Calculating the real efficiency of a pump is a notoriously difficult task, particularly in wastewater situations, where multiple variables can affect efficiency calculations. Often manufacturer test-bed calculations can be severely impacted by real-life conditions, in the same way that a car’s fuel efficiency can widely differ between clear motorway running and inner-city stop-start driving.
Pump manufacturers usually offer efficiency calculations with their pumps, but these are calculations based on perfect conditions, involving clean water, unsullied by any of the problems which sewage water brings with it.
In addition, hydraulic efficiency in centrifugal pumps is related to the number of Impeller vanes and channel depth. This is why clean water pumps have multiple impeller vanes, but these are less suitable for wastewater because the leading edge of each vane is an area where raggy material can gather thus reducing the efficiency and finally resulting in a blockage. For this reason, historically, single-vane impellers have been preferred within the water industry as they only have one leading edge where solids can attach themselves. But, this is still a compromise as it still provides an edge where solids can gather which will reduce the efficiency in running and potentially cause a blockage.
Therefore, although test-bed statistics have their place, they bear no relation to actual pump performance ‘in the field’ in live unscreened sewage pumping stations. When raggy materials get caught in the impeller, they impair the flow path and create a drop in flow, whilst still using the same or higher amounts of power, thus relating to a drop in hydraulic efficiency. Unless the drop in flow is identified pumps can run in this condition for weeks or months in an unbalanced state, which considerably adds to the energy bills for end users and puts added stress on mechanical parts within the pumps, which can result in premature failure and costly repairs.
The problem is that many companies use manufacturer test bed statistics to make purchasing decisions based on life cycle costs which take little account of the effect the pumped media has on the actual power consumption. Such calculations often take the form of purchase cost + test-bed efficiencies + subjective maintenance costs = pump whole life costs, which can be flawed in wastewater applications.
The raggy material problem has also increased in recent years, making the discrepancy between manufacturer efficiency calculations and real-life conditions all the more acute. Modern living with its emphasis on convenience has created countless products, which make life easier for the individual but significantly more difficult for our sewage network and, in particular, the pumps which power it. Disposable cleaning cloths, nappies and make-up wipes all contain cloth-like constituents which can stretch and wrap around vital components once disposed. In addition, materials such as fat, oil and grease, commonly known by the acronym FOG, help to consolidate solid matter in the system into a rag mass creating blockage problems.
The result is that companies are not getting the energy or hydraulic efficiency that they believe they will get when they purchase a pump.
Far better would be a situation where companies take into account the sustained efficiency of pumps whilst in actual live applications. Crucially, these calculations must be based on wastewater testing not clean water. The total efficiency calculation would then be based upon the following: Non-clogging performance of the pump, hydraulic efficiency, sustained efficiency, wear and motor efficiency.
The potential savings from ensuring the design of a pump is capable of passing solids without a reduction in efficiency are most clearly illustrated by trials carried out in Canada at a Pumping Station called ‘Lachenaie’.
Here, three types of impeller were run simultaneously over a nine-month period and their actual specific energies were compared against each other and to the theoretical specific energies based on clean water test curves. Specific Energy (ES) is a measure of the energy consumed per unit of pumped volume and is the most accurate way of measuring energy consumption in a system (kWh/M3).
The three Impellers were 1) two-vane design, 2) single-vane, 3) semi-open self-cleaning impeller (N-Pump).
This shows that the energy consumed by both the two-vane and single-vane impellers is much higher in a real installation than would have been calculated in a typical Life Cycle Cost Model. In fact the increase in specific energy is directly proportional to the energy cost so this would result in a 55% and 37% increase in the energy bill, which is due to the pumps running partially blocked for much of the time.
The N-Pump used in the Test is an innovative design of semi-open impeller with a self cleaning mechanism which ensures that solids cannot foul on the leading edge of the impeller but are fed through the pump eliminating any impact on the impeller efficiency or specific energy. This is demonstrated clearly in the results of the test carried out at Lachenaie shown in figure 1 where it will be seen that the N pump ran for only 120 hours to pump the same volume as the Single-Vane (163 hours) pump, although only rated at the same flow on clean water.
The diagrams below show the effects of sewage water on efficiency and energy consumption with A) Single-Vane pump running continuously, B) Single-Vane pump running intermittently and C) the N-Pump with its Self-Cleaning Impeller.
The concept of sustained efficiency is a relatively new one for many in the water industry. If customers are to truly get an understanding of the overall efficiency of their pump in wastewater situations, a measurement of sustained efficiency over time is the only true method and this can only be achieved by using data gathered from their own pumping station assets. This has historically been difficult due to the data which needs to be gathered – flow, power and running time. However the cost of equipment to measure these parameters has come down considerably in recent times and there is a good opportunity for water companies to reduce their energy costs by including these facilities into their specifications for new or refurbished pumping stations. Only then will they get a true understanding of the potential savings of using the correct pump selection.
By Andrew Barry, ITT Water & Wastewater’s Market Development Manager