Drive system for caisson refurbish

As part of the upgrade of a busy European port, the lock's caissons had to be refurbished. A weatherproof pump was required which would empty the caissons within a three hour window of opportunity. The supplied pump achieved this without the expense of having to take the lock out of service.

Figure 1. The 200 kW pumps needed to be weatherproof and capable of resisting the effects of almost continuous exposure to seawater and salt spray.
Figure 1. The 200 kW pumps needed to be weatherproof and capable of resisting the effects of almost continuous exposure to seawater and salt spray.
Figure 2. Three million litres of water were pumped out of the caisson recess.
Figure 2. Three million litres of water were pumped out of the caisson recess.

Faced with the requirement to pump three million litres of water from Liverpool's Langton Dock in just three hours – with potentially serious and costly consequences if things went wrong – Exsel Pumps decided to rely on variable speed drive systems from Ralspeed to control the pumps it would use for this challenging project.

Currently, £10 million is being invested in upgrading lock gate facilities at the Port of Liverpool, one of the largest and busiest container ports in Europe. An important part of this project involves refurbishing the caissons (sliding watertight doors) used in Langton Lock. This Lock, one of the busiest in the port, is divided into three sections, with caissons between the first and second sections, and between the second and third sections.

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, which would allow the lock to continue to function while the work was being carried out, albeit with reduced flexibility.

The first phase of the project was the removal of the 1,600 tonne outer caisson, which was then sailed to the top end of the dock and moored up. Once this was complete, the next phase involved removing the 4,000 tonne refurbished middle caisson from the recess from where it had been refurbished. The caisson was sailed down the lock to the outer recess, where it was then reinstalled and commissioned. The next stage was to sail the outer caisson into the refurbishment recess for repair and refurbishment.

To achieve this 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 was to be completed in less than three hours.

Exsel Pumps has wide knowledge and experience of supplying pumping systems for such demanding applications, and offered four 200 kW pumps, two of which were configured for emptying the dock, the other two for filling it. The engineers at Exsel knew, however, that they would need dependable variable speed drives to control these pumps not only to ensure smooth flow of water, but also to minimise the peak load on the electrical supply when the pumps started. As the electricity supply was generator fed, the use of a variable speed drive enabled a reduction in generator size and a reduction in fuel consumption.

Since the drives would be used outdoors, they needed to be weatherproof and, because of their proximity to the dock, they also needed to 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. It therefore approached Ralspeed, a company which specialises in providing drive systems for hire, and that has the resources and expertise needed to configure systems for even the most demanding of applications.

For this project, Ralspeed supplied two custom-built drive systems, each comprising a 200 kW 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. This arrangement provided a very cost-effective solution.

“Despite the drive systems having to be built specially for us, Ralspeed's hire rates were still very competitive,” said Peter Huson of Exsel Pumps, “and the company was very careful to ensure that our requirements were accurately met. We installed the drives ourselves, but Ralspeed commissioned them and, during the critical pumping operations, the company even had its technical director, Simon Finlay, on-site to make sure that everything went smoothly. All in all, we had excellent support and service from Ralspeed and, on the day, the drives and pumps operated flawlessly.”

In addition to custom drive systems of the type supplied for this project, Ralspeed also offers standard variable speed drive and soft start systems for hire and for outright purchase. The company operates a comprehensively equipped manufacturing and test facility in Blackburn, Lancashire, and provides expert support for all project stages, from the selection of the most appropriate and cost-effective equipment, right through to commissioning and on-going support throughout the life of the equipment.

“There are many instances, as the project at Langton Dock shows, where hiring a drive system is preferable to buying it,” said Peter Schaffel, managing director of Ralspeed, “yet hiring is an option that's often neglected. We're always very happy to work with potential customers to decide on the option that's best for them and, whether the answer is buy or hire, we're in a position to supply exactly what's needed.”