Flow control apparatus

A flow control valve comprising

• a housing formed with an intake passage, a drain passage and a discharge passage;
• a main spool and an auxiliary spool which are slideably housed within said housing;
• a first orifice disposed between said intake passage and said discharge passage;
• a second orifice disposed between said intake passage and said discharge passage and having an open area variable in response to displacement of said auxiliary spool;
• a drain orifice disposed between said first orifice and said drain passage and having an open area variable in response to displacement of said main spool;
• said main spool being movable in response to a differential pressure between upstream and downstream sides of said second orifice;
• said auxiliary spool having surfaces subject to pressure upstream of said first orifice, pressure between said first and second orifices and pressure downstream of said second orifice, and being movable to partially close and open said second orifice in response to differential pressures between upstream and downstream sides of said first orifice and between upstream and downstream sides of said second orifice;
• and
• a control ring secured within said housing and adapted to vary said open area of said first orifice in response to displacement of said auxiliary spool so that pressure on said upstream side of said first orifice is extremely slowly increased, the control ring having an outer surface defining grooves extending from the upstream side of said first orifice to the downstream side of said first orifice and an inner surface defining an axial bore with which said auxiliary spool is engageable.