High-low pressure pilot

A pressure pilot monitoring a process fluid pressure and for sensing predetermined high and low fluid pressure set points to control a process shutdown system when the monitored process pressure extends beyond the high or low pressure set points, said pressure pilot comprising:

• a frame;
• a Bourdon tube mounted on said frame having an input end coupled to the process fluid pressure and a moving output end responding to the process fluid pressure to move in a first direction upon sensing a decreasingly lower pressure and in a second direction upon sensing an increasingly higher pressure;
• means mounted on said frame, including a pair of pivot members for setting a respective low pressure set point and a high pressure set point;
• a lever arm pivotally mounted to said Bourdon tube moving output end between said pair of pivot members, including means enabling said lever arm to move in said first direction and contact one of said pivot members when the Bourdon tube senses a pressure reaching the low pressure set point and thereby pivoting the lever arm, and enabling the lever arm to move in a second direction and contact the other of said pivot members when the Bourdon tube senses a pressure reaching the preset high pressure set point to thereby pivot the lever arm;
• a fluid operated relay mounted on said frame having an inlet port for connection to a fluid pressure line, an outlet port, and a nozzle port, wherein the inlet port is in fluid communication with the outlet port with the nozzle port closed, and wherein the inlet port is blocked from the outlet port with the nozzle port opened;
• relay operating cap means responding to engagement with said lever arm for rapdily snap-closing said nozzle port when said sensed fluid pressure is between said low and high pressure set points and for responding to non-engagement with said lever arm for quick release opening of said nozzle port when said sensed fluid pressure has reached either said low or said high pressure set points, thereby providing a pressure level change at the relay outlet port;
• said fluid operated relay includes a piston having a piston seal at one end and a diaphragm at the other end, said piston seal being adapted to seal the inlet port from the outlet port with the outlet port being coupled through the inlet port to said pressure line with the nozzle port closed, and the diaphragm having the nozzle port on one diaphragm side and the outlet port on the other diaphragm side, and including means for for developing a higher positive pressure on the outlet port side of the diaphragm than on the nozzle port side of the diaphram in response to unsealing of said nozzle port, thereby providing a positive movement of the diapragm towards the nozzle port and a corresponding rapid, snap-acting sealing of the inlet port from the outlet port by the piston seal;
• and
• said relay operating means includes snap-acting cap means having a self-aligning cap member adapted for self-alignment with said nozzle port upon movement into engagement therewith.