Abstract:
An electrical terminal block has a heat-meltable fusible link establishing a closed electrical circuit between a conductor ans an actuator responsive to a control signal transmitted by conductor for controlling the state of an emergency process control device, such as a shut-off valve. The heat-meltable fusible link melts in response to heat exceeding a predetermined temperature to establish an open electrical circuit between the electrical conductor and the actuator thereby causing the actuator to move the emergency process control device to a safe position or state.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to electrical circuits that are normally energized to maintain emergency control devices, such as shut-off valves.  
       BACKGROUND OF THE INVENTION  
       [0002]     When devices are installed as part of safety instrumented systems (SIS), the signal from the safety programmable logic circuit (PLC) causes the device to change its position or state in such a way that will bring the process to a safe state or condition. Typically, the signals are energized during normal plant operations. Therefore, should a wire come loose, a wire break or the circuit be otherwise interrupted, the device in the field functions to move the valve to a fail-safe position. This type of design is known as a “normally energized” or “fail safe” system.  
         [0003]     When industrial control instrumentation, such as emergency isolation valve controllers, are installed in fire hazard zones, special apparatus and methods must be utilized to ensure that the necessary safety control functions will be activated, even under the worst-case fire conditions.  
         [0004]     Emergency isolation or shut-off valves (“EIV”) are installed in a wide variety of manufacturing and processing facilities, including petroleum refineries, hydrocarbon processing plants, and in pipelines and tank farms for the purpose of isolating a potential source of fuel or other hazardous materials in the event of a fire. Industrial fire safety standards require fireproofing of emergency safety valve actuators and their critical control components so they can withstand the effects of a hydrocarbon fire for at least 20 minutes and remain functional. One type of EIV is fitted with a pneumatically pressurized actuator having an internal spring that moves the valve to the fail safe position when the pressure is vented from the actuator.  
         [0005]     In lieu of, or as a supplement to an externally installed fireproof or fire and heat resistant housing, shielding, or the like, it has been known to incorporate fusible fittings or links in the pneumatic supply lines to such actuator. The fusible link in the pneumatic supply line provides this protection by venting or discharging the air supply at a predetermined temperature, thereby allowing the actuator to move the valve to the fail-safe position.  
         [0006]     When the isolation valve is so large as to preclude the use of an actuator having an internal biasing spring to move the valve to the desired safe or emergency position, double-acting actuators are utilized. In this case, the actuator and all associated controls must be made fail-safe in the event of fire. The fireproofing process increases the cost of valve actuators and controls and, in some cases, requires bulky external blankets or boxes that are both cumbersome and costly.  
       SUMMARY OF THE INVENTION  
       [0007]     In the “fusible link” terminal block housing of the invention, the link is preferably on the final control device itself. A conductive material with a known melting point is installed. Alternatively, a spring loaded switch, where a screw manufactured from a low melting-point material maintains the electrical circuit in the closed position, can be used. In either case, the function is the same: when the circuit is opened or broken under fire conditions, the actuator moves the final control element (valve) to the desired safe position.  
         [0008]     The electronic control devices associated with emergency shut-down valves are designed to fail to a safe state in a fire. Specifically, the terminal blocks that serve as the connection point between the external plant wiring and the control device open the circuit when heat from a fire reaches a predetermined level. As with the pneumatic heat fusible link or fittings and the spring return pneumatic actuators, fireproofing is no longer required. Plant safety is improved, as the control device detects the fire condition in the field independently of the Emergency Shutdown System and moves the associated emergency shut-down valve to the safe position.  
         [0009]     The fusible link terminal would serve to open the circuitry between the safety PLC and the final control device, having the effect of de-energizing the circuit under fire conditions. This allows the device to function in the intended manner to move the valve to the fail safe position before the fire destroys the capability of the device to function.  
         [0010]     The invention broadly comprehends an electrical terminal block for receiving and securing electrical conductors that is designed in accordance with the operating principles and the methods described. The terminal block connects the wiring from the safety PLC to the final valve or other process control device. Through the use of a special heat fusible link installed in the terminal block itself, in the event that causes the link to melt or otherwise fail and break the circuit, the final control element moves to the fail safe position just as it would if the safety PLC interrupted the output signal to the device.  
         [0011]     In addition to improving fire safety, an advantage of the invention is to eliminate the need for expensive and bulky fireproofing of critical electrical safety control devices.  
         [0012]     Should the fusible link terminal block open prematurely (safe failure resulting in a nuisance EIV closure) an integral “tender-timer” type indicator will alert operations to the fault to facilitate repairs to restore full EIV operations.  
         [0013]     Emergency shut-down valves are only one type of automated safety control device that operates in a “normally energized” manner. The “fusible link” terminal block has wide applications in the safety industry. Other applications include solenoid valves and smart valve positioners used to control emergency isolation valves. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is a schematic illustrating a system including a fire safety fusible link terminal block;  
         [0015]      FIG. 2  is a flowchart illustrating a method of operation of the system of  FIG. 1 ;  
         [0016]      FIG. 3  is a side cross-sectional schematic illustrating one preferred embodiment of a fire safety fusible link terminal block of the invention; and  
         [0017]      FIG. 4  is a side elevation view illustrating the fire safety fusible link terminal block of  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]     Referring to  FIGS. 1-4 , a system  10  and method  100  utilize a fire safety fusible link terminal block for maintaining emergency control devices in the event of emergencies.  
         [0019]     As shown in  FIG. 1 , the system  10  includes a safety PLC  12  which is wired through the external plant wiring  14  of a facility to a terminal block  16  having a heat-meltable fusible link  18 . The safety PLC  12  is electrically connected through the external plant wiring  14  and through the heat-meltable fusible link of the terminal block  16  to a control device  20  having an actuator  22  which controls an emergency shut-off valve  24  having a valve arm  26  in either a fail-safe position  28  (in emergencies) or a normal operating position  30 .  
         [0020]     The terminal block  16  with the heat-meltable fusible link  18  is physically positioned substantially adjacent the control device  20 . The heat-meltable fusible link  18  is composed of known materials having a predetermined temperature at which the fusible link  18  melts and creates an open electrical circuit or path across the fusible link  18 .  
         [0021]     In non-emergency operations, the heat-meltable fusible link  18  provides a closed electrical circuit through the terminal block  16 , allowing control signals from the PLC  12  to be communicated to the actuator  22  in order to control the emergency shut-off valve  24  to respond to routine safety PLC.  
         [0022]     During emergency operations when heat in the vicinity of the control device  20  exceeds a predetermined value such as a minimum temperature due to, for example, a fire, the heat-meltable fusible link  18  melts, causing an open electrical circuit condition to be present at the input from the fusible link  18  to the actuator  22 . The actuator  22  responds to the open electrical circuit condition by moving the valve arm  26  of the emergency shut-off valve  24  to the fail-safe position  28 .  
         [0023]     During emergency conditions, which can possibly prevent the safety PLC  12  from communicating with the actuator  22  to control the emergency shut-off valve  24 , the melting of the heat-meltable fusible link  18  automatically causes the actuator  22  to move the emergency shut-off valve  24  to the fail-safe position to respond to the emergency conditions.  
         [0024]     The system  10  operates according to the method  100  shown in  FIG. 2 , in which a closed circuit is provided in step  102  through the meltable fusible link  18  between a safety programmable logic circuit (PLC)  12  and the control device  20  with an emergency valve  24 . The fusible link  18  melts in response to a predetermined level of heat in step  104 , and then opens the closed circuit in step  106 . The actuator  22  detects the opened circuit in step  108 , and causes the emergency shut-off valve  24 , or the valve arm  26  thereof, to move to the fail-safe position  28  in step  110 .  
         [0025]     An example embodiment of the fire safety fusible link terminal block  16  of  FIG. 1  is illustrated in greater detail in  FIGS. 3-4 .  FIG. 3  is a side cross-sectional schematic illustrating a conceptual fire safety fusible link terminal block, and  FIG. 4  is a side elevation view illustrating the fire safety fusible link terminal block of  FIG. 3 .  
         [0026]     Referring to  FIG. 3 , the fire safety fusible link terminal block  16  includes a housing  40 , having mounting mechanisms or other devices for positioning the fire safety fusible link terminal block  16  substantially adjacent to the control device  20  and/or other equipment, for example, on the housing of such equipment where the fire sense point of the fire safety fusible link terminal block  16  is exposed to the risk area.  
         [0027]     The fire safety fusible link terminal block  16  is provided with connecors  60  adapted to receive the ends of signal conducting wires  42 ,  44  from the external plant wiring  14  and the control device  20  shown in  FIG. 1 . The wires  42 ,  44  are connected internally to the heat-meltable fusible link  18 , which can include a spring-loaded screw-type terminal  46 , having a spring-loaded switch contact  48 . The heat-meltable fusible link  18  can also include an indicator device  50 . The spring-loaded screw-type terminal  46  is composed of meltable material selected to fail at a predetermined temperature, which allows the spring-loaded contact  48  to open upon melting of the terminal  46  due to exposure to heat from a fire, thus de-energizing the safety circuit.  
         [0028]     When the spring-loaded contact  48  opens, the indicator device  50  is activated to alert operations to the fault to facilitate repairs to restore full EIV operations. In one embodiment, the indicator device  50  can include an integral “tender-timer” type indicator  52  which is partially or completely concealed within the housing  40  when the spring-loaded contact  48  is closed. When spring-loaded contact  48  opens, the movement of the spring-loaded contact  48  to the open position extends the “tender-timer” type indicator  52  outward from the housing  40  to be visible to operations personnel. The “tender-timer” type indicator  52  can also have a distinct and visible color to alert operations that the spring-loaded contact  48  has opened.  
         [0029]     Referring to  FIG. 4 , the fire safety fusible link terminal block  16  can be mounted in conjunction with emergency shutdown signal wire termination enclosure  54  on a control device such as a smart valve positioner  20 . The control device  20  is mounted on a valve that requires “fail-safe” action in the event of a fire. Thus, when the indicator device  50 , such as the “tender-timer” type indicator  52 , is activated, the smart valve positioner device  20  moves control emergency isolation valves  24  to its fail safe position.