Submersible motor pumps have become an indispensable feature of the offshore industry in recent years, largely due to the combination of technical sophistication and cost-effectiveness. Providing maintenance-free operation over extended periods, the pump is a conventional multi-stage centrifugal type in a ring-section or vulture design which allows the pumphead and output to be matched to the application by varying the number of centrifugal stages as required. The pump is driven by a submersible motor which is directly flanged below the pump.
The pump is typically suspended below the platform or vessel either in the sea or in a stilling tube, so in terms of space allocation all the installation needs is a simple head plate. The power supply and functional monitoring is via electric cables and water delivery through a simple vertical discharge column.
Submersible does not just mean that the whole motor operates in water: the inside motor space, i.e. the space between the rotor and stator, including the windings, is also filled with potable water. These water-filled motors do not require any form of lubricant as the bearings are lubricated by the water.
In addition, the water constitutes an effective cooling medium for the motor. The most important advantage of water-filled motors is that they can withstand any pressure as the internal and external pressures are always kept in balance. This means that the submersible motor pump can be installed at any depth irrespective of the prevailing pressures.
The submersible centrifugal pump and water-filled motor constitute a compact unit with the motor located below the multi-stage pump, and is a mixed flow type consisting of a suction stage, bowls and a discharge casing.
The suction casing connects the stationary parts of the pump and motor, and is fitted with a suction strainer to protect the pump against the ingress of larger sized solids. The pump shaft is connected to the motor by a rigid coupling, with the shaft being protected by sleeves and transmission of the motor torque to the impellers performed via shaft keys.
All the parts of submersible motor pumps which may come into contact with seawater are manufactured from materials that are resistant to seawater (image top left). The three-phase asynchronous motors are filled with a mixture of potable water or poly propanolglycol (antifreeze) with corrosion inhibitors. The windings, which are permanently immersed in water, are insulated either with PVC (for standard applications) or PE2 with a protective layer of PA for higher demands.
Naturally enough, corrosion is of prime importance and KSB's UPA submersible pump range is constructed from materials that give the highest protection against seawater corrosion. These materials are typically Nickelaluminiumbronze, Duplex and Super Duplex, particularly where high salinity is encountered, and should there be problems anticipated from sand, bearings will be constructed from silicon carbide.
When it comes to maintenance, submersible motor pumps are described as being maintenance free. KSB's response to questions about service intervals for its UPA pump range is not to have a planned maintenance regime but to keep the equipment under surveillance and to record all results as they occur.
Under normal working conditions, the service life of the KSB UPA pump can be anywhere between 16,000 and 25,000 hours. During this time, the pump will remain fully operational at all times.
For the platform operator, the constant availability of the pump at the touch of a switch cannot be underestimated. The technology has been well-proven in many challenging applications and operating conditions where the demands on the pumps and motors have been very high.
