FIELD OF THE DISCLOSURE 
       [0001]    The present disclosure relates to a system and method to prevent shutdown of manufacturing processes due to abnormal low temperatures encountered in a refrigeration system by the use of safety interlocks. 
       BACKGROUND 
       [0002]    Ethylene is a colorless flammable gas with a formula of C 2 H 4 . Ethylene is a basic chemical that is used widely for production of ethylene derivative chemicals. Major industrial reactions using ethylene include polymerization, oxidation, halogenation and hydro halogenation, alkylation, hydration, oligomerization and hydroformylation. 
         [0003]    Ethylene is produced in the petrochemical industries from various types of feedstocks such as ethane, propane, ethane-propane mix, butane, naphtha, etc. through the process of steam cracking or in the oil refineries by cracking over zeolite catalysts. Typical process design in the production of ethylene includes feed treating, steam cracking, heat recovery, acid gas treatment, cracked gas compression, cold fractionation and hot fractionation. 
         [0004]    In the cold fractionation process, due to the extremely cold temperatures, aluminum heat exchangers are usually used because of the compatibility of their metallurgy with various other interconnected parts of the system. In addition, aluminum heat exchangers are effective in lowering overall equipment count and capital investment required for an ethylene manufacturing plant. 
         [0005]    In general, the aluminum heat exchangers are used to transfer heat between multiple streams in a “cold box.” Heat exchangers can be used alone or in combination in the same cold box. Usually, the “hot streams” connected to the aluminum heat exchangers come from various levels of refrigeration and transfer heat to “cold streams” from cold service process equipment. 
         [0006]    While operating, temperatures across an aluminum heat exchanger can range from −350 F to 160 F, depending on the plant design. Such a great temperature range induces tremendous thermal stresses. The thermal stresses often lead to premature aging of the aluminum heat exchanger and fracture failure. 
         [0007]    In order to overcome the problem of failure, the prior art has responded by reinforcing the mechanical design of the aluminum heat exchangers. However, despite design improvements, the process piping and equipment connecting to the outlet streams of the aluminum heat exchangers are often made of interior materials, such as ductile iron. Inferior materials are not a problem so long as the system operates in normal temperature ranges. However, the inferior materials can lead to catastrophic failure when exposed to extremely low temperatures, which sometimes result from abnormal plant operating conditions. Such catastrophic failure results in plant shut down and can result in injury to plant personnel. Superior materials for process piping, such as stainless steel, are available which can operate safely across larger temperature ranges, such as below −20 F, but they are extremely expensive and difficult to fabricate. 
         [0008]    Examples of manufacturing processes which involve low temperatures are found in the prior art but they do not solve the problems inherent in abnormally low process temperatures. U.S. Pat. No. 5,361,589 to Howard, et al. discloses an ethylene recovery system with cracked gas cooled to about −20 to −40 F. However, Howard does not disclose how to control temperatures to protect process piping and equipment during abnormal operating conditions where lower temperatures are experienced. 
         [0009]    U.S. Pat. No. 5,979,177 to Summer, et al. discloses an ethylene plant refrigeration system where the gas feed is cooled to about −31 to −35 F. However, Summer does not disclose how to monitor stream temperatures and control temperatures to protect process piping and equipment during abnormal low temperature conditions. 
         [0010]    U.S. Pat. No. 4,900,347 to McCue, et al. discloses a method for recovering ethane or ethylene from cracking gas requiring low temperature refrigeration. At least one portion of the disclosed method has process temperatures below −20 F. However, McCue does not disclose how temperatures of process streams are monitored or modified in response to abnormal low temperature conditions. 
         [0011]    Other prior art demonstrates methods to monitor and control process temperatures. However, none of the methods have been entirely satisfactory in controlling abnormal low process temperatures while protecting process piping and equipment. 
         [0012]    U.S. Pat. No. 4,488,239 to Agarwal discloses a system to control temperatures in an olefin oxidation reactor by incrementally adjusting the flow rate of coolant to the chemical reactor based on measured temperatures. However, Agarwal does not disclose control of streams based on abnormally low process temperatures around the reactor to protect process lines and equipment. 
         [0013]    United States Patent Publication No. 2010-026301 filed by Schwartz, et al. discloses a method for controlling a process flow rate through an aluminum heat exchanger by adjusting a bypass. However, Schwartz does not disclose the use of redirection or flow stoppages. 
       SUMMARY 
       [0014]    Aluminum heat exchangers are often used in an ethylene processing plant for transferring heat between multiple process streams. Such aluminum heat exchangers can be operated in a single unit or in multiple units arranged in series or in parallel. Arrangement of the process streams in an aluminum heat exchanger is done so as to prevent a large temperature differential. A high temperature differential can stress the mechanical integrity of the aluminum heat exchanger, leading to fatigue and failure. 
         [0015]    Advancement in the technology has improved the design of the aluminum heat exchangers. But, such improvements fail to compensate for the impact of cold process liquid leaving the aluminum heat exchanger during transient operating conditions and the resulting impact on downstream equipment. Downstream equipment is often is constructed of carbon steel materials. Carbon steel is not compatible with extremely cold temperatures and when exposed to them becomes dangerously brittle. 
         [0016]    In order to protect process piping and downstream equipment from abnormally low process temperatures while preventing process shutdown, a method and system is disclosed which provides for monitoring and controlling the temperatures of various streams based on a set of predetermined high and low temperatures. Temperature sensors are connected through controllers to valves on the heating and cooling streams. The sensors, controllers and valves are known as “interlocks.” When abnormal process temperatures occur, the interlocks open or close in order to redirect, stop or bypass certain streams. Controllers are also provided for pumps, which, in response to abnormal temperatures, increase or reduce flow to specific pieces of equipment. Location of the interlocks and pumps in the system and careful control of their functions prevent a shutdown of flow through the cold box and allow the system to continue operating until the temperatures reach normal conditions. Hence, damage to the aluminum heat exchangers and downstream piping is avoided while also avoiding expensive system shutdown. 
         [0017]    The disclosure includes multiple temperature sensors located at different locations in the process, such as when the heating and cooling streams leave the cold box. Further, temperature sensors can be connected to multiple valves and pump controllers such that multiple actions can occur based on a single temperature reading. 
         [0018]    It should be understood that the invention is not limited to use in ethane and propane streams in an ethylene process. Other processes which can benefit from the invention include air separation, extraction of natural gas liquids, and other cryogenic processes. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    In the detailed description of the preferred embodiments presented below, reference is made to the accompanying drawings. 
           [0020]      FIG. 1  is a flow sheet of an embodiment of a cold box process. 
           [0021]      FIG. 2  is a flow sheet for an embodiment of an interlock. 
           [0022]      FIG. 3  is a flow sheet for an embodiment of an interlock. 
           [0023]      FIG. 4  is a flow sheet for an embodiment of an interlock. 
           [0024]      FIG. 5  is a flow sheet for an embodiment of an interlock. 
           [0025]      FIG. 6  is a flow sheet for an embodiment of an interlock. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0026]    In the description that follows, like parts are marked throughout the specification and figures with the same numerals, respectively. The figures are not necessarily drawn to scale and may be shown in exaggerated or generalized form in the interest of clarity and conciseness. 
         [0027]      FIG. 1  is a flow sheet for an embodiment of a cold box  100  used in an ethylene production process. Some of the streams flowing through cold box  100  are heated and some are cooled. Unless indicated otherwise, the following descriptions and conditions of the streams through cold box  100  are described at steady state conditions. 
         [0028]    Stream  102  is from the bottom of a demethanizer. As stream  102  flows through cold box  100  it is heated. The stream enters cold box  100  in a liquid form, at about −67 F, and exits cold box  100  in a liquid form, at about 14 F. In the present embodiment, the stream then proceeds to another aluminum heat exchanger before going to a deethanizer. 
         [0029]    Stream  104  is a liquid vapor ethane mixture feed. It enters cold box  100  at about −45 F and is heated as it flows through cold box  100 . The stream leaves cold box  100  and is combined with stream  106  to form stream  108 . Stream  108  has a temperature of about 89 F and is a vapor. Stream  108  proceeds to another aluminum heat exchanger before entering to the furnaces to be used as feed source. 
         [0030]    Stream  110  is a combination of streams  112 ,  114 , and  116  which is heated as it flows through cold box  100 . Stream  112  is a combination of streams  118  and  120 . Stream  118  is vapor ethane from a feed splitter reflux drum. Stream  120  is liquid ethane from feed dryers. Stream  112  consists of vaporized ethane with a temperature of about −27 F. 
         [0031]    Stream  116  is a combination of streams  122  and  124 . Stream  122  and stream  124  contain liquid ethane which is fully vaporized through an aluminum heat exchanger before combining with streams  114  and  112  to form stream  110 . Stream  122  comes from ethylene splitter. Stream  116  is a mixed vapor/liquid ethane composition having a temperature of about −47 F. Stream  114  is vaporized ethane that comes from a deethanizer overhead and acetylene converter. 
         [0032]    Stream  130  joins with stream  110  as stream  110  flows through cold box  100  and is heated. Stream  130  is a combination of streams  126  and  128 . Stream  126  consists of a mixture of liquid and vapor propane from the liquid propane recycle from propylene splitter bottom having a temperature of about 54 F. Stream  128  consists of liquid/vapor mixture of propane from a feed splitter bottom having a temperature of about 55 F. 
         [0033]    Streams  110  and  130  combine into stream  106 , which joins stream  104  to form stream  108 . 
         [0034]    Stream  132  flows through cold box  100  and is heated. Stream  132  contains hydrogen offgas going to a methanator. Stream  132  enters cold box  100  as a vapor having a temperature of about −103 F and exits cold box  100  with a temperature of about 95 F. 
         [0035]    Stream  134  flows through cold box  100  and is heated. Stream  134  contains methane which is to be used as fuel gas. The methane in stream  134  enters cold box  100  as a vapor at approximately −103 F and exits with a temperature of about 95 F. 
         [0036]    The heating source for cold box  100  is provided by a four stage propylene refrigeration system and the third stage of a three stage ethylene refrigeration system. 
         [0037]    Stream  140  contains liquid propylene having a temperature of about 100 F. Stream  140  splits into stream  138  and stream  142 . Stream  142  flows through an aluminum heat exchanger and joins stream  148  to form stream  150 . Stream  138  splits into stream  136  and stream  144 . Stream  144  flows through an aluminum heat exchanger and joins stream  146  to form stream  148 . Stream  150  flows to a tank before continuing to the third stage of the propylene refrigeration system. 
         [0038]    Stream  136  flows through cold box  100  as part of the fourth stage of the propylene refrigeration system and is cooled. Stream  136  is liquid propylene having a temperature of about 100 F when it enters cold box  100 . Stream  136  leaves cold box  100  with a temperature of about 69 F. After leaving cold box  100 , stream  136  splits into streams  152  and  146 . Stream  152  further splits into streams  154  and  156 . Streams  154  and  156  flows into two separate aluminum heat exchangers, respectively. 
         [0039]    Stream  158  flows from a tank between the fourth stage and the third stage of the propylene refrigeration system and contains liquid propylene having a temperature of about 64 F. Stream  158  flows through cold box  100  and is cooled to a temperature of about 44 F as part of the third stage of the propylene refrigeration system. After cooled, stream  158  splits into streams  159  and  161 . Stream  159  flows to a liquid drain system. Stream  161  splints into streams  160  and  164 . Stream  160  flows to multiple aluminum heat exchangers. Stream  164  splits into streams  162  and  166 . Stream  162  flows to an aluminum heat exchanger. 
         [0040]    Stream  166  is joined by stream  178  to form stream  168 . Stream  178  is propylene coming from other aluminum heat exchangers. 
         [0041]    Stream  168  splits into streams  170  and  172 . Stream  170  flows to tank  180 . Stream  172  is a bypass of tank  180  and can be closed under normal operating conditions. Stream  172  will flow and bypass tank  180  as part of one of the interlocks disclosed later. 
         [0042]    Stream  174  flows from tank  180  and joins with stream  172  to form stream  176 . Stream  176  is liquid propylene having a temperature of about 25 F. Stream  176  flows through cold box  100  as part of the second stage of the propylene refrigeration system and is cooled. The exit temperature of stream  176  is about −2 F. 
         [0043]    Stream  182  contains liquid propylene with a temperature of about 0 F. Stream  182  flows through cold box  100  as part of the first stage of the propylene refrigeration system and exits cold box  100  with a temperature of about −38 F. 
         [0044]    Stream  184  contains liquid ethylene with a temperature of about 2 F and is part of the third stage of the ethylene refrigeration system. Stream  184  flows into cold box  100  and splits into streams  186  and  188  before exiting cold box  100 . Stream  186  exits cold box  100  prior to stream  188  with a temperature of about −62 F. Stream  188  flows through cold box  100  and exits with a temperature of about −87 F. 
         [0045]      FIG. 2  is a flow sheet for a preferred embodiment of the interlock system on the ethane and propane streams heated in cold box  100 . This interlock is design to prevent a complete loss of feedstock to the downstream furnaces in the event of cold fractionation, ethylene refrigeration and or propylene refrigeration system upsets resulting in a low temperature excursion of the feed system. 
         [0046]    The piping for stream  128  further includes valve  232  and the piping for stream  126  includes valve  208 . 
         [0047]    The piping for stream  124  includes valve  210 . Valve  210  is connected to valve controller  226 . Valve controllers in this embodiment consists of a valve actuation system connected to a trip solenoid. In the preferred embodiment, the valve activation system is available from HYTORK of Netherlands. In a preferred embodiment, the temperature sensor is a Smart Sanitary Temperature type available from Smart Sensors, Inc. of Houston, Tex. The piping for stream  122  includes valve  212  connected to valve controller  214 . The piping for stream  114  includes valve  202  connected to valve controller  220 . The piping for stream  120  includes valve  206  connected to valve controller  222 . The piping for stream  118  includes valve  204  connected to valve controller  224 . 
         [0048]    The piping for stream  104  includes valve  216  connected to valve controller  228 . Valve  216  is located on the piping before stream  104  flows into cold box  100 . 
         [0049]    The piping for stream  102  includes pump  218  which is connected to pump controller  230 . 
         [0050]    The piping for stream  108  includes temperature sensor  200 . In a preferred embodiment, the temperature sensor is a Smart Sanitary Temperature type available from Smart Sensors, Inc. of Houston, Tex. Temperature sensor  200  determines the temperature of stream  108  after it has left cold box  100 . Temperature sensor  200  includes three separate sensors, each of which are determining the temperature of the stream  108  independently. A greater or lesser number of sensors may be used. Multiple sensors have the advantage of providing redundancy in the event of a failure or malfunction of one sensor. Temperature sensor  200  is connected to interlock  201 . The connection allows the temperature readings from temperature sensor  200  to be received by interlock  201 . 
         [0051]    Interlock  201  is connected to valve controllers  226 ,  214 ,  220 ,  222 ,  224 , and  228  and pump controller  230 . The connection can be an electrical wire, a data communication line, or a remote communication. The connection allows a signal to be sent from interlock  201  to valve controllers  226 ,  214 ,  220 ,  222 ,  224 , and  228  and pump controller  230  which can react to the signal. The signal can include but is not limited to the cessation of the flow of electricity or power. 
         [0052]    In the event that the temperature of stream  108  is determined to about −18 F or below by two of three sensors of temperature sensor  200 , then interlock  201  automatically sends a signal to pre-determined valve and pump controllers in order to stop the flow of certain streams while allowing other streams to continue to flow thereby raising the temperature in stream  108  such that equipment downstream of stream  108  is not exposed to temperatures below about −18 F. 
         [0053]    Specifically, in the preferred embodiment, when a temperature reading of about −18 F or below is read by temperature sensor  200 , interlock  201  sends a signal to valve controllers  226 ,  214 ,  220 ,  222 ,  224 , and  228  to close valves  210 ,  212 ,  202 ,  206 ,  204  and  216 . The interlock further sends a signal to pump controller  230  to stop operation of pump  218 . The closing of these valves will stop the flow of streams  124 ,  122 ,  114 ,  120 ,  118 ,  110 , and  104  and the stopping of pump  218  will stop the flow of stream  102  through cold box  100 . However, valves  224  and  208  will remain open allowing stream  130  to flow into cold box  100 . Stream  130  has a temperature of about 54 F when it enters cold box  100  and is higher than the streams which are stopped. This allows the temperature within cold box  100  to rise and elevate temperature of stream  108  above the about −18 F threshold, thereby protecting the equipment downstream while still allowing the process to continue. 
         [0054]      FIG. 3  is a flow sheet for an interlock system on a propylene refrigeration system which, in this embodiment, cools the propylene in cold box  100 . This interlock is designed to prevent the propylene refrigeration system from complete shut down due to low temperature excursion on the 3 rd  stage liquid propylene heat exchangers. 
         [0055]    Piping for stream  159  includes liquid drain valve  402 . The liquid drain valve is used to drain trapped liquid propylene inside the pipe pocket. During normal operating conditions, stream  159  is closed. Stream  159  piping is made up of stainless steel material, which the pipe&#39;s temperature rating is sufficient down to −350 F. 
         [0056]    Piping for stream  160  includes valve  406  which is connected to valve controller  408 . Piping for stream  162  includes valve  412  which is connected to valve controller  410 . Piping for stream  166  includes valve  414 . Piping for stream  170  includes valve  416 . 
         [0057]    Piping for stream  158  includes temperature sensor  400 . Temperature sensor  400  reads the temperature of the propylene in stream  158  after it exits cold box  100 . Temperature sensor  400  includes three separate sensors, each of which are determining the temperature of the stream  158 , independently. Temperature sensor  400  is connected to interlock  420 . The connection allows the temperature readings from temperature sensor  400  to be received by interlock  420 . 
         [0058]    Interlock  420  is connected to valve controllers  408  and  410 . The connection can be an electrical wire, a data communication line, or a remote communication. The connection allows a signal to be sent from interlock  420  to valve controllers  408  and  410  which are able to react to the signal. The signal can include but is not limited to the cessation of the flow of electricity or power. 
         [0059]    In the event that temperature sensor  400  transmits a temperature reading of about −18 F or below by at least two of the three sensors in temperature sensor  400 , interlock  420  sends a signal to valve controller  408  and  410  to close valves  406  and  412 . The closing of valves  406  and  412  prevents the cold propylene liquid with a temperature of about −18 F or less from reaching the aluminum heat exchangers located downstream. Tripping the aluminum heat exchangers will not completely stop the continuous circulation of propylene in the refrigeration system. 
         [0060]    Valves  414  and  416  stay open and stream  164  is allowed to continue to flow and joins with warmer propylene from stream  178 . The addition of warmer propylene from stream  178  allows the temperature of the propylene in stream  168  to be elevated above −18 F. Stream  168  is allowed to continue through to tank  180  and proceed through the system preventing a complete system shut down. 
         [0061]      FIG. 4  is a flow sheet for an interlock system on the third stage of a propylene refrigeration system that cools the propylene flowing through cold box  100 . This interlock is designed to protect the tank  180  from sensing cold liquid propylene temperature below −18 F by automatically bypassing it. 
         [0062]    Valve  416  is connected to valve controller  506 . The piping for stream  172  includes valve  502  which is connected to valve controller  504  and is constructed of stainless steel material with low temperature rating of −50 F. 
         [0063]    Piping for stream  168  includes temperature sensor  500 . Temperature sensor  500  determines the temperature of stream  168 . Temperature sensor  500  includes three separate sensors, each of which are determining the temperature of the stream  168  independently. Temperature sensor  500  is connected to interlock  510 . The connection allows the temperature readings from temperature sensor  500  to be received by interlock  510 . 
         [0064]    Interlock  510  is connected to valve controllers  504  and  506 . The connection can be an electrical wire, a data communication line, or a remote communication. The connection allows a signal to be sent from interlock  510  to valve controllers  504  and  506  which are able to react to the signal. The signal can include but is not limited to the cessation of the flow of electricity or power. 
         [0065]    If the temperature of stream  168  is determined to be about −18 F or below by at least two of the three sensors in temperature sensor  500 , interlock  510  sends a signal to valve controller  506  to close valve  416  and to valve controller  504  to open valve  502  allowing the propylene to bypass tank  180  and continue through the process. The bypass of tank  180  allows the propylene to continue to flow through the process while preventing exposure of tank  180  to temperatures about or below −18 F. 
         [0066]    A temperature reading of −18 F by temperature sensor  500  is most likely to occur when a temperature reading of about −18 F or below has been registered by temperature sensor  400  and the addition of warmer propylene from stream  178  is insufficient to raise the temperature of the propylene above about −18 F. 
         [0067]    Interlock  510  will often not be triggered unless interlock  420  has previously been triggered. However, interlock  510  is completely independent of interlock  420 . 
         [0068]      FIG. 5  is a flow sheet for an interlock system on the fourth stage of a propylene refrigeration system. This interlock is design to protect the fourth stage propylene users from low temperature excursion without causing a complete propylene refrigeration shutdown. 
         [0069]    Piping for stream  152  includes valve  602 . Valve  602  is connected to valve controller  604 . Piping for stream  146  includes valve  606 . Piping for stream  148  includes valve  608 . 
         [0070]    Stream  142  flows through aluminum heat exchanger  614 . Stream  144  flows through aluminum heat exchanger  610 . 
         [0071]    Piping for stream  136  includes temperature sensor  600 . Temperature sensor  600  determines the temperature of the propylene in stream  136  after it has flowed out of cold box  100 . Temperature sensor  600  includes three separate sensors, each of which are determining the temperature of the stream  136  independently. Temperature sensor  600  is connected to interlock  612 . The connection allows the temperature readings from temperature sensor  600  to be received by interlock  612 . 
         [0072]    Interlock  612  is connected to valve controller  604 . The connection can be an electrical wire, a data communication line, or a remote communication. The connection allows a signal to be sent from interlock  612  to valve controller  604  which is able to react to the signal. The signal can include but is not limited to the cessation of the flow of electricity or power. 
         [0073]    When the temperature of stream  136  is determined to be about −18 F or below by at least two of the three sensors in temperature sensor  600 , interlock  612  sends a signal to valve controller  604  to close valve  602 . The closure of valve  602  will prevent the flow of liquid propylene with a temperature of about −18 F or below from reaching heat exchangers which are not rated to handle such temperatures through streams  156  and  154 . 
         [0074]    Valve  606  and valve  608  will remain open and allow the propylene in stream  136  to mix with warmer propylene in streams  144  and  142  before proceeding through the system. The mixture of the warmer propylene will assist in raising the temperature of the propylene to above about −18 F and allow the propylene to continue to flow through the system and prevent a complete system shut down. 
         [0075]      FIG. 6  is a flow sheet for different interlock system on the fourth stage of a propylene refrigeration system. This interlock is design to protect the equipment downstream of stream  150 . 
         [0076]    Valve  606  is connected to valve controller  702 . Valve  608  is connected to valve controller  704 . 
         [0077]    Piping for stream  148  includes temperature sensor  700 . Temperature sensor  700  determines the temperature of propylene through stream  148 . Temperature sensor  700  includes three separate sensors, each of which are determining the temperature of the stream  148  independently. Temperature sensor  700  is connected to interlock  710 . The connection allows the temperature readings from temperature sensor  700  to be received by interlock  710 . 
         [0078]    The propylene in stream  148  has proceeded through cold box  100  and mixed with stream  144 . 
         [0079]    Interlock  710  is connected to valve controllers  702  and  704 . The connection can be an electrical wire, a data communication line, or a remote communication. The connection allows a signal to be sent from interlock  710  to valve controllers  702  and  704  which are able to react to the signal. The signal can include but is not limited to the cessation of the flow of electricity or power. 
         [0080]    When the temperature of stream  148  is determined to be about −18 F or below by at least two of the three sensors in temperature sensor  700 , interlock  710  sends a signal to valve controller  702  and  704  to close valves  606  and  608 . The closing of valves  606  and  608  will allow the propylene in stream  140  to bypass cold box  100  and flow into stream  142 , through aluminum heat exchanger  614  and into stream  150  where it will continue through the process. The continuing flow of propylene will allow the process to continue operating and prevent a complete system shut down. 
         [0081]    Interlock  710  often works in conjunction with the interlock  612 . However, interlock  710  is independent of the activation of interlock  612 . 
         [0082]    In a preferred embodiment, valves  210 ,  216 ,  406 ,  412 ,  416  and  608  are full port on/off globe valves. However, other types of valves may be used. Other valves described above can include but not limited to gate valves, ball valves, or plug valves. Valves in the preferred embodiment are available from Velan of Montreal, Canada. The actuation systems for valves  210 ,  216 ,  406 ,  412 ,  416  and  608  are manufactured by either BETTIS of Houston, Tex., or HYDROTORK of Netherlands. The trip solenoids attached to the actuation system are Model Nos. EV8007G1 and 258181-20 manufactured by ASCO of Florham Park, N.J. The purpose of the trip solenoid is to connect each valve with the signal triggered from the described interlocks. Valve positioner connected to each valve Model No. DXP L12GNEB and is supplied by TOPWORX of Louisville, Ky. 
         [0083]    In the preferred embodiment, valves  204 ,  206 ,  208 ,  212 ,  224 ,  414 ,  502 ,  602  and  606  are full port process control valves. The functions of these individual valves can be based on pressure, flow rate, liquid levels, temperature or operators input. Valves and actuation system are available from Emerson Fisher Control Valves of Marshalltown, Iowa. The trip solenoids attached to the actuation system is manufactured by ASCO of Florham Park, N.J. Valve positioner that is installed with the valve actuation system is available from TOPWORX of Louisville, Ky. 
         [0084]    In the preferred embodiment, actuation on the valve controllers  214 ,  220 ,  222 ,  226 ,  228 ,  408 ,  410 ,  506 ,  604 ,  702  and  704  are pneumatic controllers which are set to fail closed upon loss of signal. However, other types of valve controllers and actuation, positioning and trip solenoids can be used in the embodiment, as long as the combination of any three valve controllers is able to receive a signal from an interlock and is able to affect the position of the valve in response to the signal including but not limited to electrical, hydraulic, or pneumatic. 
         [0085]    In the preferred embodiment, pump  218  is a multi-stage submersible pumps manufactured by Sulzer of Winterthur, Switzerland. The stopping of the pumps can be done by pump controller  230  via distributed control system to the pumps&#39; control system which is connected to the associated interlock. However, other combinations of pumps and pump controllers may be used. 
         [0086]    In the preferred embodiment, temperature sensors  200 ,  400 ,  500 ,  600 , and  700  are manufactured by Smart Sensors Inc. of Houston, Tex., and have digital displays through the transmitter connected to each sensors manufactured by Rosemount of Chanhassen, Minn. However, other temperature sensors can be used. Further, while in the preferred embodiment temperature sensors  200 ,  400 ,  500 ,  600 , and  700  includes three individual sensors. 
         [0087]    In the preferred embodiment, the programming of interlocks  201 ,  420 ,  510 ,  612  and  710  are done through the safety instrumented system manufactured by AUGUST SYSTEMS of ABB Group of Zurich, Switzerland or TRICONEX of Invensys of Houston, Tex. However, other safety instrumented system can be used to program the interlock logic as long that they are able to receive temperature readings and transmit a signal to a valve or pump controller for the appropriate response. 
         [0088]    In the preferred embodiment, the composition of the streams is the majority component, however, other compounds may be found in the disclosed streams. 
         [0089]    In the preferred embodiment, the ethylene production process includes a four stage propylene refrigeration system and an ethylene refrigeration system. However, other heating streams may be used, including a single, double, or triple stage propylene refrigeration system. Further, the ethylene production process may not use an ethylene refrigeration system as a heat stream. Similarly, the ethane and propane streams may flow through the cold box from different stages in the process. 
         [0090]    The disclosed embodiment is one illustration of an interlock system with the use in an ethylene production process. A person skilled in the art will understand that other processes with similar temperature operating conditions and concerns can employ the system and method as disclosed. Further, a person skilled in the art will understand that the disclosed streams may flow to other equipment other than the equipment disclosed.
A system of interlocks for controlling flow of low temperature process streams in a manufacturing process through a cold box to equipment not specified for such temperatures by opening and closing valves and stopping pumps. At least one interlock affects streams heated in the cold box. At least one interlock affects the streams cooled in the cold box. The interlocks are activated due to temperature determinations of process lines by temperature sensors and automatically send a signal to predetermined controllers depending on the process line with the low temperature in order to prevent exposure of equipment to low temperatures while preventing the shutdown of the cold box.