Elegant solution to dewatering at Springvale

Staff Reporter

The Springvale colliery near Lithgow in New South Wales seems to have come up with an answer to one of its long-standing technical problems. Adaptation of oil industry pumping technology for mine dewatering duties is said to have enabled the mine to increase its dewatering rates at reduced cost.

Hydrological and geological hurdles have proved major stumbling blocks for Springvale, a high-capacity longwall mine in the western coal district of NSW, since the mine came on line in 1994. Geological complexities and uncertainties remain companions of the site's engineers, but the resolution of mine dewatering problems has given Springvale a major boost.

Project manager and mine electrical engineer in charge, Steve Burgess, said there had been no simple solutions to the mine's problems. The mine "water make" and layout of workings made the use of bore-hole dewatering equipment at the rear of longwall panels imperative. Water was pumped continuously from the mine's 400m-deep coal seam at rates of 20-70 litres a second, with seasonal variations.

Burgess said relocation of mine pumping equipment to a new bore-hole dewatering location in mid-1999 gave Springvale an opportunity to research wider industry developments in a bid to identify dewatering plant that offered better pump performance and reliability, and reduced overall dewatering costs.

The largest up-front cost was construction of two 16-inch boreholes. Two bore holes, with two pumps, were required to achieve required dewatering rates. However, even with two bore holes only three production rates were possible: 0, 30 and 60 litres per second. "The obvious way to reduce cost was reduce the number of bores to one and install one pump that could cover the range," Burgess said. "But to accomplish this the bore diameter needed to be increased to achieve the high rates and a variable speed drive was needed to change the flow because standard pumps could not be throttled.

"Using a variable speed drive on some of these units was questionable in any event. The larger bore and the variable speed drive resulted in a more expensive set-up than with two 16-inch bores."

The search for answers led mine personnel across borders into the gold mining and oil producing industries, and ultimately to Schlumberger business arm, Reda Production Services. Burgess said an "oil-industry-bred" bore-hole submersible pump was designed and constructed to meet Springvale's dewatering needs.

"Reda developed the original submersible pump over 80 years ago and has been in business since then. It is now part of the Schlumberger group of companies that focus primarily on the oil industry," he said.

The installed system consisted of a 14-stage centrifugal submersible pump and a 440kW motor. Between the pump and motor is an item referred to as a protector that houses a thrust bearing. Burgess said having the thrust bearing in an isolated section meant any wear on the bearing did not affect the motor. Impellers and diffusers in the pump were also designed not to rub together.

"This not only increases the life of the pump, but also allows the pump to be throttled," he said.

The unit is more than 30m long and comes up to speed in one-eighth of a second. It is designed to pump sand and up to 19% free gas by volume. The surface supply voltage chosen was 3300 volts.

According to Reda, technology advances have made the unit extremely slender; with the 20m-long motor only 142mm in diameter. The pump unit itself is 267mm in diameter. A cased and grouted hole was completed to a depth of 450m (50m below the coal seam), allowing the electric submersible pump to be installed well below the mine floor.

Because the pump was installed below the water source a shroud was installed to direct the fluid past the motor to ensure proper cooling.

Standard flanged riser (discharge) pipe could not be used because the flanges would have needed a larger bore than the pump. Line pipe could not meet the installed column weight and discharge pressure so specialised oil industry tubing, with a fine tapered thread, was used.

Burgess said initial objectives of the project had been achieved. These were:

Improved performance The mine was now being dewatered from 430m depth at a rate of up to 70 litres per second, and had the advantage of being able to control output levels across a wide flow range.

Improved reliability Burgess said the "test of time" would be the only true measure, but in the first four months of operation after final commissioning the unit's availability was at 100%. Contributing to the improved mean time to failure of an electric submersible pump was the degree to which cycling could be constrained.

"Minimising cycling with this installation is achieved as a direct result of being able to throttle the pump's output," he said.

Reduced costs Achieving a dewatering rate of 70 litres per second from a slender reliable borehole provided immediate and long-term cost advantages, Burgess said. At Springvale, the bore-hole cost reduction was in the order of $300,000 (based on the comparison of a 300mm bore-hole costed at $150,000 compared to two 450mm bore- holes costing $230,000 each).

Reda's manager for Australia, Gordon Kappelhoff, said the company had not actively promoted its products to the mining industry. Despite this, he said several installations had been completed and he was fielding an increasing number of enquiries about the Reda pumps from mines.

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