Monitoring exercise provides valuable stress data

A six week exercise monitors and records in-service loadings on scraper blades operating in the primary settlement tanks at Davyhulme Sewage Treatment Works.

The Structural Monitoring Division of Stockport based AV Technology (AVT) has successfully completed a six week exercise to monitor and record in-service loadings on scraper blades operating in the primary settlement tanks at Davyhulme Sewage Treatment Works in South Manchester.

Based on a six-channel hazardous-area strain-gauge monitoring system, the work has been carried out for Birse Water as part of a wider odour control project at the works for United Utilities.

Although Birse was able to use its in-house finite element (FE) capability to understand the stress distribution on the structure, no data existed that could quantify the actual in-service loadings on the scraper blades resulting from the different depths of settled sludge for the two operating regimes.

As a result, AVT was contracted to measure and record the actual bending forces acting on the scraper frames as they move up and down the tanks.

The project presented several practical challenges for AVT, as Special Projects Manager Neil Parkinson explains: "Firstly, in order to measure the maximum bending moment forces on the frame legs the strain gauges had to operate permanently underwater at a depth of around 1.5 metres".

"Secondly, because the scraper frame runs up and down rails either side of the 70 metre long tank, the monitoring system needed to be able to operate remotely under its own power".

"The task of measuring the forces was further complicated by the fact that the three primary settling tanks have recently been fully enclosed to provide effective odour control for the surrounding residential area".

"Due to the potential risk of explosive gases entering the sewage network, and subsequently the settling tanks, the environment inside the tanks is classified as a Zone 1 hazardous are"a".

To meet the ATEX requirements, we created a micro safe area around our equipment fitted on top of the scraper bridge".

"This consisted of a novel dual Exd-Exi environment using two explosion proof enclosures (EExd IIC T6), one to house the Campbell Scientific CR10X data logger, multiplexer and Zener barrier systems, the second the 7Ah battery pack".

"This novel solution is virtually identical to a strain monitoring system we have installed on an offshore oil tanker to monitor the integrity of the mooring chains".

To measure the maximum bending forces on the six vertical legs, pairs of waterproof single element gauges were spot-welded at the mid point on the front and back of each leg.

These were then wired back to the Exd enclosure using the integral leads.

To ensure gauge integrity in this particularly harsh environment, the gauges were protected with a further three stages of environmental coating.

Precision 120ohm resistors were used to complete a full Wheatsone bridge configuration in the enclosure for each leg and the input and outputs were wired through Zener barriers to the data logger.

In order to comply with the ATEX requirements for simple apparatus, an insulation test was carried out on each strain gauge to ensure that the insulation between the gauge circuit and the metal supporting substrate was capable of withstanding an AC RMS test voltage of 500V.

The output strain signals from the gauges have been calibrated into bending moments based on a site calibration procedure using a reference load cell.

This was achieved by attaching a winch to the end wall of the tank and applying a force to each leg in turn, using the reference load cell to measure the load applied.

The project was split into two three week tests.

Initially measurements were taken with the scraper carrying out a scrape approximately every eight hours.

To optimise storage capabilities the data logger was timed to log data during these three 'time windows'.

It was programmed to measure strains every second and to store running maximum values per channel every 10 seconds.

With a scrape time of approximately 40 minutes, this gave the unit the capacity to store data for up to 37 days with the data from each three week test fitting on a single Excel spreadsheet.

The measurements were then taken for the second three week period with the scraper only carrying out one cycle during a 24 hour period.

The data were manually collected by AVT on a weekly basis.

As the system is self-contained within the hazardous environment a mandatory gas test was carried out to ensure a safe working environment before opening the explosion proof enclosure and downloading the data to a laptop.

Collecting data is only a small part of the work AVT carry out and as Parkinson continues: "The key to successful fatigue analysis is being able to analyse and interpret large amounts of information and then present it to the client in the optimum format".

"All round good communication and the ability to work closely with other interested parties are also vital ingredients".

"In projects such as this one at Davyhulme, the working environment is far from perfect and a far cry from conditions found in the laboratory".

"However, we are used to working in difficult and often remote areas with limited facilities and have developed a high level of ingenuity to ensure that our equipment can operate without problems".

Birse Water's Chief Mechanical Engineer Ken Craven is pleased with the overall results and as he concludes: "We needed to establish the nature of the loading on the scrapers under the two operational regimes attributable to varying sludge depth to assess the long term reliability implications".

"We recognised that gathering real time data in such an awkward environment was not going to be very straightforward".

"However from the outset AVT applied a professional and pragmatic approach to the problem and their expertise has allowed us to obtain some very useful information".

"We have combined the results of their trial with data we also collected regarding the build-up of sludge within the tanks under the two operational regimes and are now working with our client to determine the best way forward".

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