Use of dedicated water drives to reduce pump blockages
Pump blockages are a growing problem for wastewater systems, where they cause unreliable operations, increased energy consumption and premature failure.
Globally, this is a problem that would cost industry and utilities billions of dollars a year.
Companies that treat wastewater are facing a growing problem with the clogging (often known as ragging) of their electric pumps. The market offers a potential solution with dedicated water variable speed drives (VSDs), which incorporate pump cleaning functions. James Chalmers, Vice President of Sales at ABB Drives, presents the challenge and the solution.
The problem arises from the increased levels of fat, oil and fat
One of the main causes of the problem is changing diets with a tendency towards increased consumption of fatty foods. This means that higher levels of fat, oil, and grease (FOG) must be carried through household and commercial drains. Once they reach the sewage system, they solidify to form floating scabs as well as to coat the surfaces of pipes and pumps.
FOG also wraps around solid objects such as wet wipes, sanitary products, cotton swabs and diapers, carelessly thrown down the toilet. The resulting masses continue to collect fog and other material, creating ‘fatbergs’ of up to enormous proportions – one in Belfast, Northern Ireland, weighed over 200 tonnes. The current trend for water-conserving toilet tanks that provide low-volume flushes makes the situation worse as they facilitate sedimentation, build-up and aggregation of fog.
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If a blockage prevents a pump from operating, it will need to be lifted mechanically, opened and cleaned manually. When the pump is in a remote location, the downtime and labor costs involved are particularly high. Even at lower levels, ragging negatively affects pump performance. This is because it produces a lower flow rate, while additional stress is placed on the pump, resulting in higher energy consumption and faster wear. In turn, the need for inspection and maintenance activities is increased.
In the worst case, a failure or loss of pump efficiency can lead to flooding. This damages property and can pollute natural waters, requiring clean-up operations and potentially resulting in fines. Additionally, whenever flooding occurs or pumps need to be removed and handled, maintenance crews and communities can be exposed to biohazards.
VSD cleaning sequences free the pumps
Precise control of pump operation is offered by the latest generation of variable speed drives developed specifically for water industry applications. They offer built-in cleaning sequences that control the pump impeller to perform a series of rapid forward and backward motions to dislodge any attached or tangled solids. The number, duration, rate of acceleration and pattern of these movements can be adjusted to suit different solid materials or mixtures.
The VSD can be programmed to initiate the cleaning action automatically when a possible blockage is indicated by a reduction in flow. Operators can also decide how many times the cleaning routine must be activated before a potential problem warning is generated.
In some cases, water industry operators have successfully resolved blockage problems by installing grinder or “chopper” pumps. In some cases, this approach can cause downstream problems, as materials are broken into smaller pieces that can pass through screens in processing plants. The resulting fragments are difficult to remove and can disrupt processes. However, automated cleaning by a VSD will keep large objects moving towards the screens, where they can be easily extracted. In some cases, they could even be used as fuel for energy conversion plants.
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VSD makes a practical difference
ABB’s dedicated ACQ580A is a proven example of a variable speed drive with cleaning functionality for the water industry. The use of this variator has made it possible to double or even triple the flow rates in certain installations, while optimizing the operation of the pumps for maximum energy efficiency.
Mittelmärkische Wasser-und Abwasser GmbH (MWA) in Brandenburg, Germany, has been using the cleaning function of the VSD pump since 2014. When the current increases due to clogging, the VSD causes the pump to shift into reverse or to a higher speed. The company has not experienced any clogging since the installation of the VSDs.
On a smaller scale, the Pietarsaaren Vesi municipal utility on the west coast of Finland serves a population of 20,000. About 80% of the region’s wastewater passes through an unmanned pumping station en route to a central treatment plant.
Controlling the flow to prevent flooding is a priority, as spills into the sea could cause serious ecological damage. Prior to installing variable speed drives with built-in cleaning capability, clogging was a major problem that frequently required manual removal and cleaning of pumps. Now, the cleaning function is activated weekly as a routine precaution, or whenever a surge in current drawn by a pump indicates a blockage.
In Queensland, Australia, Unitywater provides water and sanitation services to more than 820,000 people and operates 17 treatment plants. Its Deception Bay pumping station alone treats 1.2 million liters of wastewater per day using two pumps. These suffered from clogging and had to be removed regularly using a specialized lifting device. A simultaneous failure of both pumps would have resulted in additional expenses for transporting additional pumping equipment to keep the site operational.
The installation of variable speed drives with cleaning functionality resulted in the elimination of elevators and saved approximately $ 20,000 per year in maintenance costs. The water flow has doubled without a corresponding increase in pump speed, reducing component wear and energy consumption. Electricity bills have been cut in half. It took less than six months to get a full ROI.
Dedicated water VODs offer benefits for all stakeholders
The use of variable speed drives with cleaning functionality offers benefits to all stakeholders in the water sector. Whatever the priority – save money, save energy, protect the environment from efficient management of work, technology makes good sense.
These drives not only keep pumps running at the optimum speed for energy efficiency, but they also power process jobs at constant flow rates. This is very beneficial when it comes to managing biological and chemical processes. The sustainability benefits of reducing blockages relate to the overall reduction in carbon footprint as well as energy consumption. When considering sustainability, CO2 Emissions from vehicles traveling long distances to deal with stuck pumps can be a significant factor. This is especially true when a single incident may require the presence of a crane operator and more than one specialist engineer, each arriving in separate vehicles.
Another key advantage of dedicated water VSDs is their flexibility to adapt to changing needs. For example, the size and conditions of watersheds can change over time, rendering initial discharge estimates invalid. Oversized pumps are often specified to allow such variation, but this increases running costs. With variable speed drives, the pumps can be adjusted as needed for the best possible adaptation to the last operating speed.
VSDs can also provide a useful function in the form of flow measurement in installations where a dedicated flowmeter is not available. For example, knowing the wastewater flow rates can be very useful in allowing early warning of increased loads, clarification of daytime patterns and indication of a leak. ABB’s VSD solution has a unique integrated flow computer and is able to perform measurements without inserting sensors (which could clog and entangle solid waste).
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The critical success factor is specialized installation and commissioning
VSD solutions such as those described here are generally quick and easy to upgrade, often without significant civil engineering costs. However, buyers should always consider specialized installation, commissioning and support in their investment. This expertise can be offered directly by the supplier or through its delivery partners. Using specialist knowledge in this area will ensure that the equipment is properly specified and configured for the specific application and circumstances.
If this critical expertise is not used, there is a risk that variable speed drives will not achieve optimal results and may even increase power consumption. Most importantly, experts will ensure that the nature and needs of existing systems, infrastructure and components are understood, and that VSD operation and cleaning is scheduled to ensure smooth and reliable pump operation. To show why this matters, the Australian factory referenced above had a check valve nearby that was sensitive and vulnerable to disturbance and damage. In this case, careful management and tuning avoided the problem.
Water companies do an important job of educating consumers about the impact of flushing out inappropriate materials. However, the challenge of blockages continues to grow. Therefore, correctly applied variable speed drive technology with the right expertise in water systems can play a vital role in keeping pumps and wastewater moving.