Automatic Rotary Strainers

A robust, maintenance free, automatic self-cleaning device for the removal of solids in pipe lines. Typical applications include make-up water for cooling towers, service water systems, cooling water for small power plants etc.

Five major elements are involved:

• Body (A) (B)
  This can be of cast or fabricated construction depending upon the application. Corrosion consideration constitutes a major factor in material selection. For fresh water either carbon steel or cast iron would be chosen. Sea water installations would require stainless steel. Niresist or aluminum bronze. The body has flanged inlet and outlet nozzles which can be positioned at any preferred relative position - including one above the other. Within the lower body (B) is the sco  ur pipe (C) Connected to a scour valve. The upper body (A) is flanged to the lower body (B) and topped by a cover plate (D) incorporating the strainer basket upper bearing housing (E) and electric motor operator (F).
   

• Strainer Basket (G)
  This is divided longitudinally by 4 or more webs forming separate chambers. The basket itself is constructed from square itself is constructed from square bars equally spaced to form a cage. Attached to the cage is a perforated stainless steel sheet (H). The size of the perforations varies to suit the duty but is usually 3mm. The basket is fitted with a top plate (I). A ring (J) is provided at the bottom of the strainer basket connected to the inner hub by 4 or more spokes which comprise the base of the longitudinal dividing webs. The strainer basket incorporates an upper drive shaft (K) and a lower stub shaft. Both items are in stainless steel. The lower stub shaft bearing is positioned within the intermediate mounting plate (L).
   

•  Intermediate mounting plate (L)
   Located within the lower body this element houses the following:

  • An opening encompassing three filter chambers within the strainer basket permitting flow the lower body and out through the perforated plate the other outlet pipe.

  • Scour pipe outlet located above the fourth filter chamber.

  • Lower stub shaft bearing.
     

• Scour valve
  This device is connected to the scour pipe and is electrically operated upon initiation of the cleaning sequence.
   

• Electric motor operator (F)
  Rotation of the strainer basket is accomplished by a double reduction worm geared motor, mounted integrally to a flexible coupling and fitted with a magnetic brake which ensures accurate positioning of the strainer segments during scouring.
   

• Flow reduction during scouring
   For normal installations, where a separate source of high pressure flushing water is not provided, the percentage reduction in flow during scouring is approximately 25%. Should this be considered too large a proportion then the percentage can be reduced by increasing the number of filter chambers from 4 to 6. In such cases the reduction of flow during scouring will be only 16%.

Method of Operation

Back Flushing (Cleaning)
When a cleaning cycle is initiated the sequence is as follows:

• The basket motor and scour valve motor are started simultaneously.
• The strainer basket is rotated through 900 thus putting a clean chamber into the main flow of water and dirty chamber over the outlet to the scour valve which opens fully.
• Water then flows from the main stream back through the perforations in the basket and out through the scour valve thus cleaning the chamber by reverse flow action. Approximately 25 % of the main flow is used for flushing.
• After a flushing period of about thirty seconds, the basket motor is started again to rotate the basket a further 900 thus putting a second clean chamber in the main flow and another dirty chamber in position for scouring.
• This sequence is repeated twice more thus providing four clean chambers. Three in the main flow and one ready to come into service when the cleaning sequence is next initiated.
• The scour valve is then closed to complete the cleaning cycle. g. The time taken for complete cleaning is about 2 minutes. Once initiated the sequence is controlled automatically by a rotary cam timer.

Initiation of Cleaning Sequence :
Cleaning is normally initiated by any or all of the following means:

• Pressure Switch
  The pressure switch connection is piped upstream and down stream of the strainer. Then as the elements become blocked the rise in head loss is sensed and an electrical contact closes to start the cleaning sequence.
• Initiating Timer
  A Second timer is incorporated into the control system and can be set to operate over a wide range of times, any 2 to 48 hours, or 4 to 72 hours. When this times out after the set period a contact closes to initiate cleaning.
• Pushbutton
  Local or remote pushbuttons can be incorporated. The local one can be positioned on the control cabinet or on the strainer itself.

Hand Operation

For hand operation the shaft of the electric motor is extended through the magnetic brake to declutchable hand wheel, enabling the strainer element to be manually rotated. A limit switch prevents electrical operation when the hand wheel is engaged.