Abstract:
A method and apparatus for monitoring the quality of steam used in a process is disclosed. A pressure and temperature sensor is exposed to the steam on either side of an inline pressure drop device such as an orifice or pressure reducing valve. The measurements are transmitted to a controller that calculates the steam quality percentage or superheat value. An alarm is issued if the steam quality is beyond a predetermined tolerance.

Description:
CROSS REFERENCE TO CO-PENDING APPLICATION 
     This application is a continuation in part of U.S. patent application Ser. No. 12/498,160 filed on Jul. 6, 2009 by Walter T. Deacon, titled “Method and System for Monitoring Steam Quality,” that issued as U.S. Pat. No. 8,325,049 on Dec. 4, 2012, the contents of which are herein incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to steam control systems, and more particularly to a method and system for monitoring the quality and/or purity of steam used in a decontamination system such as a steam sterilizer or a steam injection device for pharmaceutical or food processing. 
     BACKGROUND OF THE INVENTION 
     Steam has been used in decontamination systems, such as sterilizers or autoclaves located in hospitals, manufacturing settings, and laboratories to sterilize devices such as medical and dental instruments, laboratory instruments, production equipment, manufactured products, and other articles. Steam is also used for vulcanizing, cooking, melting, humidifying, and drying in process industries such as pharmaceuticals, foods, paper, automotive, and printing, etc. 
     Steam purity and steam quality are important properties of steam that will affect the efficacy of a decontamination process, such as steam sterilization. Steam purity is an expression of the quantity of non-water components (vaporous contamination) carried in the steam. Steam quality refers to the quantity of moisture present in the steam. If there is no moisture (i.e., no liquid water), then the steam is of 100% quality. Accordingly, “pure” steam has a liquid water content of 0%. It should be appreciated that steam quality relates to steam purity because liquid droplets in steam may contain dissolved solids that may be transmitted to the process. 
     In many healthcare and pharmaceutical applications, the minimum acceptable steam quality for a steam sterilizer or autoclave is 95%. If steam quality is below 95%, then “wet packs” (i.e., moisture droplets) may develop on articles after completion of a sterilization cycle. Consequently, reprocessing will be required, and/or batches of product may have to be discarded. 
     Aside from decontamination, many processes require steam that is dry and contains no superheat. Printing presses use steam to control static electricity and to precisely control the drying and shrinkage of the paper. Poor steam quality can upset the control. Wet steam can even cause the paper to tear, ruining a press run. Dairy applications inject steam directly into milk during pasteurization. Wet steam can carry contaminants and add too much water during the injection process. 
     A steam generator used to vaporize water can introduce contaminants into the steam, thereby reducing steam purity. For example, where the steam generator is a boiler, boiler chemicals can be introduced into the steam during priming or foaming of the boiler. These contaminants may cause corrosion or staining of the product or decontamination device (e.g., steam sterilizer) or articles to be processed by exposure to the steam. 
     Attempts have been made to check the quality of steam in an effort to reduce some of the problems caused by steam with an inappropriate quality. U.S. Pat. No. 4,561,785 issued to Long claims to disclose a “method and apparatus for determining the quality of typical steam used in steam flooding for secondary recovery of petroleum” by continuously leaking a portion of high-pressure steam out of the system. Although leaking hot steam out of a system may be acceptable for oil well fields, there are many situations where such a hazard is not acceptable. 
     Other methods of analysis have been attempted such as in U.S. Pat. No. 4,547,078 issued to Long where steam quality is measured at a specific point in time by “obtaining a sample of the liquid component of steam and determining the quality of steam in a vessel or the like, such as steam flowing in a line used for steam injection in an oil well. The steam quality is determined by the known method of comparing the concentrations of dissolved solids in the liquid sample and the feedwater.” 
     These steam quality measurements are time consuming, inaccurate, and can expose operators to potentially unsafe conditions including scalding heat and deafening noise. Moreover, other approaches to measuring steam quality do not provide advanced warnings of problems with quality of the steam used in a process. 
     SUMMARY OF THE INVENTION 
     A method and system for monitoring the quality steam used in a process is disclosed. A pressure and temperature sensor is exposed to the steam on either side of a pressure drop device such as an orifice or pressure reducing valve. Pressure and temperature sensors are exposed to the steam on either side of a pressure drop. The measurements are transmitted to a controller that continuously calculates the steam quality or superheat value and issues an alarm if the steam quality is beyond a predetermined tolerance. 
     The foregoing summary does not limit the invention, which is defined by the attached claims. Similarly, neither the Title nor the Abstract is to be taken as limiting in any way the scope of the disclosed invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Each of the drawing figures now described shows an exemplary embodiment of the present invention. 
         FIG. 1  is a side view of a steam pressure reducing station with inline steam testing of steam being delivered to a sterilization chamber. 
         FIG. 2  is a schematic diagram of a sensor circuit for inline continuous monitoring of steam quality and superheat, the circuit having both pressure and temperature sensors upstream and downstream of a pressure reducing device. 
         FIG. 3  is a schematic diagram of a sensor circuit for monitoring steam quality and superheat by sensing both the pressure and temperature upstream and downstream of a process control valve. 
         FIG. 4  is a schematic diagram of a sensor circuit for monitoring steam quality and superheat by sensing both the pressure and temperature upstream and downstream of an orifice plate. 
         FIG. 5  shows a steam quality calculator continuously monitoring the quality of steam passing through a pressure reducing station 
         FIG. 6  is a schematic diagram of a steam system testing the steam quality of high, medium, and low pressure steam. 
         FIG. 7  is a diagram showing the temperature response of both high quality steam and low quality steam following heating by an inline steam heater. 
         FIG. 8  is a diagram showing the temperature response of steam being repeatedly heated by an inline steam heater. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings wherein the showings are for the purpose of illustrating the preferred embodiments.  FIG. 1  shows a steam system  10  having a first temperature sensor  300 , a first pressure sensor  400 , a second temperature sensor  500 , and a second pressure sensor  600  flanking an inline pressure reducing valve  22  for monitoring the purity and/or quality of steam used within the system. An “inline pressure reducer” is herein defined to be a mechanism structured to reduce the pressure of a gas to a pressure above atmospheric pressure, wherein the reduced pressure gas is provided to a device that performs a function other than gas analysis. In the illustrated embodiment, system  10  is a steam sterilization system for cleaning articles with steam. It should be understood that while a preferred embodiment of the present invention is described with reference to a steam sterilization system, it is contemplated that the present invention may be used in connection with other systems and facilities that utilize steam. 
     The steam sterilization system  10  generally comprises a vessel  30 , an outer jacket  40 , a steam generator  50 , a control unit  60 , and the first temperature sensor  300 . The vessel  30  defines a chamber  32  that may be any shape, but is preferably cylindrical or rectangular. Articles to be sterilized are placed into chamber  32  for exposure to steam. Steam is released from chamber  32  through an outlet conduit  16 . A steam outlet valve  26  controls the release of steam from the chamber  32 . 
     The outer jacket  40  surrounds vessel  30  and defines a steam region  42  between the vessel  30  and the outer jacket  40  for injection of steam. Like the vessel, the outer jacket  40  is preferably cylindrical or rectangular in shape. An input conduit  14  connects the steam region  42  with the chamber  32 . A steam control valve  24  regulates the flow of steam between the steam region  42  and the chamber  32 . 
     The steam generator  50  may produce steam through a variety of different heating methods. For example, the steam generator  50  may include a conventional natural gas boiler, an electrically powered resistive heating element, or the output of a high efficiency heat pump system. Water is supplied to a steam generator  50  through a water input conduit  18 . Steam produced by the steam generator  50  is supplied to the steam region  42  by a first conduit  12 . A sterilization valve  24  regulates the flow of steam into and out of the steam region  42 . Steam may be intermittently provided to the sterilization unit in order to allow operators to add or remove surgical instruments from the sterilization unit without being scalded by steam. While the steam is flowing to the sterilization unit, and when the steam is stagnant, the temperature and pressure sensors may continuously provide updates on the quality of the steam. In one embodiment of the system, steam is provided to the sterilization unit for at least five minutes (and preferably 20 minutes) before the sterilization unit is isolated from the steam. An inline steam heater  70  with an auxiliary temperature sensor  501  acts to slightly heat the low pressure steam and measures a temperature decrease in order to further test the steam quality before the steam is passed to the chamber  32 . In an alternate embodiment, the inline steam heater  70  may be positioned to use the low pressure temperature sensor  500  to record changes in temperature following a brief heating period by the inline steam heater  70 . 
     The control unit  60  is preferably a microprocessor or a microcontroller programmed on a computer readable medium to control operation of the system  10 . In this regard, the control unit  60  sends control signals to operate valves  22 ,  24  and  26 . 
     The temperature sensors ( 300  &amp;  500 ) and the pressure sensors ( 400  &amp;  600 ) may take the form of any suitable sensing device responsive to changes in the pressure and temperature of steam used within the system  10 . In  FIG. 1 , the temperature sensors are shown to be upstream of the pressure sensors, however temperature sensors may also be located downstream or parallel with the pressure sensors. Also, both pressure sensors may be between the temperature sensors and vice versa. Exemplary pressure sensors are shown flanking a pressure reducing reduction device  62  in  FIG. 2  and exemplary temperature sensors are shown around a process control valve  64  in  FIG. 3 . In  FIG. 4 , sensors are located upstream and downstream of an orifice plate  66 . In  FIGS. 2 and 3 , all of the steam mixture in the high pressure region passes through the inline pressure reducers. In the illustrated example, steam heater  70  includes a resistive heating element coupled with a thermocouple that senses the temperature near the steam heater  70 . 
     The sensors  300 ,  400 ,  500 , and  600  allow the control unit  60  to continuously compute and display (a) an indicator showing whether the steam is superheated or saturated; and (b) a second indicator showing the degree of superheat or the percentage of steam quality, also known as the dryness fraction. 
     The sensors  300 ,  400 ,  500 , and  600  are preferably located near a dedicated pressure reducing valve  22 , but it is also contemplated that they could be located in alternative locations with steam pressure differentials such as the steam control valve  24 . Furthermore, it is contemplated that multiple control units could be included in system  10 , to allow for monitoring of steam purity and/or steam quality at several locations therein. Alternatively, one control unit could calculate steam quality at a plurality of different locations having a plurality of different steam pressures and temperatures. 
     Physical properties of steam are stored on a computer readable medium in the control unit  60 . Data tables or algorithms to compute steam quality and superheat are then used during the processing function to calculate steam quality and/or the degree of superheat. In this regard, it should be appreciated that the calculation required revolves around the measured pressure and temperature versus the known physical properties. Thus if 30 psia is measured by the pressure sensor downstream of the pressure reducing valve  22 , but only 240° F. temperature is indicated by the temperature sensor downstream, then the steam quality is below 100%. The resulting quality percentage can be displayed, communicated as an alarm condition, or otherwise used for control of the process. The introduction of impurities (i.e., vaporous contaminants) into the steam will cause both the upstream and downstream conditions of the steam to change. For example, the introduction of air or carbon dioxide will result in changes to the steam temperature, but not the steam pressure. The presence of condensed water in the steam will generally cause a decrease in upstream temperature but not always, since the water can be at the saturation temperature. The presence of liquid water upstream will cause a drop in the temperature downstream. Accordingly, the control unit  60  can be used to ascertain a measure of steam purity and/or steam quality. 
       FIG. 5  illustrates an example of a steam quality calculator  72  continuously monitoring the quality of steam passing through a pressure reducing station. Thermocouples  74  and diaphragm gauges  76  for measuring pressure are located upstream and downstream of an inline steam pressure reducer  78 . Isolation valves  80  are located upstream and downstream of both the pressure reducer and the pressure and temperature gauges. In the event that the steam quality drops below an acceptable threshold, the isolation valves may be utilized to stop the flow of steam through the pressure reducing station. A strainer  82  is located proximal to the pressure reducer  78  for removing steam condensate from the line. A bypass line  84  may be connected before and after the isolation valves to allow high pressure steam to be delivered downstream of the pressure reducer if desired. In the bypass line an isolation valve  86  selectively allows steam to flow through the isolation line. 
       FIG. 6  illustrates an example of a steam system delivering high pressure, medium pressure, and low pressure steam to three different devices requiring three separate pressures of steam. A steam generator  50  provides high pressure steam (80-100 psi) to a high pressure steam artery  90 . In passing through a inline process control valve  64  the pressure of the steam is reduced to a medium pressure (50-60 psi) in a medium pressure steam artery  92 . A second inline process control valve  64  further reduces the pressure of the steam being provided to a low pressure steam artery  94 , typically 10-15 psi. From the high pressure steam artery  90 , a portion of the steam is diverted through a inline pressure reducer  62  to a first pressure line  96  connecting to a high pressure device  104  such as a washing or ironing machine. A portion of the steam from the medium pressure line is also diverted, and passes through a pressure reducer  62  to a second pressure line  98  connecting to a medium pressure device  106  such as a cleaning device for a kitchen. Finally, steam from the low pressure artery  94  passes through a third pressure reducer  62  to a third pressure line  100 . Steam in the third pressure line is delivered to a low pressure steam device  108  such as a water heater, humidifier, or air handling unit coils. In addition to issuing warnings if the steam quality falls below a predetermined threshold, the steam system may control a desuperheating device to decrease the temperature of the steam if the temperature is above the steam saturation threshold for a given pressure. In an exemplary embodiment of the invention, all of the features shown in  FIG. 6  may be enclosed with a single hospital building. In another embodiment, the steam has a pressure of 50-100 psig in the high region pressure region that drops to 25-35 psig in the low pressure region. In another exemplary system, the steam pressure drops from 60 psig in the high pressure region to 30 psig in the low pressure region. 
     A control unit (not shown) monitors and stores data signals from the plurality of pressure and temperature sensors ( 300 ,  400 ,  500 , and  600 ) located upstream and downstream of the inline pressure reducers. Using the pressure and temperature data, the control unit is able to calculate the superheat and steam quality of the steam passing through the system. In the event that the steam quality provided to the devices falls below an acceptable range, the control unit  60  may close off the inline pressure reducers and isolate a selected number of devices. Alternatively, each pressure drop may have an independent analysis module with pressure sensors, temperature sensors, steam quality calculators, and alarms. The acceptable ranges of steam quality for the high-pressure device  104 , the medium pressure device  106 , and the low pressure device  108  may not be identical so the control unit  60  may only close off the inline pressure reducer to one of the devices. For example, the high pressure device may be a laundry unit with an acceptable steam quality ranging from 80% to 100% while the low pressure device  108  is a surgical instrument sterilizer with an acceptable steam quality range of 95% to 100%. In the event that the steam quality falls to 90%, the laundry unit would continue to operate normally while an alarm would sound for the sterilization unit. Alternatively, features at the pressure reducer directly upstream of the sterilization unit would act to close off the sterilization unit. In this example, the there is more than a 5% difference in the steam quality thresholds at which alarms sound for the laundry and surgical instrument sterilizer. 
     The steam analysis system may be an original part of the steam system, or the analysis system may be added after the steam system has been installed. When the analysis system is added to an already existing steam system, any existing pressure sensors, temperature sensors, pressure drops, data cables, and computer equipment may be utilized to reduce the cost of the steam analysis system. 
     The steam sterilization system  10  is operated by placing articles in the chamber  32  that is heated by pumping saturated steam from the steam generator  50  into the steam region  42 , via the first conduit  12 . After the steam region  42  is charged, saturated steam is injected into the chamber  32  via the input conduit  14 . During a decontamination cycle, sensors  300 - 600  monitor the steam being provided to the chamber  32 . In the event that it is determined that the steam does not comply with the required steam quality, then the control unit  60  may provide an audible and/or visual indicator to the operator. Furthermore, it may be necessary to take corrective action, including reprocessing the articles in the chamber. Data collected by the sensors  300 - 600  during decontamination cycles may be stored on a computer readable medium to provide historical data for verification of appropriate decontamination processing conditions. 
     At the end of a decontamination cycle, steam is pumped out of the chamber  32  via the outlet conduit  16 , and the chamber  32  is evacuated to a pressure below atmospheric pressure to remove moisture from the chamber  32  or on articles therein. Steam leaving the chamber  32  may be condensed, and may drain down to be recycled at the steam generator  50  via the water inlet conduit  18  if the chamber is located at a higher position than the steam generator. If the steam generator is located above the chamber, the condensate may be pumped up to the steam generator. 
     The system controller may continuously calculate the steam quality at a plurality of different locations in the system by utilizing an array of pressure and temperature monitors. Utilizing data gathered from monitoring the system and stored data relating to the properties of steam mixtures, the system is able to calculate the steam quality. For example, if the measured upstream temperature is 143.32° C., the measured upstream pressure is 200 psi, the measured downstream temperature is 121.11° C., and the measured downstream pressure is 85.273 psi, then the steam quality may be determined by:
 
 Q= 200 ×P   U +143.32 ×T   U +85.273 ×P   D +121.11 ×T   D   +K  
 
wherein Q is the steam quality upstream, P U  is the upstream pressure factor, T U  is the upstream temperature factor, P D  is the downstream temperature factor, T D  is the downstream temperature factor, and K is a constant stored on the system controller.
 
       FIG. 7  illustrates the measured output of a temperature sensor following a heating period  202  of high quality steam  204  and low quality steam  206 . In the measurement testing process, the steam is allowed to reach a lower stabilized temperature  208  and is heated during the heating period  202  until the steam stabilizes at a higher temperature. Heating is discontinued and the rate at which the steam decreases in temperature during a decay period  210  is measured. By comparing the rate of temperature drop to known response curves, the quality of the steam is determined. In general, high quality steam  204  will have a slower temperature drop than low quality steam  206 .  FIG. 8  illustrates the temperature response of sequential heating  212  of steam by an inline steam heater. Continual testing of the steam by the inline system allows for quick warnings and corrections when the steam quality goes out of compliance. If the rate of steam temperature drop exceeds a predetermined threshold, the control unit is configured to generate an alert notification and/or adjust a parameter of the steam generator. 
     While the principles of the invention have been shown and described in connection with specific embodiments, it is to be understood that such embodiments are by way of example and are not limiting. Consequently, variations and modifications commensurate with the above teachings, and with the skill and knowledge of the relevant art, are within the scope of the present invention. The embodiments described herein are intended to illustrate best modes known of practicing the invention and to enable others skilled in the art to utilize the invention in such, or other embodiments and with various modifications required by the particular application(s) or use(s) of the present invention. It is intended that the appended claims be construed to include alternative embodiments to the extent permitted by the prior art.