COMALCO Alumina Refinery (CAR) is the first new alumina refinery to be built in Australia since 1984, completing construction in November 2004. It is located 15 kilometres northwest of Gladstone, Queensland. CAR has a capacity of 1.4 million tonnes per annum of smelter grade alumina.
One of the many hurdles in the ramp-up to full production rates was the reliability of six positive displacement pumps, used to charge bauxite slurry into the digestors.
Modes of failure primarily included premature failure of diaphragms, non-return valves and crankshafts. With few other installations of similar equipment in alumina refineries around the world, formed a team of internal, external and OEM resources whose specific goal was to achieve design reliability rates and improve plant production. One of the first tasks was to gather as much data as possible on the process conditions in and around the pumps.
In the initial stage, White Rock Engineering Services of Austin, Texas, US, was engaged to investigate the root causes of these failures. This was conducted using a portable datalogging system comprising two National Instruments LabView applications, a logger and an off-line analysis tool to process and display the contents of the recorded data.
The logging application collects data from up to 11 analogue channels, including slurry pressures at the suction and discharge of the pump, oil pressures in each diaphragm chamber, crankshaft position and crankshaft vibration.
The sample rate of each channel is 4000 Hz, in order to capture high frequency transient pressure fluctuations.
The analysis application arranges the raw data into sections, based upon one complete stroke of the pump. For each stroke, information is provided including precise calculations of valve opening and closing angles, rate of valve opening and closing, theoretical volumetric efficiency of each chamber, hydraulic work done, peak to peak and average suction and discharge pressure, and peak to peak crankshaft vibration.
The information is displayed providing a visual indication of the performance of the pump, as well as in text format, which allows long term trending to take place.
The initial installation was very successful. As a result, Comalco invested approximately $900K in a permanent installation of equipment on all six installed pumps that would provide the following:
• The merging of the two previous software applications into one.
• A real-time web interface to raw and processed pump data. This enables real-time evaluation of pump performance to be made, without the need for multiple applications and file manipulation.
• An OPC server interface which would provide a link to the plant process data historian. This allows direct correlation of calculated pump data and process data, which is crucial as the pump performance is directly related to the upstream and downstream process conditions.
• Discrete outputs indicating preset alarms levels for selected parameters had been breached, these outputs being scanned by the plant DCS.
• Storage of real-time data to enable detailed post event analysis.
• High reliability system in line with existing plant equipment standards.
National Instruments PXI series hardware was selected as the most suitable for this application.
White Rock Engineering Services redeveloped the software for the new hardware and provided the additional features that were required. Installation of instrumentation, electrical and networking equipment and commissioning were completed for all pumps in less than three months.
The results achieved by the system have been excellent. To date the information has been used to diagnose many pump and process problems, as well as evaluate the performance of new pump components.
Previously such analysis could only be carried out after components had been installed for some weeks or months. Following on from these investigations, a number of design changes have been instigated, a small subset of these being:
• Redesign the suction manifold of the pump to improve net positive suction head available, and remove high-frequency transient pressure fluctuations.
• Redesign of the control strategy of the level in the slurry mixing tank, which immediately precedes the pumps. The design change was aimed at preventing a drop in the level of the tank and corresponding air entrainment into the pump suction side.
• Early detection of non-return valve sealing problems due to mill screen issues.
Since the introduction and commencement of use of the condition monitoring system, pump reliability and plant productivity have been significantly improved.
A comparison of November 2005 with March 2006 shows:
• Seven diaphragm failures in November 2006, compared to one in March 2006.
• 26,000 tonnes of lost production directly attributable to charge pump unscheduled outages in November 2005 versus 5500 tonnes in March 2006.
• Total digestion production of 80,424 tonnes for November versus 100,000 tonnes for March 2006.
The condition monitoring system is being rolled out as part of a total productive maintenance plan for the charge pumps. Comalco is standardising and developing procedures and documentation for reliability and operations team members. These procedures include the use of the web interface to check the operation of the pump, and a catalogue of data from previous failures for real-time diagnostics.