Submersible motor pumps provide an effective means of lifting seawater for duties including cooling, fire-fighting, temporary ballast and emergency drain pumps.
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 (Fig.1).
By virtue of its design and under water application, the pump is 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.
Typically these pumps operate at 20-30m below sea level, but the maximum operating depth needed is 5-15m below the LAT (Lowest Atmospheric Tide) and flows up to and beyond 2,000m³/h. In the Middle East where the sea levels are much shallower and flat, depths below 8m are rarely encountered so it is necessary to ensure that the water level is always above the pump.
Submersible does not just mean that the whole motor operates in water: the inside motor space, ie 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 design of the submersible centrifugal pump, whereby the pump and water-filled motor constitute a compact unit with the motor located below the multi-stage pump, 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 (Fig.2). 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.
After completing all stator windings are subject to a Hi-Pot test which proves a high winding insulation resistance suitable for windings directly embedded in the motor liquid.
On account of the electro-magnetic considerations, the internal parts of the motors, namely the rotor and stator package, are made from cast iron and are paint protected, but are not in contact with seawater. Any ingress of seawater may not result in motor failure, but could cause corrosion to take place. For this reason, an electrolytic conductivity sensor can be fitted in the motor and this will detect any change in conductivity in the motor fill water. Or an overhead water tank with a level indicator can be added which will show liquid loss in the motor.
Should there be the possibility of the smallest ingress of the pumped medium into the submersible motor, then this can be prevented by the presence of a header tank. This is installed on the deck and connected to the submersible motor with a hose pipe to form a totally enclosed system within the inner section of the water-filled motor. The water pressure in the system is maintained at a higher pressure than the seawater surrounding the motor, and in the event of any leakage, possibly through a defective mechanical seal, water flows out of the system. This leakage is signalled by one or two level controllers in the header tank. The whole system is, therefore, protected against the ingress of the surrounding medium.
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.
The installation of submersible pumps on offshore platforms and vessels is very straightforward (Fig.3). The complete unit comes to site fully assembled with auxiliary parts fitted to the pumps, along with power and control cables. The unit is lifted into a vertical position by crane and lowered into the planned installation position and once there it can be secured onto a caisson foundation using mounting ring clamps.
The remaining installation work involves the erection of the discharge column and securing the power and control cables to the discharge pipe. On average, and depending on the pump size and installation depth, it takes between 6 and 20 hours for a two-man team to undertake the installation work.
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. Surveillance gives an exact picture of the condition of the pump at any given time and also provides a clear and timely indication as to when to preventative maintenance is necessary. Recommended routine checks during normal operating conditions include: pump capacity, delivery head, motor current, motor fill temperature, electrolytic conductivity or water level in the header tank, vibration and axial displacement if a sensor is fitted.
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. In the Arabian Gulf, KSB has operated submersible pumps for water lift duties for up to 25,000 operating hours in seawater temperatures of up to 35ºC, before being removed. Only the wear parts needed to be replaced and all the other components were in perfect condition, thus demonstrating that even under arduous conditions submersible motor pumps provide excellent service performance.
Submersible motor pumps provide many advantages, not least economy of space, absence of drive shafts and guide bearings, simple installation and alignment, and no maintenance. However, it be would be misleading to state that there are no disadvantages. Reservations have been made about the power supply to the motor, but other than possible problems in handling the power cable in sub-zero temperatures, there is no evidence that points to difficulties. In comparison to standard type motors, submersible motors are more expensive as a result of their design and single unit method of production. Also, seawater resistant materials are more expensive. Filling the motor with potable water, together with sealing and monitoring performance also adds to the overall package.
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. As a result, the submersible motor pump is replacing conventional vertical live shaft and other types of centrifugal pumps in the offshore industry.
KSB Aktiengesellschaft is based in Frankenthal, Germany. www.ksb.com