Condition Monitoring of LNG Boil-Off Gas Compressors with PROGNOST®-NT
To ensure the safety and availability of a LNG plant, continuous operational condition monitoring, data on a machine’s condition, must be permanently collected and assessed. The focus lies on data that indicate a rapid change in the condition of the machine, e.g. vibrations by segmented analyses or operating condition dependent warning threshold checks. This gives early warnings and minimises possible consequential damages and related high costs.
The mechanical safety of both stages is monitored by acceleration sensors, which are mounted on the outer frame close to the support, which connect each crosshead slide with the frame. Thereby the operator receives detailed information of the condition of each stage.
Specific modifications have to be made during the installation of the pressure sensor and vibration sensor at the cylinder. The pressure sensor measures the indicated dynamic pressure in the compression chamber and the acceleration sensor the cylinder vibration.
The suction temperature of the first stage is around –140 °C. The low gas temperature causes an ice coating of up to 100 mm around the cylinder. Due to this low temperature specialities, the accelerometer and dynamic pressure sensor needs to be located outside the cylinder ice cover. Thus the dynamic pressure sensor is attached to a drilled valve cover and the acceleration sensor is attached to the elongated valve cover bolt
Monitored parameters
Continuous monitoring of the accelerations at the crosshead slides and the continuous comparison with individual warning thresholds for each load step is necessary to detect failures in the drive trains. Damages or clearances in the drive train, e.g. a loose crosshead – piston rod connection result in higher vibrations in certain parts of the stroke. For every revolution, the crosshead slide vibration signal is subdivided into 36 segments and the RMS values are calculated for each segment and compared to the safety limit for each segment. Thus the safety protection is given for the compressor and the plant.
Waterfall diagram of both crosshead slide vibrations for a period of three hours showing the start up process and a 75 % to 50 % load change
When compressors are operated with several load steps, an effective monitoring is only given, if individual operating conditions with individual warning thresholds for each analysis can be defined.
The following online signal display the indicated dynamic pressure at the head end and the crosshead slide vibration (upper part of the figure) with the 36-segmented analysis (bottom) for the load at 50 % for one revolution.
Segmented RMS values of crosshead slide vibration at 50 % load for one revolution
Operating condition based monitoring
PROGNOST®-NT has the unique capability to detect changing operating conditions automatically. This ensures thight and precisely set thresholds for all condition monitoring tasks. After running the compressor in all possible operating conditions, the automatic detection of these conditions and the parallel setting of thresholds can be activated. The number of different operating conditions is unlimited.This system capability is available for all compressor types.