The Bode Plots in Figures 1 and 2 below were measured during the start up on the 7th December following the trim balance of the Brg No.s 2, 4, and 5. It can be seen that the Overall vibration amplitudes are dominated by a significant low frequency component. The only other plots measured during the transient survey on the 7th December are the Shaft Centreline and Polar Plots respectively presented in Figures 14 and 15 below.

Figure 1: Bode Plots - Overall Vibration Displacement – HP DE and NDE X&Y and LP DE and NDE X&Y positions. 2

1X Filtered HP vibration amplitudes are acceptable. Phase results are stable. Radial phase lag is approximately 90° between 5X and 5Y positions, and 90° lag between 6X and 6Y. Differential phase between DE and NDE X orthogonal planes also show approximately 90° lag. This suggests some form of preload acting on the HP rotor. This is possibly due to differences in the HP and LP coupling stiffnesses and torque loads.

Figure 1: Bode Plots – 1X Filtered Vibration Displacement – HP DE and NDE X&Y and LP DE and NDE X&Y positions.

The following data plots were measured during the HP compressor vibration survey on the 18th December.

The Overall Trend plots in Figures 3 and 4 below are taken from Brg No.s 5 and 6 on the HP gas Compressor DE and NDE X&Y orthogonal planes. Highest amplitudes are at No. 6 bearing in the X position.

Figure 3: Overall displacement Plot – Brg 5 and 6 X&Y

The high amplitudes shown in Figures 3 and 4 are generated by the low frequency random vibration between 7 and 11Hz frequency range.

The low amplitude areas shown in the trends coincide with Recycle Valve trips. When this occurs the forward gas flow through the HP compressor increases and recycle flow ceases.

Figure 4: Overall Displacement Trend Plot - BRG No. 5 X&Y and 6 X&Y – The high amplitudes are generated by the low frequency random vibration.

Brg No. 5 does show a small increase in vibration amplitude during the periods of the random low frequency vibration, although the peak amplitudes are very low at Brg No. 5. Predominant vibration in the low frequency band is generated at the NDE Brg 6 position.

The overall trend plot in Figure 5 below was measured form Brg No.6X position. This was the location of the dominant amplitudes measured. Figure 5: Overall Displacement Trend Plot - BRG No. 6 X&Y – The high amplitudes are generated by the low frequency random vibration between 7 and 11Hz frequency range.

The Waterfall plot and FFT spectra in figures 6 and 7 below show that the oscillations of overall vibration amplitudes at position Brg No. 6X are being generated by the random low frequency vibration in the 7 and 11Hz frequency band. There is also a small 2nd harmonic of the low frequency peak present in the spectra. Dominant vibration is measured at the Compressor NDE and position Brg 6X.

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Figure 6: Waterfall Plot Brg No. 6X Position when recycle operating.

Figure 7: FFT Spectrum – Brg 5 and 6 X&Y – When Recycle operating

When the recycle valve trips all sub-synchronous vibration disappears. This can be seen in both the Waterfall plots and FFT spectra below in Figures 8 and 9 below.

Figure 8: Waterfall Plot Brg No. 6X Position when recycle trips

Figure 9: FFT Spectrum – Brg 5 and 6 X&Y – When Recycle trips.

The Full spectrum plot in Figure 10 below was measured when the recycle was operating. The data shows the dominant peaks to be in the negative portion of the Full Spectrum (3D Spectrum). Both the 1X Peak and the random low frequency peaks are dominant in the negative portion. This suggests a preload on the NDE of the HP rotor.

Dominant peaks in the negative portion of a full spectrum can indicate a reverse precession acting on the rotor at these frequencies, although the orbit plot data below does not support this. For a reverse precession to be present, both the Full Spectrum and Orbit plot direction of rotation must concur. Therefore, there is no reverse precession acting on the hp rotor.

Figure 10: Full Spectrum Plot – Brg 6 X – When Recycle operating.

The full Spectrum Plot in Figure 11 below was measured when the recycle valve had tripped. It can be seen the sub-synchronous vibration disappears, although the 1X component s still dominant in the negative portion of the Full Spectrum plot.

Figure 11: Full Spectrum Plot – Brg 6 X – When Recycle Trips.

The orbit plots in Figures 12a, b and c below show the overall, 1X filtered , and 5Hz - 15 Hz filtered orbit plots respectively measured when the HP Recycle valve was operating. All plots show a forward precession.

The overall or unfiltered orbit plot shows the sub synchronous random orbit shape superimposed over the overall unfiltered orbit plot. It can be seen that the direction of orbit precession is forward, at both the 1X filtered, and the 5 – 15 Hz filtered frequency bands. The low frequency vibration is presented as internal loops around the annulus of the orbit, and it can be seen that the low frequency vibration rotates, in the direction of shaft rotation, approximately 1X for every 12 - 13 revolution of the rotor.

The unfiltered Overall orbit plot, 1X filtered and 5 – 15Hz Filtered orbit plots measured when the recycle valve was operating are presented below in Figures 12a, b, and c.

12a)

b)

c)

Figures 12: Brg 6X – When Recycle Valve is operating: a) Unfiltered Orbit Plot, b) 1X Filtered Orbit Plot, c) 5-15Hz Filtered Orbit Plot.

The unfiltered Overall orbit plot, 1X filtered and 5 – 15Hz Filtered orbit plots measured when the recycle valve had tripped are presented in Figures 13 a, b, and c below.

The overall or unfiltered orbit plot shows the sub synchronous random orbit shape is more circular and less random in nature. It can be seen that the direction of orbit precession is forward, at both the 1X filtered, and the 5 – 15 Hz filtered frequency bands.

The low frequency vibration is much lower amplitude and does not influence the overall orbit trajectory when the recycle valve trips. It can be seen that the trajectory of the rotor is in the direction of shaft rotation.

1X orbit shape has a slightly elliptic trajectory with a tilt angle of approximately 45° right of TDC.

13a)

b)

c)

Figure 13: Brg 6X – When Recycle trips: a) Unfiltered Orbit Plot, b) 1X Filtered Orbit Plot, c) 5-15Hz Filtered Orbit Plot.

The Shaft Centre-line plot in Figure 14 below shows the shaft lift of the HP NDE bearing during a transient start up.

a)

b)

Figure 14: Shaft Centreline Plot – a) Brg No. 6 HP NDE bearing, a) Brg No. 5 HP DE bearing.

The Polar Plots in Figure 15 below were taken from Brg No.6X HP NDE and Brg No. 5X HP DE bearing positions. They show the amplitude and phase verses speed during a transient start up.

a)

b)

Figure 15: Polar Plots – a) Brg No. 6X HP NDE bearing, b) Brg No. 5X HP NDE bearing.

None of the bearing casing vibration velocity spectra measured during the assessment showed any low frequency vibration. No frequencies coincide with the frequencies seen in the displacement measurements. All amplitudes were very low.

No low frequency vibration was measured at the HP or LP discharge and suction valves, or at the HP dump valve. Velocity spectra measured from the bearing casings and pipework will be presented in the final report. No frequencies coincide with the frequencies seen in the displacement measurements. All amplitudes were very low.

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