3.8 Boiler feed pump suction filters
Suction filters are provided to remove any foreign matter from the feedwater which could damage either the feed pumps or the downstream plant. An arrangement with 2 x 100% duty filters in parallel has been provided by the CEGB on fossil-fired stations, each with automatic backflushing facilities.
The filters prevent particles greater than 0.2 mm from passing through the filter mesh. To minimise the effects on available feed pump NPSH, the filter types adopted (Fig 3.34 and Fig 3.35) have a full flow pressure drop in the clean conditions of no greater than 0.1 bar. Backflushing is arranged to operate automatically when the pressure drop across the filter rises to 0.4 bar. Timer facilities are also provided to permit backflushing at fixed time intervals, if required.
The filter element consists of a stainless steel fine-mesh basket, which is readily removable together with the backflush assembly for maintenance and inspection. On initiation of the backflushing sequence, the back wash arm is slowly rotated against the filter mesh. At the same time a dump valve is opened, creating a back pressure over the section of the mesh covered by the arm. This causes a localised reversal of liquid flow to flush all the accumulated debris to waste.
Hydraulic instability can occur at the commencement of backflushing if the saturated liquid leaving the filter is allowed to discharge into an empty drain line. To prevent this, the line needs to be kept primed and the major pressure reduction should take place at the end of the drain line, prior to dissipation into the unit drains system. These requirements are achieved by the positioning of the dump control valve together with a pressure breakdown orifice at the disposal end of the pipeline.