Abstract:
Wear on the sealing surface of mechanical seals may be monitored by placing a sensor or conductor on the seal, at a position opposed to the sealing surface, such as on the back or opposite side of the sealing surface. Mechanical seals require the application of continuous external pressure to maintain fluid tight sealing integrity. Wear on the sealing surface may be monitored by measuring the amount of movement of the seal away from a predetermined starting point.

Description:
FIELD OF THE INVENTION 
     This invention relates to seals which are positioned between two members to maintain a fluid tight sealing relationship between the two members and the seal. More particularly, this invention relates to mechanical seals of the type in which the seal maintains a fluid tight sealing relationship in conjunction with continuous external pressure applied to the seal. 
     BACKGROUND OF THE INVENTION 
     Machinery often includes members which move with respect to each other. Bearings have long been used to facilitate such movement. Bearings may be used to aid in transmitting forces from one member to another and to reduce friction. Rolling elements may often be used in bearings to reduce friction, such as in ball bearings. Bearings may often contain lubricant to reduce the friction created within the bearing. 
     Leakage of fluid from between mechanical parts has long been a problem. Contamination of the bearings or seals with foreign material such as dirt has also long been a problem. Both loss of fluid and contamination may lead to increased friction and wear within the structure, damaging the structure and perhaps damaging the machinery. 
     Seals composed of a soft polymeric material may be used with bearings to prevent lubricant leakage and contamination. The polymeric material contacts a moving surface in the bearing and provides a seal. Friction between the polymeric material and the moving surface gradually wears the surface of the polymeric material away. If the polymeric material becomes too worn, it may no longer form a fluid tight seal against the moving surface, and fluid leakage from the bearing and contamination of the bearing may again become problems. 
     In other instances, metal or polymeric seals may be placed between moving parts. This arrangement may be particularly useful when the movement is a reciprocal, rotating or oscillating movement. Non-limiting illustrations of such seals include a piston and cylinder structure, hatches, air locks, doors, covers, lids and caps. With regard to such seals as found in hatches, air locks, doors, covers, lids and caps, the interfitting members may be said to reciprocate with respect to each other, in the sense of opening and closing with regard to each other. However, when a seal is in fluid tight sealing relationship between such interfitting members, the seal may more properly be termed a static or stationary seal, since the interfitting members and the interposed seal are all stationary with regard to each other while the integrity of the sealing relationship is maintained. In certain types of members which are reciprocating with respect to each other, the seal may be made so that it always makes contact with the piston connecting rod and the connecting rod is electrically insulated from the piston barrel. Sealing integrity or effectiveness may also be compromised by cracking, breaking, loss of flexibility or deterioration due to such conditions as repeated flexing, bending and/or compression, or length of contact with various fluids or container contents. 
     Various methods have been developed to prevent or detect leakage past a seal. As shown in U.S. Pat. No. 4,761,023, one method involves monitoring the pressure of the fluid on the high pressure side of the seal, with a loss of pressure indicating leakage past the seal. Alternatively, as shown in U.S. Pat. No. 4,290,611, the fluid pressure on the low pressure side of the seal may be monitored, with an increase in pressure indicating fluid leakage. A third method, such as that shown in U.S. Pat. No. 4,178,133, uses colored fluid and visual monitoring of leakage past a seal. Still other methods involve manual maintenance procedures, wherein seals are inspected or replaced on a regular basis. 
     These various methods to prevent or detect leakage past a seal have not proven satisfactory due to a number of problems. A problem with monitoring fluid pressure is that it is only effective to detect leakage of the fluid. Often this is too late to prevent damage. It is desired that the seal be replaced before leakage has begun. Similarly, visual monitoring of fluid leakage can only indicate that leakage has begun, not that leakage is about to begin. A problem with manual inspection of a seal is that it requires an inspector who is not only trained but also diligent. Often seals are not inspected merely due to neglect or lack of diligence. Another problem with manual inspection is that it may require the machinery to be stopped during the inspection, which can be inconvenient. Scheduled seal replacement also has problems. Seals may wear more or less quickly depending on operating conditions, and scheduled seal replacement may occur too early (before the seal needs to be replaced) or too late (after the seal starts leaking). 
     An excellent solution to these problems has been provided by the inventions disclosed in my U. S. Pat. No. 5,540,448, issued Jul. 30, 1996, entitled SEAL WITH ELECTRICAL CONDUCTOR WEAR INDICATOR and in my U.S. Pat. No. 5,246,235, issued Sep. 21, 1993, entitled SEAL WITH EMBEDDED WIRE. However, it has recently been determined that some seals may not be able to be monitored by installing a circuit on the sealing surface or embedding the circuit in the seal, in the manner described in my two previous patents. This may be because the design and/or the function of the particular seal or the sealing environment in which the seal is located will not permit any extraneous material at or on the sealing surface of the seal without compromising the fluid tight sealing integrity of the seal. In certain applications, the tolerances of the sealing surfaces may be measured in wave lengths of light. In other applications, the composition of the material of the seal body may not be compromised, without also compromising sealing integrity. Examples of seal environments in which sensors may not be placed on the sealing surface are found in apparatus such as mixers, doubled-ended pumps, vertical pumps, reactors, clarifiers, agitators, vacuum pumps and other similar equipment subject to moderate shaft runout and end play. 
     Generally speaking, such seals require the application of continuous external pressure to maintain fluid tight sealing integrity. External pressure may be applied by the use of springs, air pressure, packing nuts, hydraulic pressure, or any other similar external force. Such seals may be found in conjunction with parts which are moving, reciprocating and/or stationary with respect to each other. For convenience, a seal of this type will be referred to herein as a “mechanical seal.” 
     The present invention is particularly adapted to work with mechanical seals, which require the application of continuous external pressure to maintain fluid tight sealing integrity. According to the present invention, the wear on the sealing surface of these mechanical seals may be monitored by placing a sensor or conductor on the seal, at a position opposed to the sealing surface, such as on the back or opposite side of the sealing surface. The wear on the sealing surface may then be monitored by measuring the amount of movement of the seal away from a predetermined starting point. 
     SUMMARY OF THE INVENTION 
     The present invention provides a mechanical seal for recognizing excessive seal wear and the need for seal replacement before fluid leakage past the seal occurs. The seal is placed between members or surfaces which may be moving (e.g., rotating, reciprocating, etc.) or static with respect to each other and forms a seal with the moving member or between the static members to prevent fluid leakage. The seal is in engaged contact with and between the surfaces to prevent passage of fluid between either of the members and the seal. The seal is more readily deteriorated than at least one of the members. The seal is designed and adapted to maintain a fluid tight sealing relationship with the members in conjunction with externally applied pressure, so long as the seal has not deteriorated beyond a specified degree. An electrical conductor for determining seal wear is comprised of two mating parts. A first mating part is associated with the seal at a position opposed to the sealing surface. As the sealing surface of the seal wears, the seal will move away from the point of electrical contact between the two mating parts, thus moving the mating parts away from each other. Movement of the mating parts away from each other results in electrical discontinuity of the conductor. Thus, electrical discontinuity indicates deterioration of the sealing surface to the degree that the seal requires replacement. The electrical conductor may be, for example, a fuse or a pressure switch. 
     In another embodiment, an optical conductor for determining seal wear is comprised of two mating parts. A first mating part is associated with the seal at a position opposed to the sealing surface. As the sealing surface of the seal wears, the seal will move away from the point of optically conductive contact between two mating parts, thus moving the mating parts away from each other. Movement of the mating parts away from each other results in optical discontinuity of the conductor. Thus, optical discontinuity indicates deterioration of the sealing surface to the degree that the seal requires replacement. 
     According to another embodiment, a sonic sensor for determining seal wear is comprised of two mating parts. A first mating part is associated with the seal at a position opposed to the sealing surface. As the sealing surface of the seal wears, the seal will move away from the point of contact of the sonic sensor, thus moving the mating parts away from each other. Movement of the mating parts away from each other results in discontinuity of the sonic sensor and causes emission of a sonic signal. Thus, emission of a sonic signal indicates deterioration of the sealing means to the degree that the seal requires replacement. 
     The present invention provides a simple, cost effective device and method to detect deterioration of a mechanical seal disposed between moving or reciprocating surfaces, avoiding the problems of previous devices. Because the device detects deterioration of the seal body rather than fluid leakage, the seal may be replaced prior to leakage. The problems associated with waiting until after leakage has begun before replacing the seal may be avoided. The seal may be replaced in a timely manner before leakage begins. Because the present invention is based on optical, sonic or electrical continuity, an optical, sonic or electrical signal circuit may easily be incorporated with the invention. The signal circuit may indicate to an operator when a seal needs to be replaced, and there is no problem with diligence on the part of an inspector. Because wear of the seal body may be determined without manual inspection, problems with training inspectors and with missed or failed inspections are avoided. There is also no need to stop the equipment or machinery just to check if the seal needs to be replaced. Because the present invention monitors the actual amount of deterioration on the seal body, there is no problem with early or late replacement of the seal based on a scheduled replacement program. Replacement of the seal of the present invention may be based on the actual life of the seal body in operation, not on an average seal body life. Seals may be replaced less often and cost is reduced. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a cut-away view of a mechanical seal in position within a housing. 
     FIG. 2 is an enlargement of the mechanical seal of FIG. 1 with an electrical conductor, illustrated as a fuse, for determining seal wear. 
     FIG. 3 is an enlargement of the mechanical seal of FIG. 1, with a pressure switch as the electrical conductor for determining seal wear. 
     FIG. 4 is an enlargement of the mechanical seal of FIG. 1, with an optical sensor and optical fiber for determining seal wear. 
     FIG. 5 is an enlargement of the mechanical seal of FIG. 1, with a sonic sensor for determining seal wear. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a typical mechanical seal  10  in place against a shaft collar  12  and around a shaft  14 . The shaft  14  may rotate or oscillate. The mechanical seal  10  may be maintained in position against the shaft collar  12  by means of spring  16 . The mechanical seal body  10 , shaft collar  12  and shaft  14  may be contained within housing  18 , and may retain a fluid tight seal with shaft collar  12 , shaft  14  and housing  18 . Nut  20  may secure shaft  14  within housing  18 , while providing additional external pressure to maintain the fluid tight seal of the mechanical seal body  10  with shaft collar  12 , shaft  14  and housing  18 . 
     According to the present invention, a mechanical seal, of the type illustrated with reference to FIG. 1, is provided with a sensor for indicating when the sealing surface has deteriorated to the extent that the seal requires replacement but prior to the extent that the seal begins to leak or allow passage of fluid. FIG. 2 is an enlargement of the mechanical seal  10 , seen in FIG. 1, with like numbers indicating like parts throughout. The seal body  10  is in engaged contact with the shaft collar  12  and the shaft  14 , to prevent passage of fluid between either of these members and the seal body  10 . The seal body  10  may be composed of a polymeric material or other suitable substance and should be a dielectric or an electrical insulator. The material of the seal body  10  is more easily deteriorated than that of the shaft collar  12  or the shaft  14 , so that friction preferentially wears the sealing surface  22  of the seal  10  rather than the shaft collar  12  or the shaft  14 . The sealing surface  22  of the seal body  10  may gradually wear away due to friction between the outer surface of the shaft  14  and the confronting surface of the shaft collar  12 . As discussed above with reference to FIG. 1, the seal body  10  maintains a fluid tight sealing relationship with the shaft collar  12  and the shaft  14  in conjunction with externally applied pressure of the nut (not visible in FIG.  2 ), as long as the sealing surface  22  of the seal body  10  has not deteriorated beyond a specified degree. The surface of the seal body  10  opposite the sealing surface  22  may be treated to render it electrically conductive. This may be done by applying or coating an electrically conductive layer  24 . A pair of electrically conductive leads  26  extend outside of the housing  18  and form an electrically conductive circuit with the electrically conductive layer  24  on the seal body  10  by means of a fuse  28 . As the sealing surface  22  of the seal body  10  begins to wear from contact with the shaft collar  12 , the seal body  10  will begin to move gradually toward the shaft collar  12 , straining and finally disengaging the contact of the fuse  28  and breaking the electrical circuit. The leads  26  may be connected, external to the housing  18 , to an exterior signal-type electrical circuit, not shown. The electrical circuit may signal to an operator that the seal body  10  requires replacement based on the absence of electrical continuity through the circuit. A suitable electrical signal device is described in my U. S. Pat. Nos. 5,246,235 and 5,540,235, both of which are specifically incorporated herein by reference. 
     In the embodiment shown in FIG. 3, the mechanical seal body  110  is likewise in engaged contact with the shaft collar  112  and the shaft  114  within housing  118 , as described above with reference to FIGS. 1 and 2. Here again, the surface of the seal body  110  opposite the sealing surface  122  may be treated to render it electrically conductive. This may be done by applying or coating an electrically conductive layer  124 . A pair of electrically conductive leads  126 , as described with reference to FIG. 2, extend outside of the housing  118 . The conductive leads  126  form an electrically conductive circuit with the electrically conductive layer  124  on the seal body  110  by means of a pressure switch  130 . As the sealing surface  122  of the seal body  110  begins to wear from contact with the shaft collar  112 , the seal body  110  will begin to move gradually toward the shaft collar  112 , finally disengaging contact with the pressure switch  130  and breaking the electrical circuit. The leads  126  may be connected, external to the housing  118 , to an exterior signal-type electrical circuit of the type described above with reference to FIG.  2 . 
     Another embodiment of the present invention is shown with reference to FIG.  4 . The mechanical seal body  210  is in engaged contact with the shaft collar  212  and the shaft  214  within housing  218 , as described above with reference to FIG.  1 . An optical sensor  232  is positioned in contact with the surface of the mechanical seal body  210  which is opposite to the sealing surface  222 . The optical sensor  232  is in optically continuous contact with an optical fiber  234  which extends outside of the housing  218 . As the sealing surface  222  of the seal body  210  begins to wear from contact with the shaft collar  212 , the seal  210  will begin to move gradually toward the shaft collar  212 , finally disengaging contact of the optical sensor  232  with the optical fiber  234  and severing the optically continuous contact. The optical fiber  234  may be connected, external to the housing  218 . Suitable optical fibers and corresponding sensing mechanism may be obtained from Banner Engineering Corporation of 9714 10th Avenue North, Mpls. Minn. 55441. The Handbook of Photoelectric Sensing, particularly at pages A-8 to A-11, describes such photoelectric sensing for use in other environments. A suitable device identified as Model LP510CU is disclosed. Information from the photosensing device to circuitry that evaluates the information and warns of excessive wear of the seal. 
     A fourth embodiment of the present invention is shown with reference to FIG.  5 . The mechanical seal body  310  is in engaged contact with the shaft collar  312  and the shaft  314  within housing  318 , as described above with reference to FIG. 1. A sonic sensor  336  is positioned in contact with the surface of the mechanical seal  310  which is opposite to the sealing surface  322 . The sonic sensor  336  is in optically continuous contact with a lead  338  which extends outside of the housing  318 . As the sealing surface  322  of the seal body  310  begins to wear from contact with the shaft collar  312 , the seal body  310  will begin to move gradually toward the shaft collar  312 , finally disengaging contact of the sonic sensor  336  with the lead  338  and severing the optically continuous contact. The lead  338  may be connected, external to the housing  318 , to suitable circuitry that evaluates the sensed information warning of excessive wear of the seal. Suitable equipment is described in the Handbook of Photoelectric Sensing at pages A-11, A-12, B-22 and B-23. A suitable sensor is the Banner Omini-Beam model. The optical sensor includes an emitter and a receiver for sensing. The sensor recognizes when the surface of the seal has changed since the beam returned to the receiver either due to intensity change or by the angle of reflection change. The sonic transducer can detect a change in the amount of time of reflection or the strength of the returning signal. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be make in form and detail without departing from the spirit and scope of the invention.