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You are an expert at summarizing long articles. Proceed to summarize the following text: 
FIELD OF THE INVENTION 
     The field of this invention relates to control systems for downhole equipment, particularly subsurface safety valves (SSVs). 
     BACKGROUND OF THE INVENTION 
     Typically, production strings in wells have an SSV which is controlled from the surface. The SSV is typically a spring-loaded flapper which is pushed into the open position by downward movement of an open tube called the “flow tube.” The flow tube is actuated by an actuating piston which is, in turn, a part of a control circuit for selective opening and closing of the SSV from the surface of the well. Many different designs have been used in the past to control the opening and closing of the SSV. Typically, a control line is run from the surface to the actuating piston and a return spring acts on the actuating piston in a direction opposite the hydrostatic force put on the piston by the column of fluid in the control line to the surface. The piston is typically an annular shape or it can have a cylindrical or rod shape. The spring is made sufficiently stiff so as to withstand the anticipated hydrostatic force for the depth to which the valve is to be installed. Yet other designs have included pressurized gas chambers which act on the backside of the actuating piston to resist the hydrostatic pressures anticipated in the control line. The pressurized gas chambers contain oil so that the actuating piston seals are lubricated. 
     Annularly shaped pistons have been waning in popularity due to the numerous seals required, all of which increase the prospects for leakage and malfunctioning of the valve. Another main concern of any design for a control system for an SSV is the failure mode if certain seals malfunction. It is important to have failsafe operation of the SSV and, thus, the fewer situations that can arise where the valve fails open, the more desirable is the control system design and the valve which goes with it. 
     Some designs in the past have used pressure-balancing between the top side and bottom side of the actuating piston, coupled with fairly complex shuttle valving to allow for normal operation of the valve between an open and closed position. While use of the concept of pressure-balancing has enabled a significant reduction in the size of the return spring, other complications introduced into the system to make such a design operable have created a new set of operational issues, detracting from the desirability of the equalizing-type designs which use a complex shuttle valve. What is yet to be developed and what is an object of this invention is to provide a simple design which has minimal possibilities for fail-open operation and which is simple to build and install and reliable to operate. 
     Some of the patents which illustrate the prior designs discussed above are U.S. Pat. Nos. 5,564,501 and 4,676,307. Also of general interest in the area of SSV control systems are U.S. Pat. Nos. 4,252,197 and 4,448,254. 
     Accordingly, one of the objects of the present invention is to provide a control system where the actuating piston, which is a rod type, is in pressure balance. In combination with this objective, which is accomplished by the provision of a balance line to the surface, the actuating piston is configured in such a way so as to meet the objective of the invention of minimizing, and in certain situations eliminating, fail-open modes of the valve. These and other objectives will become apparent to one skilled in the art from a review of the preferred embodiment described below. 
     SUMMARY OF THE INVENTION 
     A control system for an SSV is disclosed. A control line from the surface is in fluid communication with the top side of an actuating piston which moves a flow tube downwardly to open the SSV. A balance line runs from the surface to the bottom side of the same actuating piston to put the actuating piston in pressure balance. A buildup of pressure in the control line overcomes a return spring to open the valve, while removal of pressure from the control line allows the return spring to close the valve. Seals and leakpaths are provided through the actuating piston so that, depending on the hydrostatic pressure in the control line and the size of the return spring, the various failure modes of the actuating piston seals and control line or balance line will preferentially result in a fail-closed situation in the SSV. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of the control system of the present invention, shown with the valve in the closed position. 
     FIG. 2 is the view of FIG. 1, with the SSV shown in the open position. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 1 and 2 illustrate a control system C for an SSV. Omitted for clarity are the tubing in which the SSV is mounted, as well as the SSV flapper and flow tube. Those skilled in the art are familiar with installation of tubing-retrievable safety valves and the basics of their operation. Those basics include a flapper with a matching seat and a reciprocating flow tube which is actuated by an actuating piston  10 . In the preferred embodiment, the actuating piston  10  of the present invention is connected to a control line  12  which runs from the location of the SSV to the surface (not shown). The piston  10  is a “rod” piston which is defined as a piston whose diameter is smaller than the wall thickness of the housing. This would exclude an annular piston. A balance line  14  also runs from the area of the SSV to the surface. Balance line  14  is connected to housing  16  at a point below the lower end  18  of piston  10 . Since there is a hydrostatic column of control fluid in the control line  12  and an essentially equal column of the identical control fluid in balance line  14 , the cylindrically shaped piston  10 , which has identical diameters at its lower end  18  and upper end  20 , is in pressure balance from the control fluids in lines  12  and  14 . A return spring  22  operates on actuating piston  10  through an opening in the housing  16 . 
     Actuating piston  10  has a lower seal  24  and a pair of upper seals  26  and  28 . An internal passage  30  extends from lower end  18  to between seals  26  and  28 . 
     The normal operation of the control system C to open the SSV simply requires build-up of pressure in control line  12  to overcome the resistance of return spring  22 . This will push actuating piston  10  downwardly to the position shown in FIG. 2, which will, in turn, push the flow tube (not shown) downwardly to rotate the flapper (not shown) 90° to the open position. Normal closure of the SSV requires removal of applied pressure in the control line  12 , which will allow the return spring  22  to push the actuating piston  10  upwardly, returning it from the position shown in FIG. 2 to the position shown in FIG.  1 . Upward movement of the actuating piston  10  will allow the flow tube (not shown) to move upwardly and will, in turn, allow the spring (not shown) attached to the flapper (not shown) to swing the flapper 90° to contact the seat (not shown) for closure of the SSV. 
     Various failure modes of the control system will now be described. A leak from the control line  12  to the annulus, with pressure being applied to the control line  12 , can occur. It can also occur when the hydrostatic pressure in the control line  12  exceeds the hydrostatic pressure in the annulus without pressure applied to control line  12 . The resulting loss of pressure from the control line  12  in this situation will close the SSV by allowing the spring  22  to shift to piston  10 . 
     A leak can occur from the balance line  14  into the flow tube around seals  24  or  28 . This kind of leakage can occur when the hydrostatic pressure in the balance line  14  exceeds the pressure in the flow tube. Such leakage can reduce the hydrostatic pressure in the balance line  14  since the hydrostatic pressure in the control line  12  becomes greater than the hydrostatic pressure in the balance line  14 . The power spring  22  must be sized strong enough to overcome the maximum pressure differential experienced by the piston  10 . If it is sized appropriately, return spring  22  will shift the piston  10  to close the SSV. If the return spring  22  in this situation is sized for a force less than the hydrostatic force on piston  10  from control line  12 , then the SSV will fail open. 
     Conversely to the above situation, a leak can occur into the balance line  14  around seals  24  or  28  if the pressure in the flow tube exceeds the hydrostatic pressure in the balance line  14 . In this situation where leakage occurs past seals  24  or  28  into the balance line  14 , a low hydrostatic pressure can occur in the balance line  14 , particularly if the application is in a gas well. The gas coming into the balance line  14  will displace the heavier fluid and reduce the hydrostatic pressure, thus potentially putting the valve in a fail-open situation unless the return spring  22  is sized sufficiently strong to overcome the hydrostatic weight and friction forces acting on piston  10 . 
     The balance line  14  can leak into the annulus if the annulus is at a lower pressure than the hydrostatic pressure in the balance line  14 . Again, with a reduction in the hydrostatic force in the balance line  14 , whether the valve fails open or closed is dependent on the sizing of the return spring  22 . If the return spring  22  is sufficiently strong to overcome hydrostatic forces from the control line  12 , as well as frictional and weight forces on the piston  10 , the valve will fail closed. Otherwise, it will fail open. 
     Seal  26  can fail. If it does, there&#39;s normally no flow across it unless pressure is applied to the control line  12 . The reason for this is that, because of the presence of the balance line  14 , there is no differential across seal  26  until the pressure is elevated in control line  12  at the surface. Once that occurs, the leakage past seal  26  will commence through passage  30  which will tend to equalize pressure on both sides of piston  10 , which allow the valve to fail closed. 
     Those skilled in the art will appreciate that there are numerous circumstances of well pressure conditions which will affect the nature of the failure of the SSV when a particular portion of the control system C fails. In general, if the return spring  22  is sized to close the valve against hydrostatic of the control line  12  and friction and weight acting on piston  10 , the SSV will fail closed in all situations of loss of seals  24 ,  26 , and  28 . A weaker spring  22  will result in some fail open situations as described above. 
     One of the advantages of the control system C of the present invention is that it is insensitive to the setting depth of the SSV. The return spring  22  can be sized for frictional and weight loads on the piston  10  independent of setting depth. By use of the balance line  14 , significant pressures in the control line  12  at the surface are unnecessary in order to open the valve. Many hydraulic systems available at the surface have upper operating limits, such as less than 5000 psi. With the balance line  14 , a stiffer return spring  22  can be used without exceeding the capacity of the surface equipment which would be required to open the valve. 
     Those skilled in the art can appreciate that the piston  10  in housing  16  is in pressure balance from the tubing pressure and, thus, is insensitive to the shut-in tubing pressure which may exist in the well. 
     In certain situations where there may be extreme sand or paraffin in the tubing, the balance line  14  can be used to assist in closing the valve by applying pressure to the balance line  14  from the surface equipment. The construction of the control system as illustrated in FIGS. 1 and 2 is substantially simpler than designs involving internal gas chambers acting on hydraulic fluid in order to resist the hydrostatic from the control line  12 . With the presence of control line  12  and balance line  14 , special constructions of the SSV which involve access into an annular chamber which is part of the control system C are not required. In some designs as a backup, access was required into the control system so that if the tubing-retrievable safety valve failed to operate, a wireline-type valve could be installed on a landing nipple and still be controlled from access to the control system C. This technique involved penetrating the wall into an annular chamber to obtain access to the control line pressure in line  12 . This technique is illustrated in U.S. Pat. No. 5,799,949. In the control system C of the present invention, the connections on the SSV body required to provide this annular chamber can be eliminated. The presence of the balance line  14  adds additional assurances in being able to close the valve if necessary. Additionally, backup lines to control line  12  can also be installed for additional security if one of them should happen to be damaged; however, redundancy in the control lines becomes more problematic with the addition of the extra line  14  which acts as the balance line. Additionally, depending on the stiffness of the return spring  22 , certain failure modes as described above may result in a fail-open situation. 
     In the preferred embodiment, the piston  10  is a rod piston, with the passage  30  extending from the lower end  18  to between two seals  26  and  28  adjacent the upper end  20 . The configuration shown in FIGS. 1 and 2 for the seals  26 ,  28 , and  24 , as well as passage  30 , can be flipped over; however, the preferred embodiment is as shown in FIGS. 1 and 2 because fewer failure modes can result in a fail-open situation in the configuration as shown in FIGS. 1 and 2. By putting the piston  10  in pressure balance, it makes it easier to use a rod piston which is the preferred shape for piston  10 . With a rod piston, the seals  24 ,  26 , and  28  are smaller and the overall design of the SSV is simpler to manufacture. With the balanced design of the control system C as shown, a fully fail-safe closed operation can be obtained, with surface equipment limited to 5000 psi by the use of a return spring  22 , which can be overcome with pressures of 5000 psi or less at the surface. As previously stated, the hydrostatic effects are eliminated with the use of the balance line  14 . 
     The foregoing disclosure and description of the invention are illustrative and explanatory thereof, and various changes in the size, shape and materials, as well as in the details of the illustrated construction, may be made without departing from the spirit of the invention.

Summary:
A control system for a subsurface safety valve (SSV) comprising of a control line from the surface in fluid communication with the top side of an actuating piston which moves a flow tube downwardly to open the SSV. A balance line runs from the surface to the bottom side of the same actuating piston to put the actuating piston in pressure balance. A buildup of pressure in the control line overcomes a return spring to open the valve, while removal of pressure from the control line allows the return spring to close the valve. Seals and leakpaths are provided through the actuating piston so that, depending on the hydrostatic pressure in the control line and the size of the return spring, the various failure modes of the actuating piston seals and control line or balance line will preferentially result in a fail-closed situation in the SSV.