A bypass valve for a plunger in a plunger lift system in an oil and gas well is disclosed. The valve has a valve body, a piston and a spring. The pressure differential across the piston keeps the valve closed during the upstroke of the plunger. The spring opens the valve when the pressure differential decreases to below a selected value.

TECHNICAL FIELD 
     The present invention relates to plunger lift systems for oil and gas wells, and more particularly to a pressure sensitive bypass valve for a gas lift plunger. 
     BACKGROUND ART 
     Plunger lift systems are artificial lift systems for oil and gas wells that are used during the producing life of the well when the bottom hole pressure and the gas to liquid ratio will no longer support natural flow. A plunger lift system includes a tubing string in the well casing with a well valve and lubricator at the top and a spring assembly at the bottom, and a plunger in the tubing string. The well is intermitted by shutting in the well for a selected time period to allow pressure build up and then opening the well valve for a selected period of time, allowing fluid to flow into the sales line. The plunger moves up the tubing string during the time the well valve is open and prevents liquid fall back. When the well valve is closed the plunger falls back to the bottom of the tubing string. The open and closed times for the well valve are usually controlled by a programmable controller. 
     Plungers are designed to seal against the interior of the tubing string during the ascent from the bottom of the well to maximize the liquid produced during the well valve open period. The plunger seal inhibits the rate of descent of the plunger back to the bottom of the well. Prior known devices, such as the plunger disclosed in U.S. Pat. No. 5,253,713 to Gregg et al., have a rod extending from the top of the plunger that opens a bypass valve that allows fluid flow through the hollow interior of a plunger. The bypass valve allows faster descent of the plunger. The rod in these prior known devices opens the valve when the rising plunger pushes with the rod against a bumper pad in the top of the lubricator. In this type of device, if the well valve is closed before the plunger reaches the top, the bypass valve does not open and the plunger descends slowly. If the controller opens the well valve before the plunger reaches the bottom of the well and the plunger surfaces without liquid on top of the plunger, the plunger could be damaged, the lubricator could be damaged and the entire wellhead could be blown off. 
     U.S. Pat. No. 5,427,504 to Dinning et al. discloses a plunger bypass valve with a ball shaped closure member and a spring loaded rod activator that pushes the ball into a valve seat to close the valve. This device opens the valve after the plunger reaches the lubricator at the top of the well and the pressures above and below the plunger are equalized. 
     DISCLOSURE OF THE INVENTION 
     A plunger for an oil or gas well with an internal bypass valve is disclosed. The plunger has a hollow plunger body, a fishing neck attached to the upper end of the plunger body and a valve attached to the lower end of the plunger body. The valve has a piston biased by a spring to an open position that allows fluid to flow through the plunger and thereby increases the velocity of the plunger during the down stroke. A rod attached to and extending downwardly from the valve piston is pushed upward and moves the valve piston to a closed position when the plunger contacts the spring assembly at the bottom of the well. Opening the well at the top reduces the pressure above the plunger and the plunger travels up the well. The higher pressure below the plunger that pushes the plunger up also keeps the valve piston in the closed position. The spring moves the valve piston to the open position when the pressure difference between the bottom of the plunger and the top of the plunger falls below a selected value. The valve opens without requiring the plunger to strike a bumper at the top of the well or reach the top of the well and without requiring equalization of the pressures above and below the plunger, thereby reducing the risk of damage to the plunger and the well. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Details of this invention are described in connection with the accompanying drawings that bear similar reference numerals in which: 
     FIG. 1 is a sectional view of a plunger embodying features of the present invention. 
     FIG. 2 is a top view of the fishing neck of the plunger of FIG.  1 . 
     FIG. 3 is a sectional view along line  3 — 3  of FIG.  2 . 
     FIG. 4 is a bottom view of the valve of the plunger of FIG.  1 . 
     FIG. 5 is a sectional view along line  5 — 5  of FIG. 4 with the valve piston in the closed position. 
     FIG. 6 is the sectional view along line  5 — 5  of FIG. 4 with the valve piston in the open position. 
     FIG. 7 is a schematic diagram of a plunger lift system for an oil and gas well that utilizes the plunger of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, the preferred embodiment of the present invention has an elongated, hollow, cylindrical plunger body  10 , a top member or fishing neck  11  attached to a top end of plunger body  10 , and a plunger valve  12  attached to a bottom end of plunger body  10 . Plunger body  10  is externally screw threaded at both top and bottom ends. 
     FIGS. 2 and 3 show fishing neck  11  with an exterior size and shape corresponding to a conventional oil and gas well plunger fishing neck. Fishing neck  11  has a lower portion  13 , an intermediate portion  14 , an upper portion  15  and a top portion  16 . Lower portion  13  is cylindrical with an exterior first diameter. The intermediate portion  14  tapers inwardly and upwardly from the lower portion  13 . Upper portion  15  extends upwardly from intermediate portion  14  and is cylindrical with a second diameter smaller than the first diameter. The top portion  16  is button shaped with a third diameter intermediate the first and second diameters and attaches to the top of upper portion  15 . The lower portion  13  is hollow and has internal screw threads for attachment to the top end of plunger body  10 . A plurality of circumferentially spaced upper fluid apertures  17  extend outwardly and upwardly through intermediate portion  14  from the interior of lower portion  13 . 
     Referring now to FIGS. 4 to  6 , plunger valve  12  has a first valve body portion  20 , a second valve body portion  21  and a valve piston  22 . The first valve body portion  20  has hollow cylindrical side wall  25  and a circular end wall  26  attached across a first end of side wall  25 . The second end of side wall  25  opposite the first end is open. Side wall  25  has a smooth lower interior surface  27  adjacent to end wall  26  and a screw threaded upper interior surface  28  extending up from lower interior surface  27  and having a larger diameter than lower interior surface  27 . The end wall  26  has an end wall interior surface  29 , an end wall exterior surface  30  and an end wall aperture  31  that extends the center of end wall  26 . A plurality of circumferentially spaced lower fluid apertures  32  extend through the side wall  25  near the top of lower interior surface  27 . 
     The second valve body portion  21  is hollow and cylindrical, and has a lower section  34  and an upper section  35 . The lower section  34  has a screw threaded exterior surface  36  sized and shaped to screw into the upper interior surface  28  of side wall  25  of first valve body portion  20 . Lower section  34  of second valve body portion  21  has a cylindrical first interior surface  37  extending up from the bottom of lower section  31  and a cylindrical second interior surface  38  extending up from the top of first interior surface  37  to the top of lower section  34 . The first interior surface  37  has a diameter corresponding to diameter of the lower interior surface  27  of first valve body portion  20 . The second interior surface  38  has a smaller diameter than the first interior surface  37  so a downward facing first step or shoulder  39  is formed between first interior surface  37  and second interior surface  38 . The upper section  35  of second valve body portion  21  extends up from the lower section  34  and is internally treaded to receive the bottom end of plunger body  10 . 
     The valve piston  22  has an elongated cylindrical rod  40 , a cylindrical intermediate portion  41  and a cylindrical upper portion  42 . Rod  40  has a diameter smaller than the diameter of end wall aperture  31 , extends therethrough and is slidable therein. The top end of rod  40  rigidly attaches to the bottom end of intermediate portion  41 . The intermediate portion  41  has an outer diameter sized to fit with a close tolerance into the lower interior surface of first valve body portion  20  so that fluid flow is restricted between piston intermediate portion  41  and lower interior surface  27 . The length of intermediate portion  41  is greater than the diameter of the lower fluid apertures  32  so that intermediate portion  41  can be disposed to completely cover lower fluid apertures  32  and thereby occlude flow through the lower fluid apertures  32 . The length of rod  40  is selected such that when the bottom end of rod  40  is flush with the end wall exterior surface  30 , piston intermediate portion  41  extends across lower fluid apertures  32  and occludes fluid flow through the lower fluid apertures  32 . The upper portion  42  of piston  22  has a diameter smaller than the diameter of intermediate portion  41  and the bottom end of upper portion  42  rigidly attaches to the center of the top end of intermediate portion  41  so that a second step or shoulder  43  is formed around the top periphery of intermediate portion  41 . The length of intermediate and upper portions  41  and  42  of valve piston  22  combined is less than the distance from the end wall interior surface  29  to the bottom of the lower fluid apertures  32  of first valve body portion  20  so that when valve piston  22  is disposed in first valve body portion  20  with the bottom of the valve piston intermediate portion  41  against the end wall interior surface  29 , the flow through lower fluid apertures  32  is not restricted. 
     Plunger valve  12  is assembled with valve piston  22  inside first valve body portion  20  and rod  40  extending through the end wall aperture  31 . The second shoulder  43  on valve piston  22  forms a seat for the bottom end of a coil spring  44 . The bottom end of second valve body portion  21  is screwed into first valve body portion  20 . The first shoulder  39  on second valve body portion  21  forms a seat for the upper end of spring  44 . The spring  44  acts as a biasing means and biases valve piston  22  down with the bottom of the valve piston intermediate portion  41  against the end wall interior surface  29 . FIG. 6 shows this open position for plunger valve  12  with lower fluid apertures  32  unobstructed and rod  40  extending beyond end wall exterior surface  30 . FIG. 5 shows the closed position for plunger valve  12  with spring  44  compressed, lower fluid apertures  32  occluded by valve piston intermediate portion  41  and the bottom of rod  40  flush with end wall exterior surface  30 . 
     Sealing means is provided that prevents fluid flow between the lower fluid apertures  32  and the interior of plunger body  10  when plunger valve  12  is in the closed position. The sealing means is shown in FIGS. 5 and 6 as an O-ring  45  around piston  22  near the top of intermediate portion  41  that seals against lower interior surface  27 . The sealing means can also be a seal that extends between the top of piston  22  and first shoulder  39  when plunger valve  12  is in the closed position. 
     Referring to FIG. 7, plunger valve  12  is initially in the open position when the plunger  9  is laced into the lubricator  50  at the top of the tubing string  51  of in the casing  54  of a well. The plunger body  10  preferably has a pad type seal but may have any conventional type of plunger sealing means that substantially restricts fluid flow between the plunger body  10  and the tubing string  51 . The upward force on the plunger  9  is the product of fluid pressure below the plunger  9  and the cross sectional area of the plunger  9 . The downward force on the plunger  9  is the weight of the plunger  9  plus the product of fluid pressure above the plunger  9  and the cross sectional area of the plunger  9 . With the plunger valve  12  in the open position, the pressure above the plunger  9  and the pressure below the plunger  9  are equal and the total force on the plunger  9  is downward force of the weight of the plunger  9 . The plunger  9  descends from the lubricator  50  to the bottom of the well with fluid flowing in through the lower fluid apertures  32  in plunger valve  12 , up through the plunger body  10  and out through the upper fluid apertures  17  in fishing neck  11 . When the plunger  9  reaches the bottom of the well, rod  40  contacts the well spring assembly  52  and pushes valve piston  22  up to the closed position. 
     When the plunger valve  12  is in the closed position, a pressure differential can be created between the top and bottom of the plunger  9 . Opening the well valve  53  at the top of tubing string  51  allows fluid to flow out of the well and reduces the pressure above the plunger  9 . When the pressure differential between the fluid above the plunger  9  and the fluid below the plunger  9  is great enough to overcome the weight of the plunger  9 , the plunger  9  begins to move up the tubing string  51 . 
     The upward force on the valve piston  22  is the product of fluid pressure below the valve piston  22  and the cross sectional area of the valve piston  22 . The downward force on the valve piston  22  is the sum of the weight of the valve piston  22 , the spring force from spring  44  and the product of fluid pressure above the valve piston  22  and the cross sectional area of the plunger. Spring  44  is selected such that sum the force of spring  44  on valve piston  22  in the closed position and the weight of valve piston  22  is less than the product of the pressure differential required to lift the plunger  9  and the horizontal area of valve piston  22 , so as the plunger  9  rises valve piston  22  of plunger valve  12  remains in the closed position. 
     The spring  44  moves the valve piston  22  to the open position when the pressure differential decreases such that the downward force on valve piston  22  is greater than the upward force. The pressure differential decreases to this extent either when the plunger  9  moves into the lubricator  50  above the well valve  53  or when the well valve  53  is closed before the plunger  9  reaches the top of the well and the plunger  9  stops moving upwards. When the plunger valve  12  opens, the plunger  9  descends to the bottom of the well where the plunger valve  12  is closed by rod  40  contacting the well spring assembly  52 . 
     Although the present invention has been described with a certain degree of particularity, it is understood that the present disclosure has been made by way of example and that changes in details of structure may be made without departing from the spirit thereof.