Variable clearance system for reciprocating compressors

An unloader system for a reciprocating compressor including a cylinder, a piston reciprocally mounted in the cylinder, a suction line, a discharge line, a suction valve assembly and a discharge valve assembly for selectively communicating the suction and discharge lines respectively with the compressor cylinder, said unloader system comprising:

• a) a clearance cavity in communication with the compressor cylinder though a passageway;
• b) an unloader valve having a valve member moveable between open and closed positions and controlling flow through said passageway, said valve member having opposed first and second ends, said first end being acted on by pressure in the compressor cylinder, said pressure producing a first force which acts to urge said valve member toward said open position;
• c) a conduit in communication with said valve member second end;
• d) a first pressure line in communication with the compressor suction line and said conduit;
• e) a first valve controlling flow through said first pressure line;
• f) a second pressure line in communication with the compressor discharge line and said conduit;
• and
• g) a second valve controlling flow through said second pressure line;
• wherein
• h) said first and second valves are adapted to selectively balance pressure from the compressor discharge line with pressure from the compressor suction line to create a control pressure which acts on said second end of said valve member to produce a second force acting in opposition to said first force, said second force acting to urge said valve member toward said closed position, said valve member moving to said open position when said first force exceeds said second force and moving to said closed position when said second force exceeds said first force.

In a multistage compressor having a downstream stage and an upstream stage, wherein each stage includes a cylinder, a piston reciprocally mounted in the cylinder, a suction line, a discharge line, a suction valve assembly and a discharge valve assembly for selectively communicating the suction and discharge lines respectively with the cylinder, an unloader system comprising:

• a) a clearance cavity in communication with the cylinder of the upstream stage though a passageway;
• b) an unloader valve having a valve member moveable between open and closed positions and controlling flow through said passageway, said valve member having opposed first and second ends, said first end being acted on by pressure in the cylinder of the upstream stage, said pressure producing a first force which acts to urge said valve member toward said open position;
• c) a pressure source;
• d) a conduit communicating said pressure source with said valve member second end;
• and
• e) a pressure regulator connected to said conduit;
• and
• f) a first sensor reading an operating condition of the downstream stage indicative of the workload of the downstream stage, said first sensor communicating a first signal to said pressure regulator indicative of the operating condition;
• wherein
• g) pressure from said pressure source is selectively varied by said pressure regulator in response to the first signal to create a control pressure which acts on said second end of said valve member to produce a second force acting in opposition to said first force, said second force acting to urge said valve member toward said closed position, said valve member moving to said open position when said first force exceeds said second force and moving to said closed position when said second force exceeds said first force, said regulator lowering said control pressure to decrease loading of said upstream stage when the first signal indicates that the workload of said downstream stage has risen above a predetermined level.