New flow monitoring system for Dublin’s sewers
Integrated Hydro Systems has completed a project to provide Dublin Corporation with a flow monitoring system for the city's sewerage network.
In November 1997, Integrated Hydro Systems (IHS) was asked to provide Dublin Corporation with details of technology available for the installation of a sewerage billing network. The criteria identified by Dublin Corporation for the proposed network were as follows;
- The system must be able to provide accurate flow data to enable Dublin Corporation to charge users outside their own area for the treatment of flows at the proposed new treatment works in Ringsend.
- The system must be compatible with Dublin Corporation’s existing Logica network.
- The system must be capable of providing accurate data from a wide spectrum of flow and site conditions.
- The system will be installed on a permanent basis and require the minimum amount of maintenance.
- The system must have potential for expansion and add-on facilities for water quality and sampling regimes.
- The system must be well proven, preferably with a history of similar applications.
Initial proposals were presented to the Corporation, which recommended a mixture of ADS 3600 peak doppler and Accusonic time-of-flight flow monitors. A pre-site survey was undertaken by IHS during January 1998 and the best technology for the project identified.
In 1999 Dublin Corporation ordered the installation of two permanent flow monitors at critical points in the network.
The first installation was undertaken in April 1999 and consisted of an ADS 3600 flow monitor in the 1,500mm diameter Dodder Valley Sewer which receives flows from South Dublin corporation.
The ADS 3600 digital peak doppler flow monitor uses a three sensor system to provide flow data; an ultrasonic peak doppler velocity sensor; an ultrasonic quad-redundant level sensor and a pressure depth sensor. Flow values are calculated from velocity and depth of flow.
The ultrasonic level sensor is positioned on the soffit (top) of the pipe and has four crystals, each of which can emit and receive a signal. The sensor is pre-configured to operate on four crystals at any one time thereby providing accurate and reliable level readings. The pressure sensor provides data if and when the pipe becomes surcharged and consequently is located as near to the invert (underside) as possible, however it can be rotated out of the flow in shallow conditions.
The velocity sensor is placed in the invert of the pipe and is very small, to minimise ragging and silt build-up which could affect the signal. Three sensor cables run from the sensors to the ‘intrinsically safe’ logging unit which is located at the top of the manhole. From here data is transferred by a communications cable to a nearby kiosk where the mains power supply, telephone lines and Logica outstation are situated.
As part of the original criteria the monitoring system would have to be capable of interfacing with Dublin Corporation’s Logica outstation network. The ADS monitor outputs data in a digital format, and in order to fulfill the criteria. IHS designed a special 4-20mA interface box for the ADS monitor.
The 4-20mA interface was specifically designed to interface the ADS protocols and provide four analogue outputs including ultrasonic level,
pressure depth, velocity and flow. One of the main reasons for Dublin installing this first ADS monitor was to ensure the interface operated as desired. The success of this interface is such that it is now in use in ADS installations worldwide.
In order to minimise the need for expensive site visits to check on the sensors and site conditions, a dedicated telephone line is installed directly to the ADS external modem unit. This line is in addition to the Dublin Corporation telemetry line installed via the Logica outstation and allows IHS staff to regularly view the data from their office in Leeds. Data is downloaded on a weekly basis and monthly reports on the quality of data and equipment performance will then be compiled and submitted to Dublin Corporation.
The second site chosen was the main lift pumping station at Ringsend, this is the second largest wastewater pumping station in Europe and will receive flows from Dublin and neighbouring catchments prior to pumping them to the STW.
The station has a design capacity of 17,000 l/s and consists of four 3,400 l/s and two 1,700 l/s static lift pumps. These pump sewage into six 1,200mm ductile iron rising mains, then into a high level collection channel and finally down into the STW.
Following investigations into the regime at the station it was decided to monitor the four main vertical 1,200 mm rising mains. The most suitable equipment was the Accusonic 7510 multiple parallel path, transit time flow meter.
The Accusonic flowmeter records the flow velocity by measuring the time differential of a pulse of sound travelling diagonally across the flow. The accuracy of the flow data obtained is a factor of the accurate positioning of the sensors and the number of paths installed. Typically a 2-path system installed on a full pipe is +/-1.5% of actual flowrate.
The 7510 FM has the capability to measure up to eight paths at once, therefore by installing 2-paths on each of the mains a single monitor could provide flow data to within +/-2% of true flow values on the four rising mains. The sensors were installed from the outside of the pipes using specially designed cold-tap assemblies. In order to obtain as accurate results as possible the sensors were placed 5-6m above ground level. This necessitated erection of scaffolding around all the pipes, and took five days to complete. Final commissioning of the installation was under-taken during the early part of January 2000.
The Accusonic monitor supplies the information as standard analogue outputs which are again taken directly into Dublin’s Logica outstation. Several additional monitors will be installed over the next year in time for the completion of the new STW at Ringsend.