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Jul 23, 2012 Special conditions apply to the cleaning of pressure pipes. This module is therefore explicitly dedicated to the procedures and methods used for cleaning this type of pipe. After completing this module, you will have knowledge of:
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There exist numerous methods for the cleaning of drainage systems. They are applied based on the cleaning objective and the structure specific, local, structural and hydraulic limiting conditions. (Image: Overview of the drain and sewer cleaning methods as per [Führb1980]) |
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Selection of cleaning measures must be in accordance with [DIN EN 14654-1] and should also take into account the technical feasibility and economic viability, along with the following boundary conditions:
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Flushing is considered to be one of the oldest sewer cleaning methods [Frühl10] [Gürsc21] and is divided into:
(Image: Schematic representation of a flush gate) (Image: Flushing shield) Both methods can be used to remove loose unconsolidated deposits in sewer lines with an unobstructed free flow. The flushing systems can either be installed permanently or temporarily. |
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According to [EN752:2008] and [DIN EN 14654-1] surge flushing is defined as “the creation of a temporarily increased discharge to remove any obstacles or deposits in drains or sewers channels.” (Image: Tiber Creek Sewer Flush Gates) |
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The surge flushing process involves temporarily backing up sewage or fresh water upstream from the section to be cleaned using specially developed flushing tanks or flushing manholes (figure). When the backed up flow volume is sufficient, the permanent or temporary flushing devices are activated and the flushing gates are abruptly opened. The wave front that is formed by the abrupt release of water stirs up the unconsolidated deposits and transports … |
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From a physical point of view, potential energy (by backing up the sewage) is transformed into kinetic energy (by a sudden release of the backwater) during surge flushing. During that energy transformation high turbulent flows and shear stresses can occur. When the surge wave has lost the required kinetic energy (transport energy), the solids will settle again [Haußm99]. (Image: Flushing chamber schematic - initial stage (Biogest-System)) (Image: Retention … |
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The flow conditions in a surge wave and their transport effect were researched by Brombach (figure). They are characterised by a distinctive secondary flow with a steady roll at the core of the wave, and heavy turbulences at the waves front and back. (Image: Secondary flows in a surge wave according to [Brombach]) |
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Dettmar characterises the wave family created during surge flushing into so-called bed load and suspension waves [Dettm2005a].
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The efficiency of surge flushing is influenced by the following factors:
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The effective range of surge flushing can be several hundred meters depending on the local conditions and the desired cleaning effect [Frühl1910]. Under favourable conditions, as in a straight section of the line with insignificant ponding, flushing lengths of more than 1,000 m can be achieved [Dettm2004a].
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According to Haußmann, the following principles apply:
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Currently, there are several different surge flushing devices used for surge flushing of drains and sewers as well as local drainage structures. According to [Dettm2004a], these devices are categorised into the following four groups based on their structure, technique, and equipment:
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When choosing the appropriate surge flushing device for a particular cleaning task, the following criteria is to be taken considered in addition to the hydraulic and structural limiting conditions [Dettm2004a]:
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Wastewater volume activated flushing systems consists of sealing devices that are especially arranged perpendicularly to the flow direction in order to backup the wastewater upstream of the section to be cleaned. Depending on the type of sealing device, the sewer cross section is blocked entirely or partly. The flush gate, arched flush gate, flushing weir, flush sluice, inflatable pipe plug and rotating elbow are among the wastewater volume activated … |
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(Image: Wastewater volume activated flushing system - Flush gate (without auxiliary energy)) (Image: Wastewater volume activated flushing system - Flush gate (with auxiliary energy)) (Image: Wastewater volume activated flushing system - Flushing weir) (Image: Wastewater volume activated flushing system - Slide gate) (Image: Wastewater volume activated flushing system - Flush sluice) (Image: Wastewater volume activated flushing system - Inflatable pipe … |
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(Image: Wastewater volume activated flushing system - Rotating elbow) (Image: Wastewater volume activated flushing system - Arched flush gate (with auxiliary energy)) |
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The sealing devices are distinguished by the type of flow release mechanisms which use lifting, lowering, and rotary gates to regulate the wastewater overflow or underflow. (Image: Lifting the flush gate into an open position) (Image: Lowering of the flush gate into the open position) (Image: Rotary flush gate) |
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Operational principle (Image: Flush gate (Hydrass Valve)) |
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Operational principle (Image: Automatic flush gate (without auxiliary energy) … |
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The Hydrass Valve or “Berlin flap“ is composed of a plate that is mounted to a metal frame, which is adjusted to the sewer profile. The length of the rotation axis can be freely selected. According to [Loren1998], it should be at the same level as the dry weather water flow. (Image: Flush gate DN 1600, closed position) (Image: Flush gate DN 1600, open position) |
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(Image: Automatic flush gate (without auxiliary energy)) At low water levels the gate remains in the vertical position. This causes the wastewater upstream to back up and increases the water level h0. So long as the wastewater pressure is greater bellow the rotational axis of the gate than above it, the gate will remain closed and continue to back up the wastewater. |
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(Image: Automatic flush gate (without auxiliary energy)) Once the hydrostatic pressure moment above the rotational axis of the gate is greater than the moment below it, the gate is rotated into a horizontal position. This releases the accumulated wastewater surge into the downstream channel area, and the water level behind the gate drops accordingly. |
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As soon as the water level falls back to the standard local levels, the gate rotates back to the closed position and the cycle starts again automatically. (Image: Automatic flush gate (without auxiliary energy) - Surge wave generation) (Image: Automatic flush gate (without auxiliary energy) - Surge wave generation) (Image: Automatic flush gate (without auxiliary energy) - Surge wave generation) (Image: Automatic flush gate (without auxiliary energy) - … |
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The use parameters for an automatic flush gate are determined by the dry weather water flow. (Image: Average shear stress produced by a flush gate released surge wave) The chart illustrates the average wall shear stresses in the invert caused by a downstream wave created by an automatic flush gate in a combined sewer DN 2,000. They are based on flow velocities and water level heights of the surge wave measured in three different locations. |
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