1. Random nature as to polarity, in that the pulse is sometimes positive and sometimes negative with respect to ground.

Transcription

1 The following data was acquired from a steam turbine driven compressor in refinery service. Ultimately a firm specializing in ESD measurement and control was called in for verification of the problem. The instrument they use to establish ESD arcing to ground had a 500 volt full scale value which was pegged in the attempt to quantify the voltage. Brushes were utilized to eliminate the problem. Readily apparent in the following plots is a spurious pulse that is typical of ESD. This type of phenomenon will typically show the following general characteristics: 1. Random nature as to polarity, in that the pulse is sometimes positive and sometimes negative with respect to ground. 2. The pulse will often occur at the same instant of time with respect to two orthogonal probes, whereas a scratch will be displaced by 90 betwe en two orthogonal probes. 3. Evidence of similar activity in other probe pairs in the machine train tends to discount the possibility of loose probes or electrical connections in the signal path. In the data that follows, the Orbits portrayed in Figure 1 and Figure 5 are from the same instant of time. It is readily apparent that spiking occurs at bearing 2 (turbine inboard) at the same instant it occurs at bearing 4 (compressor outboard). Figure 1 shows a series of direct orbits acquired from the turbine inboard probe pair over a period of approximately 2 minutes. Note the random nature in terms of phase as evidenced by the random angle at which the pulse occurs. Also, it should be noted that the transducers are mounted at 45 left and 45 right of top dead center but the pulse is not directly in line with the transducer, as would the case with a scratch. Figure 2 shows the turbine inboard Y transducer at time 13:59:10.9. Note the positive going spike within the red ellipse. This time stamp corresponds to the orbit within the red rectangle on Figure 1. Figure 3 shows the turbine inboard Y transducer at time 14:00:00.6. In contrast, here the spike, shown by the green ellipse, is negative going. This time stamp corresponds to the orbit within the green rectangle on Figure 1.

2 Figure 1. Turbine Inboard direct Orbits over 2 minute interval

3 Figure 2. Turbine inboard Y transducer at time 13:59:10.9 (note, positive spike)

4 Figure 3. Turbine inboard Y transducer at time 14:00:00.6 (note, negative spike) Another characteristic of the phenomenon is the timing of the pulse when viewed on two orthogonal probes. Shaft runout phenomenon, since the probe pair are displaced at 90 will appear displaced on the time domain signals by 90. Other phenomenon, like ESD, will occur on orthogonal probe pairs at the same instant of time. This is readily seen in Figure 4, where the spikes occur at the same instant of time. In some instances, spiking has been observed at the same instant of time at significantly different locations in the machine train. Figure 4. Turbine inboard time domain plots

5 The source of ESD is not easy to determine while a machine is in operation. The path to ground tends to be the bearing that operates with the highest eccentricity ratio, since this is the shortest path to ground. In general, steam turbines tend to be the source. In the instance of this machine train, it is seen in Figure 5 indicates that the discharge also occurs at the compressor outboard bearing, although it doesn t make itself apparent as often as at the turbine. Unfortunately, highly specialized testing techniques are necessary to ascertain the exact source. In the short term careful monitoring of the DC gap voltages may establish ongoing degradation of the bearings. If you have ESD indicators, it is thus recommended that the DC voltages be logged at least weekly until such time as preventative measures can be instituted. Additionally, the end user should consult with OEM s to ascertain if voltage mitigation equipment for either machine exists. If not, temporary brushes should be put in place. Figure 5. Compressor outboard direct Orbits over same 2 minute interval as Figure 1