In an iron and steel plant there are 20 centrifugal fans in which are permanently installed one accelerometer in each one of them in their bearing housings, coupling side on horizontal direction , in order to permanently monitoring their vibratory level.
Each accelerometer is connected to a transmitter which converse the vibratory signal of the accelerometer to an electric signal of 4 to 20 mAmp. proportional to the first one. This electric signal is sent to a PCL that shoot an alarm in presence of abnormal vibration levels.
A detailed diagram of a typical installation is shown in figure 1 (see attached file).

The accelerometer is a CTC AC 102-A model of 100 mV/g. The transmitter is a SKF CMSS530 model (velocity transmitter) with a range of 0 to 25 mm/sec. (0 mm/sec.=4mAmp.; 25 mm/sec.=20 mAmp.).

The detected problem for some of the 20 fans is the non proportionality of the electric signal at the outlet of the transmitters, in relation to the entrance of vibratory amplitude present in the ventilators (errors up to 45 %).

There is a problem that affects the current lectures at the outlet of the transmitters, but doesn’t affect the vibratory velocity lectures that are carried out from the transmitter.

According the previously indicated, we have the following questions:

- Which could be the problem that affects the current values at the outlet of the transmitter and not at the outlet of the vibratory level?
- Must the proportionality between the vibration and the current be maintained all over the transmitter’s vibration range?
- When there are low vibratory levels (mostly noise), could the proportionality relation between the vibration and the current be affected?

figure1.pdf (23 Kb, 40 downloads) figure 1

1. There may be a problem with the scale; what is normal velocity? Normal operating conditions should be between 10 – 12 mA, in this case 10 – 12 mm.
2. The proportionality is made linear through a single integration to velocity and the corresponding 4 – 20 mA output.

RE_Posting_board_question.htm (50 Kb, 10 downloads) CTC response