Impeller importance
24 Sep 2004
Engineers who design pumping stations and treatment plants, the contractors who build them, and ultimately their operators, all have to work under tremendous pressure to minimise costs. It therefore comes as no surprise that systems are often designed right on the borderline of technical and economical feasibility - and that some unfortunately cross that line.
In all of these systems pumps are the motive force and the criteria they have to meet, particularly when handling wastewater, sewage and sludge, has changed considerably in recent years.
The number of sewerage connections has increased while water consumption has decreased. As a consequence sewage now has a higher solids and fibre content. To save energy, speed control is frequently employed and pumping stations are often not provided with screens, thus allowing large solids to pass through the system.
Composition of sewage is diverse and characteristics such as fibre, gas, solids size, sand and grit, as well as dry solids content, need to be understood. As said above, the pump is the motive force with the impeller at its heart. Its design will, in the end, dictate the efficiency and reliability as well as the effectiveness of the entire plant.
A distinction must be drawn here between pumps becoming blocked with solids and blocked with fibrous material. Solids can be defined as scraps of wood, wire rope, toys and sundry household rubbish. These will cause blockages mainly because of their size.
Fibres, on the other hand, can be more problematic - particularly hygiene articles and industrial waste - as they tend to collect in the gap between the impeller and the casing, along the leading edge or at the impeller eye. (Figure 1 below shows a typical pump cross-section).
Considerable abrasion of the casing wear ring causes an increased leakage flow from the discharge side to the suction side. As the gap opens, fibrous material can get trapped and this function is often exaggerated on pumps that have open impellers. Here the efficiency of the pump relies on maintaining the clearance between the leading edge of the impeller and the casing.
Sometimes fibres collect for a short time along the leading edge of the impeller, but if it has an optimally shaped leading edge the fibres are soon swept away and pass through the pump. If, however, the leading edge geometry has not been optimised for solids handling, fibres may become trapped and completely block the impeller eye.
From the above it can be seen that it is important to try and understand the nature of the liquid we are trying to pump. Figures 2 and 3 indicate the various design forms for both open and closed impellers.
Figure 4 is an approximate classification of how these designs will cope with the various constituents found in sewage and sludge applications.
With a view to determining maximum efficiency, a comprehensive analysis was conducted on the better known hydraulics on the market. Figure 5 presents an efficiency comparison of the various impeller types in pumps of nominal size 80mm, 100mm and 150mm. From this it can be seen the maximum efficiency that can be achieved with a free flow impeller is only 55%. This efficiency, however, is hardly affected by wear and the design makes it almost immune to fibre blockage.
The efficiency of closed single vane and two-vane impellers ranges from 75% to 85%. These additional percentage points can be obtained from open single channel impellers, but only at relatively high specific speeds and high capacities (150mm pumps). Closed two-vane impellers occupy the same high levels as multichannel impellers, while open two-vane impellers are about five percentage points lower than those on a closed version. This is no doubt attributable to the losses occurring in the gap between the impeller leading edge and the casing.
Very seldom can a pump be selected that will operate at its optimum point of efficiency. It is therefore just as important to consider the low flow range. Here the impeller geometry will have a great impact. Figure 5 again shows typical efficiency curves for various impeller geometries as a function of pump capacity. All the curves are normalised for the best efficiency point Q/Qopt = 1.0.
Free flow impellers maintain constant, though low, efficiency over a broad range of pumping rates. Multichannel impellers are the most efficient at converting energy over the entire operating range, but care must be taken in selecting them for unscreened liquids where the free passage should always be greater than 100mm as a guide.
Closed impellers have flatter efficiency curves than open impellers and therefore offer better low flow efficiency. For example, the difference between a closed two-vane impeller and an open two-vane one can amount to as much as 10 percentage points even though one is just as efficient as the other at the best efficiency point.
As it is sometimes difficult to know at what flow wastewater pumps will operate in practice, it is important to consider low flow efficiency, because free passage - defined as the largest ball that can pass through the impeller - is an important feature for evaluating the ability of impellers to keep from clogging.
Few impeller geometries can provide the minimum free passage of 80-100mm often quoted. Both free flow and single vane impellers can and have served reliably for many years pumping raw sewage. Closed two-vane impellers have free passage dimensions similar to those of open single vane impellers. Open two-vane and multi-vane impellers, however, have narrower, design dependent free passages and therefore cannot guarantee non-clogging operation in the presence of sizeable solids. In Figure 6 we compare the maximum free passage of various geometries.
As was said earlier, there is tremendous pressure to minimise costs, but it is necessary to consider more than just the capital cost and efficiencies of the pump. Most sewage pumps operate infrequently and the cost of energy accounts for only 50% of the overall cycle costs. If blockages do occur, however, the cost of remedying the problem, including the consequential costs of pump failure, is a decisive factor that can easily add up to far more than the acquisition value of the pump.
It is clear that pump selection for a particular service entails a compromise between freedom from blockage, overall operating efficiency, and wear. There is no such thing as a one-design, universal solution. But, with care, we can select the right impeller geometry to cope.
Peter Maslin is the water engineering market sector manager with pump manufacturer KSB.
