The Monitoring System

(Журнал фирмы Брюль и Къер"The Monitoring System")

      The Monitoring System can be considered as made up of a number of sections: the vibration pickup section; the monitoring section that continuously monitors vibration levels both broadband and narrowband; and the datasection that is used for data storage and management of the machine condition data. Off-line data can be entered into the system from a tape recorder for periodic monitoring.
      The system was developed by the Brtiel & Kjaer System Development Group, specifically for the needs of Statoil. It is not a standard production item.

      Vibration Pickup.
      Vibration pickup is made by acceler-ometers permanently mounted on the monitored machines. These are highly sensitive, rugged transducers capable of withstanding long periods in "harsh" environments. The accelerom-eters are certified as intrinsically safe for use in the potentially explosive atmosphere present in the petrochemical plant. Signals from these acceler-ometers are routed via preamplifiers to a local control room.

      Broadband Monitoring.
      Vibration signals are routed first to a number of Local Monitors, located in the local control room, where the broadband monitoring is carried out. Each monitor has 16 parallel vibration channels and 4 logic inputs. The logic inputs are used to automatically activate and passify vibration channels on machine start up and shut down.
      The Local Monitor registers the vibration signals at one second intervals, and compares the RMS, Peak and Spike Energy levels with preset limits. If any of the preset limits are exceeded, a message is sent to the central computer and a warning given on an alarm printer.
      In addition to monitoring the signal levels every second, the Local Monitor compiles and stores 1 minute and 24 hour average levels. The 1 minute average is used for a trend alarm, where the percentage increase of the average value is compared with a preset limit, a warning being given in the event of the limit being exceeded. Both the one minute and 24 hour average values can also be called up from the central computer for trend curve displays.

      Narrowband Monitoring.
      For narrowband monitoring, the system's FFT analyzer transforms the time signal into a spectrum of narrowband frequency components. This allows monitoring with a far better resolution than with broadband monitoring. The central computer directs analogue signals from the Local Monitors to the system's Frequency Analyzer. To obtain a sufficient frequency resolution in the high as well as low frequencies, the system takes a number of narrowband spectra and combines them together to form one constant-percentage-bandwidth spectrum. In such a spectrum the bandwidth is a percentage of the centre frequency, and the same resolution (user chosen between 4% and 23%) is obtained over the whole frequency range.
      By recording one of these constant-percentage-bandwidth spectra when the machine is in "good" condition (this is then called the Reference Spectrum) and adding a tolerance, a Reference Mask is formed. Subsequent spectra are then compared to this Reference Mask (a procedure called the Spectrum Comparison), and any frequency components that increase to a level above the Mask will be reported.
      Automatic spectrum comparison normally takes place every 3 hours for each monitoring point. However, in the event of an alarm, the analysis speed will automatically be increased to 1/2 hourly intervals for the appropriate channel. To study the variation of the vibration levels with time, trend analysis can be performed. This can either be carried out at single frequencies or over frequency ranges. The system's large data-base allows a large number of spectra to be stored and used in the trend analysis, thus giving a good overview of the behaviour of the vibration level and allowing predictions of when the levels will become critical. The lead time to a preset danger level is also calculated.
      Having located a fault, the engineer in charge can interrupt the system's automatic scan mode and then use the high powered diagnostic functions of the FFT Analyzer to diagnose the cause of the fault.

      Off-line Monitoring.
      Vibration data from the periodic measurement program are recorded on a 4-channel tape recorder for later play-back into the system. Play-back is carried out via the system's FFT Analyzer, and the vibration data is then handled as with the on-line (permanent) system, i.e. by performing spectrum comparison, trend analysis etc.