For decades, submersible sewage pumps have been
a succesful means of pumping polluted liquids. They are installed in sewage
treatment processes and in private, industrial and local-authority pump stations
and are also employed, amongst other things, in lifting units in schools,
hotels, hospitals… and private homes.
The products available on the market
worldwide differ little in terms of their basic technical principles. In
the decades of submersible pump evolution, a large number of technical
innovations have become standard: mechanical seals made of silicon
carbide; components for monitoring the motors (tightness monitoring,
winding protection contacts…). Likewise all the available models use
grey cast iron for the main components such as pump and motor housing
and impeller. Exceptions to this are made for the pumping of heavily
polluted media where special alloys are employed. One can therefore
claim without reservation that all the successful and serviceable
products on the market differ little from one another and that the
competitive advantages are to be found in a company’s other activities
such as distribution and service.
However, a new development from Wilo GmbH shows that in the pump
engineering sector the limits are still far from being reached. The use
of different materials in to some extent novel applications has given
rise to a new generation of submersible pumps. As a radical departure
from all that has gone before, this new pump series uses an
unprecedented combination of materials i.e. stainless steel for the
motor housing and a polyurethane composite material for the pump
housing. For instance, a sewage pump with P2=8kW usually weighs between
about 100 tot 120 kg, depending on the manufacturer. This new
development weighs 62 kg. An advantage that pays off not only during
handling on site, but also in the design engineer’s integration of
submersible pumps in systems (e.g. rotating scraper bridges). As a
furter improvement over conventional designs, this submersible pump is
supplied with a standard detachable cable. As cables often have to be
passed trough walls or jackets, rapid "on-site" cable
detachment, e.g. to carry out preventive maintenance, is a welcome way
of saving on auxiliary materials, time and money. The non-integrated
cable is easy to detach and the motor remains watertight. The sewage
pump series is also supplied with explosion protection as standard. This
feature makes it a "universal pump" – like the standard
jacket cooling from nominal widths of DN 80 upwards. Thanks to the
standard fitting of a stainless steel cooling jacket around the motor
housing, the universal pump can be installed in wet or dry conditions.
The pump medium serves as the coolant. Thanks to certain technical
details, the medium circulates without leaving deposits and is
continuously replenished. Pump station operators are painfully aware of
the problems which occur during the handling of so-called "pump
killers" such as rags, plastic parts and various sanitary articles.
The new congestion-free pumps combine a progressively designed
single-channel impeller and a pattened spiral base plate. This
combination ensures trouble-free operation even if fibrous components of
the pump medium pass between the impeller blade and spiral holes. This
represents a major step forward on the road to reduce maintenance costs
and a longer service life.
Level controls are a frequent cause of malfunction of submersible sewage
pumps. To combat this problem, a level encoder has been developped which
can be permently installed on the pump, thus eliminating the need to fit
float switches and the like in the pump shaft. This is a closed,
unpressurized pneumatic system. The level encoder is permanently
connected to the switchgear by a hose. The switchgear can therefore be
used to remote-control the switching levels in the pump shaft
irrespective of temperature. The system operates with such precision
that hysteresis of ± 2cm is possible.
Dipl.-Ing. Carsten Krumm, Product Manager for waste water and sewage
pumps, Wilo GmbH
