Abstract:
A device intended to meet flameproof approval requirements is configured to have two compartments separated by an energy-limiting barrier. The first compartment of the device houses the wiring terminations that bear ignition-capable energy and, therefore, must be flameproof. The energy-limiting barrier is configured to limit the energy that can reach the second compartment to a level that is not ignition capable. This allows the second compartment to be safe without meeting the flameproof requirements, and allows user-interface elements such as switches and indicators to be designed in a more cost-effective manner.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on U.S. Provisional Patent Application Ser. No. 60/761,801, filed Jan. 24, 2006, the entirety of which is hereby incorporated by reference herein. 
    
    
     DESCRIPTION OF THE RELATED ART 
     The present invention relates generally to process control systems and, more specifically, to a system and devices that may be safely implemented in hazardous environments without conventional intrinsically safe barriers and without complete enclosure by a flameproof housing that would normally be required. 
     DESCRIPTION OF THE RELATED ART 
     In some process control applications, some process control devices may be used in hazardous environments presenting a significant risk of an explosion if a spark or flame is produced by the process control devices. In order to safely operate and perform process control in such hazardous environments, standards have been developed for protecting against explosions. The two most common accepted approaches are by providing flameproof housings for the process control devices, or by providing intrinsically safe energy limitation in the circuits connected to the process control devices such that sparks or flames will not be generated by the devices either during normal operations or during failures of the devices. 
     Under the flameproof housing standards, device housings are configured to prevent flames from propagating from the housings in the event of an explosion or spark within the housings. For example, the walls of the housing may be securely sealed, and the openings through the housing walls through which power and communication lines pass are configured to prevent flames from escaping the housing. To prevent flames from escaping the openings, the openings may be filled or plugged with a flame-retardant, pressure-bearing potting material that prevents a flame from passing through the opening, or the openings may be configured to provide gaps that do not provide enough room for flame to get out through the gaps. 
     Under the intrinsically safe operating standards, the voltage and/or current under which the process control devices operate is provided at a level that is not sufficient to generate a spark in the event of a failure in the connections or circuitry of the devices. Consequently, the operating voltage may be limited to a specified maximum voltage to prevent the generation of sparks. By preventing the generation of sparks, the intrinsically safe devices will not cause a fire or explosion in normal operation or in the event of a failure. 
     While process control devices operating under the standards discussed above provide the necessary fire and explosion safety in hazardous environments, the standards also create limitations on implementing and performing process control in the hazardous areas. The flameproof housing requirements are quite stringent, and make it difficult to take advantage of convenient process control and monitoring features, such as LCD displays, buttons, switches and the like. These features cannot be implemented on the exterior of the flameproof housings, and it is difficult and expensive to configure flameproof housings in a manner in which these features may be utilized. Intrinsically safety barriers typically eliminate the need for flameproof enclosures, but these barriers are expensive to purchase and install. An intrinsically safe barrier typically requires an earth ground to limit the energy available to a potential fault to earth. Providing an earth ground in many implementations is inconvenient since a solid connection to earth ground is not readily available, and provision of an earth ground may lead to excessive costs. 
     Additionally, process control systems are conventionally installed with all hazardous area equipment specified and installed according to one or the other of these methods. That is, all flameproof or all intrinsically safe. This is to avoid confusion among maintenance personnel due to the different practices involved with each approach. The flameproof approach does not require intrinsically safety barriers and their associated costs, whereas the intrinsically safety approach does not require the heavy housings and their associated access limitations. Therefore, a need exists to provide the energy-limited benefits of easier access to displays and controls in an installation that uses the flameproof approach to hazardous area safety. 
     In process control environments such as those discussed above, emergency shutdown (ESD) valves may be implemented that are configured to shutdown in the event of an emergency in the process control environment. Typically, the ESD valves are rarely operated (only in emergency situations) and may become stuck in the open or non-safe position. Hence it is desirable to implement methods to test the operation of such valves to verify their condition. In many current implementations, in particular where the ESD valves are installed in hazardous environments using the flameproof safety approach the valve control devices cannot be readily accessed to manually control or test the ESD valves. To facilitate easier, more frequent valve testing, as well as local control of the operation of the valve, it is desired to implement cost-effective local control panels that meet the requirements for use in a hazardous environment, and can be installed using flameproof installation methods. 
     Depending on the implementation, power may be supplied to the devices, including the ESD valves, by various sources. In some installations, power may be supplied by a typical power source, such as a 24-volt DC power input. In other installations, the process control devices may receive power from the process control network over which the devices may also exchange process control information. For example, in a 4-20 mA network, the wires connected to the devices may supply a DC current to power to the devices as well as communicate a process control signal. In some implementations, such as those associated with the HART® protocol, a digital communication signal is superimposed on the DC signal to provide control and diagnostic communications to the device. It can be appreciated by one of ordinary skill in the art that various power sources for the process control devices may exist within an installation and, therefore, a need exists for local process control panels to accommodate different power sources. Further, as the ESD valves may be implemented in hazardous environments, a need exists for providing lower-cost, easily accessible local control panels that meet the design standards for equipment installed in such hazardous environments. 
     SUMMARY 
     A device intended to meet flameproof approval requirements is configured to have two compartments separated by an energy-limiting barrier. The first compartment of the device houses the wiring terminations that bear ignition-capable energy and, therefore, must be flameproof. The energy-limiting barrier is configured to limit the energy that can reach the second compartment to a level that is not ignition capable. This allows the second compartment to be safe without meeting the flameproof requirements, and allows user-interface elements such as switches and indicators to be designed in a more cost-effective manner. The energy-limiting barrier may include a power supply energy-limiting circuit utilizing fuses and Zener diodes configured to ensure that the output of the energy-limiting barrier is maintained at the desired low energy level when voltage or current surges occur within the flameproof housing. The energy-limiting barrier may further include external connection energy-limiting circuits including diodes configured to ensure that any energy intentionally or accidentally supplied through the external connection is maintained within the voltage of the low energy output signal provided by the energy-limiting barrier. The energy-limiting barrier is not intended to limit the voltage between the circuitry and the housing of the device, but rather between any two points within the circuit. Energy discharge between the circuit and housing will be prevented by the provision of infallible spacing within the second housing. 
     In another aspect, a local control panel for an FSD valve is configured to receive and use power from diverse power sources. The local control panel may include communication circuitry, switches and indicators, and a communications filter configured to pass signals between the communication circuitry and the ESD valve. The local control panel may further include a plurality of power converters each configured to receive a power input signal and to convert the power input signal to a fixed voltage signal for use by the components of the local control panel. The local control panel may also include a power select switch connecting the power-converter barrier-circuit outputs to the communication circuitry, wherein the power select switch couples the output of one of the power converter barrier circuits to the components of the local control panel. In a further aspect, the power converters may be disposed within a flameproof housing having an energy-limiting barrier, while the communication circuitry, switches and indicators, and communications filter may be disposed in a non-flameproof housing and may receive the low energy output signal from the energy-limiting barrier of the flameproof housing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exemplary schematic diagram of a flameproof housing containing an energy-limiting barrier and connected to a non-flameproof housing; 
         FIG. 2  is an exemplary block diagram of an embodiment of a dual power source local control panel for an emergency shutdown valve; and 
         FIG. 3  is an exemplary block diagram of an implementation of the local control panel of  FIG. 2  implemented in a hazardous environment with the flameproof housing of  FIG. 1 . 
         FIG. 4  is an exemplary block diagram of a non-flameproof housing containing an energy-limiting barrier complying with an “Increased Safety” standard. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Although the following text sets forth a detailed description of numerous different embodiments of the invention, it should be understood that the legal scope of the invention is defined by the words of the claims set forth at the end of this patent. The detailed description is to be construed as exemplary only and does not describe every possible embodiment of the invention since describing every possible embodiment would be impractical, if not impossible. Numerous alternative embodiments could be implemented, using either current technology or technology developed after the filing date of this patent, which would still fall within the scope of the claims defining the invention. 
     It should also be understood that, unless a term is expressly defined in this patent using the sentence “As used herein, the term ‘ —————— ’ is hereby defined to mean . . . ” or a similar sentence, there is no intent to limit the meaning of that term, either expressly or by implication, beyond its plain or ordinary meaning, and such term should not be interpreted to be limited in scope based on any statement made in any section of this patent (other than the language of the claims). To the extent that any term recited in the claims at the end of this patent is referred to in this patent in a manner consistent with a single meaning, that is done for sake of clarity only so as to not confuse the reader, and it is not intended that such claim term by limited, by implication or otherwise, to that single meaning. Finally, unless a claim element is defined by reciting the word “means” and a function without the recital of any structure, it is not intended that the scope of any claim element be interpreted based on the application of 35 U.S.C. §112, sixth paragraph. 
       FIG. 1  illustrates an embodiment of a flameproof housing  100  having an internal energy-limiting barrier  102  configured to provide low energy outputs so that interface devices, such as Liquid Crystal Displays (LCDs), buttons and switches, connected to the outputs need not be considered ignition-capable (i.e. providing an ignition source in a process environment). Generally, in the illustrated embodiment, the flameproof housing  100  and energy-limiting barrier  102  are configured to ensure that a maximum voltage, such as 5 volts, is output on power supply lines having a nominal operational voltage such as 3 volts. Other external connections, for example to instruments through an external connection  136 , are also limited by the barrier  102  to a non-ignition-capable voltage within the non-flameproof housing  104 . However, those of ordinary skill in the art will understand that the components of the housing  100  and the energy-limiting barrier  102  may be configured as desired and/or necessary for a particular implementation to provide a particular maximum output voltage. 
     The flameproof housing receives power from a system, such as a distributed control system, via two wires  106 ,  108  carrying a current supply, for example, in a conventional 4-20 mA current loop wherein the first wire  106  supplies the current and the second wire  108  provides a return path that may be internally connected to instrument ground. The current loop comprising the two wires  106  and  108  may be, for example, associated with the communications loop, such as a 4-20 mA communication network, as well. As per the applicable safety standards, the wires  106 ,  108  typically enter the flameproof housing  100  through flameproof conduits. The wires  106 ,  108  are fed through a DC-to-DC converter  110  to reduce the working voltage to the desired level for input into the energy-limiting barrier  102 . 
     At the output of the converter  110 , the wires  106 ,  108  may be connected to a power source circuit  112  of the energy-limiting barrier  102  configured to ensure that the voltage output from the flameproof housing  100  to the non-flameproof housing  104  does not exceed the predetermined maximum output voltage. Accordingly, the output of the converter  110  supplies a nominal operational voltage, such as 3 volts, and is clamped to a maximum of voltage by the Zener diodes  118  and  120 , such as 5.1 volts. More specifically, the power source circuit  112  may include fuses  114 ,  116  along the wires  106 ,  108 , and may further include redundant parallel Zener diodes  118 ,  120  connected between the wires  106 ,  108 . The fuses  114 ,  116  and Zener diodes  118 ,  120  are configured to prevent any differential voltage greater than the Zener voltage from propagating to the components in the non-flameproof housing  104 . In particular, the fuses  114 ,  116  are selected with appropriate current and voltage ratings as to ensure the Zener diodes  118 ,  120  will prevent any higher voltage outputs (e.g., over their rated clamp voltage) from reaching the non-flameproof housing  104 . 
     Moreover, the Zener diodes  118 ,  120  may have a breakdown voltage, such as 5.1 volts, such that the diodes  118 ,  120  will conduct the necessary current between the wires  106  and  108  to maintain the breakdown voltage or maximum drop between the wires  106 ,  108 . One of ordinary skill in the art can further appreciate that the power source circuit  112  may further include resistors  122 A,  124 A and/or  122 B,  124 B along either or both wires  106 ,  108 , as shown, to limit the current from the energy-limiting barrier to the components of the non-flameproof housing  104 . At the outlet of the power source circuit  112 , a voltage source line  126  and an instrument ground line  128  having the specified voltage drop pass through the outer wall of the flameproof housing  100  and are connected to non-flameproof housing  104  via flameproof compliant connections  130 ,  132 , respectively. 
     In order to provide external connections between other components of the system and the components of the housings  100 ,  104 , the energy-limiting barrier  102  may further include one or more external connection circuits  134  configured to maintain the voltage of the external connection within the prescribed limits of the energy-limiting barrier  102 . For example, the devices in the non-flameproof housing  104  may be connected to remote transducer devices, switches or other devices of the process control system. In the present embodiment, an external connection line  136  passes through the flameproof housing  100  and into the non-flameproof housing  104 , with all connections, such as the connection  138  between the flameproof housing  100  and the non-flameproof housing  104 , being flameproof standards compliant. The external connection line  136  is connected to the high power line  126  by a first pair of redundant parallel diodes  140 ,  142 , and to the instrument ground line  128  by a second pair of redundant parallel diodes  144 ,  146 . One of ordinary skill in the art can appreciate that these diode pairs  140 - 146  ensure that the voltage on the external connection line  126  is maintained within the voltage limits of the output of the power source circuit  112  of the energy-limiting barrier  102 . This circuit serves to maintain a non-ignition-capable voltage between any two points in the circuit in the non-flameproof housing. Importantly, this clamping does not control the common-mode voltage on any of the circuitry in either housing. If multiple external connections are provided with the flameproof housing  100 , an external connection circuit  134  will be provided for each external connection. 
     With the flameproof housing  100  and energy-limiting barrier  102  of the present embodiment, it is possible to also contemplate placing ignition-capable components in the flameproof housing  100  in a hazardous environment and still provide low-power user interface components in an adjoining non-flameproof housing  104 . As illustrated and described, ignition-capable components are safely contained so that a flame or explosion within the flameproof housing  100  will not escape into the surrounding environment, and the non-incendive components are installed without the difficulty and expense of configuring the flameproof housing  100  such that the user interface devices are accessible by an operator in the hazardous environment, thereby facilitating local monitoring, testing and control within the hazardous environment. 
     As previously discussed, it may be desirable to install devices such as LCDs, buttons, switches and the like in hazardous environments without the complexity and expense of installing a flameproof housing configured to use such devices. In fact, one of ordinary skill in the art understands that in certain environment such an installation may not even be possible. Thus, the flameproof housing  100  and energy-limiting barrier  102 , as described above, provide a mechanism for installing these devices safely in a hazardous environment, i.e., in a non-flameproof housing such as in the housing  104  of  FIG. 1 , without the need for a custom configuration of a flameproof housing. One example of an implementation of the flameproof housing  100  and energy-limiting barrier  102  is the installation of a local control panel at an ESD valve located in a hazardous environment. In many typical installations, ESD valves are installed without local controls or monitoring, and instead are monitored at remote user interfaces. It may be desired, however, to be able to control, monitor and test the ESD valves at the valves themselves. In order to operate and to monitor and control the ESD valve, the local control panel requires connection to a power source and a communication link to the ESD valve for exchanging control signals. 
     Referring to  FIG. 2 , a functional block diagram of an embodiment of a dual power source local control panel  200  is illustrated. The local control panel  200  may include communications circuitry  202 , switches and indicators  204  and a communications filter  206 . The communications circuitry  202  may include the processing and memory capabilities for controlling the operation of the local control panel  200  to perform necessary or desired monitoring and testing functions. Consequently, the communications circuitry  202  may include an appropriate processor, memory device(s) and interface or I/O modules necessary to communicate with the other components of the local control panel  200 . The switches and indicators  204  may include the input and output devices necessary for an operator at the local control panel  200  to monitor the operation of the ESD valve, and to perform testing or manual operation of the ESD valve. 
     In one embodiment, the switches and indicators  204  may include separate switches for manually tripping the ESD valve, for resetting the ESD valve and for initiating a partial stroke test of the ESD valve, and separate status indicators, such as LCD displays or LED displays, corresponding to the switches and providing a visual indication of the status of each of the switches or to provide other information to a user. The switches are operatively connected to the communications circuitry  202  such that the communications circuitry  202  detects actuation of the switches and causes the corresponding functions to be performed, and the indicators are operatively connected to the switches and/or the communications circuitry  202  such that the indicators are illuminated in the appropriate manner to indicate the status of the switches and the ESD valve. The communication filter  206  is operatively connected between the communications circuitry  202  and the ESD valve, and is configured to facilitate communications between the control panel  200  and the ESD valve. In one embodiment, the communication filter  206  is configured to communicate with the ESD valve via a half-duplex serial bit stream. 
     As previously discussed, the local control panel  200  also requires a power source for operation of the components associated therewith. In the illustrated embodiment, the local control panel  200  is configured such that the control panel  200  may be powered by either of at least two alternate power supplies. Depending on the implementation and the availability of the power supplies at the ESD valve, the control panel  200  may be powered by either a 24-volt DC power Supply, or the control loop used to control the ESD valve. For example, the system may use a 4-20 mA control signal carried over a pair of wires and having a DC base current with, in some cases, a digital communications signal, such as HART®, superimposed over the base current. Therefore, instead of having separate power connections for each of the devices receiving a 4-20 mA control signal, the devices may be powered by the DC base current of the control signal, including the local control panel  200 . 
     Because either power input may be available, the local control panel  200  may include a DC-to-DC power converter  208  configured to receive a 24-volt DC power input and a loop power converter  210  configured to receive the 4-20 mA signal. The converters  208 ,  210  are configured to convert the corresponding input signals to a Fixed voltage signal appropriate for providing power to the components of the local control panel  200  and are well known in the art. To provide the control circuitry and other components of the control panel  200  with the available power signal, the local control panel  200  further includes a power select switch  212  receiving the outputs from the converters  208  and  210 , and having an output connected the communications circuitry  202 . The power select switch  212  provides the available one of the power inputs to the communications circuitry  202  and other components of the local control panel  200 . 
     Configured as described, a dual power supply local control panel  200  may be implemented in diverse process control systems and utilize the available power input without the necessity and expense of providing a type of power input that is not readily available in the system or portion of the system. Consequently, in the implementations where the devices are powered from the control signal, the local control panel  200  may be installed with a minimal amount of field wiring, and without the need for installing a particular power input. Moreover, because all the components of the local control panel  200  are powered by the output of the power select switch  212 , only a single pair of wires is necessary to provide power to all the local control panel  200  components. 
       FIG. 3  illustrates an implementation of the local control panel  200  in hazardous environment in conjunction with the flameproof housing  100  having the energy-limiting barrier  102 . In this embodiment, the components of the local control panel  200  may be divided between the flameproof housing  100  and the non-flameproof housing  104  as necessary to ensure safe operation of the local control panel within the hazardous environment. Consequently, the communications circuitry  202 , switches and indicators  204 , and the communication filter  206  and power select switch  212  may be disposed within the non-flameproof housing  104  and configured to operate on the 3-volt DC nominal input voltage provide by the power source circuit  112  of the energy-limiting barrier  102 , and the power converters  208  and  210  may be disposed within the flameproof housing  100 . 
     Within the non-flameproof housing  104 , the power lines  126 ,  128  enter via the flameproof compliant connections  130 ,  132 , respectively, and are connected to the power select switch  212 . The selected power source is coupled through the power select switch  212  to communications circuitry  202 , switches and indicators  204  and communications filter  206  to provide the necessary power to operate the components. 
     As the components of the local control panel  200  operate to monitor the ESD valve, the communications circuitry  202  communicates with the ESD valve via the communications filter  206 . The communications filter  206  is connected to the ESD valve through the flameproof housing  100  via a separate flameproof compliant connection  138  and a separate external connection energy circuit  134 . Moreover, the ESD valve is also connected to the flameproof housing  100  via a flameproof compliant connection, and the external connection energy circuit  134  corresponding to the communications filter  206  is also disposed the necessary internal separation or spacing denoted by dashed line  139  between the power loop and fuses. 
     One of ordinary skill in the art will understand that other configurations of the flameproof housing  100  and the local control panel  200  are possible to provide for safe implementation of the local control panel  200  in a hazardous environment. For example, the power converter  208  may be configured to provide infallible isolation of this power source such that the energy provided through this path cannot combine with the energy provided from the loop connection. One can further appreciate that additional alternate configurations of the flameproof housing  100  and local control panel  200  may also be contemplated without departing from the spirit and scope of the invention. 
     Another alternate configuration that embodies the principles disclosed herein is depicted in  FIG. 4 . In this embodiment, there is no flameproof enclosure, but rather the overall hazardous area approval is based on the “Increased Safety” standard otherwise known to those skilled in the art as “Ex e.” The enclosure in this embodiment is only required to be sealed from the environment, not flameproof. That is, there are certain spacing (separation) requirements at the wire termination points within the Ex e zone  300 . Any ignition-capable circuitry is all placed under encapsulation within the Ex m zone  302 , and the switching elements such as buttons and switches are located in the Ex ib zone  304  where there is no ignition-capable energy. The use of the energy-limiting barriers in this embodiment allows the use of ordinary switching elements that are much less expensive and smaller than their flameproof counterparts are. This embodiment requires no divided enclosure and no flameproof feed-through. However, the safety of the device is achieved with an integral energy-limiting approach very similar to the other embodiments, and providing the similar benefit of lower-cost, easier to implement indicators and switches in a hazardous environment. 
     By implementing the local control panel  200  with the flameproof housing  100  and energy-limiting barrier  102 , it is possible in a hazardous environment in which an ESD valve is installed to put the high power components of the local control panel  200  (converters  208 ,  210 ) in the flameproof housing  100  and still provide the low power user interface components (communications circuitry  202 , switches and indicators  204 , and communications filter  206  and power select switch  212 ) in an adjoining non-flameproof housing  104 . As discussed above, the high power components are safely contained and the lower power components are installed without the necessity of a customized flameproof housing, thereby facilitating local monitoring, testing and control of the ESD valve within the hazardous environment. 
     While the invention has been described with reference to specific examples, which are intended to be illustrative only and not to be limiting of the invention, it will be apparent to those of ordinary skill in the art that changes, additions or deletions may be made to the disclosed embodiments without departing from the spirit and scope of the invention.