Successful Fight Against Ragging
This technical article was published in the DWA journal “Korrespondenz Abwasser, Abfall” in issue 05/2022
Development, Construction, and Initial Operating Experience of an Optimized Impeller for Wastewater Pumps
Abstract
An increasing proportion of sanitary and wet wipes and a low water content cause a significant change in wastewater composition, leading to massive ragging in pumps. As a result, financial expenditures for cleaning, maintenance, and repair have risen enormously for wastewater management companies in recent years. To solve this problem sustainably, the Institute of Energy and Environmental Engineering at the University of Duisburg-Essen conducted a fluid mechanics investigation into the ragging behavior of single-channel impeller pumps on behalf of Pleiger Maschinenbau GmbH & Co. KG (Witten). From this research work, an optimized impeller geometry for wastewater pumps was developed. It ensures a significant improvement in the flow regarding ragging susceptibility in the impeller inlet area and thus a significantly increased operational reliability at considerably lower maintenance costs. This article presents the extensive development work on the impeller, from the installation of a prototype by the wastewater management and sewage treatment plant operator Emschergenossenschaft/Lippeverband at the Dortmund-Deusen pumping station to initial operating experience.
Keywords: ragging, wastewater disposal, single-channel impeller pump, impeller, impeller geometry, blade channel, suction mouth, computational fluid dynamics, pressure distribution, pumping station, wastewater pump
The Dortmund-Deusen Pumping Station
The Dortmund-Deusen pumping station, with a total of eight installed pumps of various nominal diameters, operates unmanned and fully automatically. The pumps are switched on and off depending on the water level in the suction chamber. Due to fluctuating water levels and irregular inflow to the pumping station, intermittent pump operating modes result.
In dry weather, only one of two wastewater pumps conveyed 90 liters/second of wastewater requiring treatment to the Dortmund-Deusen sewage treatment plant. The water is municipal wastewater from a closed sewer system that is first fed to the pumping station. No screen is installed upstream of the pumps.
A storm overflow tank (RÜB) is arranged in bypass to the pumping station (Figure 2). If the combined water inflow at the pumping station exceeds the permitted delivery rate of 90 liters/second, for example due to rain events, a separate pump conveys into the storm overflow tank. After the rain event, with decreasing inflow, the storm overflow tank is automatically emptied toward the pumping station. Remaining deposits in the storm overflow tank are loosened with the help of flushing buckets and fed to the pumping station.
Background and Initial Situation
In the reference year 2017 alone, 65 fault messages were recorded by the process control system at the Dortmund-Deusen pumping station due to contamination, silting, fibrous materials, and bulky waste. They were triggered by failed pumps. For the operator, the wastewater management and sewage treatment plant operator Emschergenossenschaft/Lippeverband, this resulted in considerable costs. In addition to material damage, the problems that occurred led to 33 unscheduled on-call deployments.
Specific Problems
The wastewater pumps installed here must be capable of pumping municipal wastewater without screening. However, in its current composition, the wastewater represents a potential challenge for any pump: fibrous materials in the form of sanitary and kitchen wipes braid together and cause blockages in the pump (Figure 1). A “wet wipe infarction” looms.
The two pumps are already installed redundantly so that one pump always delivers the base load of wastewater. The sewer network must not be backed up unnecessarily in order to keep the capacity reserves for a possible rain event in the combined sewer as large as possible. Therefore, given the stated wastewater quality, the on-call service had to act even upon the failure of a single dry-weather unit. If the inflow to the pumping station exceeds twice the dry-weather inflow (2Qt), it is temporarily stored in a storm overflow tank (RÜB). This is realized by a larger pump. In the emergency case that both base-load pumps fail, the aforementioned storm overflow tank is immediately fed with the challenging wastewater. Water collects in the storm overflow tank, and solids settle there to a greater or lesser extent. After the rain event has ended, the storm overflow tank is cleaned and emptied using flushing buckets. The deposits then re-enter the pump suction chamber via the flushing sills and are fed to the pumps in massive surges, which can lead to renewed clogging of the wastewater pumps.
In order to deliver the specified permitted volume to the sewage treatment plant and ensure that the storm overflow tank is utilized less frequently and can be emptied promptly, continuous, trouble-free pump operation is essential
In the simulation, the distribution of static pressure was represented in an axis-normal section at the suction mouth of the pump and near the pump shroud (Figure 3). The resulting pressure field led to the assumption that the shroud-side side chamber, due to the back vanes attached to the outside of the shroud, has flow passing through it centrifugally instead of centripetally as normal, at least in regions along the circumference (Figure 4). This behavior could trigger the ragging scenario.
Therefore, Pleiger Maschinenbau and the institute decided not to perform any geometry variation at first, but to expand the existing computational model by the impeller side chamber between the shroud and the housing and to run an extended simulation for this operating point with this adapted model.
The subsequent simulations showed that flow passes through the gap between the impeller and the wear ring at the suction mouth partially centrifugally and partially centripetally (Figure 5). These flow regions rotate with the impeller and are stationary in the relative frame of reference. The pressure fields responsible for this also rotate with the impeller. The centrifugal inflow into the gap is locally located in the area of the impeller where an edge is present due to the severe curvature of the impeller channel.
With this configuration, the cause of the impending ragging and subsequent clogging could be identified with high probability: long-fiber components of the wastewater are drawn both into the gap and into the main flow channel of the impeller, thus wrapping around the described edge. In this way, the blockage of the impeller gradually builds up.
As a first result of the fluid mechanics investigation, it was determined that the asymmetrical pressure field at the suction mouth of the impeller must be influenced as the cause of the clogging. Clogging can therefore only be prevented by modifying the geometry of the impeller—and possibly of the pump casing.
Parametric CAD Solid Models
For the subsequent step, Pleiger Maschinenbau and the institute decided to create a parametric CAD solid model from the available data of the impeller. This was then to be modified in such a way that the pressure field could be influenced. The defined goal was an impeller whose blade channel is shaped so that the edge critical for ragging at the impeller inlet is avoided as far as possible. In order to be able to retrofit existing single-channel impeller pumps later with minimal effort—i.e., without modifying the casing design—the external dimensions of the impeller were to remain largely unchanged. The corresponding CAD model was created at the Chair of Turbomachinery at the University of Duisburg-Essen.
Subsequently, several geometric variants of the impeller were simulated at the institute, evaluated in terms of fluid mechanics, and discussed with Pleiger Maschinenbau. Two particularly notable geometries are to be presented in more detail here.
Figure 6 again shows the pressure distribution in the axis-normal section of the pump, the streamlines in the side chamber, and the velocity vectors at the inlet to the sealing gap in the inlet side chamber for the newly developed geometry, analogous to Figure 1 of the existing initial geometry.
It can be clearly seen that the pressure distribution along the circumference is significantly more uniform. The vectors no longer show a locally strong inflow into the gap, and an analysis of the flow field for various impeller positions relative to the casing shows that the remaining inflow and outflow regions into the side chamber are stationary neither relative to the impeller nor to the casing, and thus the entrance to the side chamber should no longer cause blockages.
