NA-06 - Cleaning: Flushing Methods

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Overview of the cleaning methods
Criteria for the selection of the cleaning process...
Flushing methods
Surge flushing
Back-up flushing
End of module

Flushing shield

Operational experience (3)

Operating experience with flushing shields (cont.)

Generally, a team of 4-5 people is required for the assembly, operation and removal of the flushing shield. The cleaning performance per day strongly depends on the local limiting conditions. It varies between just a few meters in the case of low water levels and large deposit amounts to several hundred meters in the case of favorable water levels and small deposit amounts. The daily performance is on average about 100 m, involving a team of 4-5 people [Baren2006].

Baren2006

Barenthien, L.: Betrieb der Kanalisation – Reinigung, Inspektion, Bauliche Unterhaltung, 2. Auflage, Herausgeber und Vertrieb: DWA Deutsche Vereinigung für Wasserwirtschaft, Abwasser und Abfall e.V., Hennef (2006).

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Five years of trials using different self-guided flushing shields for the maintenance cleaning of man-accessible sewers in the German city of Leipzig achieved highly positive results [Winterfeld]. Compared to other cleaning methods, the required operational staff requirement was significantly lower. The staff was only needed for the assembly of the devices in the manhole and their removal from the manhole at the end of the cleaning procedure.

Also, the technology and energy use of this simple method was found to be minimal. The highlight of this process is the gentle cleaning effect which, when compared to the standard high pressure cleaning, left bricked sewers undamaged (no erosion of the joint mortar). Another advantage of the flushing shields is the large cleaning length that can be achieved, allowing for the cleaning of sections not accessible by surface equipment such as inaccessible or protected areas.

A disadvantage of this method is the necessity of a large shaft opening for the insertion of the devices. The long retention time of the devices in the sewer along with the narrowing of the available free cross section have to also be taken into consideration. The required performance capacity of the sewer has to be included in the design scenario, and a possible backup upstream should be accounted for. In the event of rainfalls, the devices are overflowed and remain in place. The rectangular flushing shield is equipped with a tilt protection via an additional guide roller, which is not required for flushing shields used in oval profiles.

Cleaning using the flushing shield is used in cases where high pressure cleaning cannot be applied efficiently due to the high partial filling level or where the structural integrity has to be protected. The sewers to be cleaned have to have a straight line course without sharp bends, positional deviations or profile changes. In order to build up the hydrostatic velocity head, the partial filling level has to be sufficient. In this context, a sufficient leak tightness has to be achieved between the sewer walls and the seal installed on the flushing shield.