Abstract:
An integrated leak detection system for a rotating union provides improved leak detection and user convenience. In an embodiment of the invention, the system provides a leak detection sensor located within the union, and in an embodiment of the invention the detection sensor is located beyond a back-up seal system. In an embodiment of the invention, the sensor and processing electronics are integral to the union housing. In yet another embodiment of the invention, the detection sensor is substantially symmetrically configured to detect leakage into a protected area regardless of orientation of the union. In an embodiment of the invention, the fluid being conveyed is electrically conductive and the detection sensor comprises an electrical conductivity sensor.

Description:
TECHNICAL FIELD OF THE INVENTION 
   The present invention relates generally to the detection of leaks with respect to rotating unions, and more specifically, to an integrated system for detecting unwanted potentially harmful leakage out of the union. 
   BACKGROUND OF THE INVENTION 
   The invention pertains generally to leak detection with respect to rotating unions. As used herein, the term “rotating union” refers primarily to a mechanical device used to transfer fluid from a stationary source such as a pipe or hose into a rotating element such as a machine tool spindle or rotating drums found on printing presses and stock calendering machines. A rotating union typically comprises a stationary member, called the housing, that has an inlet port for receiving fluid under pressure, and a rotating member, called a rotor, that has a central passage with an outlet port for delivering fluid into a rotating component. One typical feature of such rotating unions is the ability when working properly to transfer fluid without significant leakage between the stationary and rotating portions. 
   Rotating unions are used in many industrial settings, including, for example, CNC machining centers, modern printing presses, and other similar industrial environments. The primary usage in such cases is to convey high pressure and/or high volume coolant for use by the process. Coolants used may be water, water-based, or otherwise. The unions and associated equipment taken together can comprise many high precision components such as gears, bearings, couplings, electronic components, etc. that are expensive and/or difficult to replace, and that may be subject to severe corrosion or electric damage if exposed to fluid leaking from the union. In applications wherein the conveyed fluid contains chemical additives, a spill or leakage may present a health risk to operating personnel as well as an environmental hazard. 
   There are a number of different types of rotating unions on the market. The two general categories are (1) seals that are permanently closed, and (2) so-called “pop-off” seals where the seal may be designed to automatically open the contact between the seal faces when the pressure of the conveyed fluid is absent. Both types are subject to seal wear and eventual failure. The latter type of seal has the advantage of no seal wear in the absence of fluid pressure, but typically exhibits a slight amount of leakage at every shut-down and start-up cycle, such as when automatic tool change occurs in CNC machining systems. For this reason, rotating unions typically incorporate a housing that surrounds the primary seal and one or more drain ports to evacuate the leaked fluid. In addition, rotating unions generally include a back-up seal system between the primary seal (i.e., the seal normally in contact with the conveyed fluid) and any area, such as a bearing chamber, that is to be kept dry. Typical back-up seal systems include one or more labyrinths, air curtains, and lip seals mounted in association with the rotating part of the union. The following U.S. patents describe various details of several types of rotating unions, and are incorporated herein by reference for all that they teach and disclose without exclusion of any portion thereof: U.S. Pat. Nos. 6,164,316; 5,669,636; 5,617,879; 4,976,282; 4,928,997; and 4,817,995. 
   Once leaked fluid breaches the back-up seal system, the types of damage discussed above often begin to occur. To avoid unnecessary damage, it has long been a goal of manufacturers and users of rotating unions to ensure to the extent possible that rotating unions do not allow excess leakage of fluid. The initial source of such leakage, when it occurs, is the internal seal that provides an interface between rotating (spindle, draw bar, hollow shaft, etc.) and stationary (pipe, tube, hose, etc.) parts while allowing the passage of fluid between the parts. In particular, leakage is typically due to gradual or catastrophic deterioration of this seal. Since, to date, there is no such seal that is not subject to at least eventual wear and replacement, it is important in general to promptly detect leakage within the rotating union when it occurs so that appropriate maintenance may be undertaken before consequential related damage occurs. 
   At the same time, it is also desirable to minimize the degree to which the leak detecting system gives “false alarms.” That is, if the leak detecting system triggers upon the detection of acceptable levels of leakage, such as may be present during ordinary operation for purposes of lubricating the rotating seal etc., then such system will likely be deactivated or desensitized by operating personnel. This, however, creates a strong risk of eventual undetected harmful leakage. 
   There have been certain attempts, none completely successful, to solve the aforementioned problems. For example, one type of leak detection system in use as of the date of filing of this application employs a calorimetric sensor situated between the primary seal and the back-up seal system. Other systems appear to employ as of the date of filing of this application a leakage sensor that analyzes the output of the leakage drain port. As will be appreciated from the following description, none of the known existing systems of leak detection provide the necessary level of safety that many embodiments of the present invention are able to provide. In addition, commercially available leakage detection systems are awkward in that their principles of operation and basic configurations force them to rely on extensive external equipment to sense leakage and/or process detection signals. 
   BRIEF SUMMARY OF THE INVENTION 
   Embodiments of the invention generally alleviate the aforementioned shortcomings and provide the user with an improved system for detecting leakage before consequential damage can occur. As noted above, there exist different types of rotating unions, including those having permanently closed seals as well as the pop-off type unions wherein the seals automatically open in the absence of fluid pressure. Such unions typically comprise a housing surrounding the primary seal. Finally, rotating unions generally also include a back-up seal system (e.g., one or more labyrinths, air curtains, and/or lip seals) between the primary seal and the area that is to be kept dry. Early and accurate detection of unwanted leakage within rotating unions is an urgent and unmet need of modern industry. 
   In an embodiment of the invention, the leak detection system comprises a leakage sensor element (or a multi-part array) located within the housing, wherein the sensor element array is angularly symmetric about the axis of rotation in the form of a ring (or substantial portion thereof) or other substantially symmetric sensor or sensor array. In this manner, the sensor in an embodiment of the invention is able to detect the leaked medium of interest if present in the protected area regardless of the orientation of the union during use. In a preferred embodiment of the invention, the back-up seal system is located between the sensor and primary seal system, although in an alternative embodiment of the invention the sensor may be located elsewhere. Although in an embodiment of the invention the fluid (typically a liquid although the invention is useful for other substances as well, such as gaseous or misted substances) is electrically conductive and the sensing element is an electrical conductivity sensor, such is not required in every embodiment of the invention. All references herein to conductivity refer to electrical conductivity. 
   One symmetric sensor element usable in an embodiment of the invention is a substantially complete ring of conducting material having an insulating coating with a number of gaps therein spaced generally uniformly, if not necessarily precisely uniformly, about the circumference of the ring. The insulating coating separates the ring of conducting material from the union housing. However, in the event of a leak, the leaked fluid can bridge the ring of conducting material to the union housing, completing a detection circuit. 
   When the rotating shaft (e.g., rotor) is supported within the housing by two or more bearing assemblies, the sensor element may be placed between the bearings according to an embodiment of the invention. In a further embodiment of the invention, the space between the bearings also comprises a filler assembly for directing leakage to the sensor element for detection. 
   In a further embodiment of the invention, the system includes a leakage sensor located within the housing to detect leakage of the cooling liquid, and also includes a visual indicator mounted on the housing and linked to the sensor to signal the user regarding the detected leakage. In a further embodiment of the invention, the system further comprises a second visual indicator mounted on the housing to indicate that the leakage sensor is operational. For example, the second visual indicator can indicate that the sensor is not powered due to power supply failure or failure of one or more connections. In a further embodiment of the invention, the leak detecting system includes a link to a remote indicator such as a light, an LED, or a computer generated visual display. 
   In a further embodiment of the invention, the leak detecting system includes a sensor processing module that is integral with the union housing. The sensor processing module produces an electrical signal to indicate the presence of leakage of the coolant within the housing at the location of the sensor element. In an embodiment of the invention, the sensor processing module resides in an encasement secured to the housing, In an alternative embodiment of the invention, the sensor processing module resides in a cavity within the housing itself. The sensor processing module provides one or more of the types of alerts described above in various embodiments of the invention. The sensor element configuration and arrangement may be dictated by designer preference, however, in an embodiment of the invention the sensor is as described above. 
   Further features, details, and advantages of embodiments of the invention will become apparent from the following description. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       FIG. 1  is a cross-sectional side view of a housing-supported rotating union having a leakage sensor on the dry side of a labyrinth back-up seal and having an integrated sensor processing module according to an embodiment of the invention; 
       FIG. 2  is a side view of a ring-shaped leakage sensor element according to an embodiment of the invention; 
       FIG. 3  is a cross-sectional side view of a bearing-supported, rotor-mounted rotating union having a leakage sensor on the dry side of a labyrinth back-up seal as in  FIG. 1  and having an integrated sensor processing module according to an alternative embodiment of the invention; 
       FIG. 4  is a cross-sectional side view of a bearingless rotating union having as back-up seals an air curtain and labyrinth, and having an integrated sensor processing module according to an embodiment of the invention; and 
       FIG. 5  is an electrical schematic diagram showing an exemplary sensor signal processing circuit according to an embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   As discussed above, rotating unions are susceptible to leakage due to seal failure. Such failure may be due to gradual wear or to more drastic erosion, such as may be caused by particulate contamination (e.g., machining chips) in the fluid being conveyed, excessive pressure in the conveyed fluid, extended rotation without adequate seal lubrication from a conveyed fluid, or other causes. The leakage poses a strong risk of damage to associated components and machinery such as gears, bearings, couplings, electronic components, etc. that may be expensive and/or difficult to replace, and in some cases the leakage may present a health risk to operating personnel as well. 
   Existing rotating union leakage detection systems attempt to provide a warning of leakage to prevent the consequential damage that leakage can cause, however, no solution to date has effectively overcome the many problems inherent in such systems. Existing leak detection systems that provide a conductivity sensor associated with the housing drain line as described above, for example, exhibit problems with orientation-dependence and sensitivity. In particular, such systems tend to trigger too frequently due to normal allowable leakage from the rotating seal. There will almost always be a small amount of leakage even during normal operation, and this aids in lubricating the seal faces. This type of leakage does not pose a risk of damage as described above, and by being triggered by this type of leakage, the system often forces users to lower the system sensitivity. However, this raises the risk that the sensitivity will now be too low to detect abnormal leakage, i.e., leakage of an amount that may result in damage. With respect to orientation-dependent operation, such systems may malfunction, i.e., fail to detect substantial leakage, if the drain line is pointed upward, since typically the drain line is operated via gravity. 
   Other solutions exhibit similar problems. For example, a traditional calorimetric sensor installed within the housing adjacent to the primary seal will miss detection of fluid that falls past the sensor (and then out of the drain line or into the bearings or other machinery) if the union is used at a certain orientation. In addition, since the sensor is installed right next to the primary seal, it poses, however to a lesser degree compared to the drain line sensors, the opposite risk of also triggering on the detection of normal leakage and causing user interference or indifference. In addition, to the extent that this type of system is useful at all, it will only operate practically in the environment of a permanently closed seals rather than a pop-off seal. 
   The leak detecting system provided in various embodiments of the invention alleviates the disadvantages of existing systems. In particular, as will be described, in an embodiment of the invention, the sensor is designed and configured to provide orientation independent operation and to detect leakage directly in the area of interest without triggering on normal incidental leakage. Moreover, embodiments of the invention provide a unitary rotating union with integrated leak detection sensor and processing. 
     FIG. 1  is a cross-sectional side view of a housing-supported rotating union having a leakage sensor on the dry side of a labyrinth back-up seal and having an integrated sensor processing module in an attached secondary housing according to an embodiment of the invention. In greater detail, the rotating union  1  comprises a rotor  3  supported within a housing  5  for rotation with respect to the housing  5 . In the illustrated example, the rotor  3  is supported within the housing  5  by a pair of ball bearing assemblies  27 ,  29 . Although these assemblies  27 ,  29  are illustrated as comprising ball bearings, it will be appreciated that other types of bearing such as needle bearings, thrust bearings, etc. may be used additionally or alternatively. Moreover, although only two such assemblies  27 ,  29  are illustrated, it will be appreciated that the number and type of bearing assemblies will be controlled by manufacturer preference and intended use environment. 
   The rotor  3  comprises an internal passage  7  for conducting a liquid, such as a coolant, through the rotor  3 . The rotor  3  has a terminal end  9  within the housing  5  that supports an annular rotating seal  11 . The rotating seal  11  is affixed to the terminal end  9 , and coaxially abuts a stationary annular seal  13  that is fixed to a stationary conduit  15  having therein a passage  17 . The rotating seal  11  and stationary seal  13  seal against each other during normal operation such that a liquid can pass through the assembly, i.e., between the first  7  and second  17  passages without leaking substantially into the annular space  19  surrounding the seals  11 ,  13 . Herein, the combination of the two annular seals  11 ,  13  will sometimes be referred to as the “primary” seal. 
   As discussed, a rotating union may experience some level of “normal” leakage during operation and during the cycling of fluid pressure, such as during tool changes and as a result of normal seal lubrication. As such, the rotating union  1  as illustrated also comprises a secondary or “back-up” seal system  21 . In the illustrated embodiment, the back-up seal system comprises a labyrinth. However, those of skill in the art will appreciate that there are a number of such seal systems usable in embodiments of the invention, including labyrinths (also known as slingers), air curtains, lip seals, etc. The purpose of the back-up seal system  21  is to protect the dry side  23  of the system, where leakage is not desired or normally expected, from the potentially “wet” side  19  of the system, where normal leakage can be expected. 
   According to an embodiment of the invention as illustrated in  FIG. 1 , the rotating union  1  also comprises a leak detecting system comprising a sensor element  25  located within the housing  5 . The sensor element  25  is illustrated in cross-section as a ring, which will be discussed in greater detail with reference to  FIG. 2 . As illustrated, in a preferred embodiment of the invention the sensor  25  is located on the dry side  23  of the back-up seal system  21  and between the nearest bearing assembly  29  and the back-up seal system  21 . In this configuration, the sensor  25  is able to sense leakage at the earliest opportunity before it reaches the bearing assemblies  27 ,  29 . However, in alternative embodiments of the invention, the sensor  25  is located elsewhere, such as, without limitation, between the bearing assemblies  27 ,  29  or closer to the back-up seal system  21 . 
   It should be noted that the ring sensor  25  has an inner conductor that is separated from direct contact with the housing  5  by an outer layer. As will be appreciated by reference to  FIG. 2 , the sensor is configured to detect leakage, if present, in a symmetric manner, i.e., circularly symmetric or at a plurality of points at substantially angularly symmetric positions surrounding the rotor. 
   The leak detecting system also comprising an electrical conduit  31  connected to the sensor element  25  for carrying a sense signal indicating detected leakage to a sense signal processing module  33 . Although the sense signal processing module  33  is illustrated as externally integrated with the housing  5  via attached encasement  6 , it will be appreciated from the remainder of this description that the sense signal processing module  33  may also be integrated internally to the housing  5  in an embodiment of the invention. The sense signal processing module  33 , which will be discussed in greater detail with reference to  FIG. 5 , interfaces with the sensor  25  and provides an output indicating whether leakage has been detected. In the illustrated example, an LED  35  provides the leak detection output. In an embodiment of the invention, the LED  35  lights either continuously or intermittently when leakage is detected. Although the color of the LED  35  is not critical, in an embodiment of the invention the LED  35  is of a red color. 
   In an embodiment of the invention, a second LED  37  is provided by the module  33  to indicate whether the module  33  is properly powered. The module  33  may be either remotely or locally powered, and in either case, a power interruption may occur due to a connection or wiring fault or a failure of the power source. The power indicator  37  is especially desirable in an embodiment wherein the leak detection signal is a light on, since in this case, the lack of a light due to power failure might otherwise appear to signal a lack of leakage. Although the color of the LED  37  is similarly not critical, in an embodiment of the invention, the LED  37  is of a green color. 
   In an embodiment of the invention, the signal processing module  33  also comprises an external conduit  39 . The illustrated example includes three wires  41 , and the purpose of these wires  41  in an embodiment of the invention will be described in greater detail later. In general, external connections may be desired for providing power and for remote signaling of leakage. In an embodiment of the invention, the signal processing module  33  also additionally or alternatively provides a wireless link for communicating with remote computing devices, for example, to report status and/or send alarm indications. 
     FIG. 2  is a side view of a ring-shaped leakage sensor according to an embodiment of the invention. The sensor  201  comprises ring  203  of conductive material. The ring  203  is shown as open in one location  205  to facilitate handling, such as during installation, and to ease forming, but such a gap  205  is not required. The sensor  201  further comprises an insulating sheath  207 . In an embodiment of the invention, the insulating sheath  207  is provided with a number of gaps  209  that expose the inner conductor  203 . In the illustrated embodiment of the invention, the insulating sheath  207  is formed by wrapping a ribbon of insulating material about the conductor  205  in a spiral manner, leaving the gaps  209  uncovered. In an alternative embodiment of the invention, the insulating sheath  207  is formed via a series of beads or cylinders of insulating material. In a further embodiment of the invention, the insulating sheath  207  is formed via a tube of insulating material having openings cut therein. 
   The sensor  201  has a lead  211  attached thereto for connecting the sensor  201  electrically to the sense signal processing module  33 . Since the sensor  201  functions by sensing an electrical current between the sensor conductor  203  and the union housing, the lead  211  is preferably insulated so that it cannot make contact with the housing, as this would result in a false signal. 
   In an embodiment of the invention, the sensor  201  ( 25 ) is arranged within the union housing  5  as shown in  FIG. 1 , i.e., in an encircling arrangement with rotor  3 . When installed, the conductor  203  is not physically in contact with the material of the housing  5 . The housing is held at electrical ground in an embodiment of the invention and a positive potential is applied to the conductor  203  via the lead  211 . During normal operation, no current flows in the lead  211  since there is no path to ground. However, when a substantial leak of conductive fluid occurs and the leakage migrates or flows to the vicinity of the sensor  201 , the leaked fluid will form a bridge between the conductor  203  and the material of the housing  5 . In this situation, a current will flow in the lead  211  due to the short circuit and the potential difference between the conductor  203  and the housing  5 . This current is used, as will be discussed in greater detail below, to cause a leakage signal to be emitted by the sense signal processing module  33  via LED  35 . 
   Although the sensor  201  is illustrated in  FIG. 2  as a ring covered intermittently by insulation, it will be appreciated that other substantially symmetric sensors or sensor arrays are possible. For example, the sensor  201  may be replaced in an embodiment of the invention by a symmetric array of individual conductivity sensors. For example, a hexagonal or pentagonal array of individual sensors lying in the same plane as the ring  203  in the former embodiment of the invention may be used. Each individual sensor is preferably similarly configured (i.e., a partially insulated conductor held at a predetermined distance from the housing  5 ) to sense an increase in conductivity between the sensor and the housing  5 . In a further embodiment of the invention, the individual sensors of the array are connected in parallel to the lead  211  such that a current flow caused by a short of any one sensor to the housing  5  will cause a leakage signal to be emitted by the sense signal processing module  33  via LED  35 . 
   Before moving to a discussion of  FIG. 3 , it should be noted that  FIGS. 1 ,  3 , and  4  show different types of rotating unions. In particular, the union of  FIG. 1  is configured to be mounted to the associated machine (not shown) via the union housing  5 . The union in  FIG. 3  is configured to be mounted to the machine (not shown) via a threaded rotor, and both types of union are popular in the market.  FIG. 4  illustrates a rotating union wherein the back-up seal system includes both an air curtain and a labyrinth. The illustration of these different types of rotating unions is intended to illustrate an array of example environments, but is not intended to imply that the features described with respect to any of these figures is limited to use with the type of union shown in that figure. 
   Referring to  FIG. 3  more specifically now, this figure is a cross-sectional side view of a bearing-supported rotating union having a leakage sensor on the dry side of a labyrinth back-up seal as in  FIG. 1 , and having a sensor configuration according to an alternative embodiment of the invention. As with the union  1  of  FIG. 1 , the union  301  comprises a rotor  303  supported within housing  305  for rotation with respect to the housing  305  via a pair of ball bearing assemblies  327 ,  329 . The other basic elements of the union are also similar to those of  FIG. 1  and are labeled with like numbers to include: the internal passage  307 , terminal end  309 , rotating seal  311 , stationary seal  313 , stationary conduit  315 , passage  317 , annular space  319  surrounding the seals, back-up seal system  321 , sensor element  325 , electrical conduit  331  connected to the sensor element  325  for carrying a sense signal to a sense signal processing module  333 , LED  335 , second LED  337 , and external conduit  339 . The basic arrangement and functions of these elements is as described above. 
   In addition to these commonalities, there are several differences illustrated in  FIG. 3  that should be noted. In  FIG. 1 , the sensor  25  is shown as being located between the nearest bearing assembly  29  ( 329 ) and the back-up seal system  21  ( 321 ). However, in the alternative embodiment of the invention illustrated in  FIG. 3 , the sensor  325  is located instead between the bearing assemblies  327 ,  329 . In addition, the embodiment of the invention illustrated in  FIG. 3  includes a filler element  343  which partially fills the annular space between the bearing assemblies  327 ,  329 . This element  343  can be an annulus of material placed on the shaft  303  during installation of the bearing assemblies  327 ,  329 . The filler element  343  serves to direct leakage into contact with the sensor  325  to ensure detection. 
     FIG. 4  is a cross-sectional side view of a bearingless rotating union illustrating features according to further embodiments of the invention. Although not critical to the invention, the back-up seal system of the union  401  of  FIG. 4  comprises both a labyrinth  421  and an air curtain  422  to illustrate the variety of environments in which embodiments of the invention may be used. This combination shows one manner of combining two styles of barrier for additional leak protection. 
   More importantly, the union  401  of  FIG. 4  illustrates an embodiment of the invention wherein an integrated sensor processing module  433  is used. In the illustrated embodiment of the invention, the module  433  is situated within the housing  405  in a cavity  406  formed therein. Although the location of the cavity is not critical and will depend upon individual component layout and locations for a particular union, the illustrated cavity  406  is shown opening to a surface  408  of the union  401  to facilitate installation, servicing, and visibility of LEDs  435 ,  437 . The module  433  and LEDs  435 ,  437  operate in an embodiment of the invention as described above with respect to  FIG. 1 . Moreover the sensor  425  ( 25 ) and conduit  431  ( 31 ) continue to operate as described above with respect to other embodiments of the invention. 
   Although the sensor  425  is illustrated on the “dry side” of the air curtain  422  in  FIG. 4 , this location is not critical. In an alternative embodiment of the invention, the sensor  425  is located between the air curtain  422  and the labyrinth  421 . In order to avoid further repetition, the remaining elements that  FIG. 4  shares with other figures or that do not pertain to the invention will not be specifically labeled or discussed again at this point. 
     FIG. 5  is an electrical schematic diagram showing an exemplary sensor signal processing circuit  550  according to an embodiment of the invention in conjunction with the sensor/housing environment  560 . In addition, the relationship of the aforementioned system to a machine environment  570  is shown. Beginning with the sensor/housing environment  550 , this environment is preferably as described above, using a sensor conductor  561  ( 25  in  FIG. 1 ) and housing  563  ( 5  in  FIG. 1 ) configured and located as illustrated in any of  FIGS. 1-4  or the accompanying descriptions. The illustrated gap  565  between the sensor conductor  561  and electrically conductive housing  563  is maintained by the intermittent insulator (not shown) on the sensor conductor  561  ( 203  in  FIG. 2 ) as described in  FIG. 2 . 
   The sensor signal processing circuit  550  comprises an amplifier  551  for receiving and amplifying a voltage signal resulting from the current flow in the conductor  561  when leakage bridges the conductor  561  to the housing  563 . The housing  563  is connected to ground  564 . The output  553  of the amplifier  551  is received by a solid state relay  555 . The relay  555  closes in response to the received input, connecting the input  557  of a leak-indicating LED  559  ( 35  in  FIG. 1 ) to the voltage supply line  562 . Since the output of the LED  559  is linked to ground  564 , the LED lights in these conditions. 
   The sensor signal processing circuit  550  comprises a power-indicating LED  566  ( 37  in  FIG. 1 ) for indicating to the operator that the circuit  550  is properly powered. The power-indicating LED  566  has its input  567  connected to the high voltage supply line  562  and its output  568  connected to ground  564 . Thus, if the circuit  550  is receiving power via external power  571  and ground  573  leads, the power-indicating LED  566  will be lit. It will be appreciated that elements  571 ,  573 , and  575  collectively correspond to element  41  in  FIG. 1 . Thus, if the power-indicating LED  566  is dark, the operator will be aware that the circuit  550  is not powered and cannot be relied upon for leak detection. 
   The machine environment  570  represents machinery associated with the rotating union comprising the sensor  561  and the sensor signal processing circuit  550 . For example, the machine environment  570  may comprise a mill, lathe, printing presses, or other industrial environment. Although the machine environment  570  is illustrated as the source of power for the sensor signal processing circuit  550 , such is not required. In addition, the sensor signal processing circuit  550  comprises, in an embodiment of the invention, an external link  575 . The external link  575  may communicate with the machine environment  570  as shown in order to affect the machine operation (e.g., stop, start, or modify the machine operation in response to a signal from the sensor signal processing circuit  550 ) and/or to provide a remote leak indication at the machine environment  570 , such as via a warning light, LED, or computer screen notification. In an embodiment of the invention the link  575  is wireless. Although the external link  575  is shown to carry the same signal as that driving the LED  559 , in an alternative embodiment of the invention, the external link is provided with a signal other than that. For example, the signal on the external link  575  may be pulsed, inverted, or encoded. 
   In an alternative embodiment of the invention, a remote power indicator is provided so that the operator can remotely ascertain that the sensor signal processing circuit  550  is properly powered. In a further embodiment of the invention, an audible leakage warning is emitted by the sensor signal processing circuit  550  and/or remotely, such as at machine environment  570 . 
   Although embodiments of the invention have been described with reference to a conductivity sensor that senses conductivity between a sensor element and a conductive housing, it will be appreciated that in an embodiment of the invention, the housing may be non-conductive. In this embodiment of the invention, a second conductive element may be provided in proximity to the sensor element such that any leakage will bridge the gap between the two resulting in current flow. The second conductive element may be of any suitable configuration, including that shown in  FIG. 4  with respect to the sensor element. In an embodiment of the invention wherein the sensor and the second conductive element are ring-shaped, they may situated coaxially within the housing or otherwise. 
   Although the invention has been described in the context of a liquid coolant as the fluid being conveyed through the rotating union, it will be appreciated that the invention pertains to other fluids and semi-fluids (such as gaseous or misted substances) regardless of whether they serve a coolant function. It will be appreciated that a new and useful system for detecting leakage within a rotating union has been described herein. Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the claimed invention. Variations of these preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context. 
   The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.