Compressor surge counter

A surge counter for a rotating compressor is provided which detects surging by monitoring the vibration signal from an accelerometer mounted on the shaft bearing of the compressor. The circuit detects a rapid increase in the amplitude envelope of the vibration signal, e.g., 4 dB or greater in less than one second, which is associated with a surge onset and increments a counter. The circuit is rendered non-responsive for a period of about 5 seconds following the detection which corresponds to the duration of the surge condition. This prevents multiple registration of counts during the surge period due to rapid swings in vibration amplitude during the period.

BACKGROUND OF THE INVENTION 
This invention relates generally to vibration monitoring instrumentation 
for fluid compressors and more particularly to an instrument for 
monitoring compressor vibration to detect and record compressor surging. 
This invention is a result of a contract with the United States Department 
of Energy. 
In fluid-processing systems, such as a gaseous diffusion system for 
separating isotopes, which require a larger number of rotating, constant 
flow compressors, such as axial flow or centrifugal type for flow control, 
the compressors are designed to operate under certain well-defined stable 
flow conditions. When the flow becomes unstable due to shutdown of a stage 
in the system, plugging of a stage, various scheduled changes in flow 
conditions, or other flow disturbing problems in such a system, 
compressors in the system will surge. Compressor surge may be defined as a 
large amplitude, low-frequency oscillation of the total annulus-averaged 
flow through the compressor. Each time a compressor surges the compressor 
blades and seals are subjected to very high stresses. Excessive surging 
may eventually weaken the compressor blades to a point that deblading is 
possible. 
Therefore, these compressors are designed to withstand a limited number of 
surges over an estimated life span. However, the actual number of surges a 
compressor may experience in a selected period is not known. In a 
diffusion cascade, for example, the only method for counting compressor 
surges involves human interpretation of flow parameters. Therefore, in 
order to obtain a definite record as to the frequency and total number of 
times a compressor experiences surging during operation, there is a need 
for instrumentation to count surges in constant flow-type compressors to 
aid in scheduling compressor blade and seal maintenance or predict life 
expectancy of a compressor in a given system. 
SUMMARY OF THE INVENTION 
In view of the above need, it is an object of this invention to provide 
instrumentation for counting surges in a rotating compressor. 
Other objects and many of the attendant advantages of the present invention 
will be obvious from the following detailed description of a preferred 
embodiment of the invention. 
To achieve the foregoing and other objects and in accordance with the 
purpose of the present invention, as embodied and broadly described 
herein, the compressor surge counter of this invention may comprise means 
for detecting vibration in the compressor and generating an output signal 
indicative of the amplitude of vibration. A rate of change in amplitude 
envelope is monitored by a signal rate monitoring means. An amplitude 
responsive means is provided for detecting a preselected increase in the 
amplitude of the rate of change signal to qualify the detected rise in 
amplitude as a surge condition. Once a surge condition is detected, the 
circuit is rendered not responsive to further amplitude and rate change 
detections for a preselected period corresponding to a known surge period. 
During this period, an output circuit is activated to increment a counter 
which registers the surge count. 
Since it is known that a surge condition will last a definite period, 
typically, a few seconds, in a particular compressor, rendering the 
circuit non-responsive for that period will prevent false counts due to 
amplitude fluctuations during the surge condition period.

DETAILED DESCRIPTION 
Referring now to FIG. 1, there is shown schematically an axial flow turbine 
5 driven by a motor 7 through a drive bearing assembly 9. An accelerometer 
11 is mounted on the drive shaft bearing assembly 9 to monitor the 
rotating compressor vibration. The accelerometer output signal has an 
amplitude and frequency corresponding to the compressor vibration 
magnitude and frequency. The amplitude envelope associated with the signal 
produced by the accelerometer will increase typically between 4 and 6 dB 
in less than one second during the onset of surge in a typical axial flow 
diffusion cascade compressor, for example. While other events often cause 
slower amplitude envelope changes of possibly the same magnitude, these 
changes are discriminated by the surge counter circuit of this invention 
by detecting the rate of increase of the amplitude envelope. Amplitude 
increases that exceed a preselected magnitude in less than one second will 
trigger the circuit to increment a counter 13. 
Since a surge duration may contain several fast amplitude changes, the 
counter circuit, once triggered to increment the counter, ignores all 
other changes over a selected period of time, typically five seconds. This 
time interval is long enough to avoid multiple counts from a single surge 
but allows the circuit to recover in time to detect subsequent surges. 
The signal from the accelerometer is fed to an input amplifier 15 which 
raises the typical 100 mv/g input signal to 400 mv/g. The amplified signal 
is then fed to an amplitude envelope detector 17 whose output is frequency 
limited to about 2 Hz. The change in amplitude must occur in less than 1 
second to obtain a significant change in the detector 17 output level. 
The output of the detector 17 is supplied to a differentiator and buffer 
amplifier stage 19 which is biased to detect and amplify an increase in 
the envelope signal at the detector 17 output which is within the 
restricted rate limit. This signal is then applied to a delayed reset 
comparator 21 which has a preselected amplitude threshold setting 
corresponding to a preselected change in amplitude within the selected 
time frame (less than 1 second) to indicate the onset of surge in the 
compressor 5. Once the comparator threshold is exceeded, it is rendered 
non-responsive for a period of about 5 seconds, at which time the surge 
condition has subsided. 
Each time the comparator threshold is exceeded, an output driver circuit 23 
is activated to increment the counter 13. The counter 13 may take various 
forms, such as an electromechanical counter which is driven by the output 
drive circuit 23. 
Referring now to FIG. 2, the circuits of the system components of FIG. 1 
(outlined by like numbered dashed lines) are shown in detail. The 
vibration signal from the accelerometer 11 of FIG. 1 is applied through a 
coupling capacitor 31 to a non-inverting amplifier 33 which has a gain of 
about 4 to raise the typical 100 mv/g signal at the input to 400 mv/g at 
the output. 
The output of amplifier 33 is connected to the amplitude envelope detector 
17. A series resistor 35 and coupling capacitor 37 are series connected at 
the input to a diode rectifier circuit formed of diodes 39 and 41. The 
negative portions of the input signals are shunted to ground through diode 
41 while the positive portions pass through diode 39 to charge a capacitor 
43 connected between the cathode of diode 39 and ground potential. A 
resistor 45 is connected in parallel with capacitor 43 to provide the 
proper rate of discharge for capacitor 43. 
The ungrounded side of capacitor 43 is connected through a capacitor 47 to 
the inverting input of an operational amplifier 49. The amplifier 49 is 
connected to form a differentiator by means of a feedback resistor 51 
connected between the output and the inverting input thereof and the 
series input capacitor 47. The non-inverting input of amplifier 49 is 
connected to a voltage-biasing circuit 53. The output of amplifier 49 is 
connected through a voltage follower 55 to buffer the output of the 
differentiator. 
The output of the voltage follower 55 is connected to the cathode of a 
diode 57 in the input of the comparator circuit 21. The anode of diode 57 
is connected to the non-inverting input of an operational amplifier 59. 
The amplifier 59 is connected as a delayed reset comparator by means of a 
feedback capacitor 61 connected between the output and the non-inverting 
input thereof. Further, the output is connected to the inverting input 
through a feedback resistor 63. 
Voltage-biasing circuits are connected to the inputs of amplifier 59 to 
control the preselected input voltage threshold at which the comparator is 
triggered. A light emitting diode 65 may be connected in an output circuit 
of the amplifier 59 which is turned "on" each time the comparator is 
triggered to indicate that the monitored compressor is in surge. 
The output of amplifier 59 is connected to the output driver 23. The driver 
circuit includes a transistor switch 67 which conducts each time the 
comparator 21 is triggered and applies a signal to the counter 13 to 
increment the count. 
In operation, the amplitude envelope of the vibration signal from an 
operating compressor 5 is detected by the envelope detector 17. The 
capacitor 43 of the envelope detector is charged through the resistor 35. 
The capacitor 43 and resistor 35 RC charging time constant and the 
discharging resistor 45, forming the RC discharging time constant, are 
selected so that the capacitor voltage essentially corresponds to the 
positive amplitude envelope. A rapid increase in the envelope voltage 
indicative of the onset of surge (a 4 dB increasde in less than 1 second), 
coupled through capacitor 47 of the differentiator 19, will cause a 
correspondingly large negative-going derivative signal to be passed to the 
input of the comparator 21. This large negative-going input to the 
comparator will exceed the comparator threshold and trigger the comparator 
21. Thus, it will be seen that a more gradual change in the amplitude 
envelope of the vibration signal from changes in operating conditions not 
related to surge will not produce a large derivative signal and, 
therefore, not trigger the comparator. 
Once the output of the differentiator goes sufficiently low, it causes the 
non-inverting input of the comparator amplifier 59 to go below the 
threshold level determined by the bias applied to the comparator amplifier 
59. This causes the output of amplifier 59 to go low, turning "on" the 
transistor 67 to increment the counter 13. The comparator is rendered 
non-responsive for at least 5 seconds by means of the value of the timing 
capacitor 61. The low going output voltage of amplifier 59 is impressed on 
the non-inverting input, holding it low until the capacitor 61 discharges 
through resistor 69. Since the discharge resistance is very high, the 
capacitor 61 discharges very slowly, thus, easily providing the 5 second 
delayed reset period. The diode 57 prevents discharging of the capacitor 
through the buffer 55 output. 
Thus, it will be seen that a surge counter for use with a rotating 
compressor is provided that detects a surge condition from the vibration 
pattern of the compressor monitored by an accelerometer mounted on the 
compressor shaft bearing. 
The foregoing description of a preferred embodiment of the invention has 
been presented for purposes of illustration and description. It is not 
intended to be exhaustive or to limit the invention to the precise form 
disclosed, and obviously many modifications and variations are possible in 
light of the above teaching. The embodiment was chosen and described in 
order to best explain the principles of the invention and its practical 
application to thereby enable others skilled in the art to best utilize 
the invention in various embodiments and with various modifications as are 
suited to the particular use contemplated. It is intended that the scope 
of the invention be defined by the claims appended hereto.