PURPOSE:To automate drain system by a mechanism wherein filter clogging is automatically detected, drain valve is opened by the liquid pressure at filter inlet side, high conc. drain is returned to drain tank, and back-wash liquid is returned to oil tank through drain valve. CONSTITUTION:When the differential pressure at filter outlet and inlet exceeds the specified vaule, the differential pressure valve 7 detects it. This causes the valve body 17 to be released from valve seat 14, and part of oil at filter inlet side flows through oil chamber 10, oil routes 37, 38 into oil chamber 21, 29. This releases the main valve body 24 from the main valve seat 22 in the drain stop valve 8 to cause it to open. Spool valve body 31 of drain-change-valve 9, which cuts off drain route 25 and oil route 26, slides to the side of holding fixture 34. When small diameter section 28 comes to the route 25, the route 25 opens to discharge drain through chamber 18, connection mouth 46, to drain tank. When the small diameter section 28 comes to the route 26, the route 25 is cut off the cause drain to be returned through connection mouth 45 to oil sump tank.
