The heart of impeller design
Peter Maslin of KSB reports on advantages and disadvantages of a range of impellers when selecting pumps for various applicationsEngineers who design pumping stations and treatment plants, the contractors who build them and ultimately the operators all have to work under tremendous pressure to minimise costs. It therefore comes as no surprise systems are often designed right on the borderline of technical and economical feasibility but some unfortunately cross that line.
In all of these systems the pump is 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 and, as a consequence, sewage now has a higher solids and fibre content. To save energy, speed control is frequently employed and often pumping systems are not provided with screens, 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 if the designer is to avoid problems with the operation of the system. As stated, the pump is the motive force with the impeller at the heart. Pump design will, in the end, dictate the efficiency and reliability as well as the effectiveness of the entire plant. A distinction must be drawn 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 blockage mainly because of their size. Fibres, particularly hygiene articles and industrial waste, can be problematic because they tend to collect in the gap between the impeller and the casing, along the leading edge or at the impeller eye. Figure 1 illustrates a typical pump impeller cross-section. Considerable abrasion of the casing 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, where the efficiency of the pump relies on maintaining the clearance between the leading edges 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 soon get 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 we can see it is important to try and understand the nature of the liquid we are trying to pump. Figure 2 and figure 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 ascertaining 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 illustration it can be seen that the maximum efficiency which 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 imm-une to blockage by fibres. The efficiency of closed single-vane and two-vane impellers ranges from 75-85%. 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 multi-channel 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 loading edge and the casing. Very seldom is it possible to select a pump 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 6 shows typical efficiency curves for various impeller geometrics 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. Multi-channel 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 impeller can amount to as much as ten percentage points even though one is just as efficient as the other at the best efficiency point.
As it is sometimes difficult to ascertain 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 geometrics can provide the minimum free passage of 80/100mm often quoted. Both free-flow and single-vane impellers can meet this criteria 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 impellers and multi-vane impellers, however, have narrower, design-dependant free passages and therefore cannot guarantee non-clogging operation in the presence of sizeable solids. In Figure 7 we compare the maximum free passage of various impeller geometrics in pumps with discharge connections of 80, 100 and 150mm. At the start of this article we said there is tremendous pressure to minimise costs. 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, however, blockages do occur, the cost of remedying the problem including the consequential costs of pump failure is a decisive cost factor that can easily add up to far more than the acquisition value of the pump. It is clear 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 needed to cope with the application at hand