12.5 Shaft seals
Mechanical seals are included to prevent water escaping from the casing along the pump shaft. Split type seal designs are used (Fig 4.51). All the components which are subject to wear, are split into two sections to permit inspection and replacement of worn components to be carried out without any major dismantling of the pumpset.
The seal needs to be provided with a clean supply of flushing water which can either be filtered water taken from the pump discharge or a separate towns water supply. Figure 4.52 shows a typical seal flushing arrangement on a coastal station, where the filtered water is normally taken from the pump discharge for flushing purposes while the pump is running. A separate towns water supply is integrated into the system which enables the seals to be flushed prior to start-up and following shutdown. The towns water supply is also available as a back-up supply during pump operation should the seawater system fail as a result of excessive pressure losses across the filtration system.
The seal assembly also includes an additional inflatable static seal. This is operated by air and forms a watertight joint around the shaft to allow dismantling of the main seal unit without dewatering the pump. A secure source of air is required and the use of a standby accumulator eliminates the need for the station air supply to be available at all times.
Any water which leaks into the access well of the concrete volute pumps is removed to the drainage sump by either a small submersible pump or an air-operated ejector system. These can be controlled from float switches or an adjustable timer.