Drives eliminate £10,000 in pump repairs for Alkane Energy
ABB is helping a major producer of energy from coal mine methane (CMM) to cut its pumping costs by using variable-speed drives (VSDs) to make its process more reliable. The drives ensure that the pumps are not over stressed, allowing them to last longer.
Alkane Energy is one of the UK's fastest growing independent power generators. It operates mid-sized 'gas to power' electricity plants providing both predictable and fast response capacity to the grid. Alkane now has a total of 81MW of installed generating capacity and an electricity grid capacity of 100MW.
The company uses VSDs to control the vacuum pumps on new sites but was keen to see if they could be retrofitted to existing sites. The challenge with these is that the pumps are run direct-on-line and operate at full capacity all the time. This causes extreme wear on pumps, which tend to fail after only a few months.
Keith Mitchell of Alkane says: "As we pump gas deeper from the mine, the pumps need to work even harder. A failed pump can cost around £10,000 to repair. We can also lose generation revenue, as well as the cost to remove the failed pump, so it is important we know that retrofits of drives are possible on existing sites."
ABB Drives Alliance member Inverter Drive Systems (IDS) already supplies Alkane with VSDs for the new sites so was asked to investigate the possibility of retrofits, with a trial to be carried out at the Old Mill Lane site in Nottinghamshire.
Phil Nightingale of IDS says: "The existing set up uses soft starts to slowly start the pumps. When the pump is at full speed, the soft start is bypassed with a contactor. The pressure is then controlled with a bypass valve. The inverter installation does away with this valve and modulates the pump to keep the pressure constant and saves the wasted by-pass energy."
The drive chosen was a 75kW ABB industrial drive. IDS designed a bespoke door as well as a back panel to fit the existing cabinet so that the drives' switches and connections could be accommodated. Says Nightingale: "One of the major challenges of the project was the small space available at the site. We needed to fit the drives and panel into the space previously occupied by the soft starters and added fans to cool them. We also had only one day to fit the drive panels so we did all work off-site to minimise disruption and ensure that Alkane could achieve maximum gas pumping."
Says Mitchell: "We have received many benefits form the application. The VSDs give us a fine control of the gas pumping, compared to the coarse control of the soft starts. As well as the money saved from cutting the number of pump repairs and the lost production, the VSDs also save around 12kW an hour in energy that we can sell to the grid. We also now know that the older sites can be successfully retrofitted with VSDs."
IDS have so far completed two retrofits, with another three to do and a potential to complete more in the future. In further news, ABB reports that one of the UK's major producers of ready meals is saving 25% a year on its cooling costs by employing a single 55kW ABB variable-speed drive (VSD).
Saladworks of Leicester supplies 135 different ready-meal products to four of the UK's major retailers. The company asked IDS, to recommend ways to reduce its energy use. A major part of its process is chilling the finished products, for which Saladworks employs two refrigeration circuits, each with a duty and a standby glycol pump and a condenser.
Paul Hampton, engineering manager for the plant, says: "We knew that variable-speed drives could save us money by cutting our energy use. IDS confirmed our instinct by taking a more statistical approach. Their energy appraisal produced a detailed list of all the motors on site and flagged up two glycol pumps that could save particularly high amounts of energy. We look for a payback of under a year and these applications met that criterion."
An ABB drive was selected on one glycol pump in each refrigeration circuit, one of 55kW, the other of 11kW. The circuits are in constant operation, with the VSD controlled pump motor running 98 percent of the time. The direct-on-line pump is run for the other two percent of the time to ensure it remains in running order ready to take over the load if the main pump suffers a breakdown.
The VSD controlled pump motor is operated at 40 Hz, producing a saving of 25% of the original energy usage. This equates to a payback time of less than a year, meeting Saladworks' target. Saladworks has the option of running it at 43 to 44Hz to account for elevated temperatures in the middle of summer. Using the drives also has the added benefit of causing less stress on the motors, reducing the maintenance effort needed to keep then in working order.
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