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You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND 
   Flapper valves and other types of downhole valves can be damaged if a tool string is allowed to pass through them when they are in the closed condition. For example, the tool string run through the valve can damage the valve&#39;s flapper when closed. Also, the tool string even if capable of passing through the closed flapper may not be allowed to pass back up through the flapper so that the tool sting becomes trapped by the valve. 
   From the surface, operators do not always know if the flapper in these type of valves is open or not. Typically, operators must run a wireline drift tool through the valve to determine if the flapper is open or closed. If the flapper is open, then the drift tool is able to pass through. If the flapper is closed, the drift tool will stick through the flapper which then activates an artificial hold open sleeve to allow for the drift tool to be retrieved. Another way operators can determine whether a flapper is open or closed involves running a camera downhole and feeding back images to the surface. 
   Sometimes, a downhole valve may have dogs that engage a specifically designed stringer used to open and close the valve. For example, Weatherford&#39;s completion isolation valve (CIV) is a ball type valve actuated by a stinger. Dogs in the CIV engage the stinger and allow the stinger to move internal components to open and close the valve&#39;s ball seal. In this instance, these dogs move with the internal components of the valve that operate the ball seal. Therefore, these dogs are directly used to operate the valve by engaging the stinger and not to passively prevent a generic type of tool from being passed through the valve when closed. 
   What is needed is a way to reliably and easily prevent potential damage to a downhole valve by a generic tool string and to prevent entrapment of the tool string in the valve by passively preventing mechanical passage of the tool string in the valve when closed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  illustrates a passable no-go device for a downhole flow control tool in a no-go condition. 
       FIG. 1B  illustrates the passable no-go device in the no-go condition preventing a tool string from passing into the downhole flow control tool. 
       FIG. 2A  illustrates the passable no-go device in a passable condition. 
       FIG. 2B  illustrates the passable no-go device in the passable condition allowing a tool string to pass into the downhole flow control tool. 
       FIGS. 3A-3B  show an alternative arrangement between the tool&#39;s flow tube and the passable no-go device. 
       FIGS. 3C-3D  show another alternative arrangement between the tool&#39;s flow tube and the passable no-go device. 
       FIG. 4A  illustrates the passable no-go device in the no-go condition used on a hydraulically actuated flapper valve when closed. 
       FIG. 4B  illustrates the passable no-go device in the passable condition used on the hydraulically actuated flapper valve when opened. 
   

   DETAILED DESCRIPTION 
   A passable no-go device  50  illustrated in  FIG. 1A  is used with a downhole flow control tool having a housing  10  and an actuator  20 . The flow control tool can be a downhole valve, and the actuator  20  can a flow tube hydraulically actuated between first and second positions to control fluid flow through the housing  10  by actuating a flapper (not shown) in the tool. The device  50  includes an activation member  60 , a support  70 , and dogs  80 . The support  70  fits into the housing&#39;s flow passage  12 , and an end piece  30  fits on the end of the housing  10  and holds the support  70  therein. The support  70  has windows  72  that hold the dogs  80  therein. The dogs  80  are movable into and out of the windows  72  relative to the housing&#39;s flow passage  12 , and springs  74  connect the dogs  80  to the support  70  and bias the dogs  80  to a retracted position in the windows  72 . 
   As shown in  FIG. 1A , the activation member  60  attaches to a distal end of the actuator  20  (e.g., flow tube) by threads  62 , although other forms of affixing the member  60  to the flow tube  20  can be used. The member  60  has a distal lip  64  that is passable in a space behind the dogs  80  to force the dogs  80  to a no-go condition through the windows  72 . In this no-go condition as shown, the dogs  80  extend partially into the housing&#39;s flow passage  12 . To prevent the dogs  80  from passing completely through the windows  72 , the dogs  80  can have ledges or shoulders (not shown) along their back edges that engage sides of the windows  72 . Preferably, the lip  64  has a smaller inner diameter to fit in the space behind the support  70 . Also, the support  70  preferably has the same diameter bore as the tool&#39;s flow passage  12  so that as little flow restriction occurs as possible. 
   As shown in  FIG. 1B , the extended dogs  80  inhibit or restrict a tool string T attempting to pass through the device  50  in the housing&#39;s flow passage  12  while the lip  64  on the flow tube  20  forces the dogs  80  into their no-go condition. As discussed below, this no-go condition may occur when the flow tube  20  is moved into a first (uppermost) position in the tool&#39;s housing  10 , for example, when a hydraulically activated flapper valve is closed. 
   As shown in  FIG. 2A , the passable no-go device  50  has a passable condition when the activation member  60  on the flow tube  20  is moved away from the support  70 . In this condition, the lip  64  is removed from the space around the support  70 , and the springs  74  bias the dogs  80  out of the support&#39;s windows  72 . Therefore, the dogs  80  are allowed to move into the free space around the support  70 . In this passable condition, a tool string T is mechanically uninhibited or unrestricted by the dogs  80  as the tool string T passes through the tool&#39;s housing  10  as shown in  FIG. 2B . As discussed below, this passable condition may occur when the flow tube  20  is moved into a second (lower) position in the tool&#39;s housing  10 , for example, when a hydraulically activated flapper valve is opened. 
   In addition to preventing a tool string T from passing through a closed flow control tool, the passable no-go device  50  may advantageously prevent (full) closure of the downhole flow control tool when the tool string T is positioned through the opened tool. When the tool is open as shown in  FIG. 2B  with the tool string T passing through, the retracted dogs  80  positioned in the space around the support  70  can stop the upward movement of the flow tube  20  by engaging the lip  64 . Even though the lip  64  is made to fit behind and push the dogs  80  and may have a beveled edge, the dogs  80  may be prevented from extending through the windows  72  by engaging the profile of the tool string T passing through the flow passage  12 . In this way, the lip  64  and flow tube  20  are not allowed to reach their uppermost position because the dogs  80  cannot extend. This can prevent full closure of the flow control tool and can prevent some forms of damage to the tool. 
   Although the activation member  60  is shown as a separate component from the flow tube  20  in  FIGS. 1A through 2B , the features of the lip  64  can be integrally formed with the flow tube  20 . As shown in  FIGS. 3A-3B , for example, the lip  64  can be integrally formed on the end of the flow tube  20  and can operate in the same way discussed above to move behind and away from the space around the support to move the dogs  80 . In another alternative shown in  FIGS. 3C-3D , the lip  64  can be part of an independent activation member  60  unattached to the flow tube  20 . In this way, abutting engagement of the flow tube  20  with the activation member  60  can move the lip  64  in the space behind the support  70 . In this arrangement, the member  60  may have ridges  66  or the like that are held within slots  14  defined in the housing&#39;s flow passage  12  to guide the member&#39;s movement. In addition, the member  60  may be biased away from the support  70  by one or more springs  68  (shown here as extension springs) when the flow tube  20  is moved away from the member  60 . 
   The passable no-go device  50  can be used with any downhole flow control tool that controls fluid flow therethrough but must also allow tool strings to pass through the tool when opened. Some suitable downhole flow control tools for use with the passable no-go device  50  include safety valves, downhole control valves, downhole deployment valves, fluid loss valves, and the like. 
   As shown in  FIGS. 4A-4B , for example, the passable no-go device  50  is shown used with a hydraulically actuated flapper valve  100 . In this example, the valve  100  has a housing  110  with a flow passage  111 . In the valve  100 , a flow tube  120  in the flow passage  111  acts as an actuator and is hydraulically actuated between first and second conditions. A flapper  118  acts as a closure member for the flow passage  110  and is mechanically operated between opened and closed conditions by the flow tube  120 . 
   In operation, the absence of hydraulic pressure at a hydraulic port  112  allows the flapper  118  to pivot closed and restrict fluid flow through the valve  100  as shown in  FIG. 4A . When hydraulic pressure is applied as shown in  FIG. 4B , the hydraulic control fluid communicated through the port  112  actuates a piston  114  connected to the flow tube  120  by a coupling  116 . The hydraulically actuated piston  114  thereby moves the flow tube  120  downward in the housing  110  to open the flapper  118  and permit fluid flow through the valve  100 . A spring (not shown) may be provided in the space around the flow tube  120  to bias the flow tube  120  to its uppermost position so that the flapper  118  is biased closed. 
   When the flapper  118  is closed as shown in  FIG. 4A , it is preferred that a tool string is not allowed to pass through the valve  100  because the tool string could damage the closed flapper  118 . Even if a tool string were allowed to pass the closed flapper  118 , operators may not be able to back out the tool string because the flapper  118  may catch on portions of the tool string preventing its retrieval from the valve. To overcome these problems, the passable no-go device  50  installed as part of the flapper valve  100  passively reacts to the opened or closed condition of the valve  100  to either permit or restrict mechanical passage of a tool string through the valve  100 . 
   To do this, the passable no-go device  50  prevents the tool string from reaching the flapper  118  when closed by tying its operation to the independent operation of the flapper valve  100 , which is hydraulically actuated by separate means. In other words, the passable no-go device  50  acts as a restricting member mechanically operated by the flow tube  120  and responds to the closing of the flapper  118  by the upward moving flow tube  120  so that the dogs  80  extend and prevent mechanical passage of the tool string through the valve  100 . As shown in  FIG. 4A , the flapper  118  is closed because the flow tube  120  is moved to its uppermost position in the housing  100 . The activation member  60  moved by the flow tube  120  fits behind the support  70  and pushes the dogs  80  into the valve&#39;s flow passage  111 . In this restrictive no-go condition, the device  50  at least partially restricts mechanical passage through the flow passage  111  because the dogs  80  can mechanically restrict a tool string from getting to the flapper  118  while closed. 
   On the other hand, the passable no-go device  50  responds to the opening of the flapper  118  by the downward movement of the flow tube  120  when the valve  100  is opened so that the dogs  80  retract and allow mechanical passage of the tool string through the valve  100 . As shown in  FIG. 4B , the flapper  118  is opened because the flow tube  120  has been hydraulically moved to its lowermost position in the housing  100 . The activation member  60  on the upper end of the flow tube  120  is moved away from the support  70  and allows the dogs  80  to retract from the valve&#39;s passage  111 . In this unrestrictive passable condition, the dogs  80  will not restrict a tool string from passing through the valve  100 , removes the mechanical restriction through the valve  100 . 
   As will be appreciated, the passable no-go device  50  eliminates the need for an initial discovery run with a camera or a drift tool to be performed to determine if the flapper  118  is first open before running a tool string through the flapper valve  100 . Instead, the tool string can be run down hole. If the valve  100  is inadvertently left closed or is inoperable for some reason, then the passable no-go device  50  can prevent further passage of the tool string to the valve  100 . This can speed up running in and out of the wellbore and can reliably reduce the potential of damage to the flapper  118  or a stuck tool string. 
   The foregoing description of preferred and other embodiments is not intended to limit or restrict the scope or applicability of the inventive concepts conceived of by the Applicants. In exchange for disclosing the inventive concepts contained herein, the Applicants desire all patent rights afforded by the appended claims. Therefore, it is intended that the appended claims include all modifications and alterations to the full extent that they come within the scope of the following claims or the equivalents thereof.

Summary:
A downhole valve has a flow tube hydraulically actuated to open and close a flapper. The flow tube moved to a first position closes the flapper to restrict flow through the valve. The flow tube moved to a second position opens the flapper. A passable no-go device disposed in the valve permits or restricts mechanical passage through the valve in response to the position of the flow tube. The apparatus has a support and one or more dogs supported in windows of the support and biased by springs. The flow tube in the first position pushes the dogs to an extended position that restricts mechanical passage through valve so that a tool cannot be passed through the valve while the flapper is closed. When the flow tube is in the second position, however, the dogs retract so the tool can be passed through the valve while the flapper is open.