Abstract:
An emergency shutdown system for a process control system includes an emergency shutdown (ESD) valve and an associated valve actuator. An emergency shutdown (ESD) controller provides output signals to the ESD valve in the event of a failure in the process control system. A solenoid valve responds to the ESD controller to vent the actuator to a fail state. A digital valve controller (DVC) test strokes the ESD valve. An impedance booster device enables the dc powering of the solenoid valve and the DVC over a two wire line while still permitting digital communication over the same two wire line.

Description:
This invention relates to emergency shutdown systems used in process control systems and to the testing of emergency shutdown valves used in such systems. 
     BACKGROUND OF THE INVENTION 
     Process control systems incorporate emergency shutdown valves which are normally in a fully opened or fully closed state. In order to assure that such emergency shutdown valves can properly function, they are periodically tested by partially opening or closing these valves. Since these tests are typically performed with pressure in the pipeline of the process control system, it is important to perform any test quickly and then return the valve to its normal state. 
     In such prior emergency shutdown systems, in order to perform a short test, a complicated panel of pneumatic valves and switches are normally used by an operator to partially open or close the emergency shutdown valve under test. In particular, this requires an operator to observe and understand a panel of pneumatic valves and switches and then to operate the particular associated valves and switches in order to partially open or close the desired emergency shutdown valve being tested. It is also important that any emergency shutdown system provide the ability to be able to move the emergency shutdown valve to its safe condition when commanded by the emergency shutdown controller, in the unlikely, but possible situation where a failure event has occurred during a valve test interval, and the valve under test must now be properly commanded by the emergency shutdown system. 
     SUMMARY OF THE INVENTION 
     In accordance with the principles of the present invention, there is provided an emergency shutdown system which includes an emergency shutdown controller for commanding the operation of an emergency shutdown valve between a normally fully opened or fully closed state to an emergency state when a failure event is detected in the process control system. The emergency shutdown system includes a solenoid valve coupled to the emergency shutdown controller for responding to the controller and venting the emergency shutdown valve actuator to a fail state. A digital valve controller or positioner includes an output pressure coupled through the solenoid valve to the valve actuator of the emergency shutdown valve so that the emergency shutdown valve can be quickly ramped up and down in order to test the emergency shutdown valve. As an example, a digital signal sent to the digital valve controller moves the emergency shutdown valve from its normal fully opened or fully closed state to a partially opened or partially closed state, and then quickly returns the valve to its normal state. If an emergency occurs during the test procedure or at any other time, the emergency shutdown system acts through the emergency shutdown controller to de-energize the solenoid valve, causing the solenoid valve to vent the emergency shutdown valve actuator to a fail state. 
     In a preferred embodiment of the invention, the powering of the solenoid valve and of the digital valve controller is provided by the emergency shutdown controller on the same pair of wires. Also, it is preferred that a digital communicating device, such as a hand held digital unit is coupled to the digital valve controller to provide the required digital signals for moving the emergency shutdown valve during the test procedure. The digital communicator can be coupled to the same pair of wires powering both the solenoid valve and the digital valve controller with the use of an impedance boosting device inserted on the pair of wires between the emergency shutdown controller and the digital units. The impedance boosting device enables dc power such as 24 volts dc to be supplied to power the solenoid valve and the digital valve controller on the same pair of wires, while also permitting the digital communicator to reliably communicate with the digital valve controller over the same pair of wires. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of this invention which are believed to be novel are set forth with particularity in the appended claims. The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements in the several figures and in which: 
     FIG. 1 is a schematic diagram illustrating a system for testing emergency shutdown fluid control valves; and 
     FIG. 2 is a schematic diagram illustrating a impedance boosting device useful in the system of FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates an emergency shutdown test system  10  for testing an emergency shutdown valve  12 . The emergency shutdown valve  12  may be located for instance in a process control system including a pipeline  14  supplying fluid at the inlet to valve  12  and outlet pipeline  16  leading fluid from the outlet of valve  12 . 
     The emergency shutdown valve  12  is normally in a wide open state permitting fluid to flow freely between pipeline  14  and pipeline  16  or in a fully closed state preventing any fluid communication between pipeline  14  and pipeline  16 . In order to ensure that the emergency shutdown valve  12  will properly function when desired in a true emergency condition, the valve  12  must be periodically tested by partially opening or closing the emergency shutdown valve. The emergency shutdown test system  10  includes means for periodically testing the operation of the valve  12  with pressure in the pipelines  14 ,  16 , and to perform the testing of the valve  12  quickly and then returning the valve to its normal state. Means are also provided in the emergency shutdown test system  10  to move the emergency shutdown valve  12  to its safe condition in the event an actual emergency condition exists during the periodic testing of the valve  12 . 
     A valve actuator  18  includes a pneumatic input coupled to a pneumatic line  20  and an actuator output line  22  providing the actuator output to move the valve  12  in response to the pneumatic pressure in pneumatic line  20 . A solenoid valve  24  includes a solenoid control portion  26  receiving dc power and electrical control signals on a two wire line  28 ,  30 . The control portion  26  provides an output on line  32  coupled to a solenoid valve portion  34  to control the flow of pneumatic pressure through the solenoid valve portion  34 . 
     A digital valve controller  36  is mounted to the emergency shutdown valve  12  and includes a pneumatic output line  38  coupled to the solenoid valve portion  34 . Thus, depending on the signal on control output line  32  from the solenoid control portion  26 , the pneumatic pressure on line  38  from the digital valve controller  36  can be either directed to the pneumatic line  20  and then to valve actuator  18 , or to an exhaust line  39 . The digital valve controller  36  also includes electrical lines  40 ,  42  connected respectively to the two wire lines  28 ,  30 . A digital communications device such as a handheld unit  44  or a personal computer is coupled to the two wire line  28 ,  30  for supplying digital control signals to the digital valve controller  36  during periodic testing of the emergency shutdown valve  12 . 
     The emergency shutdown test system  10  includes a monitor system including an emergency shutdown controller  46  having a two wire line  48 ,  50  coupled respectively to the two wire line  28 ,  30  via a impedance boosting device  52 . The emergency shutdown controller  46  provides dc power, such as 24 volts dc, to the two wire line  48 ,  50  and to the two wire line  28 ,  30  and eventually to the solenoid valve control portion  26  to enable the proper operation of the valve  12  as will be described in more detail hereinafter. 
     The impedance boosting device  52  enables dc power from the emergency shut down controller  46  to be supplied to power the solenoid valve  24  and the digital valve controller  36  on the same pair of wires  28 ,  30 , with minimal power loss, while also permitting the digital communicator  44  to reliably communicate with the digital valve controller  36 . The impedance boosting device  52  provides efficient high ac impedance isolation of the emergency shutdown controller  46  from the digital communication units  44  and  36  while still enabling a low dc impedance level to the dc power which must be supplied from the controller  46  to the controller  36  and the solenoid valve  24 . 
     In particular, a field effect transistor  56  is biased by resistor  58  to conduct sufficient dc power to operate the digital valve controller  36  and solenoid  24  while at the same time capacitor  60  holds the gate voltage of transistor  56  relatively constant such that the ac impedance on two wire lines  28 ,  30  is high enough to facilitate digital communication between device  44  and the digital valve controller  36 . 
     The operation of the emergency shutdown system  10  may now be described as follows. During normal operation of the emergency shut down monitoring system, the emergency shut down controller  46  supplies 24 volts dc which is coupled on two wire lines  48 ,  50  to the impedance boosting device  52  and with minimal loss the 24 volts dc is coupled to two wire lines  28 ,  30  to power the solenoid valve  24 . The solenoid valve  24  is thereby maintained in a normal state to provide fluid communication between pneumatic lines  38  and  20 . The digital valve controller is also coupled to wires  28 ,  30  by means of lines  40 ,  42  which provides power to the digital valve controller. The digital valve controller  36  receives a pressure supply and senses a valve stem position through the linkage  62 . The digital valve controller compares a digital set point which is internal to the digital valve controller to the valve stem position to maintain the desired emergency shutdown valve position during normal operation. 
     When it is desired to periodically perform a short test on the emergency shut down valve  12 , digital command signals from the communication device  44 , to the digital valve controller  36  over the two wire line  28 ,  30 , command the digital valve controller  36  to control the pneumatic output on line  38  coupled to the valve actuator  18  to ramp the valve  12  from the normal 100 percent open condition to a partially opened condition and then back again to the normally  100  percent open condition. It is to be appreciated that the ramp testing of emergency shutdown valve  12  is accomplished on the same two wire line  28 ,  30  which supplies the operating power from the controller  46  to the solenoid valve  24  for placing the valve  12  in the desired position. 
     If a true emergency occurs during the valve ramp test sequence or at another time, the emergency shut down controller  46  will remove the 24 volts dc power source from the two wire lines  28 ,  30  and  40 ,  42 . Removal of the power source from the solenoid valve  24  causes the solenoid valve to vent the valve actuator  18  to the exhaust line  39  so as to vent the actuator to a fail state, and thereby place the valve  12  in the desired emergency shut down position. This also removes power from the digital valve controller which causes it to vent pressure in line  38 , thereby providing a redundant method in moving the valve  12  to the desired emergency shutdown position. 
     Rather than the field effect transistor  56  utilized in the impedance boosting device  52 , other devices may be utilized in accordance with the teachings herein. For example, a suitable inductor and resistor in parallel may be utilized if desired. The purpose of the impedance boosting device  52  is to enable passage of the 24 volts dc power source from the controller  46  on the two wire line  28 ,  30  to power the solenoid valve  24  as well as the digital valve controller  36 , while also enabling the digital communication of the communicating device  44  and the digital valve controller  36  with the same two wire line  28 ,  30  during periodic testing and other diagnostic procedures conducted with valve  12 . Accordingly, other devices can be utilized with the teaching herein to pass the 24 volts dc power source from the controller  46  to the two wire line  28 ,  30  while blocking passage of the digital signals from the two wire line to the controller  46 . 
     In utilizing the present invention the operator could visually observe movement of the emergency shutdown valve during testing if the operator is in viewing distance of the valve. On the other hand, the operator could transmit commands from a remote location with a personal computer acting as the digital communicator  44 . The digital valve controller  36  then could provide diagnostic information to indicate how well the emergency shutdown valve performed during the test. 
     The foregoing detailed description has been given for clearance of understanding only, and no unnecessary limitations should be understood therefrom, as modifications will be obvious to those skilled in the art.