Lower maintenance costs:
Inverters to prevent clogging on pumps

The water supplier and disposal company Hamburger Wasser is using more and more for the pumps in its sewage network frequency converter a. Because the lower flushing water quantities with increasing solids content make an intelligent actuator necessary. In the Randersweide pumping station, the converter, thanks to its backflushing mode, saves on the previously very often necessary interference suppression through clogging.

The initial situation: This is how Hamburg's wastewater flows

Hamburg Wasser is the water supplier and disposal company for the citizens of Hamburg. Around two million people are currently connected to the company's network. In addition to the extraction and treatment of drinking water, Hamburg Wasser also treats the Hanseatic city's waste water. Here the supplier has one goal: to achieve lower maintenance costs in the pumping stations. At the same time, the safe operation of the waste water management in the Hanseatic city is guaranteed.

Hamburg's wastewater flows in a pipeline network that is around 5.800 kilometers long. The house lines, through which the waste water enters the public sewer system, have a diameter of only approx. 15 centimetres. The public channels are usually two to five meters deep underground and are up to 150 centimeters high. Even larger canals, the transport sewers and collectors, as well as the older Hamburg main sewers, run at depths of up to 27 meters and can be up to 4,7 meters wide and 3,85 meters high. Unlike the street sewers, these transport sewers have no connection to the surface waters, but instead lead directly to the Hamburg sewage treatment plant – mostly in free fall.

Transport silos and collectors have the task of relieving the local sewage system, especially when it rains, and thus protecting against water overflows onto properties and into the waterways. The construction of underground mixed water retention basins also serves to relieve the sewage system when it rains: if the sewers are full, the water runs into the retention basins via so-called overflows. It is temporarily stored there and only returned to the system when the sewer system is ready to absorb again, either by automatic pumps or by gravity.

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No more old braids...

In this Danfoss video podcast, Michael Rix from Hamburg Wasser tells how the Hamburg water supplier uses the integrated backwash function of the VLT® AQUA Drive frequency converter uses to clear clogs and other debris from the pumps at its sewage plant, reducing maintenance costs.

Pumping station Randersweide: Lower maintenance costs to the goal

Since there are many substances in the wastewater and a lot of things that don't really belong in the water, from hair to bags to cleaning rags, swim with it, the pumps used have a correspondingly difficult life. At Hamburg Wasser, the increasing viscosity of the waste water led to more and more critical operating states in the waste water pumping stations: The particles and fibers contained in the water were deposited on the impellers, which led to a reduction in performance and excessive wear on the pumps.

Lower maintenance costs: Smooth pump operation with frequency converter

The Randersweide pumping station was recommissioned in 2014 after renovation. Due to the growing inflow due to numerous new buildings in the catchment area, there were 39 fault clearance calls due to pump blockages in the year after the plant was put back into operation. Servicing a clogged pump takes a long time: from driving to site and lifting the pump to cleaning and reassembly. In this respect, the many failures caused high costs. A solution was needed for this.

Since the problem continued to increase, an attempt was initially made to optimize the pump level. In addition, attempts were made to operate the pumps in reverse after a fault was reported manually via the control center. Unfortunately not always successful because the pumps were already heavily clogged. That's why Hamburg Wasser's industrial engineering department started looking for an automated solution on the market. The aim was to have the pumps reliably and automatically rotate backwards via a frequency converter while at the same time being economical to operate.

Makes money liquid: deragging

The fouling of impellers in sewage pumps through deposits or tangle formation is a general problem because the performance of the pump is reduced and it can become blocked. That is why the VLT® AQUA Drive has one as standard deragging, which reverses the direction of rotation of the impeller in order to loosen deposits on the impeller. Since not every type of pump can be operated in opposite directions, the operator must consult the pump manufacturer before using this function.

At the start of a backwash process, the frequency converter first ramps off to stop, then a turn off delay elapses before the first cycle begins. If the deragging is triggered from a stopped condition of the drive, the first switch-off delay is skipped. The backwash event can consist of several cycles. A cycle consists of a reverse pulse followed by a forward pulse. The backwash process is complete after the specified number of cycles have been completed. Between pulses, the drive coasts for the specified switch-off delay time to allow debris to settle in the pump.

Backwash mode: Individually adjustable to the application

The deragging can be parameterized individually to the experience of a user with his system by configuring delay times, backwash speed and performance monitoring. The user determines how often, how fast and how long the frequency converter rotate the impeller in the opposite direction at start or stop, timed by a digital input, by serial communication, or using a set of programmable logic rules.

Smooth pump operation with frequency converter allows lower maintenance costs

Another approach is reactive cleaning. Here he leads frequency converter cleaning when the pump draws more energy from the frequency converter than it normally should. At this stage, the pump is already beginning to clog. The user can combine the functions for proactive and reactive cleaning.

In Randersweide, due to the high full-load times of the pump and the already low output of 9,2 kW, the focus is not on energy-saving effects. For that he brings drive controllers Significant added value thanks to the extended data acquisition of condition and engine data via value corridors. First and foremost, however, the integrated backflushing function of the Aqua Drive has a positive impact: Whereas the maintenance team previously had to go out four times a month, the number of times the Aqua Drive was used to remove blockages has fallen to zero. As the operator, Hamburg Wasser calculates that the purchase of the frequency frequency converters for 5800 euros has clearly amortized over a period of 7500 months thanks to the cost savings of 6 euros.

Mission accomplished: Reduced maintenance costs in Randersweide

Thanks to the system optimization enabled by the backwash function, the Hamburg saves a considerable amount of water in terms of maintenance and repair costs in the Randersweide pumping station and also increases the operating time of the pump. This and the extended monitoring options in the direction of the pump and motor make it easy to get over the small energetic disadvantage. In every application in which clogging occurs again and again and a high level of operational reliability is important, it is worth taking a close look at the topic and using the backwash function as a preventive and reactive program if necessary. Even if there are no energy-saving effects due to speed control.

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