Pipe Plate Wear Plate Main Valve Bridge Block Cake Form Conn. Cake Dry Conn.

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The clip below shows the internal drum piping of the "two row" manifold. The trail pipes shown in red are normally handling the mother filtrate on the ascending side of the drum up to the 12 o’clock position and then the lead pipes shown in blue handle the wash filtrate on the descending side. The trail pipes are always connected to the outer row and have a bigger diameter than the lead pipes that are connected to the inner row. The reason for this arrangement is that the trail pipes handle more liquid than the lead pipes so require a bigger cross section to avoid vacuum losses.

The animation on the left shows a partial section of the cycle of a single compartment as it passes from cake washing down to cake formation on the descending side of the drum. Note the point when the vacuum is cut-off and the trailing pipe opens to purge. At this point the leading pipe evacuates the filtrate that remains in the piping and compartment prior to blowing off the dry cake.

The Drum Deck The drum deck is divided into separately isolated compartments each subjected to vacuum or blow while the drum is in rotation. The timing of vacuum or blow depends on the bridge setting of the main valve. The compartments are divided with grooved division strips along the drum face and around the circumference of the drum heads. These division strips are holding synthetic grids shown on the right that cover the entire drum and serve to support the filter cloth. The filter cloth itself is fastened to the drum face by inserting special caulking ropes into the grooves.

The Filter Cloth The filter cloth retains the cake and is fastened to the drum face by inserting special caulking ropes into the grooved division strips. Nowadays, with some exceptions, they are made from synthetic materials such as polypropylene or polyester with monofilament or multifilament yarns and with sophisticated weaves and layers. The image on the right shows the method of joining the cloth ends with clippers and to retain the fines from passing through to the filtrate multifilament strings are threaded across the entire cloth width. Another option quite often used on belt discharge filters is to join the ends with a special sewing machine. The entire subject of filter cloth and its selection will be discussed in a separate section that was not yet constructed.

• The Cake Discharge Mechanism

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The cake discharge mechanism that can be either a scraper, belt, roll and in very rare cases a string discharge. Blow is applied only to filters with scraper and roll discharge mechanisms but not to filters with a belt or string discharge. The images on the left illustrate the various mechanisms.

The selection of a suitable type of mechanism depends largely on the release characteristics of the cake from the filter media and will vary from process to process. Scraper discharge mechanisms will suit cakes that release readily and roller discharge mechanism are better for thixotropic cakes.

• The Drum Speed Variation

The drum filter has a drive with a variable speed that rotates the drum at cycle times that normally range from 1 to 10 MPR.

• The Agitator

An agitator keeps gently the slurry in suspension and reciprocates between the drum face and tank bottom at 16 or so CPM.

• The Tank

The tank that houses the drum and agitator has baffled slurry feed connections, an adjustable overflow box to set a desired drum submergence and a drain connection. The tanks are normally designed for an "apparent submergence" of 33-35% however on certain applications 50% and more is possible. With these special designs the tank ends are higher in order to accommodate stuffing boxes on both the drive shaft and valve end trunnion.

• The Cake Washing Manifold

On applications where cake washing is required, 2 or 3 manifolds with overlapping nozzles are mounted to a pair of splash guards bolted to the tank ends. The position of the manifolds and the quantity of wash liquid are adjustable depending on the wash characteristics of the cake.

• Control Instrumentation

  Optional controls may be used to automate settings such as drum speed, applied wash liquid and drum submergence for a desired cake thickness or throughput. The monitoring of drum submergence controls the slurry feed valves so an adjustable overflow weir is not necessary except for a fixed connection in case of emergency.

The flow scheme of a Rotary Drum Filter Station will generally look like this: 

 

Operational Sequence

The entire filtration cycle on a rotary drum filter must be completed within a geometry of 360 degrees. Let us follow the cycle sequence of a single sector assuming that the drum rotates in a clockwise direction while viewing the valve end:

• Cake Formation
  With the overflow weir set to a maximum the "apparent submergence" is normally 33-35% so the slurry levels between 0400 and 0800 hrs. Once a sector enters submergence vacuum is applied and a cake starts to form up to a point where the sector emerges from the slurry. The portion of the cycle available for formation is the "effective submergence" and its duration depends on the number of sectors, the slurry level in the tank and the bridge setting which controls the form to dry ratio.

• Cake Washing and Drying
  After emerging from submergence the drying portion of the cycle commences and for non-wash applications continues to about 0130 hrs where the vacuum is cut-off. If cake washing is required the wash manifolds will be located from about 1030 to 1130 hrs and the remaining time to vacuum cut-off at 0130 is the portion allocated to final cake drying.

• Cake Discharge
  After vacuum for the entire sector is cut-off air blow commences at about 0200 hrs in order to facilitate cake discharge. The blow, depending on the position of the tip of the scraper blade, will cut-off at approximately 0300 hrs. Drum filters are normally operated with a low pressure blow but on certain applications a snap blow is applied and to avoid the snapping out of the caulking bars or ropes wire winding of the cloth is recommended . Blow is used on scraper and roll discharge mechanisms but on belt discharge filters vacuum cuts-off when the filter media leaves the drum.

• Dead Zone
  Once the blow is cut-off the sector passes through a zone blocked with bridges so that no air is drawn through the exposed filter media which might cause the loss of vacuum on the entire drum surface.

 

Selection Criteria

In broad terms drum filters are suitable to the following process requirements:

Slurries with solids that do not tend to settle rapidly and will remain in a uniform suspension under gentle agitation. Cakes when a single washing stage is sufficient to remove residual contaminants from the cake or yield maximum recovery of filtrate. Cakes which do not require long drying times to reach asymptotic moisture values. Filtrates that generally do not require a sharp separation between the mother and wash filtrates. Some complex valves, however, enable atmospheric purging of the sectors and internal piping to facilitate a sharp separation of filtrates. Filtrates that are acceptable with a low quantity of fines that pass trough the filter cloth in the first few seconds of cake formation. Broadly, and depending on particle size and cloth permeability, the filtrate may contain 1000 to 5000 ppm insolubles. For very corrosive applications plastic drum filters are available with up to 10-15 m2 filtration area.

 

Maintenance

The slow rotation of the drum and reciprocation of the agitator reduce maintenance requirements to a minimum but the following should be inspected periodically:

• The strip liner of the trunnion bearing at the valve end will normally wear at the lower half. However, in cases when the slurry has a high specific gravity, the drum may become buoyant causing a wear to the upper half. At this point it should be mentioned that one way to remove the lower half of the liner, when hoisting facilities are not available or operational, is to float the drum by filling the tank with a sufficiently concentrated solution.

• The stuffing boxes on high submergence filters should be inspected for leakage and, if necessary, the stud nuts should be tightened. It should be noted that excess tightening can increase substantially the load on the drum drive so the use of a torque wrench is recommended.

• The face of the wear plate should be checked periodically and remachined if necessary. A whistling noise during operation is an indication the wear plate is worn out or the valve spring requires tensioning.

• The drum has a bailer tube that protrudes from the drive end shaft and must be kept open at all times since its blockage may cause the collapse of the drum. The bailer tube is a tell-tale indication to the following:

  • If a lighter flame is drawn through the bailer tube to the inside of the drum it indicates that a vacuum leak exists in the drum shell or the internal piping. It should be noted that in certain instances there is a possibility that explosive gases build-up inside the drum and may pose a safety hazard. In such cases the use of aerosol type smokes or a light tissue paper should be used instead of an open flame to identify a vacuum leak.

  • If liquid leakage is observed from the bailer tube it indicates that a hole exists in the drum head causing penetration of slurry from the tank into the drum.

• The on-line filter on the wash headers manifold should be checked periodically for pressure build-up due to progressive blockage. Likewise, the nozzles on the wash headers should be kept clean in order to ensure overlapping for full coverage of the washed cake.