Advantages of screw pumps and Disadvantages of Propeller / Centrifugal Pumps

Advantages of screw pumps:

+ Slow Speed, Simple and Rugged design

Probably the main and overall advantage of a screw pump is its superb reliability. The simple design, open structure and slow rotation speed makes it a heavy duty pump with minimal wear that operates for years without trouble.

+ Pumps raw water with heavy solids and floating debris

Because of the open structure and large passage between the flights a screw pump can pump raw sewage without the need for a coarse screen before the pump. Both floating debris and heavy solids are simply lifted up. This saves considerably on equipment costs for a coarse screen or maintenance!

+ Can run without water

A screw pump can operate even when there is no water in the inlet. Therefore it is not necessary to install expensive measures (level control etc) to prevent ‘dry-running’’. The lower bearing does not need cooling.

+ No collection sump required = minimum head

A screw pump ‘scoops’ the water directly from the surface and does not need a collection sump. This keeps the pump head to a minimum.

+ Constant high efficiency with variable capacity

The efficiency-curve of a screw pump is flat on the top. Due to that efficiency characteristic, the screw pump offers even high efficiency when it works at 50% of its capacity.

+ Pump capacity is self-regulating with incoming level

When incoming water-level goes down, at dry weather flow, the screw pump ‘automatically’ pumps less water. Ergo: no control system required to adapt pump performance.

+ 'Gentle handling' of biological flock

The activated return sludge on STP’s is a delicate biological substance. Because of the low rotational speed and large opening between the flights, screw pumps do not damage this biological flock (whereas the high speed rotating centrifugal pumps will completely shred the biological flock).

+ Easy maintenance (no 'high skilled' staff required)

A screw pump requires very little maintenance. Compared to (submersed) centrifugal pumps it is next to nothing. Besides that no ‘highly skilled’ maintenance staff are required which makes this type of pump very suitable for remote locations.

+Long lifetime ( > 20-40 years)

Screw pumps with typical lifetimes of between 20-40 years are not unusual.

Disadvantages of Propeller / Centrifugal Pumps:

- High speed = increased wear

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.

-Possible blockages at reduced capacities

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

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 pumped

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.

- Dry running is fatal

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 friction losses in pipes

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.

- Mechanical seals need regular adjustment

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.

- Lifting facilities required with each maintenance

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.

- Higher skilled maintenance staff required

The submerged pumps and motors require higher educational skill of both operators and maintenance staff.

- Submerged motors cause more trouble than dry motors

The nature of the design requires the use of submerged motors; problems are encountered with leakage and short circuiting.