The seating ring is the primary bearing housing, which defines the spacing between the upper guide bearing in the motor lantern and the lower support bearing. It is manufactured as welded construction. Adjustment screws for mounting are provided in the top flange of the seating ring. These are also used for precise levelling during mounting. In the variant as concrete foundation, the seating ring is inserted in the digester tank and cast. Subsequently, the seat ring remains in the digester tank and only the rotor can be uninstalled. In the version with gas hood or support plate, the seating ring is screwed on the flange of the mechanically machined gas hood flange/support plate. In between is a perfectly fitting flange connection with an O-ring groove for sealing. This allows uninstalling the mixer at any time and reinstalling it without realignment of the impeller gap.
Drive engineering
The drive is realized by a three-phase motor with ATEX certification designed for clockwise and counter-clockwise rotation. The motor is mounted on a steel-welded motor lantern, which accepts the top guide bearing and which is arranged over the seating ring. An elastic coupling serves as connector for motor and mixer shaft.
Draft tube pipe
The vertical draft tube consists of flanged, ductile cast iron pipes, which have based on the material characteristics a vibration-absorbing effect on the system. The inlet and outlet piece consists of grey cast iron or ductile cast. A supporting frame with three strong support props serves as mounting device at the bottom of the digester tank. The legs are screwed onto plates placed in concrete or they are fastened with anchors.
Guying
The draft tube pipe is fastened in the area of the impellor with four side bracings. Greater draft tube heights may require additional guying levels. The required steel ropes or tension rods are screwed onto the digester tank wall on plates set in concrete or they are mounted with special bonded anchors.
Coating
Parts in contact with air and sludge are receive long-term corrosion protection. Components made of non-corrosive materials are stained and neutralised.
Gas-tightness
Mechanical components along the shaft are sealed against the explosive digester gas. This is accomplished by arranging multiple lip seals with barrier grease chambers and loss lubrication. The lubrication grease is pumped continuously to the seals through the electric grease pump. The protective sleeve of the shaft are hard-armoured in this area forming a hard material bond with the base material. Furthermore, a centrifugal disc with a small sealing gap is arranged to protect the bearings. The wide film of grease protects the bearing area permanently against the penetration of digester gas and contaminations by splashing sludge.
System monitoring
The bearing temperatures are monitored by installed temperature sensors. The values measured through the bearing temperature sensors are transferred to an ex-zone protected terminal box located on the motor lantern. These measurements can be fed to the control system of the sewage treatment plant. This avoids generating ignitable temperatures during operation and in the event of system malfunctions. It also prevents thermal decomposition of the grease used and therefore, that it looses its sealing and lubrication characteristic. The lubrication of the bearings is ensured through an electrical grease pump with individual grease lines. With the fill level monitoring of the grease reservoir, the mixer becomes a secured unit acc. to ATEX.
System efficiency
High reliability, robust design and regular maintenance of the mixer ensure an operating duration of up to 80,000 operating hours before the next inspection is recommended. A maximum gas yield is achieved through the strong homogenisation and mixing of the sludge. This yield is more than twice as high than if no forced circulation is installed. In addition, remediation and cleaning measures required by the sedimentation of digester tanks are nearly unnecessary.
By default, the speed of each drive is fixed as standard. If an adjustment is desired, the speed can be reduced by up to 70%.