Manhole access covers are provided, as well as faced pads for all inlet and outlet connections in the front domed end, and for the air release valves in the top of the filter cylinder. The front dome end has connections, which serve for admitting unfiltered water air for air scour, and discharging used wash water from the compartment formed by a vertical weir plate. The outlet connection on the underside of the filter serves for the discharge of filtered water, the admission of up wash, and for draining out. An air release, automatic ball type or hand operated, is provided in the top of the filter. A pressure gauge is fitted on the front of the shell, connected through a four- way single lever valve to a point just above the filter media and to the outlet, to indicate the filtering head loss. A nozzle discharge is also provided for drawing samples of unfiltered & filtered water.

Filter Floors
Concrete Construction

The Filter floor consists of a set of parallel hard burned clay laterals running at right angles to the long axis of the filter and discharging into a central common channel. The laterals, which are embedded in concrete, have holes along their soffits. Nozzle plates are fitted to these holes and polypropylene dome headed nozzles with radial slots are screwed into the nozzle plates. These nozzles fit flush with the concrete floor with only the slotted domes projecting above the surface. The complete floor, (which is specially designed to provide for even distribution of filtering, up wash and air scour flow) is entirely non-corrodible. The air scour is distributed internally through an air header system supported above the concrete floor. Concrete filling for surrounding the laterals and for formation of the central channel is not supplied by PCI unless specifically requested.

Steel Construction
Alternatively, the filter media is supported on a steel plate welded in position, reinforced with heavy beams and fitted with polypropylene nozzles as before. In this case the filter floor is supplied welded into position. The air scour is distributed internally through a pipe suspended from the underside of the steel plate floor.

Filter Media
The standard filter media usually has a total depth of 2' 6" (760 mm) and the grading of the sand and/or other material is selected according to the working conditions. The filter media is cleaned by first admitting air below the media for about 5 minutes to scour the sand grains followed by an upward flow of filtered water for about 6-8 minutes to wash away the loosened impurities.

Air Scour Supply
The compressed air for air scouring the filter is required at a pressure of 5 p.s.i. (0.35 bars) and at the rate of 1 cubic feet (0.47 litres/sec.) of free air per minute per square foot of filter bed. It is usually provided by a motor driven rotary compressor designed to deliver the required flow at the correct pressure.

Washwater Supply
In a supply system where the water is pumped direct through the filters into a reservoir at a higher level, the washwater can often be drawn back from the reservoir provided there is a connection to the bottom. In this case the number of filters is immaterial as far as the washing is concerned, provided that the filtered water main is large enough for the reverse flow at the required rate. This applies to gravitational as well as pumping systems provided that the hydraulic conditions are suitable and that the temporary reduction in the output of the plant can be tolerated. Where the washwater supply is to be independent of the system, a storage tank must be provided at a height to give a head of not less than 20 ft. (0.6bars) at the filter outlet. This may be charged by aconnection from the filtered water main or bypump. Alternatively, the tank may be a ground level and the water pumped from it. In either case a separate upwash main with branch to each filter is necessary.

Choice Of Site
The main factors to be taken into consideration when selecting the site for a pressure filter installation are:

The pressure of the water at the site; the higher the pressure the greater the cost of the filters;

The conditions that will occur when filters are being washed and the arrangements for providing the washwater - as referred to previously;

The availability of sufficient head for filter and friction in station pipework - usually 15 to 20 ft. (0.45 to 0.6 bars);

The control of flow through the plant;

The avoidance of the filters being subjected to a negative head.
     
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