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Exsel Pumps used variable-speed drive systems from Ralspeed to control the pumps it would use to pump three million litres of water from Liverpool’s Langton Dock in just three hours.

Currently, GBP10m is being invested in upgrading lock-gate facilities at the Port of Liverpool, a large and busy container port.

It would be too costly and disruptive to take the lock out of service for the whole duration of the project, which will take around three years.

The decision was therefore taken to refurbish the caissons one at a time, so as to allow the lock to continue to function while the work was being carried out, albeit with reduced flexibility.

To achieve the changeover of the caissons it was necessary to pump out and refill the caissons recess, which holds three million litres of water, several times.

And, in order to limit the amount of time for which the lock was out of service and to cater for variations in the tides around the dock, the time available for each of the emptying and filling operations had to be completed in less than three hours.

Exsel Pumps offered four 200kW pumps, two of which were configured for emptying the dock, the other two for filling it.

As the electricity supply was generator fed, the use of a variable-speed drive enabled a reduction in generator size and also in fuel consumption.

As the drives would be used outdoors, they needed to be weatherproof and be capable of resisting the effects of almost continuous exposure to seawater and salt spray.

Further, since the caisson exchange was taking place in the middle of winter, the drives had to able to operate reliably at low temperatures.

Since Exsel Pumps required the drives only for the duration of the project, it decided that it would be more cost effective to hire them rather than buy them.

For this project, Ralspeed supplied two custom-built drive systems, each comprising a 200kW Vacon NX drive and ancillary components mounted in a rugged weatherproof enclosure.

Since filling and emptying operations would never be carried out simultaneously, rather than providing four separate drives Ralspeed supplied a changeover switching system, which would allow the drives to be switched almost instantly to work with either the filling or the emptying pumps.

For Exsel Pumps, this arrangement provided a cost-effective solution.

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