A filtration system which may include multiple filters capable of being individually washed of clogging filtrate material by a "blow-down" or flushing operation, is regulated by a blow-down controller device. Each filter has a blow-down discharge valve which is normally maintained closed by a required...http://www.google.com.hk/patents/US4005013?utm_source=gb-gplus-share專利 US4005013 - Differential controller valve and sequencer
