Seal for a joint or juncture – Seal between relatively movable parts – Relatively rotatable radially extending sealing face member
Reexamination Certificate
2001-08-23
2003-08-05
Swann, J. J. (Department: 3677)
Seal for a joint or juncture
Seal between relatively movable parts
Relatively rotatable radially extending sealing face member
C277S358000, C277S512000
Reexamination Certificate
active
06601854
ABSTRACT:
The invention relates to a shaft seal for rotating shafts in turbo-machines or other pressurized machine. In particular, the present invention, in common with WO-A-96/33357 provides a shaft seal comprising a sealing element, a rotary sealing part mounted coaxially with the sealing element and forming therewith a contactless primary seal between opposed faces of the sealing element and rotary sealing part to substantially prevent fluid flow across the primary seal from a high pressure radial side to a low-pressure radial side, a seal housing, a pusher sleeve disposed, between the seal housing and the sealing element, coaxially with and in contact with the sealing element, biasing means urging the pusher sleeve away from the seal housing and against the sealing element to urge the sealing element axially towards the rotary sealing part, and a first sealing member disposed about the pusher sleeve and located, in a channel, in communication with the high-pressure radial side to provide a secondary seal for the pusher sleeve between the high-pressure and low-pressure radial sides. Such a shaft seal is disclosed in WO-A-96/33357.
Non-contacting shaft seals are often used with machinery for the compression or expansion of gas (hydrogen, natural gas, air, etc.) where the transmission of gas along the shaft needs to be prevented. Due to the high-pressure, high-speed machinery which is normally used, the shaft seals are chosen to be of non-contact type, in order to reduce heat build up in the seals and the wear of the sealing parts and/or in order to avoid the complexity of oil seals and their associated systems.
Non-contacting operation avoids this undesirable face contact when the shaft is rotating above a certain minimum speed, which is often called a lift-off speed.
Non-contacting shaft seals provide advantages over seals where the sealing surfaces contact one another, due to reduction in wear and the lower heat generation. Articles entitled “Fundamentals of Spiral Groove Non-contacting Face Seals” by (Gabriel, Ralph P. (Journal of American Society of Lubrication Engineers Volume 35, 7, pages 367-375), and “Improved Performance of Film-Riding Gas Seals Through Enhancement of Hydrodynamic Effects” by Sedy, Joseph (Transaction of the American Society of Lubrication Engineers, Volume 23, 1 pages 35-44) describe non-contacting seal technology and design criteria and are incorporated herein by reference.
As with ordinary mechanical seals, a non-contacting face seal consists of two principal sealing elements. At least one of the sealing elements is provided with shallow surface recesses.
These recesses are taper-shaped perpendicular to and concentric with the axis of rotation, the tapering being in the direction opposite to the direction of rotation of the shaft. In known contactless face seals, both sealing elements, in the form of rings, are positioned adjacent to each other with the sealing surfaces in contact at conditions of zero pressure differential and zero speed of rotation. One of the rings is normally fixed to the rotatable shaft by means of a shaft sleeve, the other being located within the seal housing structure and allowed to move axially. The shaft seal is designed to enable axial movement of the sealing ring and yet prevent or substantially minimize leakage of the sealed fluid. For this reason, a sealing member is placed between the ring and the housing.
As mentioned above, to achieve non-contacting operation of the seal, one of the two sealing surfaces is provided with shallow surface recesses, which act to generate pressure fields that force the two sealing surfaces apart. When the magnitude of the forces resulting from these pressure fields is large enough to overcome the forces that urge the seal faces closed, the sealing surfaces will separate and form a clearance, resulting in non-contacting operation.
As explained in detail in the above-referenced articles, the character of the separation forces is such that their magnitude decreases with the increase of face separation. Opposing or closing forces, on the other hand, depend on sealed pressure level and as such are independent of face separation. They result from the sealed pressure and the spring force acting on the back surface of the axially movable sealing ring. Since the separation or opening force depends on the separation distance between sealing surfaces, during the operation of the seal or on imposition of sufficient pressure, differential equilibrium separation between both surfaces will establish itself. This occurs when closing and opening forces are in equilibrium and equal to each other. Equilibrium separation constantly changes within the range of gaps. The goal is to have the low limit of this range above zero. Another goal is to make this range as narrow as possible, because on its high end the separation between the faces will lead to increased seal leakage. Since non-contacting seals operate by definition with a clearance between sealing surfaces, their leakage will be higher than that of a contacting seal of similar geometry. Yet, the absence of contact will mean zero wear on the sealing surfaces and therefore a relatively low amount of heat generated between them. It is this low generated heat and lack of wear that enables the application of non-contacting seals to high-speed turbo machinery and other pressure machines, where the sealed fluid is gas. Turbo compressors are used to compress this fluid and since gas has a relatively low mass, they normally operate at very high speeds and with a number of compression stages in series.
As explained in the above-referenced articles, the effectiveness of the seal is largely dependent upon the so-called balance diameter of the seal. This is also true for contact seals.
When pressure is applied from the outside diameter of the seal, reduction of the balance diameter results in a greater force pushing the two sealing faces together and so a smaller gap between the faces. Thus, less gas is leaked from the system.
Known compressors have been used for compressing gas at inlet pressures of some 200 bar to delivery pressures of some 500 bar. Contactless shaft seals of the kind described above are typically used to seal against the compressor inlet pressure. The trend in compressor requirements nowadays is towards higher inlet and delivery pressures. However, such pressure levels give rise to a problem with the contactless shaft seals described above, as is now explained with reference to
FIGS. 1
,
1
a
and
2
,
2
a.
FIG. 1
is a partial longitudinal sectional view through the shaft seal showing the relevant structural elements of a non-contacting shaft seal of the type described above. The shaft seal is incorporated in a turbo-machine (not shown) such as a compressor in this example. There is shown a shaft seal
1
having a (non-rotating) sealing element or ring
2
mounted coaxially with the shaft axis (denoted by reference numeral
3
), and a rotary sealing part or ring
4
located coaxially with the sealing ring
2
, and therefore also with the shaft axis
3
. It will he appreciated that the vertical sectional view of
FIG. 1
, for simplicity, shows only the portion of the shaft seal located above the shaft axis. The sealing ring
4
is mounted on an inner sleeve
5
having a radial flange
5
a
against which the sealing ring
4
abuts, the sleeve
5
being mounted on the shaft
6
such that the shaft
6
, inner sleeve
5
and rotary sealing ring
4
co-rotate as a single rotary element. In addition, a locating sleeve
7
is bolted to inner sleeve
5
. The assembly comprising components
4
,
5
and
7
is prevented from displacement in one axial direction by a locating ring
21
and in the opposite axial direction by the high pressure acting inside the compressor.
The shaft seal also has a seal housing
8
and a pusher sleeve
9
disposed between a radially inward flange
8
b
of the seal housing
8
and sealing ring
2
. The pusher sleeve has a radial flange
9
b
against which a plurality of biasing springs (one of which,
10
, is shown in FIG.
1
,)
Dresser-Rand S.A.
Haynes and Boone, LLP.
Lugo Carlos
Swann J. J.
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