Pneumatic actuators with liquid seals for oil and gas well swabs
RELATED APPLICATIONS 
     This is a continuation-in-part of my Application Ser. No. 832,442 filed Feb. 24, 1986, now abandoned, and incorporates by reference the disclosures of my Applications Ser. Nos. 832,443 and 832,444, also filed Feb. 24, 1986. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to pneumatic actuators, especially those comprising a cylinder, piston and one or more piston rods, for oil and gas well swabs where they are used to actuate valves for opening internal valves for allowing the fluid to pass through the swabs and closing such valves due to head pressure acting on the pistons as the swabs sink into the petroleum and other liquids in the well pipe. 
     2. Description of Prior Art 
     Sealing means for pneumatic apparatus for use in oil and gas well swabs as disclosed in U.S. Pat. No. 4,070,134 have been limited to the lipseals and O-rings surrounding the pistons. 
     SUMMARY OF THE INVENTION 
     The invention involves apparatus and method for sealing gasoperated pistons sliding in pneumatic cylinders that control valves in oil and gas wells of a type disclosed in U.S. Pat. No. 4,070,134, which is incorporated by reference herein. Seals composed of rubber or other resilient materials on pistons to retain a charge of gas, especially low-pressure gas, in a cylinder have been unsatisfactory for extended use because of &#34;bubbling&#34;, or the slow leakage of gas past minute imperfections in the seals. Liquids, especially the very high viscosity oils and greases, have large molecules or, in any event, molecules which interact so they do not pass through those imperfections. The present invention places a second &#34;sealing&#34; piston adjacent to the actuating piston, separated by a slug of oil or grease. The sealing piston is slideable on the piston rod and has appropriate O-rings or lip seals which engage the piston rod and cylinder wall. A spring mounted on the piston rod applies slight pressure against the sealing piston. In another configuration, two sealing pistons are used, one on each side of the actuating piston. The pressure of an activating charge of gas applied to the actuating piston is, in reality, on the sealing piston, which, in turn, applies pressure through the trapped oil or grease to the actuating piston. The present invention precludes the actuating gas from coming into contact with the actuating piston&#39;s seals. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates an actuating piston between two slugs of oil or grease and two sealing pistons; and 
     FIG. 2 illustrates an actuating piston in an upright cylinder with a layer of oil riding on its upper surface and a slug of grease between the actuating piston and an underlying sealing piston. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is especially useful where a trapped chamber of gas actuates, through a piston and piston rod, a mechanism which depends on a change of ambient pressure in a gas or oil well to work. Normally, a bellows would be used in such a situation but the stroke of a bellows is too limited for the apparatus and method involved. Over a period of time, however, with prior art structures the gas charge in the chamber tends to leak away, rendering the mechanism useless. 
     Referring to FIG. 1, a charge of gas is applied to the left chamber 4 of cylinder 2. The increased pressure in left chamber 4 is directed against sealing piston 5 and the force is transmitted through the slug of oil or grease 7 to the actuating piston 9, and then to a piston rod 12 which controls a valve (not shown) in the swab involved. A spring retainer 14 and spring 15 mounted on piston rod 12 apply a small constant pressure to the sealing piston 5. Actuating piston 9 is secured to piston rod 12 so they reciprocate in cylinder 2 together whereas sealing pistons 5 and 20 are slideably received on piston rod 12. 
     When the right chamber is subjected to the ambient pressures through the port 18, such pressures are applied in a like manner to sealing piston 20, the slug of oil or grease 21, and the actuating piston 9. Thus, the gas trapped in chamber 4 controls the extension of rod 12 which in turn opens or closes a valve in the oil and gas swab involved making it operative to pump petroleum and other fluids from the well in which the swab has been installed. 
     Gas may be charged in chamber 4 by simply inserting pistons 5, 9 and 20 aligned on rod 12 therein at atmospheric pressure with grease or oil slugs 7 and 21 filling the spaces between the pistons. A grease which is available and has proved practical is No. 2 cup grease which is the same as that normally used for automobile lubrication. 
     When the apparatus is used to control the valve in a swab as illustrated more specifically in FIG. 2 and such as disclosed in U.S. Pat. No. 4,070,134, the cylinder, here designated 2a, is generally vertical with the rod, here designated 12a, depending downwardly and the components above piston 9a are eliminated so that there is only one grease slug 21a. As a substitute for grease 7, a viscous liquid 7a is provided on top of piston 9a which rises up and down with piston 9a as the swab travels up and down the well pipe and fluid under pressure entering the lower chamber 3a through port 18a forces piston 9a via piston 20a (similar to pistons 5 and 20) and grease slug 21a upwardly relative to cylinder 2a. O-rings 6a and 8a ensure that grease slug 21a and viscous liquid 7a are retained in the locations illustrated relative to pistons 20a and 9a. Spring retainer 14a  and spring 15a function essentially the same as parts 14 and 15 in FIG. 1. A workable and readily available viscous liquid for this purpose is STP Oil Treatment liquid or other similar lubricating oil additives which increase in viscosity as their temperatures are increased by ambient temperatures experienced in oil or gas wells. Its viscosity is preferably at least one hundred centistokes. The swab falls generally about one hundred feet in petroleum and/or other liquid in the oil or gas well before rod 12a closes the swab valve, causing the swab to commence its ascent carrying the petroleum and other fluids trapped above it due to the build-up in gas pressure under the swab. At its uppermost position in the well pipe, after the petroleum and other fluids have been discharged, the valve is caused to open wherein gas pressure in chamber 4a forces the pistons 9a and 20a and piston rod 12a downwardly for another cycle.