Project Focus: Shaft solution for sewer monitoring in Athens
Greek utility EYDAP required an innovative installation solution when it put in UK-made flow monitoring equipment into the Athens sewer network
—Â To fulfil the expectations of its customers and regulators, the Athens Water Supply and Sewerage Company (EYDAP SA) needed more information on the operation of its sewer network
—Â Real-time monitoring, with alarms unusually high or low flows, was needed in order to respond quickly to sewer leaks, blockages and floods
—Â EYDAP also wanted to understand how much surface run-off water was entering the sewers, in order to know how to target future infrastructure investment
—Â To overcome the challenge of not being able to enter the sewers to install the monitors, an excavation method was developed, drilling into the crown of each pipe and installing a shaft for maintenance access
—Â The sensor was fitted in barrel housing that would retract with buoyancy if the water level reaches the sensor, protecting the electronics
—Â Unusual egg-shaped and bonnet-shaped pipes meant extra calculations needed to be built into software to effectively calculate flow
by James Brockett
An innovative project to install flow monitoring stations has given the Athens Water Supply and Sewerage Company (EYDAP SA) a valuable source of information about the operation of its sewer network.
The work, carried out by Greek contractor Metrica together with UK manufacturer Detectronic, has seen 15 monitoring stations put in on the main sewers and other important locations in the sewer network of the Greek capital. The utility now plans to extend the project to many other locations in the city after the stations, which give detailed data on sewer flow and provide warning alarms for sewer blockages, leakage and flooding, were judged a success.
Serving 4.3 million people, EYDAP is the largest water supply and sewerage company in Greece and has 8,000 km of sewers in its network. With heavy rain presenting the periodic threat of sewer flooding, and with European regulatory standards to maintain, the company decided eighteen months ago that it needed to know more about the way its sewers were operating. After winning a competitive tender, Metrica and Detectronic were engaged in summer 2014 with a five-year contract for the installation and maintenance of suitable monitoring equipment.
“The ultimate target is to improve operational network performance, but this starts with understanding how the sewage network works: how it works during normal operations, during floods, blockages and so on,” explains Hippokrates Pappas, director at Metrica. “They were interested in in having a real-time monitoring network, with alarms for unusually low or high flow that would help them react quickly to any problems that develop. Another important aspect is that although they do have a separate network for wastewater and surface run-off water, they wanted a clearer idea of the proportion of run-off water that actually ends up in the sewers.”
Detectronic’s flow and velocity meters were identified as suitable for the job. EYDAP’s preference was for the monitors not to be in contact with the sewage flow, to eliminate the need for frequent cleaning; this requirement meant that Detectronic’s MSFM Lite, a non-contact monitoring system consisting of an ultrasonic sensor and a GSM/GPRS data logger, was chosen.
However, their installation presented a number of unique challenges. To ensure the best and most useful readings, the sensors had to be installed in the main section of the pipes, away from intersections and access chambers. But EYDAP did not want contractors to access these points from within the sewer, for both health and safety reasons and for ease of maintenance afterwards.
The solution was to dig down to the required locations and drill into the top of the pipes; the sensors could then be inserted in such a way that they can be brought in and out through a shaft, Pappas explains.
“The engineered solution means we are a suitable distance from the turbulent flows and therefore measuring laminar flow patterns. After excavating and digging down to the sewer pipe, the engineers drill into the crown of the pipe using a 200mm core-drill. They then insert a vertical shaft and feed the sensor, which is fixed on a special bracket.”
The sensor, and the accuracy of its readings, is protected from contact with the sewage even in flood conditions because of the unique mounting arrangement of the sensor, he continues.
“The ultrasonic sensor is fitted inside an IP68 barrel-type housing that, if the water level reaches the sensor, will retract via buoyancy to protect the electronics.”
Meanwhile, the data logging unit is permanently installed at ground level in a compartment beneath a manhole. Special antennae with the ability to withstand more than two tonnes in weight were required in order to secure a good mobile signal for the upload of the data.
Flow calculation challenge
The variations in the size and shape of the sewers in the network presented a second major challenge. The pipes vary from 1m to 3.5m in diameter, and while most are around 2-3m below ground, some are as deep as 9m. However, most problematically, not all of the sewers are circular: some have an egg-shaped cross section while others have an even rarer bonnet shape.
Dave Walker, commercial director at Detectronic, said that this made calculating the flow rate through the relevant formula - the Manning Flow Calculation – much more challenging and required the company to carry out significant work on adapting its software.
“To calculate the flow rate, you need to take the wetted cross-sectional area and multiply that by the velocity,” says Walker. “Wetted cross sectional area in a rectangular channel is basic maths, while in a circular channel there is a formula to use which is relatively straightforward. But whenever you start getting irregular shaped channels, you have to work out the geometry of that cross-sectional area and it really takes some quite clever mathematical calculations.
“As a manufacturer, we are fortunate that as well as our team of electronic engineers we have a team of software engineers and were able to do everything in-house.”
Not being able to access the pipes from inside also placed an additional constraint: while flow level meters can be deployed in such a way that they are not in contact with the sewage, this is not the case for velocity meters. For this reason, the team opted to use a single MSFM 12 Area velocity meter which could be placed in the flow at a more accessible point, and moved periodically to a number of locations. This will provide velocity data which can be used to make the Manning Flow Calculation as reliable as possible.
The team will also use portable meters to help fine-tune the assumptions it has made about the pipes, as Pappas explains.
“We will take measurements in the day and night operations, and use portable instrumentation to get a more concrete idea of the velocities and flows involved. The coefficients that we have used for the inclination and roughness of the pipes are based on assumptions or old designs, and this will help us calibrate the systems better.”
EYDAP has invested around €90,000 (£70,000) in the 15 monitoring stations so far, and have been sufficiently impressed to order five more. If all goes according to plan, there is scope for an enlarged network of around 100 stations in the next five years, according to Pappas.
The data produced from the station is uploaded to a web-based software platform so it can viewed both in EYDAP’s control room and in Detectronic’s UK data centre. Alarms notify EYDAP of unusually low or high readings, and Detectronic and Metrica continue to work with the utility to interpret and analyse the data received and co-ordinate the best response.
“All data monitored directly from EYDAP SA and the oracle database of EYDAP SA will be continuously updated for historical data purposes,” says Walker. “Specific attention has been paid to setting up alarm regimes that are sent directly to EYDAP SA. Whenever the thresholds are exceeded, an alarm is activated and the latest data is forwarded to EYDAP SA Database and the Detectronic Data Centre. In normal conditions, the data is sent daily to optimise power usage.
“The network of sensors will facilitate monitoring for EYDAP SA and enable the team to predict blockages and improve operational network performance,” he continues. “To achieve this, they will use our specialist, web-based DetecDataPro platform to access and analyse their data. Web access ensures that Network Management data are available to each relevant person within EYDAP SA.”
The network of 15 stations was officially handed over to EYDAP in December. Pappas says that effective teamworking between EYDAP, Metrica and Detectronic, and flexible thinking from all the parties, has been key to the project’s success so far. He expects similar projects to be carried out in Greece and says the experience gained of overcoming the challenges involved will stand all the partners in good stead.
“It was very challenging for all of us – and very difficult in the beginning – but once you climb the mountain you feel very happy about it,” he concludes.
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