Abstract:
A method and apparatus for switching a supply to a process from a first vessel to a second vessel. Liquid is flowed from the first vessel through a first outlet conduit through a vessel selection valve, including first and second switch valves, to the process. A conductivity sensor measures the conductivity level of the liquid in the first outlet conduit at a point upstream of the selection valve and a computer compares the conductivity level to a predetermined range. The computer signals an isolation valve, in a second outlet conduit from the second vessel, to open and allow liquid to displace air in the second outlet conduit from the second vessel to the selection valve. The computer determines a delay to allow liquid below the first conductivity sensor to reach the first switch valve, then closes the first switch valve and opens the second switch valve.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to valve switching manifolds and, more particularly, to methods and apparatus for online switching between supply vessels for continuous operations. 
     BACKGROUND OF THE INVENTION 
     Modern methods to continuously manufacture complex photographic products require a constant source of coating solution. These photographic products typically involve the uniform coating of photosensitive compositions on a substrate, such as, for example, a continuous web of paper, cellulose acetate, polyethylene terephthalate, or PEN. Traditionally, a particular coating solution is prepared in one vessel, and coating is performed using that one vessel as the source for that particular coating solution. Coating of that solution from that source continues until the vessel is nearly empty. At that point, the supply to the coating operation is switched to a second vessel, such that the second vessel containing the same coating solution becomes the source for that coating solution. The remaining coating solution in the old or first vessel and the piping associated therewith becomes waste. 
     In the past, switching between source vessels has been performed using level sensors to sense when the level of coating solution in the vessel is approaching depletion. When using such a method to determine when to switch from one supply vessel to another in a coating operation, there is considerable waste of usable material in the vessel being superseded. Some of the materials used in making photographic coating compositions are very costly and such waste of usable material can represents a great expense. 
     In current practice, switching may be performed in accordance with method and apparatus disclosed in U.S. Pat. No. 5,156,298 issued Oct. 20, 1992 to LaRue, the relevant disclosure of which is incorporated herein by reference. LaRue provides a change or switching valve at a juncture between the conduits leading from the first and second containers, and further includes a commercially-available conductivity sensor in each of the conduits between the containers and the change valve. A third conduit from the switching valve leads to a coating hopper. The two conductivity sensors are connected by electric leads to a computer. Stored within the computer is a range of values which represents values of the conductivity of composition when it is acceptable for coating. It will be recognized that the conductivity of composition froth is different from the conductivity of composition free of air bubbles, or of air itself, so that it is possible to use conductivity as a metric for determining acceptability or unacceptability of composition for coating. Thus, when the conductivity detected by the sensor goes outside of the range of acceptable values, it can be taken that the composition passing through the sensor is no longer usable for coating. At such time, the computer sends a signal to the switching valve to close off flow from the first vessel and to simultaneously open flow from the second vessel. 
     It will be recognized that the volume of usable composition, which is wasted each time the switching valve switches from taking supply from one container to taking supply from the other container, is approximately the volume of the length of conduit between the sensor and the switching valve. In some prior art composition delivery systems however, this may still amount to up to several liters of wasted good composition. 
     Further in the prior art, typically the conduit leading from the second vessel to the switching valve is prepared for introduction of composition from the second container by being back-filled with water from a port in the switching valve to a port in the container valve to purge air from the conduit. Then, and again prior to the actual switching, the container valve is opened and composition is allowed to flow downwards through the conduit, displacing the backfill water through a drain port in the switching valve. Because the composition typically is water-miscible and generally has a specific gravity that is greater than water, there can be considerable mixing of the composition with the backfill water during this downwards purging of water by composition. Thus, an excess of good composition must be diverted to the drain in order to be sure that all the backfill water has been displaced. Otherwise, the first composition sent to the hopper from the second container after switching over will be diluted, resulting in coating defects. Again, several liters of usable material from the second container may be wasted. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to minimize the waste volume of usable coating composition resulting from an online changeover from a first vessel depleted of coating composition to a second vessel containing a fresh supply of coating composition. 
     It is a further object of the present invention to provide for automatic changeover from a depleted to source vessel to a fresh source vessel. 
     Another object of the invention is to provide such a changeover without introducing air bubbles into the conduit leading from the switching valve to the coating apparatus. 
     Yet another object of the present invention is to provide such a changeover between vessels wherein the amount of usable composition wasted by the changeover is substantially zero. 
     Briefly stated, the foregoing and numerous other features, objects and advantages of the present invention will become readily apparent upon a review of the detailed description, claims and drawings set forth herein. These features, objects and advantages are accomplished by connecting each of two source vessels to a vessel-switching valve via respective outlet conduits. The vessel-switching valve has a single delivery conduit for delivering coating solution to a coating apparatus. There is a conductivity sensor located in each vessel outlet conduit between a respective vessel isolation valve and the vessel-switching valve. The conductivity sensors are used to determine whether the contents of the outlet conduits are suitable for delivery to the coating apparatus. An acceptable range of conductivity for the composition type is predetermined. At a predetermined time during delivery of the composition from the first vessel to the switching valve, the vessel isolation valve of the second vessel is opened. The then empty outlet conduit from the second vessel is allowed to fill by gravity to the switching valve thereby displacing the air in the outlet conduit upwards by buoyancy through the coating composition in the second vessel. The volume of that portion of the conduit from the vessel first between the conductivity sensor and the switching valve as well as the volumetric flow rate of the liquid composition is provided to a computer or programmable logic controller. Using this information, the computer can calculate the period of time that it will take to exhaust the volume of coating solution in that portion of the outlet conduit. When the sensor in the outlet conduit from the first vessel indicates a conductivity that is outside the predetermined range, the computer begins a timing operation based on the calculated period of time. At the expiration of that period of time substantially the last of the usable coating composition has reached the switch valve. The computer then opens the switch valve controlling flow from the second vessel thereby allowing coating composition to begin flowing from the second vessel. Shortly thereafter, the valve controlling flow from the first vessel is closed thereby shutting off further flow from the outlet conduit from the first vessel and also preventing coating composition from the outlet conduit of the second vessel from backing up into the outlet conduit from the first vessel. Flow is thus changed over from the first vessel to the second vessel. This is accomplished without introduction of any air into the outlet conduit of the second vessel and with substantially no usable coating composition remaining in the outlet conduit from the first vessel. Further, no waste of usable coating composition has been generated in preparing the outlet conduit from the second vessel for delivery of composition to the vessel-switching valve. 
     This method allows vessels (kettles or any other continuous source of supply) to be switched online with zero liquid waste and without the introduction of bubbles or flow perturbations to coating. A single bubble, 30 microns or larger can cause a coated defect. Flow perturbations as low as ±2.0% of aim flow rate can also cause coated waste. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The FIGURE is a process flow schematic depicting a system for switching from a first source vessel to a second source vessel, each containing a liquid composition to be supplied to a downstream process. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the FIGURE, a vessel selection valve  10  is schematically depicted in combination with a first supply vessel  12  and a second supply vessel  14 . First supply vessel  12  and second supply vessel  14  contain liquid such as, for example, liquid coating solution to be applied to a moving web in a downstream coating operation for the manufacture of photographic films and papers. First supply vessel  12  contains a first quantity  16  of a particular coating composition and second supply vessel  14  contains a second quantity  18  of the same coating composition. Vessel selection valve  10  allows for switching between flow of the liquid composition from the first supply vessel  12  and flow of the liquid composition from the second supply vessel  14 . First supply vessel  12  has an outlet  20  with an isolation valve  22  attached thereto. There is an outlet conduit  24  connecting isolation valve  22  to vessel selection valve  10 . Similarly, second supply vessel  14  has an outlet  26  with an isolation valve  28  attached thereto. There is an outlet conduit  30  connecting isolation valve  28  to vessel selection valve  10 . There is a first conductivity sensor  32  in outlet conduit  24  and a second conductivity sensor  34  in outlet conduit  30 . There is a delivery conduit  36  exiting vessel selection valve  10  that delivers liquid solution to the downstream process which includes a coating apparatus. The remainder of a composition delivery system downstream (not shown) of the vessel selection valve  10  may be, for example, substantially as disclosed in U.S. Pat. No. 5,156,298 which is hereby incorporated herein by reference. 
     Conduit  24  connects to valve  40  in vessel selection valve  10  and conduit  30  connects to valve  42  in vessel selection valve  10 . When valve  40  is open, liquid flows through valve  40  from conduit  24  and into delivery conduit  36 . Similarly, when valve  42  is open, liquid flows through valve  42  from conduit  30  and into delivery conduit  36 . Thus, switching between valves  40 ,  42  allows selection of either vessel  12 ,  14  as the source for supplying liquid to the downstream operation. 
     Vessel selection valve  10  further includes flush/drain valves  44 ,  46  associated with switch valves  40 ,  42 , respectively, and with a flush/purge valve  48 . A utility supply pipe  50  for supplying flush water, for example, and having a control valve  52  therein, is in fluid communication with flush/drain valves  44 ,  46 . A drain line  54  having a valve  56  therein is connected to flush/drain valve  44 . Flush/purge valve  48  connects to switch valves  40 ,  42  to permit reverse flushing and air purging with water through delivery conduit  36 . 
     The two conductivity sensors  32 ,  34  are preferably identical and may be, for example, as described and illustrated in the incorporated reference. Each sensor  32 ,  34  is intended for use in determining the conductivity of the fluent composition flowing through it. A suitable sensor is offered as Model No. 871 AB-3 by Foxboro Instrument Corp, Foxboro, Mass., USA. The two sensors  32 ,  34  are connected to the process control computer or programmable logic controller (PLC)  58  which is programmed to monitor the signals therefrom, representing the conductivity levels of material within the sensors  32 ,  34  at any given time. Computer  58  is further connected to vessel selection valve  10 . Vessel selection valve  10  is controlled by computer or programmable logic controller (PLC)  58 . Each valve  40 ,  42 ,  44 ,  46 ,  48  in vessel selection valve  10  is independently controllable by computer  58  to open and close a respective flow path gate  41 ,  43 ,  45 ,  47 ,  49 . Computer  58  also controls isolation valves  22 ,  28 . 
     In operation, as the last of the usable composition  16  from first vessel  12  (defined by a previously specified conductivity) passes beyond sensor  32 , an unacceptable change in composition conductivity is sensed by sensor  32 . The signal from sensor  32  is monitored by computer  58 . Computer  58  is also provided with input representing the flow rate of composition through conduit  24 . Using the volume of conduit  24  between sensor  32  and switch valve  40 , computer  58  calculates the time required for the last of the usable composition to reach switch valve  40 . Computer  58  then executes a timing function to delay changing over flow from switch valve  40  to switch valve  42  until, preferably, the precise moment at which the last of the usable composition reaches switch valve  40 . In changing over, preferably the switch valve  42  controlling flow from the second vessel  14  is opened momentarily before the switch valve  40  controlling flow from the expiring first vessel  12  is closed, to ensure that there is no momentary loss of flow through delivery conduit  36 . Of course, in practice it may be desirable to make the changeover slightly sooner than the calculated time to ensure that no unusable composition enters delivery conduit  36 . Thus, the changeover would typically be made less than about 5 seconds (depending on flow rate) prior to the expiration of the calculated period of time required for the last of the usable composition to reach switch valve  40 . The actual period of delay can, therefore, be something slightly less than the actual calculated period of time. 
     When first vessel  12  is cleaned and recharged with another batch of composition  16 , the liquid composition  18  in second vessel  14  is being consumed. Thus, when the composition within vessel  14  is about to be exhausted the changeover process is repeated understanding that the depleted vessel  14  is now the first vessel and recharged vessel  12  is the second vessel. 
     After flow has been changed over from vessel  12  to vessel  14 , any remaining unusable composition in vessel  12  may be drawn off for further use or recycling via valves  22 ,  60 . Similarly, after flow has been changed over from vessel  14  to vessel  12 , any remaining unusable composition in vessel  14  may be drawn off for further use or recycling via valves  28 ,  62 . Also, conduit  24  may be flush cleaned, as is customary between batches, even of the same formula composition, by opening valve  52 , flush/drain valve  44 , and valve  60 . Conduit  24  may subsequently be drained by closing valve  52  and opening drain valve  56 . 
     Preferably, vessel selection valve  10  is mounted at an elevation below vessels  12 ,  14  such that all runs of conduits  24 ,  30  have no reverse-direction bends and, therefore, no bubble traps. Following flush cleaning as described above, a conduit  24 ,  30  may be drained of flush water by opening the appropriate valve  60 ,  62 , the appropriate flush/drain valve  44 ,  46 , and drain valve  56 . As a conduit drains, it fills with air. The just-mentioned valves are then closed, and the appropriate vessel isolation valve  22 ,  28  for the fresh vessel  12 ,  14  is opened, preferably at a signal from computer  58  which is timed to occur near the expiration of the then-flowing batch from the other vessel  12 ,  14 . In the manufacture of photographic products, it is considered good practice to fill the new conduit only a short time prior to changeover. When a particular isolation valve  22 ,  28  is opened, the air in the respective conduit  24 ,  30  is displaced by fresh composition, the displaced air bubbling up through the isolation valve  22 ,  28  and being expelled through the vessel  12 ,  14  to atmosphere. Fresh, bubble-free composition is then in place in the chamber of the appropriate switch valve  40 ,  42 , ready for changeover, and no usable composition is wasted to the drain. 
     Each vessel  12 ,  14  includes a respective level sensor  64 ,  66 . The respective outlet conduit  24 ,  30  is prepared in advance of the expiration of the opposite outlet conduit  24 ,  30  such that line preparation is not a factor in vessel switching. Because vessel-level sensors  64 ,  66  are not used in the process, except to determine when to prepare the new outlet conduit  24 ,  30 , errors in such sensors in the range of ±5-10% errors are not significant. 
     The embodiment of the invention, as described herein, comprises apparatus and method for changing between two alternating vessels. However, those skilled in the art will recognize that an arrangement involving three of more containers can benefit from the invention. The only requirements for using the present invention with three or more vessels are that each vessel be provided with an independent outlet conduit, conductivity sensor, and switch valve in the vessel selection valve. 
     The method of the present invention not only fully utilizes vessel contents, but also fully utilizes line contents from each vessel  12 ,  14  to the vessel selection valve  10 . There are no flow perturbations associated with the method. No air is introduced into the downstream process, which would subsequently have to be removed. 
     Vessel selection valve  10  is schematically depicted as a single multiport valve. While a single multiport valve is preferred, those skilled in the art will recognize that vessel selection valve  10  can comprise a plurality of interconnected individual two and three way valves, or a combination of multiport valve(s) and two and three way valves. 
     From the foregoing, it will be seen that this invention is one well adapted to obtain all of the ends and objects hereinabove set forth together with other advantages which are apparent and which are inherent to the apparatus. 
     It will be understood that certain features and subcombinations are of utility and may be employed with reference to other features and subcombinations. This is contemplated by and is within the scope of the claims. 
     As many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth and shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense. 
     PARTS LIST 
       10  vessel selection valve 
       12  first supply vessel 
       14  second supply vessel 
       16  first quantity 
       18  second quantity 
       20  outlet 
       22  isolation valve 
       24  outlet conduit 
       26  outlet 
       28  isolation valve 
       30  outlet conduit 
       32  first conductivity sensor 
       34  second conductivity sensor 
       36  delivery conduit 
       40  Valve 
       41  flow path gate 
       42  valve 
       43  flow path gate 
       44  flush/drain valves 
       45  flow path gate 
       46  flush/drain valves 
       47  flow path gate 
       48  flush/purge valve 
       49  flow path gate 
       50  utility supply pipe 
       52  control valve 
       54  drain line 
       56  valve 
       58  process control computer or programmable logic controller 
       60  valve 
       62  valve 
       64  respective level sensor 
       66  respective level sensor