GB2354825A

GB2354825A - Plant condition monitoring using vibrational measurements

Info

Publication number: GB2354825A
Application number: GB9917877A
Authority: GB
Inventors: Eric Atherton
Original assignee: Individual
Current assignee: Individual
Priority date: 1999-07-30
Filing date: 1999-07-30
Publication date: 2001-04-04
Also published as: AU6298700A; GB9917877D0; WO2001009576A1

Abstract

To remotely monitor the condition of machinery 1 and 2 this system monitors vibration by way of an accelerometer attached to that machinery. This accelerometer then transfers its measured data to a local device 4 on or near the accelerometer which converts the data into a digital signal and then logs the signal in memory. The local processor may perform a fast Fourier transform (FET) on the digital data to identify key elements of the data such as the highest peaks of amplitude and frequency. This digital signal is transmitted to a remote data processing system 11 via a transmission line or a wireless link 6. Trends may be displayed on the screen of this processor or stored in the processor. This system has particular relevance to Electrical Submersible Pumps (ESP) used to raise water in bore holes.

Description

2354825 PLANT CONDITION MEASUREMENT This invention relates to plant

condition measurement. It is particularly, but not exclusively, concerned with electrical submersible pumps (ESPs) which are typically used to raise water or oil from boreholes. The ESP is supplied with power from the surface via an electrical cable. In some boreholes, no flow of fluid to the surface occurs without some sort of pumping mechanism such as an ESP. In other boreholes, the natural flow to surface is very slow, and can be enhanced by an ESP.

ESP's can be very expensive devices, and furthermore, the loss of production of an unscheduled failure may also be very costly. For this reason, attempts have been made to monitor the performance and condition of ESP's while in place at the bottom of a borehole. Such monitoring if successful may extend the life of the ESP, or at the very least provide an early warning of failure, so that a scheduled replacement may be undertaken.

Monitoring systems for ESP"s can currently detect motor temperature, pump inlet and outlet pressures and also a vibration measurement, indicating the average vibration at one point in the ESP. The pressure values provide useful diagnostic information to monitor the efficiency of the pump in situ. The temperature of the motor can provide an early indication of a emerging problem. The averaged, single point, vibration reading however often proves less useful in detecting and diagnosing problems.

Condition monitoring of machinery at the surface is well known. Typically an accelerometer is mounted close to each likely area of failure, such as a bearing, so that many points on a machine are monitored. In addition, a broad range of frequencies is monitored, typically from a few cycles a second (Hz) up to 10kHz. The information is obtained by rapidly sampling the vibration signal from the accelerometer, and then performing a mathematical manipulation known as a Fast Fourier Transform (FFT) on this time domain data. The FFT is performed using a computer system, and the result is frequency domain data ( the frequency spectrum). Analysis and trending of the 2 frequency spectrum can often yield diagnostic information on the condition of the machine, and provide early warnings of impending failure.

To obtain a useful FFT for diagnostic purposes, high frequencies need to be transmitted from the accelerometer to the data acquisition equipment and computer system. This requires a relatively high bandwidth cable, such as coaxial cable. Unfortunately, it is difficult and expensive to obtain such cable suitable for permanent submersion, and so high bandwidth monitoring of ESPs has not been undertaken. In addition, if several points on the ESP are to be monitored, several high bandwidth cables to surface would be required.

The present invention allows multiple high bandwidth monitoring points on an ESP, using a standard submersible cable to convey the data to surface.

According to one embodiment of the present invention, the signal from an accelerometer is rapidly sampled using an analogue to digital converter (ADC) in the same package as the accelerometer. The signal is sampled for a relatively short period of time. Typically the sample rate would be of the order of 20,000 times per second, and the sample period would be of the order of a tenth of a second, giving about 2000 samples. The data gathered during this period is stored in a local temporary memory, also in the same physical package. The data is then transmitted to a remote data logging or display instrument, The data is transmitted in digital format, allowing optional error correcting protocols to be used during transmission. The rate of data transmission can be tailored to the quality of the transmission medium. For example, a slow radio telemetry link may take many seconds to transmit the data gathered during a small fraction of a second. In this way, the remote data logging or display instrument can obtain short bursts of high sample rate vibration data, containing information over a wide frequency range. This enables an FFT to be performed on the data, and standard vibrational analysis methods to be used that require high frequencies to be analyzed, even though the transmission medium between the 3 accelerometer and the remote data logging or display instrument has only a low bandwidth capability.

In one further embodiment of the invention, the FFT is performed locally on the data stored in the temporary memory, by a local microprocessor. The total energy in specified frequency bands is then transmitted to the remote data logging or display instrument. This enables trends to be monitored, while only transmitting a fraction of the data contained in the entire set of samples, or full FFT. This takes even lower data transmission bandwidth. The local microprocessor may also be programmed to identify key elements in the FFT, and transmit an appropriate summary of the information. For example, the local microprocessor may identify the highest peaks in the FFT, and transmit their amplitude and frequency. In the case of monitoring rotating machinery, the frequency of the fundamental vibrational frequency can be used to indicate the rotational speed of the machine, which can be a useful diagnostic parameter.

In a further embodiment of the invention, a multi-drop transmission medium is used, so that many accelerometers, with their associated rapid sampling ADC and local temporary memory, can all share the same network.

A further embodiment of the invention enables the recording of vibrational information down a borehole, when there is no cable connection to the surface. In this case, the signal from a battery powered accelerometer is rapidly sampled using an analogue to digital converter (ADC), for a relatively short period of time. The data gathered during this period is transferred to a battery powered data logger. A set of samples is only taken at infrequent intervals, so as to conserve the memory in the data logger. Typically, the accelerometer and ADC may be powered off between sets of samples, to conserve battery power. It is important to note that each set of samples contains high frequency vibrational information. Data logger memory is held to practical levels by only gathering sets of samples at long time intervals. The battery, 4 accelerometer, memory and ADC are all housed in a waterproof case, suitable for submersion in boreholes.

Of course the current invention has many applications other than for use in monitoring the condition of equipment in boreholes. The current invention may usefully be applied to the monitoring of vibration in any location where it is not desirable to locate a computer system, and it is also not desirable to connect a high bandwidth data link to that location.

The present invention will be illustrated by way of example in which figure I shows a submersible pump, consisting of motor 1, connected to pump 2. The submersible pump is located in a borehole, 3, and water from the borehole is pumped to surface through tube 5. The power cable to the submersible pump is not shown. Six vibration monitoring packages, 4, are positioned at strategic points on the motor, 1, and pump.2. typically at bearing locations. The vibration monitoring packages, 4, are connected together and to the surface interface box, 7, by cable, 6. The surface interface box, 7, is connected to computer, 11 - The surface interface box, 7, provides power to the vibration monitoring packages, 4. The surface interface box, 7, also interfaces the standard RS232 serial interface in the computer, 11, to the multi-drop RS485 interface to the vibration monitoring packages, 4, in a manor well known to those skilled in the art.

Figure 2 shows the inside of a vibration monitoring package, 4. Accelerometer, 8, outputs an analogue voltage signal proportional to the acceleration being experienced by the accelerometer. This voltage signal is convert.ed into a digital value, by the analogue to digital converter, 9. The accelerometer has a frequency response from 1OHz to 10,000 Hz. The ADC,9, takes readings at the rate of 20,000 times per second. These readings are stored in the microprocessor 10 that also contains memory. 2048 successive readings are taken and stored in microprocessor 10. After a set of 2048 readings have been taken, the microprocessor, 10, performs a FFT on the readings, storing the results in it's memory. The microprocessor, 10, then scans the frequency spectrum, picking out the frequency and amplitude of the 10 largest peaks. The amplitude and frequency of these peaks are also stored in the memory.

The computer, 11, interrogates each vibration monitoring package, 4, in turn, and requests the amplitude and frequency data of the 10 largest peaks. The computer 11, then stores these values in a database, and displays trends of these values on the screen.

If a vibration fault trend is located at one particular vibration monitoring package, 4, the operator of the computer, 11, can request the full FFT from that location for detailed analysis on the surface.

I I

Claims

1 A method of monitoring remotely the condition of a machine which in operation generates vibration comprising the steps of: generating by means of an accelerometer located on a selected position on the machinery an output analogue signal representing a detected frequency of vibration as time domain data; converting the analogue signal to a digital signal and providing a sampled output of time domain data; all the aforesaid steps being undertaken by means located on or by way of the machine; and linking at least periodically the sampled output as aforesaid as aforesaid or data based on the sampled output to be transferred by a link to a remote data processing system.

2 A method of monitoring remotely as claimed in Claim 1 including the further steps following the converting step of undertaking a transform process such as a fast Fourier transform on the time domain data; and storing the results of the transform process; prior to the linking step.

3 A method of monitoring remotely as claimed in Claim 1 or Claim 2 wherein the linking step involves the transmission of data in digital format.

4 A method of monitoring remotely as claimed in any preceding claim wherein the linking step is undertaken by way of a transmission line or a wireless link.

Apparatus for monitoring remotely the condition of a machine comprising a unit for location on a selected position on the machine including:

1) an accelerometer having an output analogue signal representing a detected frequency of vibration at the location on the machine as time domain data; 4) an analogue to digital signal converter adapted to receive the output analogue signal and to provide a digital signal output representing the sampled data; and 5) a data storage device for the storage of the digital signal output; and 4) a link output whereby, at least periodically, the data storage device or data based thereon can be transferred by way of the link to a remote data processing system.

6 Apparatus as claimed in Claim 5 wherein the unit further includes a processor adapted to undertake a transform process such as a fast Fourier transform on digital signal output representing frequency domain data time domain data; prior to submission to the link output.

7 Apparatus as claimed inClaim. 5 or Claim 6 wherein the processor is programmed to identify key elements or particular results following the transform process such as amplitude and frequency of highest peaks in the transform.

8 Apparatus as claimed Claim 5, 6 or 7 wherein the processor is used to establish operating para meters of the machine such as rotational speed by way of fundamental vibration frequency.

9 Apparatus as claimed in' Claim 5, 6, 7 or 8 wherein the unit includes a data logger.

e Apparatus as claimed in Claims 5, 6, 7, 8 or 9 adapted to provide information to a location remote from the unit by way of a transmission line or a wireless link.

11 A method of measuring plant condition as hereinbefore described with reference to the accompanying drawings.

12 Apparatus for measuring plant condition as hereinbefore described with reference to the accompanying drawings.