month or two during the late spring snowmelt. To calculate flow during both the normal season and the wet season, the utility uses a noncontact radar flow meter and a submersible area velocity flow meter. For most of the year, the flow is shallow and fast, a perfect scenario for the noncontact flow meter. When the pipe fills in late spring, the utility switches to submersible flow meters, and because there is often debris in the form of leaves and sediment, these flow meters are mounted on the side of the pipe, rather than the usual bottom location, to avoid them getting buried. Since they will be used only when the pipe is full, the utility can side-mount them without concern that readings will be compromised by an empty pipe.
It is also important to note that even if a submersible is buried under sediment or a noncontact is submerged under water, these impacts do not necessarily permanently damage the device. Some products are built to withstand such effects, even if their operational capabilities are temporarily offline. This is why having both types of devices installed—and being able to pull data from either type—can best prepare a facility for any flow situation.
The Benefits of Open Protocol
While most flow meter vendors have proprietary products that require the use of their entire flow monitoring system, some now provide flow meters with an open protocol. Data from these flow meters can output to any remote data loggers, which can save capital expenses and offer great flexibility. When an operation only needs to upgrade its flow meters, products with open protocol can be a fraction of the cost of replacing the entire system.
Matching Conditions to Equipment for Best Outcomes
When selecting a flow meter, especially when replacing other flow meters that have failed, it is important to understand the challenges and reasons for the failure so the right flow meter for the job can be brought into service. Whether the problem is frequent overflows, a caustic industrial environment, enormous interceptor pipes or maintaining a massive inventory of flow meters, there is a well-suited product ready to assist.
