Abstract:
A seal suitable for a driveshaft of a pump for cryogenic media. The seal can operate without extraneous confining gas, runs frictionless and assures a long service life, which was unattainable so far. The seal is designed as a tandem axial face seal. A sliding ring has sliding faces on both sides with spiral-shaped grooves terminating at an outer periphery, and the sliding ring is fixedly mounted on a driveshaft. The sliding faces each is adjoined by a sliding ring. The sliding ring is tightly connected via a metal bellows with the housing cover on the pump side, and the sliding ring via the metal bellows with the housing cover on the motor side. With the tandem axial face seal, the pump process pressure is sealed with respect to atmospheric pressure, and only a small, controlled gas leakage occurs. The process gas remains 100% pure.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a tandem axial face seal for installation in pumps used for pumping cryogenic media, in order to seal a driveshaft against an interior of the pump. 
     2. Description of Prior Art 
     In what follows, supercooled liquids, starting approximately at −100° C., are understood to be cryogenic media, the components of air in the liquid state, for example, such as nitrogen (N 2 ) at lower than −196° C., oxygen (O 2 ) at lower than −183° C. and argon (Ar) at lower than −186° C., as well as hydrogen (H 2 ) at lower than −253° C. In technical terms, the above mentioned elements in liquid form are called liquid nitrogen (LiN), liquid oxygen (Lox) and liquid argon (LAr) and liquid hydrogen (LH2). Such supercooled liquids are produced on a large industrial scale when atmospheric air is split into its components by cooling and cleaning or, in the case of hydrogen, water is split into its components. The individual pure and liquid components are then stored in special cryogenic tanks under atmospheric pressure and are transported by trucks which are equipped with special cryogenic tanks. A portion of the cryogenic liquid, which is close to the boiling state, evaporates continuously because of a certain unavoidable heat input from the environment. Over a sufficiently long period of time the entire contents of the tank evaporate at unchanged temperatures and with an increasing pressure in the interior of the tank. While transferring cryogenic liquids, for example by pumping from one tank into another, or also when removing liquid, for example for using in an industrial process, it is continuously necessary to battle the undesired evaporation of the liquid. The more cryogenic liquid that evaporates, the more that must be considered a loss. The pumps are particular weak points during the transfer. Upon entering the pump, the conveyed liquid is close to a boiling pressure. Therefore a cryogenic pump must be constructed so that it pumps with a comparatively high suction pressure, and so that the pressure does not fall in the interior of the pump, because in case of miscellaneous underpressures the aspirated liquid would immediately evaporate. The cavitations created can cause the pump to run dry and become damaged. Heat flows continuously and unavoidably into the pump through the housing and the driveshaft of the pump, so that the supercooled medium can easily evaporate, in particular into the chamber of the motor side of the pump wheel, where only the approximate suction pressure of the pump in the pump medium prevails. 
     Therefore the seal which seals the rotating driveshaft against the pump interior is surrounded by gas, not by liquid. This is a dry-running seal. These seals are customarily known as labyrinth seals. The locally evaporated pump medium at the suction pressure pushes into the seal from the inside of the pump and tries to flow along the driveshaft in the direction of the motor. To minimize this leakage, a filtered confining gas at a slightly lower pressure, for example lower by 0.2 bar than the suction pressure of the pump, is pumped from the motor side of the shaft into the labyrinth seal for building up a counterpressure. In connection with a small cryogenic pump of approximately 40 kW output, approximately 15 standard m 3 , for example m 3  at atmospheric pressure, of nitrogen per hour are required as the confining gas, and with large pumps more than twice that amount is required, just to mention an order of magnitude. This method therefore has one disadvantage that it is necessary to employ a separate confining gas, namely preferably nitrogen, so that a complete unit with a tank, filter and pressure regulator is necessary. Secondly, the use of a confining gas has the inevitable result that the cryogenic process liquid to be pumped is contaminated by confining gas, even though only slightly. These contaminations are more and more objected to by the users and are no longer tolerated in some cases. 
     Axial face seals with a spiral groove surface have also been used as alternatives to labyrinth seals. On one of their seal ring surfaces, such seals have a number of flat depressions of only about half a hundredths of a millimeter, which lead outward in a spiral shape. If the seal ring with the spiral-shaped grooves rotates in such a rotating direction that the mouths extending obliquely with respect to the ring periphery point in the direction of rotation, then ambient gas enters into the grooves and a dynamic pressure is created between the sliding rings, which provides a permanent gas cushion between the sliding rings, so that these run with essentially no friction. However, it is still necessary to pump a confining gas into the chamber in which the seal is located in order to compensate the suction pressure of the pump. Thus this the disadvantages remain, namely the requirement for installations for making available, filtering and pumping the confining gas, as well as the contamination of the process liquid with confining gas. 
     SUMMARY OF THE INVENTION 
     It is one the object of this invention to produce a seal for pumps for cryogenic media which can operate without extraneous confining gas, so that contamination of the pumped medium is impossible, which runs essentially frictionless, and which achieves a long service life, which is unattainable so far. 
     This object is achieved with a seal for the driveshaft of a pump for pumping cryogenic liquids, which is designed as a tandem seal. A sliding ring, with sliding faces on both sides have spiral-shaped grooves terminating at the outer periphery, is fixedly mounted on the driveshaft. The sliding faces are each adjoined by a sliding ring, which on one side is tightly connected via a metal bellows with the pump housing to be sealed, and on the other side with the pump housing on the motor side. 
     The seal is shown in the drawings in a longitudinal section taken through the driveshaft of a cryogenic pump. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This invention will be described in detail in view of the drawings, and its function will be explained, wherein: 
     FIG. 1 shows a sliding ring in a perspective top view from a direction of its side, with a view of one sliding face, on which a partially cut open seal ring is seated; 
     FIG. 2 shows a cryogenic pump with a seal in accordance with this invention, in longitudinal section taken through the center of the driveshaft; and 
     FIG. 3 is a diagram for providing a seal in accordance with this invention, with gas from the pumped liquid for the gas cushion to be built up between the sealing faces. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The basis of the seal of this invention is a sliding ring  14  with flat grooves  29  cut in a spiral shape out of the sliding faces  15 ,  16  of the latter, as shown in FIG. 1, where the sliding ring  14  is shown from the direction of one side. The grooves  29  are only approximately half a hundredths of a millimeter deep. If the sliding ring  14  is turned on its other side, the spiral-shaped grooves  29  provided there would be arranged in a reversed direction. Thus, on the one face, which is shown, the grooves  29  extend outward in a clockwise direction toward a periphery of the sliding ring  14 , and exactly reversed on the opposite sliding ring side  15  in a counterclockwise direction outward toward the periphery of the sliding ring  14 . If the sliding ring  14  in the drawing rotates in a clockwise direction, the spiral-shaped flat grooves  29  arranged on each of the two sides of the sliding ring  14  extend with their respective mouths leading. If a seal ring  18  is placed against each of the sliding ring sides  14 ,  15 , and if the sliding ring  14  turns with a sufficient speed with the outer mouths of the grooves  29  leading, the outer mouths catch the ambient gas, channel it toward the center of the sliding ring  14  and form a gas cushion each between the sliding ring  14  and the seal ring  18 , so that mechanical friction is cancelled. Such seals are basically known. 
     FIG. 2 represents the pump with the essential parts of the novel seal in longitudinal section through the pump driveshaft. Thus the pump motor which is not shown is arranged on the left side, and the shaft extends toward the right and supports the pump wheel  2 , which rotates inside of the pump housing  3 . The aspirating port of the pump therefore is located on the front end  4  of the pump housing  3 . Towards the front, the pump wheel  2  has a worm wheel  5 , the outside of which forms a large screw thread, so to speak, so that an Archimedean screw  6  is formed. The corresponding spiral-shaped vane ends in a sharp edge at the front. During rotation, this edge cuts off cryogenic liquid and screws it in the direction toward the pump wheel  2 . The cryogenic liquid is accelerated in the conduit  7  of the pump wheel by centrifugal forces acting because of the rotation of the pump wheel  2 , and is therefore pushed radially outward. Pumped liquid at an increased pressure is therefore present in the chamber  8 , which extends all around the impeller wheel  2  of the pump inside the pump housing  3 , and is then conveyed out of the pump housing  3  through the pump outlet  25 . The impeller wheel  2  itself is sealed in labyrinth bushings  10 ,  11  in the pump housing. Cryogenic liquid is also present on the side  12 , here on the left, of the pump wheel  2 , but can partially evaporate there. The pressure prevailing in it moves around the suction pressure. However, the suction pressure already is higher than the atmospheric pressure, for example 2 bar, otherwise the medium would immediately evaporate because of the aspiration. The pressure in the medium is typically increased to approximately 4 to 6 bar by the pumping. The problem now lies in sealing the chamber to the left of the rotating impeller wheel  2  with respect to the rotating driveshaft, namely in respect to the motor. 
     To achieve the object proposed, a tandem axial face seal is used for this, which is constructed as follows. First, the sliding face  15 ,  16  located on both sides of a respective plane of rotation of a sliding ring  14  is inverted on the driveshaft  1  and is clamped together with the worm wheel  5  via all elements  21 ,  2  located to the right of it, so that the sliding ring  14  rotates together with the driveshaft  1 . A sealing ring  17 ,  18  adjoins each of the sliding faces  15 ,  16  and is fixedly connected with a metal bellows  19 ,  20  via respective connecting elements. The metal bellows  19  to the left is stationarily installed in a set screw  13  on the motor side, while the metal bellows  20  on the right side of the sliding ring  14  is stationarily connected with the housing cover  26 . It is necessary to obtain as gas-tight a seal as possible on the one side between the chamber  12  to the right of the metal bellows  20 , which is mounted under prestress and then extends inside of it toward the left, and the chamber  22  outside of the rotating sliding ring  14 , and on the other side between the chamber  22  and the one inside of the sliding ring  14  on its left side, which then extends inside the metal bellows  19  along the driveshaft. To the left of this tandem axial face seal, the set screw  13  is only sealed against the housing cover  27  by means of some O-rings  23 . Ideally, for example except for a small unavoidable leakage, no gas should flow from the chamber  12  to the left of the impeller wheel  2  of the pump into the chamber between the rotating driveshaft  1  and the pump housing to the left of the tandem axial face seal. However, the seal and bearing would not function with the arrangement so far described, since a pressure of 2 bar and more prevails to the right of the right metal bellows  20 , while atmospheric pressure prevails to the left of the right metal bellows  20 . Under these conditions no gas cushion would be built up during rotation, because no gas could get from the periphery between the sealing faces against the pressure applied from the inside, and act as a gas cushion there. Moreover, large amounts of evaporated cryogenic liquid would flow from the pump interior, i.e. from the chamber  12 , first inside the metal bellows  20  along the driveshaft toward the left, and then between the stationary seal  18  and the rotating sliding ring  14  radially outward into the chamber  22 , and from there on the left side of the sliding ring  14  between the sliding ring  14  and the sealing ring  17  to the driveshaft  1 , and finally to the left along the driveshaft  1  to the outside. So that a gas cushion for seating or for eliminating friction can be built up between the sliding ring  14  and the sealing rings  17 ,  18  adjoining it on both sides, approximately the same pressure must prevail in the chamber  22  as on the inside of the sealing rings  17 ,  18 . Moreover, the sliding ring  14  must be equipped on both sides with grooves, which are arranged in a spiral shape and terminate toward the outside, and the grooves must extend in the correct direction. Let it be assumed that the driveshaft  1  rotates in such a way that it turns into the drawing plane on the upper side in the drawing, for example when viewed from the right, it rotates in a clockwise direction. In this case the spiral-shaped grooves in the right sliding face must extend in a clockwise direction toward the outside, and those in the left sliding face spirally in a counterclockwise direction toward the outside. To provide the seating without the use of separate confining gas, a small amount of liquid is diverted at the pressure connector  9  of the pump, which is evaporated by supplying heat, for example by means of a heater, and this gas, such as filtered process gas, is pumped through the inlet  24  into the chamber outside of the sliding ring  14  in order to achieve a slight overpressure in the chamber  22  on this side of the sealing ring  18  over the chamber  12  on the other side of the sealing ring  18 . Under these conditions the sliding ring  14  can catch gas with its forward rotating groove mouths in the course of rotation and can build up a gas cushion between its faces and each of the adjoining sealing rings  17 ,  18 , so that seating free of friction is assured. By means of the set screw  13 , which can be finely adjusted, the sealing ring  17  mounted on the metal bellows  19  to the left of the sliding ring  14  can be mechanically pressed more or less strongly against the sliding ring  14  by the metal bellows being mechanically acted upon by means of pressure from the left in order to compensate the higher pressure prevailing on the other side of the sealing ring  17  as best as possible. In spite of this, a microscopic amount of leakage through the seal formed between the sealing ring  17  and the sliding ring  14  cannot be avoided. This is channeled through the connector  28 , but is so small that it can be easily accepted. It approximately amounts to a standard liter per hour, which is a quite negligible amount in comparison with the pump output. 
     To automatically regulate the optimum pressure in the chamber  22 , and to be also to set the correct contact pressure of the sealing ring  17  against the sliding ring  14 , a control circuit is provided, which will be described in connection with FIG.  3 . FIG. 3 shows a simplified diagram with only the elements required for producing the control circuit. The drive motor of the pump, in this case an electric motor, is identified by element reference numeral  30 , and the pump itself by element reference numeral  40 . The pump housing shelters the tandem axial face seal with the already described elements, namely the central, rotating sliding ring  14 , the two stationary sealing rings  17 ,  18  resting against it, as well as the stationary metal bellows  19 ,  20 , with which the sealing rings  17 ,  18  are connected. The adjusting device, by means of which the contact pressure of the left sealing ring  17  against the sliding ring  14  can be regulated, is indicated by element reference numeral  13 . The intention here is to build up a counter pressure in the chamber outside the sliding ring  14  for compensating the pressure prevailing in the pump interior, which corresponds to the suction pressure, so that a gas cushion is built up on both sides of the sliding ring  14  during rotation. The pump  40  aspirates cryogenic liquid via the line  41 , which can also be a flexible hose  42 , at a suction pressure of 2 bar, for example, and conveys it at a pressure of 4 to 6 bar, for example, through the line  43 , possibly also a flexible hose  44 , as indicated. Liquid cryogenic medium is now removed upstream of the pump by means of the line  45 , and is thereafter conducted through a pressure difference indicator  80 . The line  47  is connected on the opposite side of this pressure difference indicator  80  with the pressure line  48 , which diverts cryogenic medium downstream of the pump, for example at pump pressure, from the pump line  43 . This cryogenic liquid is then conducted through an electric heater  50 , so that it evaporates. Thereafter, the process gas thus obtained is conducted through a pressure difference controller  60 , and downstream of that also through a gas filter  70 . Thus, the pressure of the evaporated pumped liquid medium prevails at the lower side of the pressure difference indicator  80 , while the suction pressure of the pump prevails on the upper side of the pressure difference indicator  80 . The pressure difference controller  60  permits more or less gas to flow from the direction of the electric heater  50  through the filter  70  and finally through the line  46  into the chamber outside of the sliding ring  14 , in order to build therein a counterpressure to the respectively prevailing pressure in the pump housing. The pressure prevailing corresponds to the suction pressure of the pump. The pressure difference controller  60  therefore must let just a sufficient amount of gas pass, so that the pressure difference at the pressure difference indicator  80  is zero or approximately zero. The gas supply on the other side of the tandem axial face seal is determined by means of a flow-through meter  96 , and the gas supplied in this way is vented to the outside. The unavoidable leakage to the outside is detected by an additional flow-through meter  90 . However, this measurement is not absolutely necessary and remains optional from case to case. A temperature sensor  91  is installed as an emergency stop. If the leakage gas should rise to a temperature of −30° C., the pump motor is stopped. Finally, the pressure with which the left metal bellows  19 , and therefore the sealing ring  17  resting at the left of the sliding ring  15 , is acted upon is set in such a way that the measured leakage flow is minimal.