Product Profile

Why the right flow meter matters in wastewater

Owning flow meters is a challenge of its own for water authorities and companies that deal in water management and wastewater treatment. This is especially so as the volume increases and consequently pipe size gets larger to serve high water demand in plants or an area with high population density. The current go-to technology for measuring water flow is usually a full bore, inline, magnetic flow meter. However, due to the aforementioned challenges, the cost of ownership of such a flow meter can be unexpectedly expensive. Installing, calibrating, maintaining and replacing a new meter might mean shutting down the pipe, which may compromise water availability and attracts scrutiny.

Andrew Aditya is a sales engineer who works for instrumentation specialist AMS Instrumentation & Calibration (AMS) and knows that it is important for wastewater facilities, water networks and other facilities to have performing equipment that can be relied on to manage plant and resources in the most effective and efficient way.

His company specialises in such equipment, one being a full-profile insertion magnetic flowmeter, the McCrometer FPI Mag.

“A full-profile insertion magnetic flow meter becomes a good solution because we do not need to shut down the whole pipe during installation and maintenance,” he said. “In Australia, we started introducing the technology three years ago. There are several departments that are looking into using these units. The people that will be initially interested are instrumentation engineers. Secondly, it will be water network managers then non-regulative water managers, waste treatment plant managers, water quality managers and others who care.”

One of its key features is that it has virtually no back pressure. Aditya said that similarly to a full-bore, inline magnetic flow meter, it does not make a pump work harder to maintain good water pressure at the end destination. Having a specially designed insertion probe with a diameter of around 40mm installed straight down vertically from the top to the bottom of the pipe, allows water to flow around it easily.

“Because water authorities need to deliver the water from far flung areas and they also have to factor in multiple kinds of terrain, the water needs to go uphill as well, not just a certain distance,” said Aditya. “They need enough pressure generated by the pump – very big pumps – and that brings a large amount of energy requirement to get the water from one point to the other side. They need to have a substantial amount of pressure so they can be used for getting into large facility or a high rise.”

“Similarly in industrial plants, there are processes with machines that require sufficient water pressure to flow through a 3-D maze of pipes network,” he added.

Using a flow meter that does not cause additional back pressure to the water delivery system, does not add extra load to the pumps and thus saves energy and cost. When it comes to getting water or wastewater from one place to another, the water network needs to be controlled properly, according to Aditya. This is why the right meter is needed.

Water and wastewater managers have to ask themselves, ‘how much water or wastewater is the network managing – from getting it from the reservoir or treatment plant, or getting it from natural resources and getting it into their network, and then into the market?’

“They have multiple ways to compare data but nothing really matches to each other,” according to Aditya. “There are several issues as to why they are not matching. One, the meters are inaccurate because they are old and have never been calibrated, particularly the large ones. Number two is leakage. When there is leakage that means the water downstream is less than upstream. There is less water delivered
than supplied.”

Once you have those numbers combined or reported back into the SCADA, people who are running the whole network are confused, said Aditya.

“Where did the water go? All electronics drift as they age. There is no exception for flow meters. A full bore in line mag flow meter’s electrode’s coating and coils deteriorate over time,” he said. “It has been a standard practice where flow meter manufacturers supply a calibration certificate, usually valid for one year. The manufacturer would also recommend calibration frequency to ensure that the flow meter performs within its published specification. The dilemma is to justify the total cost including all the special services, equipment and loss due to shut down, when there is no bypass facility. The fact that many of these water pipes are buried underground, makes life even tougher.”   

To better manage leakage, managers need more meters to cover more areas.

“Ageing pipes are rarely maintained. It is hard to locate leaks before making any fixes,” said Aditya. “In the case of long running water mains, one can add more full-bore inline magnetic flow meters. However, imagine if we have 100s of kilometres of water pipes services 100s of 1000s of people. Shutting down the pipe? For how long? People would be screaming.”

According to Aditya, a lot of flow meters need a straight pipe run in order to do their job. Most of the standard requirements are 20 internal diameters upstream and 10 internal diameters of the pipe downstream from the flow meter location. A full bore inline magnetic flow meters would ask for one or two diameters upstream with one diameter of downstream. That is the same general requirement of the McCrometer FPI Mag flow meter. McCrometer has produced a table of required straight-run distances for various flow obstructions. In some cases, the straight requirement is none – zero internal diameter upstream and downstream.

Why there is a need for that straight run is because it is ideal to  have the water or wastewater flowing uniformly as much as possible. Depending on the size of the internal diameter, McCrometer’s FPI Mag is designed with multiple electrode sensing points on the probe rod. Those points are allocated at the traverse points upon which the FPI Mag averages the reading, representing the averaged flow profile of the water. These sensing points employ Faraday’s Law, where a conductive media will disrupt the electromagnetic field produced by the electrodes and cause voltage variation across the electrodes, which is proportional to the velocity of the water flowing around the probe.

On the waste treatment side, the FPI Mag is best used at the effluent, where the water is “cleaner” and the pipe tends to be larger. Here, the FPI Mag also offers accuracy, maintainability for calibration and servicing, and no shut down.

“For other waste treatment application, the pipe size tends to be smaller,” said Aditya. “In general, I’d still prefer a full-bore, inline magnetic flow meter. We do not want the FPI Mag probe to catch debris, hair, etc, and would like to avoid heavier sludgy water to ‘hammer’ the probe. We need to consider it case by case to ensure that the FPI Mag fits for the application.”