Balancing piston for centrifugal compressors with a seal...

Details Balancing piston for centrifugal compressors with a seal... Balancing piston for centrifugal compressors with a seal...

2002-06-11

2004-08-24

Verdier, Christopher (Department: 3745)

Rotary kinetic fluid motors or pumps

With shaft connected fluid force subjected thrust balancing...

C415S119000, C415S173500, C415S174500, C415S230000

Reexamination Certificate

active

06779970

ABSTRACT:

The present invention relates to a device to control and damp the subsynchronous vibrations of centrifugal compressors.
BACKGROUND OF THE INVENTION
In general, centrifugal compressors are machines which impart to a compressible fluid a pressure which is greater than that at which it receives the fluid, providing the latter with the energy necessary for this pressure increase with one or more impellers or rotors disposed in series, which are provided with radial blades and are activated at a high speed, by means of a motor which in general is keyed onto the compressor shaft itself.
Typically the centrifugal compressors fulfill a great variety of uses with requirements for high capacities and medium-low pressures, such as in refrigeration systems, in the petrochemicals industry, for example ethylene systems, catalytic cracking systems, and units for compression of CO
2
in urea systems, in the energy industry, such as systems for LPG, for oxygen and the units for compression and release to the gas pipeline service. The power levels installed are in general substantial and the pressures are as much as 40 bars.
A general drawing of the structure of a centrifugal compressor is illustrated in
FIG. 1
, in order to illustrate the technical problem to which the present invention relates. The blades of the rotor
1
, i.e. of the rotary part, co-operate with a stator
2
, i.e. a fixed part which is interposed between the adjacent impellers and through which there passes the shaft
3
, which connects and supports the impellers. The blades of the rotor can have various shapes, and in
FIG. 1
impellers
4
are shown which have as blades closed channels
5
, which make possible a structure with greater strength and improved guiding of the flows. The fixed part or stator consists of a body
6
which surrounds the outlet openings of the closed channels
5
of the impeller
4
and is subdivided into a plurality of deflector vanes
8
which, in the various stages, co-operate with the blades or channels of the preceding impeller in order to receive the fluid at a high speed, transform into pressure energy part of the speed which is impelling the fluid, and deflect and convey it into the innermost part of the blades of the successive compression impeller. The pressure of the fluid thus increases from stage to stage until it reaches its final value.
In the axial direction, there is generation of pressure differential between the various stages, which thus requires insertion on the shaft
3
of the rotor
1
of a system of seals between the rotor and stator of each stage, which limits as far as possible the phenomenon of reflux of the compressed fluid to the preceding stages, in order to maintain the compression performance at appropriate values.
As the pressure value progresses from upstream in the downstream direction, on the rotor body there is thus generation of axial and radial forces which must be equilibrated and balanced both statically and dynamically, owing to the presence of the inevitable irregularities over a period of time in the system as a whole.
One of the most sought-after characteristics of the rotors of centrifugal compressors, and of rotary machines which operate at high speed and with fluids at high pressures, is their dimensional stability even in the presence of operating variations caused by temporal irregularities of flow upstream or downstream or of the real density or pressure of the gas on which work is taking place. The forces which are generated by these irregularities of density or pressure of the fluid give rise, in the labyrinths of the seals at the ends of the machines and of the passages from stage to stage, to subsynchronous vibrations on the rotor which are extremely detrimental to the operation and efficiency of the machine. This tangential course of the fluid in the machine or wherever there is a part which rotates relative to a fixed part, with limited tolerances, such as in the sealing units or balancing pistons, gives rise to destabilizing forces and vibrations which must be balanced and damped.
The dynamic performance of the rotor must therefore always be controlled and included at the design stage of the installation for each specific application, with reference to its stability in terms both of flexure and torsion. For this reason, and taking into consideration the fact that the negative influence of these vibrations on the rotor depends directly on the flexure and torsion induced on it, as a solution there is a tendency to increase the rigidity of the rotor and decrease its flexibility, both by using shafts with a large diameter and by shortening their section with free flexure, by reducing as far as possible the pitch between the support bearings.
A consequence of the step of increasing the diameter of the shaft and shortening the pitch of the supports consists of the complication of the problem of the seals on the rotary shaft, and especially the seals to be produced between the stages with the highest pressure of the compressor and the surrounding atmosphere. An increase in the diameter thus corresponds to a greater linear development of the play to be controlled, thus limiting the flows of the compressed fluid which escapes via the play of the seals.
In recently designed compressors, in order to damp and restrict the phenomena of vibration, in addition to the normal bearings for support, and to withstand the axial thrust, balancing pistons are inserted in the end parts of the drive shaft of the compressor.
With reference to the play which exists between the elements which constitute the seals or the balancing pistons and which are interposed between the impeller or rotor part and the fixed part or stator, account must be taken of the fact that this play is necessary and its dimensions must be appropriate, taking into consideration both the foreseeable deformations caused by the mechanical stresses and the expansions/contractions caused by the temperature variations. When evaluating the flexure deformations of the various parts, account must also be taken of the parts own frequencies and of the modes of natural vibration in terms of flexure and torsion at the various speeds of the rotor. On the other hand as far as the thermal ranges are concerned, account must be taken of the dimensional variations caused by the temperature during the transient phases, in which the stator and rotor can heat up and cool down, and thus expand or contract, with a greater or lesser speed relative to one another.
Excessively narrow play causes friction, heating and wear which detracts from the efficiency, the factor of use and the technical service life of the machine. Excessive play detracts from the performance of the machine.
The balancing pistons which are placed at the ends of the compressor shafts can be of various types, with bodies provided with cavities or venting openings, and are used to create an action of damping, which assists control of the subsynchronous vibrations of the compressor. In recent times, in the balancing pistons for compressors, considerable preference has been given to the use of sealing bodies which are commonly known as honeycomb by persons skilled in the art. This type of balancing piston is shown by way of example in the detail in FIG.
1
A.
On the rotor
1
side, on the body of the shaft
3
, there is keyed a cylindrical body
10
which acts as a balancing piston and separates a space
11
inside the compressor in which a greater pressure exists, from a space
12
in which a lower pressure exists. The outer cylindrical surface
13
of the body
10
is smooth and is parallel to the axis of the shaft
3
. On the stator
2
side, at the axial correspondence of the balancing piston
10
, there is fitted a hollow cylindrical structure
15
which has in its interior a seal with an annular body
16
provided with a large number of small hollow cells, in the form of a honeycomb. The honeycomb seal
16
can be made of metal material in a known manner, and can be secured to the structure
15
by conventional means, for example by being brazed.

Affiliated with

Also associated with

Rating

Balancing piston for centrifugal compressors with a seal... does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Balancing piston for centrifugal compressors with a seal..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Balancing piston for centrifugal compressors with a seal... will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3351304