Rotating compressor surge detection using Variable Speed Drive signals Michal Orkisz, Piotr Lipnicki ABB Corporate Research, Krakow, Poland
[email protected]
Abstract— Early surge detection in rotating gas compressors is an important part of any compressor control strategy. To minimize potential equipment damage the surge detection needs to be as quick as possible. Traditionally it is done by looking at process signals, such as pressures and temperatures. Powering compressors by variable speed drives gives opportunity to use additional drive signals for surge detection. This paper focuses on combining process and drive signals to detect a surge. Based on measurements from an experimental compressor rig several surge indicators are proposed. Keywords—Rotating compressors, surge detection, variable speed drives
I.
INTRODUCTION
A. Industrial perspective Gas compressors play an important role in many industrial processes, e.g., in the Oil, Gas & Petrochemicals business. Large rotating compressors have typically been driven by gas turbines, but solutions based on electrical motors and variable speed drives are gaining popularity due to their enhanced flexibility and reliability. Gas flow through a compressor system can undergo various instabilities, such as rotating stall, light surge, but most notably a full surge, where the compressor works against too high pressure, forcing the gas flow to oscillate, leading, potentially, to destroying the system. On the other hand, compressor is most efficient when operating near the surge region. Surge avoidance is thus an important part of compressor control. Surge detection is a necessary part of that strategy. Typical surge detection – distance from surge line, increase in temperature, oscillations of pressure, unmistakable noise can be augmented by considering variable speed drive signals [1,2]. In this paper we combine them with process signals available on an experimental compressor rig. B. Theoretical bakcground In a classical approach surge detection is performed by observing the outlet pressure measurement [3]. One has to notice that using only the outlet pressure measurement one can detect a severe surge, which takes form of rapid pressure oscillations. Considering the above, this approach may be too slow. This is because the compressor is already in surge and the flow reversal, as well as axial thrust and radial vibrations could already have harmed the machine and perturbed the downstream process. Surge protection requires an inlet flow
measurement and a differential head pressure measurement (for getting the operating point on the compressor characteristic curve (map) and the proximity to the surge curve). The goal is to perform a preventive opening up of the safety anti-surge valve(s) before the operating point crosses the surge line [3]. In case of having a multistage compression system, any single stage may jump into surge. In this case differential pressure measurements are needed across each stage. In a variant where the detection of surge at an individual stage is not a priority and the inlet pressure is negligible – compared to the discharge pressure – the measurements of the latter two would be sufficient to localize the current operating point on the compressor curve (map). When a discharge pressure measurement (not the inlet flow measurement) is available, the readings for flow measurements must be compensated for pressure and temperature. This is done to recover the inlet flow that is the X-axis of a traditional compressor map. The Y-axis is the differential pressure. When a compressor is in a surge state already, the feedback [3] surge controller may encounter problems to get the machine out of it. The reason are fast and severe oscillations. One of the solutions is an open backup loop, which opens and holds the states of surge valve(s). It is usually triggered by a steep drop in flow or drastic oscillations. This state is kept until the systems goes back into stable conditions – then the feedback control is turned on again. Active anti-surge controllers are designed as classic PID controllers or modern active controllers (fuzzy-logic, model predictive). In [4] the performance of such controllers is evaluated. The conclusions are that an active fuzzy logic surge control system is able to stabilize a centrifugal compressor with a minimum Overshoot (in dimensionless flow and pressure diagrams in terms of time) or even without Overshoot (in dimensionless pressure diagrams in terms of time). Thus, the capability and accuracy of an active fuzzy logic control system is better than active classic control systems (PID) in controlling of the surge phenomenon in centrifugal compressors. To be able to implement good active anti-surge detection, and thus protection, mechanisms, the incipient indicators are needed. This study examines the variable speed Drives signals in order to determine the usefulness of each signal. The procedures are focused on an experimental setup, which allows for fast verification of the proposed approaches. The next section describes the details of performed experiments.
II.
EXPERIMENTS
A. Experimental Setup The measurements were executed on an experimental compressor rig presented in Fig. 1. The rig consists of a 15 kW electric motor supplied by a 15 kW variable speed drive (ACS800 series, from ABB), powering a 5-stage industrial blower. The coupling is direct (no gearbox). Multiple sensors, such as temperature, pressure (absolute and differential) and flow meters, as well as accelerometers (for vibrations) are connected, as illustrated. The gas flow path consists of an inlet (controlled by a manual valve), compressor, tank, a recycle path controlled by a recycle valve (pneumatically actuated), and an electrically actuated output valve.
High speed data collection of these signals has not been effectuated (though feasible), in order to model realistically what may happen at an actual industrial installation, where process variables are collected at a (relatively) low rate. CompressorMonitor vector data collection (using drive’s loggers sampled at 1ms) was triggered manually, in order to coincide with the steps of the experimental procedure. The logger lengths were 255, 511 or 1023 points, depending on the number of simultaneously monitored signals. Where good frequency resolution was required, multiple consecutive logger readings (sampled every 2ms) were combined forming time waveform vectors of up to 13 thousand points. C. Experimental procedure The measurements were carried out in two stages. First, several system startups/coastdowns were performed. Then, three different speeds were used: 3600, 4200 and 4800 RPM (top graph in Fig. 2). For each speed the output valve position was changed in steps, as the middle graph in Fig. 2 illustrates. The output valve started as fully open, and was closed, in steps, until a surge occurred. After each step the system was allowed to equilibrate for several minutes. The step size was 10% away from surge and 5% close to the surge. For the last two runs different recycle valve positions were tried.
Fig. 1. Piping and Instrumentation Diagram of the experimental setup
B. Data Collection Data has been collected using ABB’s DriveMonitor™ [5] system with a CompressorMonitor extension, supplemented by reading process values from an AC800PEC controller, to which the process sensors have been attached. TABLE I. lists the process variables that have been recorded with 2s resolution. TABLE I. PROCESS VARIABLES MEASURED Variable
Comment
Process.DiffPressure
Pressure drop across orifice (between compressor and tank)
Process.DriveSpeed
From drive via DataSet
Process.DriveTorque
From drive via DataSet
Process.InletFlow Process.InletPressure Process.InletTemp Process.OutputValvePos
Feedback signal from valve
Process.RecycleValvePos
Feedback signal from valve
Process.TankPressure Process.TankTemp Monitor.BearingDSTemp
Compressor Drive Side bearing temperature
Monitor.BearingNDSTemp
Compressor Non-Drive Side bearing temperature
Monitor.MotorPhaseTemp
Motor stator temperature
Fig. 2. Top: Speed range: 3600, 4200 and 4800 RPM. Middle: Output valve closing for each corresponding speed. Bottom: Recycle valve positions (final
part of the graph): 0.05 and 0.1. Note the “spikes” in recycle valve positions: they were induced to get out of surge.
to be able to detect the surge based on drive-provided information.
Process.TankPressure [kPa]
30
4800 RPM 20
4200 RPM
3600 RPM
10
0
0
10
20
30
40
50
60
70
Process.DriveTorque [%]
Fig. 4. Above: Regulated variable (frequency). Below: “free” variable (power). Notice the range of changes (small above, large below).
Fig. 3. “Compressor map” plotted using torque and tank (or outlet) pressure. Surge loops clearly visible. Solid lines provided to guide the eye.
D. Compressor Map Compressor map normally depicts the relationship between flow and pressure of gas through the compressor at various speed settings [6]. It allows one to track the operating point, the efficiency, the distance from the surge line, etc. However, the flow quantities are related to the operating parameters of the drive system, such as the speed, power, torque, together with valve settings, so drive signals can be used directly. Below we demonstrate an “alternative” way of presenting a compressor map, using a combination of drive and process signals. It is to be noted that the medium being pumped in our compressor rig is air, therefore we do not observe variabilities such as gas composition/molecular weight. Fig. 3 shows a “compressor map” obtained by plotting tank pressure vs. drive torque. NB, the raw drive torque signal is quite noisy, thus a spread in the horizontal direction when multiple samples are gathered for a given operating point. The surge loops are clearly visible. For 4200 RPM three different output valve settings were used resulting in three different loops. For 4800 RPM two settings were used (top of Fig. 3).
III. DETECTING ONGOING SURGE Surge detection has two aspects [6]. One is detecting the surge once it started occurring. This is necessary in order to take any actions to get out of surge. Normally the detection is done by observing variations in the flow, pressure and temperature. Here we consider various drive signals as well. As they are available continuously and “for free” (drive uses/generates them for control and operation), it is interesting
Another aspect is detecting the approach to the surge line before full surge occurs. Traditionally this is done by considering the compressor map and marking on it a surge control line. Crossing of this line is an indication of being dangerously close to the surge. We shall call this “pre-surge detection” and examine it separately A. Signals with surge signature It is known that failures can occur in different signals as fault signatures [7]. Evidence of surge was obvious in many signals, both internal to the drive and related to the process. This signature was seen both in the process (regulated) variables (such as speed, frequency, even the DC link voltage), as well as the manipulated variables (RMS current, torque, power). Fig. 4 illustrates both. The difference in the range of amplitudes (small for process, large for manipulated variables) is apparent. In high-frequency data the surge occurrence is also obvious, as illustrated for the drive’s speed signal in Fig. 5. TABLE II. below summarizes the usefulness of different signals for determining when the system is in surge.
Fig. 5. Speed signal from drive’s data loggers sampled at 200Hz. The sinusoidal signal corresponds to surge.
TABLE II.
SIGNALS WITH SURGE SIGNATURE
Variable
Comment
Drive.CurrentRMS
Large oscillations of the value
Drive.Power
Large oscillations
Drive.TorqueRef
Large oscillations
Drive.Torque
Large oscillations
Process.DriveTorque
Large oscillations
Process.InletFlow
Large oscillations
Process.InletPressure
Large oscillations
Process.TankPressure
Large oscillations
Process.DiffPressure
Visible oscillations
Drive.DCVoltage.Log Drive.Frequency.Log Drive.Phase?Current.Log Drive.Power.Log Drive.Speed.Log Drive.SpeedRef.Log Drive.Torque.Log Drive.TorqueRef.Log
These Basic Logs (255-element vectors) are too short (1/3 of the surge period) to observe the surge oscillations. Large Standard Deviation may indicate a surge, but may also be a process point change.
Drive.PhaseUCurrentVLong.Log
Evident “beating”-modulation at the surge frequency (AM modulation).
Drive.Frequency
Noticeable oscillations (larger than for speed)
Drive.Speed
Small oscillations
Process.DriveSpeed
Small oscillations
Drive.DCVoltage
Very slight indication of oscillations
Drive.SpeedSetpoint
No oscillations
Process.InletTemp
No oscillations
Process.OutputValvePos
No oscillations
Process.RecycleValvePos
No oscillations
Process.TankTemp
No oscillations
Monitor.BearingDSTemp
No oscillations
Monitor.BearingNDSTemp
No oscillations
Monitor.MotorPhaseTemp
No oscillations
Note that the time waveforms in the Basic Logs, which on smaller drives have only 255 values, are too short to uncover the low-frequency features associated with the surge. The period of the surge oscillations is about 0.8s, while the logger time span is 0.255s, roughly one-third of the surge period.
Fig. 6. Surge loops at 4200 RPM (fragment of Fig. 3). Three different output valve openings are represented here. Solid lines drawn to guide the eye. 25 16%
20 11% 15
M P R
21%
10
5
0 0.9
1.0
1.1
Fig. 6 shows a magnification of the surge loops from Fig. 3. An obvious question is what the frequencies of the surge oscillations are, and do they differ for different conditions? To answer this, long-term waveforms of speed drive data have been constructed from consecutive logger time waveforms (about 13000 points), and then a spectrum taken. Fig. 7 shows a magnification of the resulting spectra, corresponding to the loops in Fig. 6. The variation in surge frequency is minimal, even though the loops in Fig. 6 look very different.
1.3
1.4
1.5
1.6
Hz Fig. 7. Drive Speed spectra for 4200 RPM and different output valve openings (indicated as percentage values on the graph). Sample length = 53.2s
TABLE III. summarizes the surge frequencies for different speeds, output valve and inlet valve positions. Only the recycle valve has a significant effect on the surge frequency, as could be expected. TABLE III.
B. Surge frequency
1.2
SURGE FREQUENCIES
RPM
Output Valve
Recycle Valve
Surge Frequency [Hz]
3600
20.8%
-
1.201
4200
20.9%
-
1.226
4200
15.6%
-
1.280
4200
10.9%
-
1.266
4800
20.9%
-
1.259
4800
15.6%
-
1.278
4800
15.6%
5%
1.247
4800
10.9%
10%
1.063
C. Fast surge detection In order to undertake surge countermeasures, the surge needs to be detected quickly. In fact – the quicker the better. There are different methods for surge detection [8, 9]. The natural timescale to compare to is the period of the surge oscillations. In our case the period is ca. 1.2 Hz. Generally it is known to be in the range of 0.3-3 Hz. It is straightforward to detect the surge after several oscillations: some examples are described above. However, several seconds may elapse before we are certain – posing a risk to the equipment. Therefore it is desirable to detect the surge at the earliest possible stage. Additionally, the surge “signature” needs to be distinguishable from normal drive operations, such as startup, or step change of the operating point. Several pairs of drive signals were examined. A promising pair is presented in Fig. 8, plotting drive power vs. drive frequency. Each point/curve represents 0.254 s, or about 1/5 cycle. The pronounced line at the bottom left corner corresponds to a surge – it is a fragment of a surge loop, analogous to those presented in Fig. 6. It is significantly different from the “normal” points. Furthermore, it is clearly different from an operating point change, where power and frequency increase in tandem (Fig. 9).
An indicator for being in surge can be constructed based on the above observations. For example, when the axes are scaled so that the surge loop is at -45°, the magnitude of the projection onto that line can be taken as a surge indicator (with appropriate limit values).
IV. PRE-SURGE DETECTION – ATTEMPTS It would be preferable to tell if the surge is going to occur before it actually has a chance to happen. In other words, it would be great to be able to tell the actual distance to the surge based on actual measurements (rather than on pre-determined surge line). In order to investigate such possibility, several hypotheses were put forward, and then measurements were analyzed to verify these hypotheses. At this stage no positive indicators were identified. The hypotheses were as follows: · Information is contained in statistical distributions of signal fluctuations · Information is contained in cross-correlations between signal fluctuations · Information is contained in the frequencies present in the system A. Hypothesis 1 – statistical distributions of signal fluctuations It is conceivable that the shape of signal fluctuations distribution of drive or process signals may depend on the gradients on the compressor map, and thus on the distance from the surge line. Indeed, from among the signals and distribution moments tried, skewness of torque fluctuations distribution seems to change with the changes of output valve position. However, the correlation was at best tenuous, and at this stage it cannot be used as a surge proximity indicator.
Fig. 8. Detecting surge in high-speed (1kHz-sampled) signals of power and frequency (sampled simultaneously) Each point/curve represents a time series of 0.255s.
Power – Min(Power) [%]
15
10
5
0
0
0.1
0.2
0.3
0.4
0.5
Frequency - Min(Frequency) [Hz]
Fig. 9. Difference between surge lines (slanted) and a speed-up line (almost horizontal)
B. Hypothesis 2 – cross-correlations between signal fluctuations It is conceivable that fluctuations of pairs of signals related to the operating point should be different at different points on the compressor map. This would be the case if the fluctuations were related to some physical quantities, and not purely to measurement noise. In order to verify this hypothesis, high frequency drive signals (1ms sampling rate, collected simultaneously), were plotted against each other. Fig. 10 shows an example for instantaneous power vs. instantaneous speed, corresponding to the top branch in Fig. 3 (4800 RPM). Each “cloud” consists of 511 points spanning 2.55s. Decreasing the power corresponds to approaching the surge. It is easy to see the surge evidence at the bottom of the figure (power < 50%). However, there is no evidence of the shapes of the “clouds” changing with the proximity to the surge, as would be expected if the hypothesis were true. Perhaps high-speed process variables would better reflect surge proximity. This, however, was beyond the scope of this study (limited to low-frequency acquisition of process variables).
REFERENCES [1] 100
Power [%]
[2]
[3] [4] 50
[5] [6] 0
4760
4780
4800
4820
4840
Speed [RPM] Fig. 10. Instantaneous power vs. speed as the compressor moves towards surge
C. Hypothesis 3 – frequencies present in the system We have observed that the spectra of drive signals (such as speed, torque, power) contain low-frequency oscillations (in the range of a few Hz). Can the frequency of these oscillations be indicative of the distance to the surge? While the oscillations frequency seems to decrease with the approach to a surge condition, it is not obvious whether it is possible to extract some measure of the distance. Again, further investigation is needed.
CONCLUSIONS This report has investigated the possibilities provided by examining rotating compressor process signals along with signals from a Variable Speed Drive powering the compressor for the purpose of surge detection. Experiments were carried out on an experimental compressor rig. Process signals included were limited to low sampling rate in order to mimic a real-world case, where high-frequency signals are likely not to be available. Drive signals were recorded with the full available resolution (up to 1 kHz sampling rate). Speed-up and coast-down conditions were examined along with steady-state operation. Alternative “compressor maps” were investigated, demonstrating some useful properties. Several ways to discover surge, after it has started, have been presented. Among the most useful is one that allows to detect a surge within ¼ of the surge cycle. Investigation of real-time measures for the distance to the surge line yielded no obvious candidates, but indicated some hope that, with further effort, such indicators may be found. It is suggested that further experiments, not limited to lowsampling-rate signals should be performed.
[7]
[8]
[9]
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