Large shaft displacement deviation
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It is a multi-stage horizontal pump; the static measurement of shaft play meets the manufacturer’s specified values. However, the static measurement of mechanical shaft play does not match the value displayed by the DCS, with the DCS value being higher. After multiple calibrations, the values became consistent. However, after the pump was started, the readings displayed on the DCS became increasingly larger; the difference between the two gauges was 0.13 mm, and an alarm was triggered. I have a few questions that I would like fellow sailors to help analyze. 1. The mechanical shaft shift is 0.53 mm; the alarm threshold set by the manufacturer is 0.75 mm, while the shutdown threshold is 0.9 mm. I think there is a logical inconsistency in these values. I checked with the manufacturer several times, and they said there’s no problem. 2. The static test showed that the mechanical displacement was consistent with that of the DCS; once the pump was started, the value displayed on the gauge kept increasing, and there was a significant difference between the two gauges. The instruments have been calibrated and are considered to be in good condition, but this issue occurs as soon as the pump is started. What suggestions do you have regarding the above two issues?
The first issue: the alarm values and shutdown values are fine. The normal serial value is 0.53; an alarm is triggered when wear increases to 0.75, and shutdown occurs when it further increases to 0.9. Secondly, the zero point for serial communication should be specified clearly; generally, the rotor is pushed into position in the direction of positive thrust (with the positive thrust bearing in contact with the thrust plate) as the zero point. An alarm is triggered when the reading reaches 0.22, and the system shuts down when it reaches 0.37. There are also those that use the middle position of the total serial value as the zero point; in such cases, the alarm value and trip value should be obtained by adding 0.265 to the corresponding numbers. For reference only, to see if it is accurate.
Alarm: the jump-out value is ±0.75； 0.9 millimeters. Personally, I think the value provided by the manufacturer is already too high; based on that value, the moving and stationary parts should collide. I’m not sure if my understanding is correct
The motion trend can only move in one direction or the other; to go from alarm to vehicle exit, it is sufficient to increase the displacement by another 0.15. There is a difference between the amount of play caused by manual operation in a static state and that in a pump operating in practice; the thrust during operation is greater than in a static state, so the amount of play is also larger. This error needs to be eliminated.
Under normal operating conditions, the movement of some bearing shells is greater than the amount observed during static tests; this may be due to 1) the force applied during static testing being less than that during operation, especially in pumps with high axial forces； 2. If there are adjusting shims or similar items at the thrust bearing, they may be further compressed under high axial forces, which causes the value to increase.
A large error between the two gauges may also be due to the degree of tightening of the threads on the two probes, as well as the verticality of the measuring disk. It still needs to be determined in conjunction with temperature.
Thrust clearance and stringing amount are two different concepts~~~ Thrust clearance applies only to thrust bearings~~~
@OP, if there’s a difference of 13 microns between the readings on the gauge and those from the DCS, it’s very likely that there’s a gap in the thrust plate fastening system~~~ Please check that carefully.