1. Field of the Invention
The present invention relates generally to a centrifugal pump, and more specifically to a rotor thrust balance of the centrifugal pump.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Rotary machines such as a centrifugal pump are used to pressurize a fluid such as a liquid or a gas. In a centrifugal pump, the fluid flows axially into the inlet of the pump and radially outward at the exit. The outlet also has a tangential component of velocity due to the rotation of the radial directed outlet. A typical single stage or multiple staged rotary pump or compressor contains a rotor surrounded by a stationary shroud or housing. An active part of the rotor is sometimes referred to as an impeller which typically contains an arrangement of vanes, disks or other components forming a pumping element that transforms its kinetic rotational energy to the pumping fluid.
In a rotary machine, such as a centrifugal compressor or pump, the presence of an axial force which is also known as an axial thrust is produced on the rotor disk. The axial thrust can impact the performance of the rotor. Depending on the rotational speed, the rotor diameter, fluid dynamics, annular gap leakage flows and other parameters, the axial thrust produced may reach such significant levels and as such present a challenge to the longevity and reliability of the rotary machine operation. Axial load is especially harmful for the axial thrust bearings. Failure of the axial thrust bearing can cause general failure of the rotary machine. Expensive procedures of bearing replacement comprises a significant part of the overall maintenance of the rotary machine, especially for a turbojet engine and similar machines in which access to the axial bearings is quite difficult.
It is also known in the art of rotary machines that the level of axial thrust forces depends on the wear state of the rotor seals of the machine. As the seals wear out, the annular gap leakage flow increases which changes unfavorably the hydrodynamic nature of the vortex flows in the cavities between the rotor and the housing of the rotary machine and typically causes the increase in the axial thrust. That in turn causes higher loads on the axial thrust bearings and may bring about their premature failure.
The challenge of reducing the axial thrust has been long recognized by the designers of the rotary machines. A variety of concepts has been proposed in the prior art in attempt to solve this problem. One of the most popular methods of reducing the axial thrust is the use of a balancing disk or drum. A balancing drum or disk is added in the back of the rotor and placed in its own balancing cavity in such a way that one side of the disk is subjected to high fluid pressure in order to compensate for the axial thrust cumulatively developed in all the prior stages of the machine. Examples of various designs of such balancing disks can be found in U.S. Pat. No. 5,591,016 by Kubota; U.S. Pat. No. 5,102,295 by Pope; U.S. Pat. No. 4,892,459 by Guelich; as well as U.S. Pat. Nos. 4,538,960 and 4,493,610 by Iino. Although capable of reducing the axial thrust to a certain extent, these devices are not generally capable of eliminating the problem over a wide range of rotor speeds and pumping conditions. In addition, they are not as simple to implement, require their own maintenance service and increase the size, inertia and weight of the rotary machine which ultimately reduces its efficiency of operation. They also increase the annular gap leakage and in addition can not compensate for the increasing axial thrust due to the wear of the rotary machine seals.
Another method of axial thrust compensation is to increase the fluid pressure in the appropriate cavity of the rotary machine to exert higher pressure on the rotor and therefore to compensate for the axial thrust. Various additional fluid passages have been proposed in the rotary machines of the prior art for the purposes of creating conditions of changing the fluid pressure against the certain areas of the rotor. Examples of single- and multi-staged rotary machines utilizing these devices are described in U.S. Pat. No. 5,862,666 by Liu; U.S. Pat. No. 5,358,378 by Holscher; U.S. Pat. No. 5,104,284 by Hustak; and U.S. Pat. No. 4,170,435 by Swearingen. Rotary machines of this type employ complicated monitoring and control devices designed to adjust the leakage rates and the pressure values of the additional fluid passages in order to compensate for the axial thrust over a wide range of operating parameters. In addition to complexity, another limitation of this approach is the hydraulic losses associated with these compensating fluid passages which negatively affect the hydraulic and overall efficiency of the rotary machine. As with balancing disks, these devices require separate maintenance and thus increase the operation costs of the machine.
One of the simplest and quite efficient ways to address the problem of the axial thrust is the use of so called swirl brakes described for example in the U.S. Pat. No. 5,320,482 by Palmer or in the article by J. M. Sivo entitled “The influence of swirl brakes on the rotor dynamic forces generated by discharge-to-suction leakage flows in centrifugal pumps” (Transactions of ASME, Volume 117, March 1995, pages 104-108). A plurality of stationary ribs, grooves, cavities or vanes located along the housing of the rotary machine are utilized to change favorably the fluid pressure distribution outside the rotor in order to reduce the axial thrust. Although simple and reliable, this method has its own limitations such as creating additional localized vortexes and areas of hydraulic disturbances in the rotary machine which reduces its hydraulic efficiency.
Another method of axial thrust reduction is proposed in the U.S. Pat. No. 4,867,633 by Gravelle. Hydraulic thrust balance is achieved and continuously maintained according to that patent by the controlled axial movement of the rotor shaft and the rotor in order to modulate the gap at the rear seal and therefore control the pressure acting on the back side of the rotor. In that case, an outward thrust force resulting from the rotor operation counterbalances an inward thrust force resulting from the pressure acting on the front side of the rotor. This device is quite complicated and delicate and requires careful adjustment for proper operation. It also reduces the hydraulic efficiency of the machine.
The centrifugal pump in the U.S. Pat. No. 1,020,699 issued to Kieser on Mar. 19, 1912 shows a discharge portion of the pump with parallel outer walls lying in planes perpendicular to the axis. The hub has a series of steps formed by perpendicular and circumferential surfaces. A narrow clearance space is formed between the adjacent surfaces formed between the stationary pedestal and the rotating stepped surfaces. FIG. 6 shows the arrangement of the Kieser stepped seal. When the impeller shifts in the axial direction, the clearance between the surfaces will remain unchanged while those on one side of the impeller and between the surfaces perpendicular to the axis will decrease and those on the opposite side increase. As a result of this axial shift, pressure will build up on one side of the rotor and balance the axial force. However, in this prior art rotor thrust balancing seal, the gap clearance remains the same as the rotor shifts axially and therefore the pressure on that side of the rotor will not vary.
Another prior art centrifugal pump rotor thrust balancing arrangement is disclosed in U.S. Pat. No. 6,129,507 issued to Ganelin on Oct. 10, 2000 which discloses a method and device for reducing or eliminating axial thrust in a rotary machine such as a centrifugal pump or compressor by altering the fluid pressure in a cavity formed between a rotor and a housing. The Ganelin patent is incorporated herein by reference. The device contains a disk placed along the rotor for subdividing the fluid in the cavity in such a way that all annular gap leakage flow is channeled and pumped through the space between that disk and the rotor from the center of the pump towards the periphery. As a result, the pressure in the cavity is altered to reduce and control the axial thrust on the rotor which becomes independent of the wear state of the shaft seals. In another embodiment, the step of flow subdividing is achieved by providing a set of braking vanes along the periphery of the cavity for reducing the rotational speed of the fluid coming from the cavity as well as from the annular gap and a stationary disk placed along the interior wall of the housing for directing the radial flow of that fluid towards the center of the pump.
U.S. Pat. No. 5,385,442 issued to Lehe et al on Jan. 31, 1995 discloses a centrifugal pump with an open-faced impeller in which the balancing chamber communicating with the delivery pipe via a first nozzle whose axial clearance is kept invariable in operation and which is defined by the peripheral end of the impeller itself acting as a balancing turntable, and a nozzle piece secured to the outer rear portion of the casing and interposed between the diffuser and said peripheral end of the impeller. The balancing chamber communicating directly or indirectly with the suction pipe of the pump via a second nozzle. FIG. 7 shows impeller tip seal used in the Lehe et al patent.
The need exists therefore for a device to reduce axial thrust that is simple in design, is easy to install in existing rotary machines, does not require monitoring and control devices in order to work properly, and is effective in its function over a wide range of operating parameters of the rotary machine.
The need also exists for a device to reduce and control axial thrust that would allow reducing or preferably eliminating completely the dependency of the axial thrust forces on the wear state of the seals in a rotary machine.