DE20103013U1 - Liquid seal assembly - Google Patents

Description

DE2374 Liquid seal arrangement

The invention relates to a liquid sealing arrangement for sealing a rotating component with respect to a stationary component, e.g. a shaft with respect to a housing, according to the preamble of claim 1.

It relates in particular to an arrangement with leakage return, as is known in principle from EP-A-864 756. The leakage return of the known liquid sealing arrangement is intended to prevent a leakage, which cannot be completely avoided but is always very small, from reaching the environment along a main seal. The main seal is therefore designed to seal the medium as extensively as possible. In addition to a mechanical seal, a labyrinth seal or stuffing box packing can also be provided as the main seal. The design of the main seal to minimize leakage means that more or less significant friction occurs between the sealing surfaces that engage with one another and move relative to one another, and thus wear that limits the service life. Even in the case of mechanical seals, where a lubricating film is formed from the medium to be sealed between the interacting sealing surfaces when operating at a certain rotational speed, significant wear can be observed under the conditions mentioned, especially if the medium contains impurities, e.g. foreign bodies.

The invention is based on the object of creating a liquid sealing arrangement of the generic type in which wear during operation as a result of solid body friction or the like is completely or at least largely avoided. At the same time, a high sealing effect of the liquid sealing arrangement should be ensured so that even toxic or aggressive environmentally harmful liquids are reliably prevented from entering the environment.

This object is achieved according to the invention by the features according to the characterizing part of claim 1. An essential feature of the inventive

The liquid seal arrangement is a design of the main seal in such a way that it does not have only a small or negligible leakage, as was previously the case, but rather an increased leakage flow is deliberately created and maintained between the sealing surfaces moving relative to one another. This reliably prevents the interacting sealing surfaces from touching each other and any foreign particles in the medium to be sealed are continuously flushed out of the area between the sealing surfaces. By taking suitable measures listed in the subclaims, it can be achieved that the contactless operation of the main seal occurs even at low rotational speeds and thus the main seal runs practically wear-free not only during normal operation but also when the relevant equipment is started up and shut down. The high leakage along the main seal, which is deliberately accepted, is prevented from escaping into the environment by combining the main seal with a leakage return system, which ensures that the leakage is completely returned to the main flow of the liquid medium to be sealed. Despite the increased leakage along the main seal, toxic or aggressive liquid media can therefore also be reliably sealed against the environment using the sealing arrangement according to the invention. The service life of the liquid sealing arrangement according to the invention exceeds that of the known generic sealing arrangements by a multiple, with the lower requirements for the accuracy of fit of the structural parts of the main seal being an advantageous side effect. Although the main seal can be designed as a labyrinth seal or throttle gap seal, a single-acting mechanical seal is preferably provided for this purpose, which can basically have a conventional structure but is specifically modified for increased leakage.

The invention is explained in more detail below using embodiments and the drawing. They show:

Fig. 1 shows a partially schematic sectional view of a liquid sealing arrangement according to an embodiment of the invention,

Fig. 2 in perspective fragmentary view a sliding ring of a

Mechanical seal as the main seal of the liquid sealing arrangement according to Fig. 1 according to a modification of the invention, and

Fig. 3 in cross-sectional view a mechanical ring pairing of a mechanical seal as

Main seal of the liquid sealing arrangement according to Fig. 1 according to a further modification of the invention.

Although the invention is described below in connection with its application to a centrifugal pump, it is to be understood that this field of application is only exemplary and not restrictive.

In the drawing, reference numeral 1 refers to the impeller of the centrifugal pump and reference numeral 2 to its housing. The housing 2 defines a pressure chamber 3 in which the impeller 1 is rotatably accommodated. When the impeller 1 rotates, a liquid conveying medium present in an intake port or low-pressure area 4 of the centrifugal pump 1 is sucked in and accelerated to a radially outer discharge area 5 and pressurized in the process. As shown, the impeller 1 is mounted on a horizontal shaft 6 which is guided through the housing 2 to the outside to a drive device (not shown).

To seal the shaft 6 against the housing 2, an externally loaded main seal 7 is provided, which is designed according to the invention in such a way that, compared to known devices of this type, a comparatively high leakage of the liquid conveying medium to be sealed is permitted. The main seal 7 separates the pressure chamber 3 of the centrifugal pump from a pressure-relieved annular leakage collection chamber 9, which extends around the circumference of the shaft 6 downstream of the main seal 7 and in a

penetrated attachment part 8. The attachment part 8 can be attached to the housing 2, e.g. by a screw connection. The relatively high leakage of the conveying medium along the main seal 7 reaches the leakage collection chamber 9 and will preferably collect in its lower area. Any gaseous portions of the leakage that may be present can be discharged to the outside, e.g. to a flaring point, via a pressure relief line 22 that is connected to the upper area of the leakage collection chamber 9. If desired, a further seal (not shown), e.g. a labyrinth or mechanical seal, that interacts with the shaft 6 can be provided on the attachment part 8 in order to additionally seal the leakage collection chamber 9 on the downstream side from the outside.

The leakage collection chamber 9 is connected to the interior of a storage tank 12 via a connecting line 10 and a shut-off valve 11 arranged therein. The storage tank 12 forms an intermediate storage into which the leakage from the collection chamber 9 can be introduced. The shut-off valve 11 enables the leakage to flow from the collection chamber 9 into the storage tank 12, while preventing a flow in the opposite direction.

A closable outlet 13 near the bottom of the storage tank 12 is connected via a line 17 to the intake side of a pumping device 18. Although other suitable pumping devices can be used, one that can be designed in an encapsulated manner and does not require an external drive is preferred. For further details of such a pumping device, reference can be made to EP-A-0 864 756. On the discharge side, the pumping device 18 is connected via a line 19 to the intake port 4 of the centrifugal pump 1.

To open and close the outlet 13 of the storage tank 12, a closing element is provided, indicated at 14, which can be actuated depending on the level of leakage in the storage tank 12. For this purpose, a float 15 can be arranged in the storage tank 12, which is connected to the closing element 14 via a rod mechanism 16. As soon as the leakage in the storage tank 15 reaches a certain

has reached the upper filling level, the float 15 moves the closing element 14 into the release position so that the interior of the storage tank 12 can be connected to the pumping device 18 via the line 17 and then to the intake port 4 of the centrifugal pump via the line 19. The closing element 14 closes the outlet 13 again when the filling level has fallen to a lower level so that a residual leakage is always retained in the storage tank 12. This prevents the pumping device 18 from sucking in gaseous matter which would contaminate the conveying medium.

A monitoring device 20 can be provided to detect an excessive filling level in the storage tank 12 and to emit an alarm signal if the amount of leakage introduced into the storage tank 12 is greater than that which can be discharged via the outlet 13. 21 indicates a line which connects an upper region of the storage tank 12 to the pressure relief line 22. Gaseous components of the leakage which have accumulated in the upper region of the storage tank 12 can therefore be guided to the flaring point via the lines 21 and 22. At the same time, this prevents a pressure build-up in the storage tank 12.

Although a leakage return with the aforementioned safety features is preferred, the invention is not limited to this. Rather, a continuous leakage return without intermediate storage could also be provided.

According to the invention, the main seal 7 is designed for operation with minimal wear on the sealing surfaces. This is achieved by the sealing surfaces being brought out of contact with one another by hydrodynamic effects during operation even at low shaft rotation speeds, such as those that occur when the centrifugal pump starts and stops, and a cushion or reinforced lubricating film is formed between them from the medium to be sealed. The resulting high leakage along the main seal 7 is harmless even with toxic or aggressive media, since it is prevented thanks to the aforementioned measures.

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does not escape into the environment, but can be returned to the conveying system of the centrifugal pump.

It has been found within the scope of the invention that the desired wear minimization is achieved if a leakage along the main seal 7 is permitted which is several times, e.g. 10 times, greater than the leakage which was previously considered permissible under comparable conditions (shaft diameter, rotational speed, pressure and viscosity of the medium to be sealed, design of the main seal, etc.).

Although a labyrinth or throttle gap seal could in principle be used for the main seal 7, this is preferably designed as a single-acting externally loaded mechanical seal, the basic structure of which is known to those skilled in the art and therefore does not need to be described in more detail here. Reference can be made, for example, to BURGMANN, ABC of mechanical seals, 1988 self-published, page 46. In such a gel ring seal, measures are provided according to the invention which cause a gap to form between the sealing surfaces of a pair of interacting mechanical seal rings 23, 24 even at low rotational speeds, which results in the mechanical seal running without contact at an early stage. On the other hand, the mechanical seal must seal as leak-free as possible when the shaft 6 is at a standstill. To this end, the mechanical seal rings 23, 24 can be permanently loaded with a static preload force so that the sealing surfaces are pressed into a sealing relationship with one another when hydrodynamic forces are absent.

One measure to prevent early gap formation is to set the so-called load factor k of the mechanical seal, defined as the ratio of the hydraulically loaded surface of the mechanical seal to the mechanical seal surface (see also BURGMANN, ABC of the mechanical seal, op. cit., page 25), to a low value, so that a significantly relieved mechanical seal is created. While conventional single-acting mechanical seals have a load factor k of up to 2.0, reduced load factors k < 0.8, e.g. 0.4 to 0.5, are selected for the present invention, whereby the value for

the load factor k is influenced by the pressure and viscosity of the medium to be sealed.

Example:

In a conventional single-acting mechanical seal with flat sealing surfaces and a load factor k = 1.2, the leakage was a typical value of 3 to 5 ml/h in the presence of an aqueous medium, with a shaft diameter of 50 mm, a rotational speed of 3000 min" and a medium pressure of 3.5 bar. By reducing the k-factor to 0.5 with otherwise unchanged conditions, the leakage increased to 150 ml/h. The mechanical seal equipped with increased leakage showed no signs of wear even after long periods of operation.

Early gap formation can also be initiated by introducing effective conveying recesses into at least one of the sealing surfaces. Even at low rotational speeds of the shaft 6, these pump the liquid medium to be sealed between the sealing surfaces in order to build up pressure between them and thus space them apart from one another. Such recesses can be designed in a similar way to the effective conveying grooves frequently used in sealing gaseous media, e.g. spiral grooves, as described in BURGMANN, Gasdichtungen, Selbstverlag 1997, page 17. The grooves extend from the outer circumference of a sealing surface to an intermediate radial point, so that a groove-free annular dam area remains near the inner circumference of the sealing surface, which is pressed into a sealing relationship with a flat counter surface on the adjacent sliding ring 24' (not shown) under the axial preload force when the shaft is at a standstill.

Another possibility for early gap formation is to introduce a wave structure into the sealing surface of one or both of the sliding rings, as shown schematically in Fig. 2. In Fig. 2, the reference numeral 23' refers to one of the interacting sliding rings 23', 24' and the reference numeral 25 refers to the sealing surface of the respective sliding ring 23'. In a

The sealing surface has a wave-shaped structure in the region 26 of the sealing surface 25, which extends between the outer circumference, i.e. the circumference exposed to the medium to be sealed, and a radially intermediate circumferential boundary. This creates alternating, radially running wave crests and troughs, which promote the entry of the medium to be sealed between the sealing surfaces of the interacting sliding rings 23', 24' in order to bring about the desired early gap formation. Near the inner circumference of the sealing surface 25, a dam region 27 with an annular flat surface is provided, which ensures a sealing relationship between the sealing surfaces when the shaft is at a standstill.

Finally, as a further alternative, the sealing surfaces can be arranged or designed in such a way that a V-gap 30 is formed between them, as shown schematically in Fig. 3. The V-gap 30 also results in increased leakage. For this purpose, the sealing surfaces 28, 29 of the interacting sliding rings 23", 24" are preferably designed to be spherical, so that they progressively become further apart from each other from the inner to the outer circumference. When the shaft is at a standstill, ring-shaped surface areas near the inner circumferences of the sealing surfaces 28, 29 come into contact with each other in order to seal the gap.

The invention makes it possible for a gap to be formed between the sealing surfaces of the sliding rings during operation of the mechanical seal, which results in a leakage of at least 25 ml/h at a pressure of the medium to be sealed between 1 and 25 bar. The formation of a gap with such a leakage is guaranteed at low rotational speeds of the shaft up to a maximum of 5000 min" ^1. If a labyrinth or throttle gap seal is provided as the main seal 7 instead of a mechanical seal, the leakage must be designed according to the aforementioned values. Such main seals 7 should also be combined with a preferably speed-controlled standstill seal, as described in more detail in BURGMANN, ABC of the mechanical seal, op. cit., page 232, to which reference can therefore be made.

Claims (9)

1. Liquid sealing arrangement for sealing a rotating component with respect to a stationary component with an externally loaded main seal ( 7 ) with a stationary sealing surface which interacts with a sealing surface rotating with the rotating component, and a device ( 8 , 9 , 10 , 12 , 17 , 19 ) for absorbing the leakage along the main seal and returning it to a main flow of the liquid to be sealed, characterized in that the main seal is designed to enable a continuous leakage flow of at least 25 ml/h between the sealing surfaces when the rotating component rotates, 2. Liquid sealing arrangement according to claim 1, characterized in that the main seal ( 7 ) comprises a single-acting mechanical seal. 3. Liquid sealing arrangement according to claim 2, characterized in that the mechanical seal has a load factor k between 0.4 and 0.8, preferably 0.4 and 0.6, most preferably about 0.5. 4. Liquid sealing arrangement according to claim 2, characterized in that liquid-conveying effective recesses are provided in at least one of the cooperating sealing surfaces of the mechanical seal. 5. Liquid sealing arrangement according to claim 2, characterized in that a wave structure ( 26 ) is provided in at least one ( 25 ) of the cooperating sealing surfaces of the sliding rings ( 23 ', 24 '). 6. Liquid sealing arrangement according to claim 2, characterized in that the cooperating sealing surfaces ( 28 , 29 ) of the mechanical seal are spherical, so that a gap ( 30 ) is formed between the sealing surfaces, which gap increases from the inner to the outer circumference of the sliding rings ( 23 ", 24 "). 7. Liquid sealing arrangement according to claim 1, characterized in that the main seal ( 7 ) comprises a labyrinth seal. 8. Liquid sealing arrangement according to claim 1, characterized in that the main seal ( 7 ) comprises a throttle gap seal. 9. Liquid sealing arrangement according to one of the preceding claims, characterized in that at least annular partial regions of the sealing surfaces of the main seal ( 7 ) are held in a sealing relationship to one another by a prestressing force when the rotating component is at a standstill.