Vibr Axial Disp

1. INTRODUCTION It is normal for all machines, even in peak performance conditions, to vibrate and make noise. The level of vibration is dependent on the operating condition of a mechanical system: when vibration increases, deviating from an acceptable level, it is a sure indication that some component is deteriorating. Development of reliable vibration measurement instrumentation now gives us sufficient information for a detailed analysis of the state of health of any mechanical component. Analysis of vibration signals, combined with experience and familiarity with the machine are the keys to relatively accurate troubleshooting. 2. BENEFITS Although machinery protection alone is sufficient reason for equipping critical machine parts with supervisory instrumentation systems, monitoring systems can be of advantage for other reasons. They are essential for diagnostic and predictive maintenance, which is a growing industry trend and replacing preventive maintenance wherever possible as preventive maintenance generally has a high economic penalty which is not always justified in technical terms. A predictive instead of a preventive approach means operating on a machine or component just when needed, and no service as long as the system is running reliably. Early identification of machinery problems therefore offers a number of benefits, including:

GE Power Systems Oil & Gas Nuovo Pignone

CM&

1) shutdowns can be timed; 2) replacement parts can be prepared in advance, saving time; 3) elimination of defects causing vibration significantly reduces noise. 3. RADIAL VIBRATION Two no-contact proximity probes are mounted, offset 90°, on the front support and two on the rear support (or directly on the exhaust casing cap); when the existing arrangement doesn’t allow such an installation, probes are mounted on the journal. The proximity probe signals are sent to a control room monitor, which: • displays the peak-peak value of the signal from each probe • performs alarm and trip functions on each probe • provides the power supply for field probes. A third probe, a Keyphasor probe, is installed on the toothed wheel cover plate. The Keyphasor, acts as a phase reference, ie, it generates a pulse every time a notch made in the shaft passes under the probe. By correlating the signals from the two proximity probes and the keyphasor, an elliptical wave shape is displayed (for instance on a 2-channel oscilloscope) representing the shaft orbit. By simply observing deviations from the ideal orbit pattern, it is possible to identify any defects or irregularities. Every defect generates a characteristic vibration mode and is represented in the form of a particular

CM&Us

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STEAM STEAM TURBINES TURBINES

VIBRATION AND AXIAL DISPLACEMENT MONITORING

TU/VA08-9/2002

VIBRATION AND AXIAL DISPLACEMENT MONITORING vibration frequency. Analysis of vibration frequencies identifies many types of problems including, for example, unbalance of rotating parts and reduction of the oil film which lubricates journal bearings. Fig. 1 shows the string of instruments from field to control room. 4. AXIAL DISPLACEMENT No-contact probes are also used to measure rotor axial displacement. The installation comprises two probes which guarantee redundancy; the probes are installed on the toothed wheel cover plate (inside a protection box) or on the front support. The average values of the signals from the probes can be displayed on a dedicated monitor integrated in the previously described radial vibration monitoring system. 5. INSTALLATION AND MACHINE MODIFICATIONS All the modifications necessary for probes installation (included shaft burnishing in the area exposed to the vibration

probes) can be done on site with modification drawings supplied by Nuovo Pignone. 6. SCOPE OF SUPPLY • 2 axial displacement probes on the bracket cover plate or on the front support; • 4 radial vibration probes for both journal bearings; • 1 keyphasor; • materials for probe installation; • extension cables and proximitors for all probes; • local junction box for installing proximitors; • cable sheathing up to the junction box; • control room monitor to suit the number of probes; • Zener barriers for each probe (if necessary); • mechanical modification drawings for installing the probes and wiring diagram. Interconnecting cables between the local junction box, barrier and monitors can be supplied on request.

FIELD

CONTROL ROOM

SYSTEM MONITOR

DUAL THRUST MONITOR

X

NO CONTACT PROBES

Y POWER SUPPLY

ORBIT

Nuovo Pignone S.p.A. via F. Matteucci, 2 - 50127 Florence (Italy) Tel. +39-055-423-211- Fax +39-055-423-2800 E-mail: [email protected] www.gepower.com/geoilandgas

DUAL VIBRATION MONITOR

ZENER BARRIER

LOCAL PROXIMITOR

COMK/MARK 704/II- Studio Tre Fasi - 9-2002 ©2002 Nuovo Pignone S.p.A. all rights reserved

Figure 2