22 Oct 2018
Unitywater owns 17 sewerage treatment plants (STPs) across Queensland which includes the Moreton Bay, Sunshine Coast and Noosa regions. They provide its customers with a high-quality, safe and reliable water and sewerage service that is economically and environmentally sustainable. Today water authorities are facing ever-increasing pressures to increase their quality of service and reduce maintenance costs.
With more blockages pushing up costs, and customers generally unwilling to absorb these costs, Unitywater sought the assistance of Control Logic to conduct a trial to reduce their pump blockages at their most troublesome pump station located in Deception Bay Queensland. Imagine having to pump 1.2 million litres of sewerage per day and your pump required manual removal and cleaning monthly.
This particular pump station consisted of two redundant 160 kW submersible pumps with a weight of 1.3 tons each. A VSD mounted in an outdoor enclosure was coupled to each pump and was controlled by a SCADA system connected to a local RTU which was hard wired into the drive providing a 4-20 mA speed reference and start command. The pumps operated in a duty standby arrangement, pumping into a single pipeline with a head of 70m, with an expected flow rate of one hundred litres per second. The challenge provided by the water authority, was to reduce the operating costs of this pump station.
Based on a number of factors, Control Logic recommended and utilised ABB’s ACQ580 water drive. Designed for the water industry with specific pump clean functions built-in, it was the ideal solution for this task. To ensure that it was as accurate as possible, ‘torque’ was selected as the reference trigger to activate the pump clean function. The actual torque was recorded from the drive’s inbuilt torque display which established a pump talk curve.
The pump clean function was set to trigger at a narrow torque band above and below this curve. The pump clean function itself consisted of a series of forward and reverse pumping sequences. The challenge here was the reflux valve which was situated 2m upstream of the pump itself. This problem was mitigated by ensuring the torque on the reverse pumping sequence was low and very brief.
After running the pump clean sequence four times, the flow increased from 98 L/second to 145 L/second. It was obvious at this point that the problem was solved and the water clean function was switched off. One month later, the pump blocked again. This time the auto pump clean function was activated and left to run. The following month there was an improved flow rate of 178 L/second which was left active for additional month, with a flow rate of 201 L/second. Since leaving the pump clean function active there has been no further blockages or manual pump cleaning required.
Why had the flow rate doubled? Looking at pump curve four, flow rates of 100L/s will have a head of 68m and 201L/s has a head of 48m. The static head is constant, however the dynamic head was now 20m less. This indicates that not only was the pump cleaned, but the downstream pipeline as well.
By reducing the manual clean requirements, maintenance costs reduced by $26,000 per annum. By doubling the flow rate the pumping times reduced by half, lowering their energy bills. Energy bills were further reduced by ensuring that the peak demand did not exceed the time trigger threshold. Their total return on investment was less than six months.
For more information on ABB’s ACQ580 water/waste water drive, contact the Control Logic team on 1800 557 705 or by emailing firstname.lastname@example.org.