How to pick the right impeller

As wastewater now tends to be more full of pump-clogging solids than ever, Peter Maslin, general manager of pump manufacturer KSB, weighs up the pros and cons of impeller designs

Water consumption has greatly reduced over the last few years. Numerous measures have been promoted to reduce water consumption with many households opting for metered supplies.

This has had the effect of substantially increasing the solid and fibre content in wastewater. This is further exacerbated during dry spells because fibres and other solids tend to collect in the sewers only to be dislodged by the next heavy rainfall and passed through the system to the pumping station. Also, in the past, inlet screens protected pumps from the worst of the solid material. But, in an attempt to reduce maintenance costs, many water utilities are removing these screens.

The result of these factors is a change in the character of wastewater, which poses an additional challenge to the designers of wastewater pumps if their designs are not to block.

Two areas in a pump particularly prone to clogging are the wear ring and the impeller leading edge. In pumps with a closed impeller, fibrous material and hygiene articles are prone to collect in the clearance between the pump casing and the impeller.

Added risk
In extreme cases where for example the wastewater contains high concentrations of sand and grit, the wear ring will be abraded and the increased recirculation of flow from the high-pressure side back to the suction will add to the risk. The design of vortex impellers is such that no wear ring is necessary, eliminating this potentially problematic area because there are no spaces where solids might accumulate. These impeller types offer considerable advantages in terms of operating reliability.

The other critical area is the impeller eye and the position and shape of the vane leading edges. It is here that textile material, long fibres, sundry household waste and even scraps of wood can become trapped and impair the pump's proper function.

This is where one impeller design may look similar to another, but the better ones will be constructed so fibres depositing on the vane are flung off as quickly as possible.

Another factor is the number of vanes because this and the size of the pump dictates the free passage.

This is defined as the diameter of the largest sphere that can pass through the impeller. Few impeller geometries can provide the minimum free passage of 80mm or even 100mm that many users stipulate for handling raw sewage.

Both vortex and single-vane impellers provide relatively large free passages and both have provided many years of satisfactory service on difficult applications. A pump with a 100mm discharge connection fitted with a vortex impeller should be able to handle a 100mm sphere, but this will reduce for a single-vane impeller pump to about 75mm or 80mm. Open two-vane and multi-vane impellers will have much smaller free passages.

Open impellers are also at an advantage when fluids with a high gas content have to be handled, for example, digester recirculation. When the gas content rises much above 5% closed impellers carry the risk of the flows being blocked by the gas.

Life cycle cost considerations are becoming increasingly important and here the pump designer faces the difficult task of providing a highly efficient pump that does not block. In the past, high efficiencies have only been possible when the internal clearances, including free passage are small.

Also, due to the way wastewater pumps are operated, the run of the efficiency curve in the low-flow range is of equal importance to the best efficiency point. It is here impeller geometry has the greatest impact.

Closed impellers have flatter efficiency curves than open impellers and therefore offer better low-flow efficiency. For example, the difference between a closed single-vane impeller and an open single-vane impeller can be as much as 10 percentage points, even though they both have the same best efficiency.

It is clear from this that the concept of efficiency should not always be considered a question of best efficiency, but also as one of low-flow efficiency because the tank is often what counts most in the actual service environment. At this point, we should mention another open impeller design: the diagonal spiral vane or screw impeller.

With this the designer has tried to bring together high efficiency with a relatively large free passage. It is now possible to provide a 100mm pump with a 100mm free passage and the ability to handle large concentrations of fibrous material.

On the face of it, the only disadvantage would appear to be that the high efficiency is dependent on the small clearance between the impeller leading edge and the pump casing. Hence, any liquids containing large amount of abrasive sand and grit will produce wear across this area resulting in a drop in efficiency. It is therefore important in such circumstances that the impeller leading edge and the wear plate are made of very hard materials. It should also be mentioned that progressive wear also alters the load curve. But this generally goes unnoticed because most stations are not provided with any means of flow measurement.

Life cycle costs
Today, life cycle costs are important selection criteria for operators wanting to buy wastewater pumps. Even when pumps have low utilisation, which is very frequently the case in wastewater pumping stations, the cost of energy can amount to 50% of the cost of ownership.

If blockages occur, remedying the problem can easily cost more than the acquisition value of the pump. Hence, wastewater pump station operators should attach maximum importance to operating reliability with efficiency only the second decision-making criteria.

Although lower in efficiency, vortex impellers are the best choice where low flow rates are being handled, because of their high operating reliability. Open impellers are generally less susceptible to operating problems caused by solids, fibres and gas.

The highest efficiency can be achieved with closed multi-vane impellers, but care must be taken to ensure their free passage is suitable for raw sewage otherwise screens must be installed. In conclusion, choosing a particular impeller
for a particular wastewater pumping application always entails a compromise between freedom from blockage, efficiency and wear behaviour.

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