The comments below were received independently from various clients/engineers who operate both Archimedean Screw Pumps and the (Submersed) Propeller Centrifugal system.
The comments below were received independently from various clients/engineers who operate both Screw Pumps and the (Submersed) Propeller Centrifugal system.
The relatively high operational speed (450 - 950 rpm) causes wear and damage in the pump housing, this is particularly so when pumping waste water containing sand and stones. This wear results in regular expensive repairs to the housings.
At reduced capacities the speed in the vertical pipe reduces to such an extent that the solids fall out of suspension to the bottom of the pipe causing blockages which eventually stop the pump.
Heavy solids cannot be pumped; the sump eventually fills with the solids which have to be removed by hand by maintenance staff. Alternatively a coarse screen would need to be installed which increases the total equipment costs!
Floating debris is not removed; this collects in the sump and has to be removed by hand.
Moreover, at lower water levels in the sump when the spirals of the pump are not completely covered, floating (wooden) debris can enter the conical spiral causing the pump to block. Repair is difficult necessitating complete dismantling of the pump.
A centrifugal pump will be severely damaged when running dry; it is therefore necessary to install expensive measures (level control etc) to prevent 'dry-running'.
High speed is required in the vertical discharge pipe in order to lift the solids with the waste water, obtaining this high speed requires the use of small diameter piping. Using small diameter piping causes high friction losses in the pipe which increases energy consumption.
The mechanical seal between the pump and motor requires regular adjustment or replacement which is time consuming (isolation and wash down required) and hence expensive.
Even low capacity pumps (100 l/s) are too heavy to lift by hand, therefore every time repair is required a mobile crane must be used or permanent lifting facilities must be installed - either option being expensive.
The submerged pumps and motors require higher educational skill of both operators and maintenance staff.
The nature of the design requires the use of submerged motors; problems are encountered with leakage and short circuiting.
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