The main function of a bulk meter is to measure the flow rate
of water and record the totalised volumes that can then be
used for invoicing. Depending on the metering technology, other
functions such as pressure, temperature and water quality
parameters can also be monitored.
Strategically positioned along the water network, bulk flow meters allow monitoring of the volume of water distributed in the various sections (districts) for analysis of consumption and cross checks of potential discrepancies (e.g. leakages, illegal spillages, NRW). Different metering technologies also allow for analysing consumption peaks, night flows, flow rate fluctuations, and evaluation of network performance. The choice depends on the flow measurement method, the type of end user, the required flow rates, the accuracy requirements, and the analytical techniques that are to be used.
Many types of bulk meters are available, such as mechanical bulk flow meters (turbines, gears, oval wheels, etc.) Electromagnetic or ultrasonic flow meters along with Insertion flow meters that are low cost to install but which have greater uncertainty of measurement. The selection of suitable metering technology must take into account the specific service and application: district metering, leakage detection, custody transfer, regulation, distribution or processing. For district metering and water leak detection, particular attention should be paid to the low flow, data logging, and communication capabilities.
Mechanical meters are direct volume meters, based on determined volume displacement. The measuring element consists of a rotating element (e.g. gears, oval wheels, turbine, vanes, nutating disk) moved by the liquid flowing through the metering chamber. Each revolution of the rotating element allows a specific volume of liquid to pass, which is determined by the volume of the metering chamber. Mechanical meters are generally characterised by a heavy body with bulky dimensions that are potentially subject to natural wear by the moving elements with gradual deterioration of accuracy over time. These meters need to be protected against entry of solid particles and debris by the use of filters or strainers. Pressure loss across each measuring point can be significant.
Electromagnetic Flow Meters
Electromagnetic flow meters are typically a velocity-type meter based on Faraday’s law of induction. Electromagnetic flow meters measure the flow speed in a pipe section by measuring, through electrodes, the voltage generated by the liquid passing through a magnetic field which is proportional to the flow velocity. An essential condition is that the liquid must be conductive. Full-bore electromagnetic flow meters are characterised by free passage, no moving parts, no wear, a long life time, insensitivity to the presence of suspended particles (no need for strainers), and negligible pressure loss. Electronics associated with the flow sensor introduce features that support the concept of smart water networks, such as bi-directional volume measurement, flow rate monitoring, pressure and temperature monitoring, data logging, and wireless data transmission. Versions with “all-in-one” features are available that don’t require additional equipment. These meters can be either mains or battery powered.
Ultrasonic Flow Meters
Ultrasonic flow meters are based on transit time measurement of an ultrasound pulse crossing the flowing media. The pulse is accelerated (with flow direction) or decelerated (opposing flow direction) by the flowing media and the time difference measured. The difference in time taken by the upstream and downstream pulses is proportional to the fluid velocity. Similar to electromagnetic flow meters, the sensor presents no restriction, no moving parts, and negligible pressure losses. It can work on non-conductive liquids but non-homogeneous liquids, the presence of solid particles, and changes in density all affect the measurement uncertainty. Ultrasonic Flow meters are generally more sensitive to flow profile disturbances than electromagnetic flow meters and therefore require longer straight sections of pipe to ensure operation within the stated uncertainty of measurement. Smaller sizes can be powered by battery with acceptable asset life, but as the size increases more power is required to trigger the ultrasound pulse. In these cases mains power is required.
Some of the benefits of using bulk meters in a water network include:
Facilitate data-driven management of the main asset for a water utility
Provide data to manage the water networks
Provide data to facilitate invoicing of a water company’s customers
Provide data for water efficiency initiatives
Monitor potential losses and leakages within the network to assist in the estimation of NRW
Bulk meters are generally installed directly in line within the piping (full bore style), flanged or wafer style. For correct operation and long service, a water meter should be entirely full of water and protected from risk of damage by shock or vibrations from the surroundings.
As indicated by international recommendation such as ISO 4064: “Many types of meter are sensitive mainly to upstream flow disturbances, which cause large errors and premature wear. They are likewise, though to a lesser extent, sensitive to downstream flow disturbances. It should be noted that proper functioning of different water meters is related not only to their construction but to their installation conditions as well.”
Installation conditions introducing velocity profile distortion and swirl must be avoided. It is a common practice to allow for straight sections of pipe upstream and downstream of the water meter; straight length requirements are different depending on metering technology, but “the longer the straight length of pipe, the better, particularly in the upstream side of the water meter”. There are bulk meters tested to work with “zero diameters” upstream and downstream. However, this is a condition tested in a lab according to specific conditions provided by the reference standard and cannot be taken as valid under all circumstances.
Proper sizing of the flow sensor is fundamental for acceptable measurement and the best achievable uncertainty. The flow sensor should be sized in accordance with the flow range to be measured and, in particular, to the low flow regime in order to ensure the flow meter performance. Temporary measurement using clamp on ultrasonic or insertion electromagnetic flow meters can be a good method to indicate the local flow velocities and define the most appropriate flow meter size.