Turbocompressor antisurge control by vibration monitoring

Abstract

The proposed mechanical method of turbocompressor surge detection uses vibration signals from vibration monitoring equipment mounted on the compressor components to detect a surge event and provide antisurge control thereby. This method utilizes only mechanical information to identify surge, as compared to present day antisurge controllers that use compressor thermodynamic information such as flow, pressure, and temperature to locate a compressor's operating point on a compressor map compared to a surge region.

Claims

1. A method of antisurge control for a turbocompressor, the method comprising: receiving, by an antisurge controller and from at least one sensor, displacement data associated with a rotor shaft of the turbocompressor, wherein the displacement data is received over a period of time in which the turbocompressor is not in a surge condition; calculating, by the antisurge controller, a background displacement level based on the displacement data; receiving, by the antisurge controller, a current displacement level from the at least one sensor; calculating, by the antisurge controller, a value based on the background displacement level and the current displacement level; comparing, by the antisurge controller, the value to a set point; and controlling an antisurge valve based on a comparison of the value to the set point.

2. The method of claim 1, wherein the calculating a value comprises calculating a difference between the current displacement level and the background displacement level.

3. The method of claim 2, wherein controlling the antisurge valve comprises: determining if the difference between the current displacement level and the background displacement level exceeds the background displacement level by more than a predetermined amount or percentage; and opening the antisurge valve in response to determining that the difference exceeds the background displacement level by more than the predetermined amount or percentage.

4. The method of claim 1, wherein controlling the antisurge valve comprises opening the antisurge valve based on the comparison.

5. The method of claim 1, further comprising: selecting a frequency band based on a rotational speed of the turbocompressor.

6. The method of claim 5, wherein the receiving displacement data and receiving a current displacement level comprises: receiving displacement information associated with the selected frequency band.

7. The method of claim 1, wherein the displacement data comprises at least one of radial shaft displacement or axial shaft displacement associated with the rotor shaft.

8. An apparatus for providing antisurge control for a turbocompressor, the apparatus comprising: at least one sensor comprising at least one of a displacement sensor, a velocity sensor or an acceleration sensor configured to measure data associated with a rotor shaft of the turbocompressor; and a controller configured to: receive data from the at least one sensor over a period of time in which the turbocompressor is not in a surge condition, calculate a background level based on the data, receive a current level from the at least one sensor, calculate a value based on the background level and the current level, compare the value to a set point, and control an antisurge valve based on a comparison of the value to the set point.

9. The apparatus of claim 8 wherein the at least one sensor is configured to monitor axial displacement or vibration of the rotor shaft.

10. The apparatus of claim 8, wherein the at least one sensor is configured to monitor radial displacement or vibration of the rotor shaft.

11. The apparatus of claim 8, wherein the at least one sensor is configured to measure at least one of velocity or acceleration of the rotor shaft.

12. The apparatus of claim 8, further comprising: the turbocompressor.

13. The apparatus of claim 8, wherein receiving data and receiving a current level comprises: receiving the data and the current level for a selected frequency band.

14. A method of antisurge control for a turbocompressor, the method comprising: receiving, by an antisurge controller and from at least one sensor, vibration data associated with a rotor shaft of the turbocompressor, wherein the vibration data is received over a period of time in which the turbocompressor is not in a surge condition; calculating, by the antisurge controller, a background vibration level based on the vibration data; receiving, by the antisurge controller, a current vibration level from the at least one sensor; calculating, by the antisurge controller, a value based on the background vibration level and the current vibration level; comparing, by the antisurge controller, the value to a set point; and controlling an antisurge valve based on a comparison of the value to the set point.

15. The method of claim 14, wherein the vibration data comprises one of displacement, vibration or acceleration data associated with operation of the rotor shaft.

16. The method of claim 14, wherein controlling the antisurge valve comprises: determining if a difference between the current vibration level and the background vibration level exceeds the background vibration level by more than a predetermined amount or percentage; and opening the antisurge valve in response to determining that the difference exceeds the background vibration level by more than the predetermined amount or percentage.

17. The method of claim 14, further comprising: selecting a frequency band based on a rotational speed of the turbocompressor.

18. The method of claim 17, wherein the receiving vibration data and receiving a current vibration level comprises: receiving vibration information associated with the selected frequency band.

19. The method of claim 14, wherein the vibration data comprises at least one of radial shaft displacement data or axial shaft displacement data associated with the rotor shaft.

20. The method of claim 14, further comprising: generating an operating point for the antisurge valve based on the background vibration level.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) FIG. 1 is a schematic of a compressor and antisurge control system of the prior art;

(2) FIG. 2 is a schematic of a compressor and vibration monitoring system;

(3) FIG. 3 is a schematic of the antisurge control system and the vibration monitoring system;

(4) FIG. 4a is a logic diagram of a scheme for detecting surge using displacement data;

(5) FIG. 4b is a logic diagram illustrating the calculation of background displacement;

(6) FIG. 4c is a logic diagram of a scheme for detecting surge using vibration data;

(7) FIG. 4d is a logic diagram illustrating the calculation of background vibration;

(8) FIG. 5 is a representative compressor map showing a surge limit and a surge control curve;

(9) FIG. 6 is a representative compressor map showing an original and a new surge limit and an original and a new surge control curve;

(10) FIG. 7a is a logic diagram showing the calculation of a process variable for a PID loop from displacement data;

(11) FIG. 7b is a logic diagram showing the calculation of a process variable for a PID loop from vibration data;

(12) FIG. 8 is a frequency plot of compressor rotor shaft axial displacement;

(13) FIG. 9 is a frequency plot of compressor rotor shaft radial displacement; and

(14) FIG. 10 illustrates a vibration sensor affixed externally to the compressor.

DETAILED DESCRIPTION OF THE INVENTION

(15) The compressor 100 is equipped with a vibration monitoring system, including a vibration monitor 200 and one or more vibration sensors 210, 220, such as an axial displacement, velocity, or acceleration sensor 210, and radial displacement, velocity, or acceleration sensors 220. The vibration monitor 200 provides signal conditioning for the purpose of more accurately detecting surge. Additionally, the vibration monitor provides a signal that may be conveyed to an antisurge controller 140, or directly as a set point to the antisurge valve 150, 250 to avoid, prevent, or recover from a compressor surge. Thus, the vibration monitor 200 may be part of a monitoring system that generates a compressor stability indication based on the mechanical measurements described above. The sensors 210, 220 may include sensors 210, 220 operatively attached to the bearings of compressor rotor shaft 230. A thrust bearing 240 as well as a plurality of radial bearings 245, are illustrated along the compressor rotor or impeller shaft 230 in FIG. 2. The thrust bearing 240 is intended to provide a variable axial force to counter a resultant force due to the pressure forces and the axial component of the substantial derivative of the momentum of the fluid through the compressor. The radial bearings 245 are intended to provide for relatively friction-free rotation of the compressor shaft 230 and to restrict radial displacement of the shaft 230 by presenting a radial-directed force countering any radial component of force presented by the rotor shaft 230.

(16) The axial vibration sensor 210 senses axial displacement, velocity, or acceleration of the compressor shaft 230 at the thrust bearing 240. A signal representing this measurement is transmitted to the vibration monitor 200. Similarly, the radial beatings 245 are shown with radial sensors 220 operatively attached thereto. The radial displacement sensors 220 for the radial bearings 245 transmit radial shaft displacement, velocity, or acceleration signals to the vibration monitor 200. Generally, a rotational speed sensor 260 is provided to sense the compressor shaft's angular speed. The signal from the speed sensor 260 is transmitted to the vibration monitor 200. This signal may be unnecessary, especially for a constant speed driver, such as many electric motors.

(17) Ultimately, an antisurge valve 350 must be actuated under surge conditions to increase the flow rate through the turbocompressor. The antisurge valve 350 may be a recycle valve 150 or a blowoff valve 250. On rare occasion, a compressor's purpose is to provide a vacuum, in which case the antisurge valve is disposed on the suction side of the air compressor, and is actuated the same as the blowoff valve 250. The vibration monitor 200 may provide the antisurge valve position set point directly, as indicated in FIG. 2. Alternatively, the vibration monitor 200 may provide information to the antisurge controller 140 and the antisurge controller then provides the antisurge valve position set point as shown in FIG. 3.

(18) FIG. 4a illustrates a logic chart where the current displacement 405 and background displacement 410 are both calculated using only frequencies within the same predetermined frequency band, as indicated. The predetermined frequency band may be a function of the compressor's rotational speed. This simple logic chart is used to determine if the compressor 100 is in surge or not. As illustrated in FIG. 4b, the background displacement, D.sub.b 410, is calculated by continuously monitoring the shaft displacement in the predetermined frequency band during a predetermined duration of time, t, while the compressor 100 is not in surge and a mean value, D.sub.b 410, is calculated as follows in an averaging operation, D 450:

(19) $D_b=\frac{1}{t}{}_{t_0}^{t_0+t}\mathrm{Ddt}$
where D is a current displacement level, calculated as a suitable vector norm such as a Root Mean Squared (RMS) value of displacement. The background displacement, D.sub.b 410, may he recalculated at different operating conditions any time the compressor is not in surge.

(20) The difference between the current displacement level, D.sub.c 405, and the background displacement level, D.sub.b 410, is determined in a difference operation 415. In other words, d=D.sub.cD.sub.b. The absolute value of d is found in the absolute value operation 420, or |d|=|D.sub.cD.sub.b|. The background displacement level, D.sub.b 410, is divided into the absolute value of d, as:

(21) $r=\frac{.Math.D_c-D_b.Math.}{D_b},$
in the division operation 425. A set point, R, may be a function 427 of the background displacement level, D.sub.b 410, such as (l+n)D.sub.b, where n is a number greater than zero. For instance, if n=0.1. When the absolute value of d exceeds the background displacement level, D.sub.b 410, by 10%, then r=R.

(22) As long as r<R, the comparator function 430 returns a false, thus concluding the compressor 100 is not in surge. When rR, the comparator function 430 returns a true, thus concluding the compressor 100 is in surge.

(23) FIG. 4c illustrates a logic chart where vibrationvelocity or accelerationis used to detect surge. The current vibration 435 and background vibration 440 are both restricted within the same predetermined frequency band, as indicated. The predetermined frequency band may be a function of the compressor rotational speed. As illustrated in FIG. 4d, the background vibration, V.sub.b 440, which may be velocity or acceleration, is calculated by continuously monitoring the shaft vibration in the predetermined frequency band during a predetermined duration of time, t, while the compressor 100 is not in surge and a mean value, V.sub.b 440, is calculated as follows in an averaging operation, V 455:

(24) $V_b=\frac{1}{t}{}_{t_0}^{t_0+t}\mathrm{Vdt}$
where V is a current vibration level, calculated as a suitable vector norm such as an RMS value of velocity or acceleration. Those of ordinary skill in the art are well aware of the calculation of an RMS value:

(25) $V=\sqrt{\frac{1}{N}\underset{i}{\overset{N}{.Math.}}{V}_i^2}$
The background vibration, V.sub.b 440, may be recalculated at different operating conditions any time the compressor is not in surge.

(26) The ratio of the current vibration level, V.sub.c, 435, to the background vibration level, V.sub.b 440, is determined in a division operation 445. In other words,

(27) $r=\frac{V_c}{V_b}.$
A set point, R, may be a function 427 of the background vibration level, V.sub.b 440, such as (1+n)V.sub.b, where n is a number greater than zero. For instance, if n=0.1, when the current vibration level, V.sub.c, 435, exceeded the background vibration level, V.sub.b 440, by 10%, then r=R.

(28) In As long as r<R, the comparator function 430 returns a false, thus concluding compressor 100 is not in surge. When rR, the comparator function 430 returns a true, thus concluding the compressor 100 is in surge.

(29) The above conclusions may be used as illustrated in FIGS. 5 and 6. FIG. 5 illustrates a representative compressor performance map, commonly referred to as a compressor map. Those of ordinary skill in this art are familiar with compressor maps. The abscissa and ordinate variables are preferably dimensionless parameters or derived from dimensionless parameters obtained from similitude. The abscissa variable, is frequently related to the flow rate through the compressor 100. The ordinate variable, .sub.c, is frequently a static pressure ratio or related to a mass specific energy added to the compressed fluid. A more complete list of possible coordinate systems may be found in U.S. Pat. No. 5,508,943, which is hereby incorporated in its entirety by reference.

(30) The individual curves having non-positive slopes in FIG. 5 are performance curves at different compressor rotational speeds. Each curve is for a different value of corrected speed, N.sub.c, which is a function of the compressor rotational speed, N. The left-most curve 500 is the surge limit curve, surge limit line, or simply surge limit. To the left of and above the surge limit 500, the compressor's operation is unstable, and is characterized by periodic reversals of flow direction. This is surge as defined previously. The actual surge limit is sometimes unknown or only guessed at. In this case, the best guessed location of the curve is used in designing an antisurge control system for the compressor. The other curve having a positive slope in FIG. 5 is known as the surge control curve or surge control line. It is displaced toward the stable operating region from the surge limit by a safety margin. This curve is defined by an antisurge control system designer or field engineer based on experience or tests.

(31) A typical antisurge control system will incorporate a digital depiction of the compressor map such that the control system can compare the location of the compressor's operating point to the surge control curve.

(32) Consider a compressor operating point 620 as illustrated in FIG. 6. The surge limit 500 is assumed correct by the antisurge control designer, but may be inaccurate for a number of reasons, including compressor performance degradation over time. The surge control curve 510 is defined based on the assumed location of the surge limit 500. If the analysis illustrated in FIG. 4a concludes the compressor 100 is in surge when its operating point 620 is as shown in FIG. 6, the antisurge control system as schematically illustrated in FIG. 1 may use that information to automatically relocate its surge limit curve 600 and surge control curve 610 as shown.

(33) Other uses of the conclusions drawn from the logic diagram of FIG. 4a include initiating an alarm, either visual or audible, to notify operators of a surge condition; and initiating the recording of operating parameters, such a record being archived for analysis to determine the cause of the surge event.

(34) FIG. 7a illustrates another logic chart where the background displacement and current displacement are both quantified within the same predetermined frequency band, as indicated. The value of r is calculated in exactly the same fashion as detailed for FIG. 4a:

(35) $r=\frac{.Math.D_c-D_b.Math.}{D_b}.$
The set point, R, is used in a difference operation 720, to calculate the value rR. As above, R may be a function 725 of the background displacement, D.sub.b, as illustrated. The value, rR, is used in two separate branches of the logic path, in the lower branch, the absolute valve of rR is determined in an absolute value operation 730. In the upper branch, the value rR remains unchanged. In a summation operation 740, the sum of these two values, i.e., rR|rR| is found. This value must be nonnegative. This last sum is halved in a halving operation 750 before it is used as a process variable, PV, in a Proportional, Integral, Differential (PID) loop. The PID loop then calculates the set point for the recycle valve 350.

(36) In the PID loop, the process variable, PV, signal may be processed to, for instance, reduce noise. Then an output of the PID loop is calculated as:

(37) $X_{\mathrm{sp}}=P.Math.\mathrm{PV}+I{}_t^{t+t_i}\left(\mathrm{PV}\right)d+D\frac{d\left(\mathrm{PV}\right)}{\mathrm{dt}}$
which is used as the set point for the antisurge valve. In this equation, P is the coefficient for the proportional term, I is the coefficient for the integral term, D is the coefficient for the derivative term, and t.sub.t is the loop time of the control loop. Those of ordinary skill in the art are well familiar with PID loops.

(38) FIG. 7b illustrates still another logic chart where the background vibration and current vibration are both quantified within the same predetermined frequency band, as indicated. Here, vibration connotes velocity or acceleration. The ratio of the current vibration level, V.sub.c, to the background vibration level, V.sub.b, is determined in a division operation 710,

(39) $r=\frac{V_c}{V_b}.$
The set point, R, is used in a difference operation 720, to calculate the value rR. As above, R may be a function 725 of the background vibration, V.sub.b, as illustrated. The value, rR, is used in two separate branches of the logic path. In the lower branch, the absolute value of rR is determined in an absolute value operation 730. In the upper branch, the value rR remains unchanged. In a summation operation 740, the sum of these two values, i.e., rR+|rR| is found. This value must be nonnegative. This last sum is halved in a halving operation 750 before it is used as a process variable, PV, in a Proportional, Integral, Differential (PID) loop. The PID loop then calculates the set point for the recycle valve 350.

(40) In the PID loop., the process variable, PV, signal may be processed to, for instance, reduce noise. Then an output is calculated as above, and is used as the set point for the antisurge valve.

(41) A plot of a Fourier transform of the axial vibration data taken from a compressor is shown in FIG. 8. The spike at about 412 Hz represents the rotational speed of the compressor. When surge occurs, the axial vibration level in a range 800 of low frequencies, i.e. 10-70 Hz increases dramatically. Hence, a band of frequencies in this range 800 proves most useful for monitoring for surge in a preferred embodiment of this invention.

(42) The Fourier transform of radial vibration of a compressor is plotted in FIG. 9. Compressor surge will result in increased vibration in a frequency band 900 of 40-60% of the compressor's rotational speed. A band of radial frequencies in this frequency range 900 is used for monitoring for surge in a preferred embodiment of this invention. The exact band is preferably determined individually for each compressor by test.

(43) Another preferred embodiment of the present invention is shown in FIG. 10. This embodiment is particularly suited to retrofits. Where a compressor 100 was not originally outfitted with a vibration monitoring system. A sensor 1010, which may sense position, velocity, or acceleration, is installed on an external component of the compressor 100, such as the housing, volute, piping, etc. where it will provide accurate measurements of the vibrations caused by compressor surge. The same calculation methods and application of the results as explained and illustrated herein are used with the data gathered from such an externally mounted sensor. The location of the externally mounted sensor should be chosen to minimize background noise.

(44) The above embodiments are the preferred embodiment, but this invention is not limited thereto, nor to the figures and examples given above. It is, therefore, apparent that many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.