Shaft seal

Details Shaft seal Shaft seal

1997-12-08

2001-04-10

Knight, Anthony (Department: 3626)

Seal for a joint or juncture

Seal between relatively movable parts

Relatively rotatable radially extending sealing face member

C277S400000

Reexamination Certificate

active

06213472

ABSTRACT:

The invention relates to a non-contacting shaft seal for rotating shafts which can be used in turbo-machines.
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. Article 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 (Transactions 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.
U.S. Pat. No. 4,212,475 describes a shaft seal of turbo-machine comprising the spiral-loaded sealing element and the rotary sealing ring, one end face of which is provided with the sealing face and plurality of the spiral grooves. The grooves are formed by two lateral arcs connected by the third one of the circle which is the boundary between the grooves and the sealing face forming four peaks: two peaks on the boundary with the sealing face and two peaks on the inlet end faces of the rotary ring or circular element which may be made on the outside or inside edge of one of the mentioned rings. In this case, the width of the sealing band (dam) and the width of entire end face of the ring are determined according to the formula:
GD
-
ID
OD
-
ID
=
0.5
-
0.8
,
if the groove is made from the outside edge of the rotary ring or the circular element
and
OD
-
GD
OD
-
ID
=
0.5
-
0.8
,
if the groove is made from the inside edge of one of the rings,
where:
OD is the outside diameter;
ID is the inside diameter;
GD is the diameter of a circle, defined by the boundary of the grooves and the sealing face.
The formation of the grooves is caused by the necessity to prevent wearing of the end faces of the element and the ring which occurs due to friction against each other. Upon ring rotation, gas transferred from the chamber is trapped into the grooves and is forced to move towards the centre. During rotation, gas inside the grooves is compressed, its pressure is increased and the force is created affecting the element and promoting the creation of the gap between the end faces of the circular element and the ring. This force and gap are approximately constant under condition when the compressor mode of operation is constant and specified speed of the shaft rotation and gap pressure in the chamber are ensured. Upon transient conditions when, e.g. speed of rotation is reduced and gas pressure inside the chamber remains high, the balance of forces affecting the ring and the element outside and inside the gap is disturbed. At this time we can observe distortion, friction of the end faces of the ring, and the element against each other, their wear and failure. Changes in the mode of operation result in increased vibration of the rotor, angular oscillations of the rotary sealing ring which lead to wear of the sealing end faces and failure of the seal. Thereby, the disadvantage of the said seal of the shaft lies in its low reliability when running at transient conditions because of instability of the gap between the sealing end faces the ring and the element (instability of the working medium layer in the gap).
Russian Inventors Certificate 1,535,122 describes a shaft seal of a spring-loaded circular sealing element and the rotary sealing ring, one end face of the sealing ring is provided with a sealing face having a plurality of the spiral grooves provided therein each with a dividing partition. Therein, the dividing partition in each spiral groove is made in the shape of the substantially rectangular bulge. The bulge divides each groove into two cavities terminating in obtuse peaks. The dividing partition is made integral with sealing face and flush with the end face of the ring. The said partitions are intended to improve the gas flow in the grooves compared to grooves with circular peaks and, therefore, for stabilisation of radial layer of working medium inside the end gap. However, shaft seals with partitions of this nature, provide an unstable layer of the gas layer during transient conditions such as start-up, shut-down or speed variation of the motor. This leads to disturbance of stabilisation of radial layer of working medium at the end gap during transient conditions (e.g. start-up and shut-down) and this results in the distortion and vibration of the ring and the element and, therefore, in local abrasion of the end faces of the element and the seal failure.
EP-A-O 595 437 discloses a non-contacting gas seal with a triangular groove pattern. The groove pattern allows bidirectional operation of the seal.
The present invention seeks to provide a stable radial layer of the working medium at the end gap under different modes of operation including transient conditions to provide the work of turbo-machine by means of improving the shape of the groove. The working medium will normally be a gas.
According to the present invention, there is provided the shaft seal described in claim
1
. Preferably, the partitions are shaped so as to form only acute angled faces to the gas flow, thereby only causing slight disruption to the gas flow.
The present invention provides a partition which allows smooth gas flow around the partition and/or in the channels. A marked improvement in the stability of the gas layer, especially during transient conditions, is thus provided compared to the rectangular-like partitions of the prior art. The present inventors speculatively consider that the partitions of the prior art acted as obstacles to the gas flow in the grooves due to the blunt face provided by the partition. Consequently, the partition caused turbulence in the gas flow and acted to resist gas flow thereby reducing the stability of the gas layer, especially during transient conditions.
The partition may take any suitable form but preferably the separating partition is made in the shape of the wedge for example, formed by intersection of arcs connecting pairs the opposite peaks.
According to preferred embodiments of the invention, the seal will have the following characteristics. The sealing ring has an outside diameter (DH) and an inner diameter (DO). In the case where the spiral grooves extend from the outside edge of the sealing ring, the circumference of diameter (DK) upon which peaks of wedge partitions are placed and circumference of diameter (DB) which is the boundary between grooves and the sealing face is defined according to the formula:
DK
-
DB
DH
-
DB
=
0.2
-
0.8
,
The width of sealing face and the whole end face surface of the sealing ring is defined according to the formula:
DB
-
DO
DH
-
DO
=
0.3
-
0.5
,
when the grooves extend from the outside the edge of the sealing ring,
and
DB
-
DK
DB
-
DO
=
0.2
-
0.8
,
and the width of the sealing face and the whole end face of the sealing ring:
DH
-
DB
DH
-
DO
=
0.3
-
0.5
,
in case grooves are made from the inside edge of the ring element,
where:
DH is the sealing ring outside edge diameter;
DK is the diameter of the circle upon which peaks of wedges partitions of grooves;
DB is the diameter of the circumference which is the boundary between grooves and the sealing face;
DO is the sealing ring inside diameter.
In addition arcs forming side edges of grooves can be formed by radius R. R is equal to the half of sealing ring outside diameter (DH) in case grooves are made from the outside diameter. R is equal to the half of inside diameter (DO) in case grooves are made from the inner edge of the ring. R is measured from a point on a circumference the diameter of which is connected with outside diameter (DH) by means of the following relationship:
D
DH
=
0.25
-
0.4
,
where D is the diameter of circumference upon which the radius centre of grooves side edges arcs are placed.
In addition, or alternatively, the outside diameter of the face of the sealing element opposed to the spiral grooves in the sealing ring may have

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