maintenance procedures given here apply. For a more detailed discussion of these procedures, consult the manufacturer's instructions.

The following maintenance operations are required on a monthly basis:

1. Purge the diaphragm cases of air, and check the cable to be sure that it leaves the segment at a tangent to the lower end when a zero reading exists on the unit.

2. Remove dirt from the knife edges; when necessary, tighten the cam hubs on their shafts.

3. Drain mud from the mud leg. In doing this, flush the mud out of the water pipeline running from above the sand to the loss-of-head gauge. Drain the mud leg until the water runs free of sediment.

Annually, inspect the diaphragms for leakage, and replace when necessary.

NOTE

Diaphragms in stock should be stored underwater.

Also, disassemble the unit to clean and lubricate it when necessary. Check the working parts and the cables. (They should be free of knots, splices, or fraying.) Repack the stuffing box when it is leaking. Make certain that the knife edges rest solely on their edges where the pendulum is hung vertically, and be sure that all cable ends are knotted tightly.

PRESSURE FILTERS

Except where the filter medium is housed in an enclosed pressure shell, pressure filters (fig. 8-6) are constructed like gravity filters with respect to the underdrain system, gravel, and the filter medium (sand or anthrafilt). Pressure filters need the same care and attention as gravity filters. Since their backwashing operations cannot be observed, the filter must be opened regularly and inspected carefully. The recommended maintenance procedures are as follows:

Weekly, inspect piping and valves for leaks. Lubricate and repack valves if necessary.

Quarterly, open the pressure shell and inspect the filter bed surface. The inspection procedures are as follows:

1. Use a garden rake during backwashing while the manhole is open to test for mud balls in the lower part of the filter bed and for evenness of the gravel layer surface.

2. Determine when the sand bed level has changed since the last inspection by comparing the bed surface elevation with some reference point.

3. When the filter does not have a surface wash system and shows evidence of mud balls, backwash it at the highest rate possible while jetting the surface with a stream of water from a high-pressure hose nozzle.

Annual maintenance requirements are as follows:

1. Open the filter, remove the sand from an area large enough to permit the inspection of the gravel. When the sand or gravel distribution indicates nonuniform distribution of backwash water, the filter media and gravel may need to be removed, and the underdrain system checked.

2. Clean and paint the exterior of the shell.

Every 3 years (or more often ifnecessary), the filter medium and gravel should be removed and the underdrain system checked for the distribution of wash water, and repaired if necessary. Clean the under-drain system, and paint it or apply a protective coating to all parts subject to corrosion, including the inside shell. Replace the gravel and the filter medium.

DIATOMITE FILTERS

Most diatomite filter installations in potable water supply plants are of the pressure type, although there are vacuum-type filters that can be used in certain installations. In general, the maintenance procedures for cleaning the filter element are the same for both types. The following procedures apply:

At monthly intervals, or as often as operating conditions show the need, check the filter elements. Cleaning is needed if the precoat has apparent bare spots on the elements. Causes of element clogging are iron oxide, manganese dioxide deposits, and algae growths.