Thermal retracting actuator

Abstract

A thermal actuator for a rotating shaft shutdown seal that has a piston with a portion of its axial length enclosed within a chamber shell with a material that expands upon a rise in temperature. The portion of the actual length of the piston within the chamber has at least two different diameters with the larger diameter leading in the direction of travel of the piston. Upon a rise in temperature, expansion of the material surrounding the piston within the chamber creates a force on the piston in the desired direction of travel. Below a preselected temperature the piston is positively locked with a passive release when the preselected temperature is reached.

What is claimed is: 
     
         1 . An actuator comprising:
 a cylinder having an axial dimension;   a piston axially movable within the cylinder with the cylinder having an upper and lower end which is sealed around the piston, the piston having one end connectable to an operator;   a cavity occupying a space within the cylinder between the upper and lower ends and which space at least partially surrounds a portion of the piston, an axial dimension of the piston extending through the space within the cavity when the piston is in a first position of its travel, the axial dimension of the piston having at least two separate diameters with a largest of the diameters leading a smaller of the diameters in a direction of travel of the piston to move the piston to a second position, the two diameters occupying at least a portion of the space when the piston is in the first position;   a material occupying at least a portion of the space within the cavity, the material expanding upon an increase in temperature to exert a force on the piston that causes the piston to move to the second position when the material rises above a preselected temperature; and   wherein the piston extends axially through the material occupying a portion of the space within the cavity.   
     
     
         2 . The actuator of  claim 1  including a safety lock configured to prevent the piston from moving from the first position to the second position when the material is substantially below the preselected temperature and free the piston to move in a direction toward the second position when the material rises approximately above the preselected temperature. 
     
     
         3 . The actuator of  claim 2  wherein the lock is thermally activated and configured to passively unlock the piston when the fluid is above the preselected temperature. 
     
     
         4 . The actuator of  claim 2  wherein the thermally activated safety lock comprises a pin that is suspended from one end of the cylinder and extends in a direction that the piston moves toward the second position, the pin extending partially within a recess in an end of the piston, a substantial remainder of the recess being substantially filled with a thermally activated material, wherein the thermally activated material has a viscosity at temperatures below the preselected temperature that prevents the thermally activated material from flowing alongside a side of the pin and out of the recess and at temperatures substantially at or above the preselected temperature the thermally activated material has a reduced viscosity that enables it to flow alongside the side of the pin and out of the recess enabling the piston to move in a direction toward the second position. 
     
     
         5 . The actuator of  claim 4  wherein the thermally activated material is a polymer. 
     
     
         6 . The actuator of  claim 5  wherein the polymer is polyethylene. 
     
     
         7 . The actuator of  claim 3  wherein the safety lock comprises a shear pin that is configured to form an obstruction that prevents the piston from substantially moving toward the second position when the material is substantially below the preselected temperature and when the force on the piston is large enough to move the piston in a direction toward the second position, the shear pin ruptures or otherwise deforms to remove the obstruction. 
     
     
         8 . The actuator of  claim 7  wherein the safety lock comprises a pin that is suspended from one end of the cylinder, in a direction that the piston moves toward the second position, wherein substantially below the preselected temperature the pin extends partially within a recess in an end of the piston, a substantial remainder of the recess being substantially accessible to accommodate piston travel, the pin having projections in the form of one or more shear pins or rings which prevent the pin from substantially moving into the substantial remainder of the recess when the material is substantially below the preselected temperature, wherein the projections are configured to rupture or otherwise deform to enable pin travel into the substantial remainder of the recess when the force on the piston is large enough to move the piston in a direction toward the second position. 
     
     
         9 . The actuator of  claim 1  wherein the force is exerted over an area around a circumference of the piston wherein at least a portion of the at least two separate diameters of the piston extend. 
     
     
         10 . The actuator of  claim 1  including a first seal supported between the cavity and the piston at a lower end of the cavity and a second seal supported between the cavity and the piston at an upper end of the cavity, the first and second seals being operable to substantially confine the material to the cavity. 
     
     
         11 . The actuator of  claim 10  wherein at least one of the first seal and second seal are cup seals. 
     
     
         12 . The actuator of  claim 11  wherein the cup seals are constructed out of PEEK. 
     
     
         13 . The actuator of  claim 10  including a backup seal for either or both of the first and second seals. 
     
     
         14 . The actuator of  claim 13  wherein the backup seal is an o-ring seal. 
     
     
         15 . The actuator of  claim 14  wherein the o-ring seal is formed from EPDM or HNBR. 
     
     
         16 . The actuator of  claim 10  wherein either a support for the first seal or a support for the second seal is designed to relieve a pressure within the cavity when the pressure exceeds a predetermined value. 
     
     
         17 . The actuator of  claim 1  including a spring supported within the cylinder and configured to bias the piston in the first position while the material remains below the preselected temperature. 
     
     
         18 . The actuator of  claim 17  wherein the force exerted on the piston is substantially greater than a force exerted by the spring. 
     
     
         19 . The actuator of  claim 1  wherein at least a portion of a wall of the cavity has a reduced thickness that will bulge outward when pressure within the cavity exceeds a design value to reduce the pressure within the cavity to a safe level.