There is no doubt that the pressures on a wastewater business have changed regardless of whether you are a publicly funded municipality, privately owned water company or a private operator of a wastewater collection system. We clearly see an increased focus on energy use with regard to cost and CO2 footprint, along with the demand to reduce the number of blockages and the associated risk of overflows.
Why is this such a problem? Well, traditional thinking says that if you want to reduce energy consumption you ought to install higherefficiency equipment. But we also know that, in many cases, higher efficiency is associated with increased risk of blockage resulting from manufacturers taking short cuts to high hydraulic efficiency by increasing the number of impeller vanes, reducing free solid passage and using vane profiles that are not suitable for handling rag.
When we look at a typical number of municipal wastewater pumping stations, we see that just under 60% of all call outs to the site are a direct result of a pump-related problems. If we investigate further we also see that of these 60% of pump-related calls, over 60% originate from blockage problems. From investigations it is clear that blockage still represents a major challenge to wastewater collection operators. This is encouraging users to choose impeller designs with even lower efficiencies but reduced risk of blockage, the end result being higher energy costs and greater environmental impact from CO2 generation.
Options for improved efficiency
When we talk about efficiency regarding pumping equipment, we need to be clear about exactly what we are talking about. In reality there is only one final efficiency value that we need to increase and that is the total efficiency. This is what gives us the P1 absorbed kW which represents the true amount of electricity consumed. That being said, we also need to understand what contributes to total efficiency and its impact on the risk of blockage.
With conventional pumping equipment the user has more flexibility when choosing the motor and pump efficiency. Normally two separate components are purchased, of which the motors fall into various efficiency categories ranging from the lowest level IE1 efficiencies up to the IE3 premium efficiency ranges.
What is clear is that when trying to achieve the best balance between blockage risk and energy consumption, the starting point should always be to select the best commercially viable motor efficiency available. The reason for this is that motor efficiency is a win-win situation giving better total efficiency without any impact on blockage risk.
Working with (IE3) submersible motors
So how does this work when it comes to submersible wastewater pumps? Unfortunately, most submersible pump manufacturers have been slow to keep up with advancing motor technology and efficiency standards, and still make use of old technology that results in some of the lowest efficiency levels, such as IE1 (EFF 3). The good news is that the next generation of wastewater pumps is now available with premium-efficiency IE3 motors that allow wastewater collection operators to optimise the motor efficiency as a first step in improving energy consumption without increasing the risk of blockage.
The benefits of working with motor efficiency:
• Quick win, no risk of increased blockage
• Recognised international standard including test procedure
• Higher quality motor with lower temperature rise, reduced need for cooling and higher reliability
• Easy to benchmark different suppliers
• Capital Allowance Scheme
“a consideration when working with motor efficiency is the added benefit of a cooler motor, resulting in a reduced need for cooling and significant increases in reliability”
A further consideration that should not be overlooked when working with motor efficiency is the added benefit of a cooler motor, resulting in a reduced need for cooling and significant increases in reliability as a result of the optimised conditions for the motor, bearings and mechanical seals.
Having selected a submersible pump with IE3 premium-efficiency motors, what is the next step to get the best total efficiency? Not only do we need the best motor efficiency, but we also need the best hydraulic efficiency. This is where things get complicated.
Regrettably, it is not possible to simply use selection software to choose the pump with the highest efficiency because, if you do, there is every chance of an increased risk of blockage. You first have to understand a little about your application and pumping station – what is the size of the station, how close is it to operator support in the event of a blockage, what is the implication of a blockage, do you want to do regular maintenance / adjustment, is it a high-rag or high-wear station, etc?
If the above all sounds too complicated, there are easy-to-use wastewater pumping station assessment tools that can help with this process or you can contact your application support department for further advice. When you have the relevant information you can start to think about impeller selection and hydraulic efficiency.
How to select the best impeller
When we look at wastewater today we can see that things are gradually changing. Whether we are talking about the reduction in water consumption or changes in personal hygiene habits, it is clear that the rag content in wastewater is increasing. More cases of soft blockage (rag trapped in the impeller) are occurring and the number of pumping stations considered to be at high risk as a result of rag content is increasing.
“more cases of soft blockage are occurring and the number of pumping stations considered to be at high risk is increasing”
Impeller selection should therefore focus on two main areas, the first being the efficiency of the impeller and secondly its resistance to blockage resulting from either hard or soft blockage. In terms of assessing your intended choice it is relatively easy when it comes to efficiency and hard blockage since both can be measured using agreed measuring methods on a test bed or by looking at the free solids passage of the selected impeller.
Based on our experience we can see that if you want to go below a free solids passage of 75mm you will increase the risk of hard blockage unless you have screens fitted at the pumping station. In many cases, free solids passage is the first sacrifice in achieving high efficiency for some of the leading submersible manufacturers.
We should be clear in stating that it is possible to purchase highefficiency wastewater impellers that do not go below 75mm free solids passage and this should always be the first choice. If you have two impellers with a similar hydraulic efficiency, one design with > 75mm free solids passage and the other < 75mm, there is no good reason for selecting the impeller giving an increased risk of hard blockage.
When looking at literature covering wastewater submersible pumps you will see that everybody claims to produce impeller designs that can handle rag. The problem is that there is not one agreed method of testing that can confirm these statements, so basically it comes down to personal experience and very subjective selection.
However, companies such as ABS have now invested considerable time in developing an agreed testing method to enable benchmarking of different impeller designs and presenting a quantitative measure of the blockage resistance of each impeller design. This can then be used in conjunction with the free solids passage and hydraulic efficiency to make sure that the impeller selected is optimised for a particular application.
This extensive testing also clearly demonstrates that there is a very large variance in blockage resistance when comparing different existing impeller designs available for wastewater applications. This method of quantitative testing and advanced CFD modelling techniques have also been used to develop next-generation wastewater impellers making use of optimised vane profiles and impeller designs to achieve the best balance between efficiency and blockage resistance.
Impeller selection is not just a matter of efficiency when the equipment is new. Submersible sewage pumps are installed in a very aggressive high-wear environment and if attention is not paid to the effects of this, you will not retain the as-new efficiency for very long. You must decide if you want to maintain equipment to regain the as-new efficiency. The golden rule here is not to select a high-efficiency impeller that requires regular maintenance if you do not plan to carry it out. If you ignore the golden rule, you will end up with even higher energy consumption and increased blockage than if you had selected a lower-efficiency design with better blockage resistance.
For smaller pumps (<= DN150) also be aware of impeller designs that rely on a cutting action. As the name implies, the use of a cutting action demands a sharp edge which must be maintained over time. Again, if this is ignored, increased blockage will result from larger pieces of rag being unable to pass through the impeller.
The design of next-generation, smaller wastewater impellers is focused on passing rag through the impeller without the need for a cutting action. This is achieved through optimising the impeller and vane profiles to give a best-of-class blockage resistance while maintaining very high hydraulic efficiency levels.
Turning theory into reality
To summarise, when trying to achieve the best balance between energy optimisation and pump blockage one must follow a number of important steps:
• Start with the best-efficiency, commercially viable motor available, preferably IE3 premium efficiency
• Understand your application and operating requirements
• Select an impeller that has been designed based on quantitative blockage testing, has at least 75mm free solids passage and offers high hydraulic efficiency
• Consider the ongoing maintenance requirements, and if required, select a pump with easily adjustable features
Marc Redit, ABS Group
Published: 10th Dec 2009 in AWE International