DYNAMIC THRUST BALANCING FOR CENTRIFUGAL
COMPRESSORS
TECHNICAL FIELD
[0001] The present invention relates generally to centrifugal compressors and,
more specifically, to balancing thrust in such compressors.
BACKGROUND
[0002] A compressor is a machine which increases the pressure of a compressible
fluid, e.g., a gas, through the use of mechanical energy. Compressors are used in a
number of different applications, including operating as an initial stage of a gas
turbine engine. Gas turbine engines, in turn, are themselves used in a large number of
industrial processes, including power generation, natural gas Hquification and other
processes. Among the various types of compressors used in such processes and
process plants are the so-called centrifugal compressors, in which the mechanical
energy operates on gas input to the compressor by way of centrifugal acceleration
which accelerates the gas particles, e.g., by rotating a centrifugal impeller or rotor
through which the gas passes.
[0003] Centrifugal compressors can be fitted with a single rotor, i.e., a single stage
configuration, or with a plurality of rotors in series, in which case they are frequently
referred to as multistage compressors. Each of the stages of a centrifugal compressor
typically includes an inlet conduit for gas to be compressed, a rotor which is capable
of providing kinetic energy to the input gas and an exit pipe which converts the
kinetic energy of the gas leaving the rotor into pressure energy.
[0004] Multistage centrifugal compressors are subjected to an axial thrust on the
rotor caused by the differential pressure across the stages and the change of
momentum of the gas turning from the horizontal to the vertical direction. This axial
thrust is normally compensated by a balance piston and an axial thrust bearing. Since
the axial thrust bearing cannot be loaded by the entire thrust of the rotor, a balance
piston is designed to compensate for most of the thrust, leaving the bearing to handle
any remaining, residual thrust. The balance piston is normally implemented as a
rotating disc or drum which is fitted onto the compressor shaft, such that each side of
the balance disc or drum is subjected to different pressures during operation. The
diameter of the balance piston is chosen to have a desired axial load to avoid its
residual load from overloading the axial bearing. Conventional oil-lubricated
bearings are typically designed to withstand axial thrust forces on the order of four
times the maximum residual axial thrust which are expected to occur during
abnormal, e.g., surging, conditions.
[0005] However, when the gas conditions change during operation in the compressor,
the compensation provided by the balance piston may not be sufficient to avoid
bearing overload. Indeed some types of centrifugal compressors are more likely than
others to experience such gas condition variations, e.g., in gas storage applications for
multistage centrifugal compressors employing parallel operation, wherein the
difference in axial thrust between the first and second sections of the compressor,
linked to flow coefficient difference, may not be readily compensated for by the
balance piston. Thus, conventional oil-lubricated bearings are typically designed to
withstand axial thrust forces on the order of four times the maximum residual axial
thrust which are expected to occur during abnormal, e.g., surging, conditions.
[0006] Another recent development involves substituting active magnetic bearings
(AMBs) for conventional oil-lubricated bearings as the axial (and radial) rotatable
support for the compressor shaft. AMBs operate by based on electromagnetic
principles to control axial and radial displacements within the compressor. Briefly,
AMBs include an electromagnet driven by a power amplifier which regulates the
voltage (and therefore the current) into the coils of the electromagnet as a function of
a feedback signal which indicates displacement of the compressor's rotor inside the
device. AMBs have the desirable attribute that they do not require oil as a lubricant,
reducing overall maintenance of the compressor system and, potentially, removing the
requirement to provide seals between the impellers and the bearing. However AMBs
also have the drawback that they are not capable of handling as much axial thrust as
the conventional oil-lubricated bearings.
[0007] Accordingly, it would be desirable to design and provide methods and systems
for dynamic thrust balancing in such compressors which overcome the
aforementioned drawbacks of existing balancing systems.
SUMMARY
[0008] Exemplary embodiments relate to systems and methods for dynamically
balancing axial loads in centrifugal compressors to reduce residual axial loads on the
bearings used therein. A sensor or probe detects a parameter associated with the axial
load acting on the bearing. Based on the detected parameter, the pressure in a balance
chamber is controlled to adjust the compensating axial force generated by a balance
drum. Advantages according to exemplary embodiments described herein include, for
example, a reduction in the residual axial forces acting on the bearings across
different operating conditions. However, it will be appreciated by those skilled in the
art that such advantages are not to be construed as limitations of the present invention
except to the extent that they are explicitly recited in one or more of the appended
claims.
[0009] According to an exemplary embodiment, a centrifugal compressor includes a
rotor assembly including at least one impeller, a bearing connected to, and for
rotatably supporting, the rotor assembly, a stator, a balance drum disposed between
said at least one impeller and said bearing, a balance chamber, defined at least in part
by an outboard side of said balance drum, and having a balance line connected
thereto, a sensor for sensing a parameter which is associated with an axial load on said
bearing, and a control valve for varying a pressure within the balance chamber based
on the sensed parameter.
[0010] According to another exemplary embodiment, a method for dynamically
balancing axial load acting on a bearing in a centrifugal compressor includes the steps
of detecting a parameter associated with the axial load, and controlling a pressure in a
balance chamber proximate a balance drum in the centrifugal compressor based on the
detected parameter to dynamically balance the axial load acting on the bearing.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The accompanying drawings illustrate exemplary embodiments, wherein:
[0012] Figure 1 is a schematic view of a multistage-type centrifugal compressor,
which can be provided with dynamic balancing mechanisms according to exemplary
embodiments;
[0013] Figure 2 depicts static axial load balancing in a centrifugal compressor;
[0014] Figure 3 depicts dynamic axial load balancing in a centrifugal compressor
according to an exemplary embodiment; and
[0015] Figure 4 is a flowchart illustrating a method for dynamic load balancing
according to an exemplary embodiment.
DETAILED DESCRIPTION
[0016] The following detailed description of the exemplary embodiments refers to the
accompanying drawings. The same reference numbers in different drawings identify
the same or similar elements. Also, the following detailed description does not limit
the invention. Instead, the scope of the invention is defined by the appended claims.
[0017] To provide some context for the subsequent discussion relating to thrust
balancing systems according to these exemplary embodiments, Figure 1 schematically
illustrates a multistage, centrifugal compressor 10 in which such thrust balancing
systems may be employed. Therein, the compressor 10 includes a box or housing
(stator) 12 within which is mounted a rotating compressor shaft 14 that is provided
with a plurality of centrifugal impellers 16. The rotor assembly 18 includes the shaft
14 and impellers 16 and is supported radially and axially through bearings 20 which
are disposed on either side of the rotor assembly 18.
[0018] The multistage centrifugal compressor operates to take an input process gas
from duct inlet 22, to increase the process gas' pressure through operation of the rotor
assembly 18, and to subsequently expel the process gas through outlet duct 24 at an
output pressure which is higher than its input pressure. The process gas may, for
example, be any one of carbon dioxide, hydrogen sulfide, butane, methane, ethane,
propane, liquefied natural gas, or a combination thereof. Between the rotors 1 and
the bearings 20, sealing systems 26 are provided to prevent the process gas from
flowing to the bearings 20. The housing 12 is configured to cover both the bearings 20
and the sealing systems 26, to prevent the escape of gas from the centrifugal
compressor 10. According to different exemplary embodiments of the present
invention, the bearings 20 may be implemented as either oil-lubricated bearings or
active magnetic bearings. If active magnetic bearings are used as bearings 20, then
the sealing mechanisms 26 may be omitted.
0019] The centrifugal compressor 10 also includes the afore-described balance piston
(drum) 28 along with its corresponding labyrinth seal 30. A balance line 32 maintains
the pressure in a balance chamber 34 on the outboard side of the balance drum at the
same (or substantially the same) pressure as that of the process gas entering via the
inlet duct 22. However, according to exemplary embodiments described below, this
balance line 32 includes a control valve which can modulate the pressure in the
balance chamber 34 based upon, for example, sensed axial loading on or near the
bearing 20 as will be described below with respect to Figure 3.
[0020] Initially, however, it will be useful to describe the interaction of the various
elements shown in Figure 1 as they relate to axial loading in general by discussing
Figure 2. Therein, the various axial loading forces associated with operation of the
centrifugal compressor 10 are illustrated conceptually. As shown in Figure 2, the
impellers 16 place an axial load (force) on the bearings 20 in the direction of the
inboard (low pressure) side of the compressor 10 due to, e.g., differences between
stages, changes in gas momentum, etc.. Although not shown in Figure 2, the motor
which rotates the compressor shaft 18 will place a (substantially constant) axial load
in the opposite direction, i.e., toward the outboard (high pressure) side of the
centrifugal compressor 10. To counteract the remaining axial load of the impellers 16,
the balancing drum 28 is designed to exert an axial force in the outboard direction, the
magnitude of which is based on the expected axial load of the impellers minus that of
the motor. This is accomplished by, for example, designing the system such that the
pressure Pu of the process gas on the inboard side of the balancing drum 28 is greater
than the pressure Pe on the outboard side of the balancing drum 28, and by selecting a
balancing drum of an appropriate size (diameter) to generate the desired balancing
force. The pressure imbalance is developed and maintained by providing the balance
line 32 between the balance chamber 34 and the main suction line associated with
inlet duct 22 such that the pressure in the balance chamber is substantially the same as
that on the inboard side of the impellers 16.
[0021] Ideally, the axial thrust compensation provided by the balance drum 28 would
substantially offset the axial load placed on the bearings 20 by the impellers 16, or at
least offset such an axial load enough that any residual load is within the design
specifications of the bearings 20. However, as described above, operational variances
within such compressors and/or the use of AMBs as bearings 20 may cause the
residual loading to exceed the design tolerances of the bearings 20 for axial loading.
Consider, for example, the following Table 1 which illustrates results from an axial
loading test for an exemplary six impeller centrifugal compressor 10 having a balance
drum 28 with a diameter of 231 mm rotating at 17000 rpm. This test compressor was
equipped with AMBs as the bearings 20 which were nominally rated for axial loading
of between +/- 9000 N.
TABLE 1
From Table 1, it can be seen that for flow rates of 73%, 130%, 140% and 141% of the
designed nominal flow rate, the residual axial load, i.e., the axial load placed on the
AMBs 20 of the centrifugal compressor, exceeded the +/- 9000 N rating of the
bearings using the configuration shown in Figure 2, i.e., with an uncontrolled balance
line 32.
[0022] According to exemplary embodiments of the present invention, a control valve
40 is placed on the balance line 32 to enable an automated control of the pressure Pe
which is exerted on the outboard side of the balancing drum 28 as shown in Figure 3.
Therein, the same reference numerals as used in Figures and 2 refer to the same or
similar components of a centrifugal compressor 10. The control valve 40 regulates
the pressure in the balance chamber 34 to vary the reaction force generated by the
balancing drum 28 as a function of, for example, the bearing 20' s displacement or the
axial load on the bearing 20 as measured by a sensor or probe 42.
[0023] The control valve 40 thus controls the value of the pressure Pe and,
accordingly, the amount of compensatory axial load provided by the balancing drum
28. More specifically, by closing the control valve 40, the pressure Pe increases
thereby reducing the amount of compensatory axial load provided by the balancing
drum 28. Alternatively, by opening the control valve 40, the pressure Pe decreases
thereby increasing the amount of axial load provided by the balancing drum 28.
When the control valve 40 is completely open, the maximum amount of compensatory
axial load is generated by the balancing drum 28. Since the amount of load provided
by the balancing drum 28 is, according to exemplary embodiments, controllably
variable, it may be desirable to design the balancing drum 28 such that its maximum
compensatory axial load is larger than that provided by conventional static balancing
drums (i.e., by providing a larger balancing drum 28 to the system) since it is possible
in these exemplary embodiments to reduce the amount of compensation being
provided by closing the control valve 40 as desired.
[0024] As mentioned above, the control valve 40 is controlled based on a feedback
signal from probe or sensor 42 regarding the amount of axial loading that the bearing
20 is experiencing at a given time. Measurements can be made periodically by the
probe or sensor 42 and reported back to control logic 44 which is connected to control
valve 40 to implement any desired control algorithm to open and close the valve 40 as
needed to adjust for operating changes which result in more (or less) residual loading
of the bearings 20. An exemplary relationship between the sensed axial loading and
the operation of control logic 44 to control the gas pressure using valve 40 is
discussed below with respect to Table 2. Control logic 44 may be implanted as an
ASIC, FPGA, computer, or other type of processor and may be implemented purely in
hardware, purely in software or in some combination thereof. The sensor or probe 42
may be any of a number of different types. For example, if the bearing 20 is an AMB,
an induction sensor or probe such as a linear potentiometric displacement transducer
(LPDT) can be used to measure displacement of the bearing 20 due to axial loading.
Alternatively, if the bearing 20 is an oil-lubricated type of bearing, an eddy current
sensor or probe may be a more appropriate implementation of sensor or probe 42.
Other types of sensors, e.g., piezoelectric or sensors which measure pressure at the oil
film in the bearing, may be used as alternatives.
[0025] According to one exemplary embodiment, control logic 44 can include a
proportional integral derivative (PID) controller which automatically, in a closed loop,
changes the pressure in the balancing drum chamber 34 as a function of the measured
thrust on the machine. For example, for AMBs the currents on the coils of the AMBs
are representative of the thrust that is controlled by the system. In particular if the
current in the thrust bearing coil of an AMB exceeds a given value (threshold), the
control logic 44 can act on the valve 40 via a simple PID controller. According to
exemplary embodiments, the control system can be designed with a bias (hysteresis
value) to avoid any thrust valve hunting.
[0026] A test was performed to evaluate the arrangement according to exemplary
embodiments illustrated in Figure 3 and to determine its ability to better control the
residual load on the bearings 20. The test used the same type of centrifugal
compressor 10 which was evaluated above to generate the results in Table 1, i.e., a six
impeller centrifugal compressor running at 17000 rpm, except that the balance drum
28 was increased in size to have a diameter of 247 mm to provide for a slightly greater
maximum compensatory axial load capability in this dynamic balancing arrangement.
The results of the test are shown below in Table 2.
TABLE 2
%Flow Motor Axial Stages Inlet Drum Residual
Rate Load [N] Axial Pressure Axial Load [N]
Load [N] [bar] Load [N]
141 11055 -74644 60 63141 -448
140 11055 -76773 60 66470 752
130 11055 -103554 65 92105 -394
120 11055 -122331 69 109779 -1497
110 11055 -137399 72 125865 -479
100 11055 -149200 75 137265 -881
90 11055 -157029 77 146551 577
80 11055 -161512 79 150734 277
73 11055 -162875 80 152146 326
[0027] It can be seen in Table 2 that the pressure Pe in the balance chamber 34 varies
for at least most of the different flow rates in the table under the control of valve 40.
The control valve is controlled by the sensor or probe 42 and control logic 44 such
that is closed more (lower Pe pressure) for higher flow rates and more open (higher Pe
pressure) for lower flow rates. As can be seen in the residual load column, this has the
effect of controlling the residual load on the bearing 20 to within a much narrower
range than was possible without the dynamic controls according to the exemplary
embodiments. In fact the values are now easily within the design specifications for
axial load handling of the AMBs (+/- 9000 N). Note that in this example the nominal
pressure in the balance chamber (i.e., when the control valve 40 is completely open)
for this test setup is 52 bar. Those skilled in the art will appreciate that the parameters
used in the test setups associated with Tables 1 and 2 are in all respects purely
illustrative.
[0028] It will thus be further appreciated that exemplary embodiments enable for
centrifugal compressors to be outfitted with smaller thrust bearings, since the axial
loading on such bearings can be better controlled. In addition, such compressors are
expected to have higher availability by reducing the residual load on such bearings. A
method for controlling residual axial loading in such compressor systems according to
exemplary embodiments can be performed as illustrated in the flowchart of Figure 4.
Therein, at step 100, a parameter associated with the axial load on the bearing is
detected. Then, at step 102, a pressure in the balance chamber proximate the balance
drum in the centrifugal compressor is controlled based on the detected parameter to
dynamically balance the axial load acting on the bearing.
[0029] The above-described exemplary embodiments are intended to be illustrative in
all respects, rather than restrictive, of the present invention. Thus the present invention
is capable of many variations in detailed implementation that can be derived from the
description contained herein by a person skilled in the art. All such variations and
modifications are considered to be within the scope and spirit of the present invention
as defined by the following claims. No element, act, or instruction used in the
description of the present application should be construed as critical or essential to the
invention unless explicitly described as such. Also, as used herein, the article "a" is
intended to include one or more items.

CLAIMS:
1. A centrifugal compressor (1 ) comprising:
a rotor assembly (18) including at least one impeller (16);
a bearing (20) connected to, and for rotatably supporting, the rotor assembly
(18);
a stator (12);
a balance drum (28) disposed between said at least one impeller (16) and said
bearing (20);
a balance chamber (34), defined at least in part by an outboard side of said
balance drum (28), and having a balance line (32) connected thereto;
a sensor (42) for sensing a parameter which is associated with an axial load on
said bearing (20); and
a control valve (40) for varying a pressure within said balance chamber (34)
based on said sensed parameter.
2. The centrifugal compressor of claim 1, further comprising:
control logic configured to receive an output of said sensor and to control said
control valve according to a predetermined function.
3. The centrifugal compressor of claim 2, wherein said predetermined function
operates to increase pressure in said balance chamber when said axial load on said
bearing exceeds a predetermined value.
4. The centrifugal compressor of any preceding claim, wherein said bearing is an
active magnetic bearing.
5. The centrifugal compressor of any preceding claim, wherein said bearing is an
oil-lubricated bearing.
6. The centrifugal compressor of any preceding claim, wherein said sensed
parameter is displacement of said bearing.
7. The centrifugal compressor of any preceding claim, wherein said sensed
parameter is axial load on said bearing.
8. The centrifugal compressor of any preceding claim, wherein said sensor is an
induction sensor.
9. The centrifugal compressor of any preceding claim, wherein said sensor is a
piezoelectric sensor.
10. The centrifugal compressor of any preceding claim, wherein said sensor is an
eddy current sensor.
11. A method for dynamically balancing axial load acting on a bearing (20) in a
centrifugal compressor (10), the method comprising:
detecting a parameter associated with said axial load; and
controlling a pressure in a balance chamber (34) proximate a balance drum
(28) in said centrifugal compressor (10) based on said detected parameter to
dynamically balance said axial load acting on said bearing (20).
12. The method of claim 11, wherein said step of controlling further comprises:
opening or closing a valve connected to a balance line which controls said
pressure in said balance chamber.
13. The method of claim 1 or claim 12, wherein said step of controlling operates
to increase pressure in said balance chamber when said axial load on said bearing
exceeds a predetermined value.
14. The method of any of claims 11 to 13, wherein said bearing is an active
magnetic bearing.
15. The method of any of claims 11 to 14, wherein said bearing is an oil-lubricated
bearing.