Introduction teria based on rms levels of vibration within unspecified fre-

V quency bands can be misleading.

IBRATION levels on machine frames and bearing The objectives of the work described in this paper were firstly, housings provide a useful guide to such things as state to review the general problems of vibration measurement and of balance and alignment of installations, particularly for criteria making full use of the more sophisticated techniques in commissioning and acceptance trials. In recent years, also, general use for research and development work, and secondly, continuous vibration monitoring of process plant has become to produce routine procedures based on the techniques which accepted practice, sometimes by means of permanent electronic would enable operators to predict rate of wear and deterioration installations coupled to alarm systems, but more often by means of machine components and to diagnose malfunctioning of raa-of periodic checks with hand-held vibration level meters. chines. Thus, it was thought, it would be possible to improve

However, vibration criteria used for the comparative assess- the information supplied to control room operators from con-ment of plant performance are extremely generalized in concep- tinuously monitored alarm systems and, also, to build up sufficient tion, being based entirely on subjective opinions and having little experience on machines with inherent malfunction to enable regard to the dynamic response of particular machines. Dy- plant to be operated to the limit of component failure and, thus namic forces arising from the rotating elements inside a machine reduce the number of unscheduled shut-downs which are so costly produce bearing and seal failure, for instance, and are in lost production on continuously operated process plant, often disguised by extremely stiff bearing supports. Also, cri-

Vibration Criteria Contributed by the Design Engineering Division for presentation . ., . .

at the Vibrations Conference, Toronto, Canada, September 8-10, Operational criteria representing vibration boundary levels 1971, of THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS. for satisfactory or unsatisfactory running conditions are many Manuscript received at ASME Headquarters, June 11, 1971. Paper a n c | varied. Of the more widely quoted criteria, those of Rath-No. 71-Vibr-96. b o n G j Y a t e g ) a n d v D j 2056 are worthy of mention with par-

Copies will be available until June 12, 1972. ticular regard to the way in which these criteria were obtained.

Discussion on this paper will be accepted at ASME Headquarters until October 11, 1971

Paper No. 71-Vibr-96

E. DOWNHAM Professor of Mechanica l Engineering,

University of Aston in Birmingham, Birmingham, England

R. WOODS Engineering Technical Services,

Shell Chemical (U.K.) Ltd., Carr ington, England

The Rationale of Monitoring Vibration on Rotating Machinery in Continuously Operating Process Plant This paper presents a factual discourse on the problems associated with vibration monitor­ing of continuously operating process machinery. The shortcomings of existing vibration criteria are discussed with reference to operational experience and case studies on chemical process plant operated by Shell Chemicals U. K. Ltd. and with particular regard to the wide variations in machine frame impedances which have been measured on a representa­tive number of machines used for ethylene and propylene production, A technique is de­scribed which enables bearing impedances to be measured quickly and safely on opera­tional machines on site. The information thus obtained is used in conjunction with operational vibration levels to interpret general vibration criteria in a more realistic manner, having regard to the dynamic characteristics of particular machines. The philosophy of machine wear diagnosis in relation to prediction of component failure is discussed with a view to the extension of running periods between overhaul and to enable predictions to be made of replacement components requirements prior to dismantling machines, and thereby to reduce overhaul periods. This philosophy is being applied at present to a number of operational machines, and details of the results to date are given. Finally, the type of monitoring system required to provide this additional information, as well as providing the conventional day-to-day alarm facility, is discussed.

Fig, 3 Simplified model of rotating machine for vibration analysis

Transactions of the ASME

Fig. 2 Vibration criteria for rotating machines; V . D. I. 2056 and Shell Chemicals U.K. Ltd., Carrington

Fig. 1 shows the Rathbone and Yates criteria; in both cases the assessment of acceptability was obtained by subjective opinions of several practical engineers and inspectors, followed by vibra­tion level measurements with relatively crude instruments.

Rathbone [ l ] 1 limited his assessments to machinery running at speeds less than 5000 rpm and considered it reasonable to extrapolate his curves for higher rotational speeds. He was also conscious of the fact that the measurement of bearing levels only would not necessarily produce valid criteria for all machines.

Yates [2] produced his criterion by numerous tests on marine

1 Numbers in brackets designate References at end of paper.

2

Fig. 1 Vibration criteria for rotating machines proposed by Rathbone and Yates

geared turbine installations. The fact that in this case the machinery was installed in relatively flexible steel shells, as opposed to the more massive foundations of Rathbone's machines, would account in part for the differences between the two cri­teria. For example, at 22 Hz 'too rough' on the Rathbone curve is classified as only slightly rough on the Yates curve.

I t is also interesting to note that the line classifying an un­pleasant level of vibration from the Reiher and Meister curves of human sensitivity [3] to vibration compare closely with the ac­ceptable boundary levels on machines.

There is also a similarity between these curves and those for building vibrations.

The V.D.I. 2056 specification [4], Fig. 2, is one of the most exhaustive in present use and was compiled from previously pub­lished criteria for five separate classes of machines. Group G machines for power generation and process machinery on heavy rigid foundations were taken from Rathbone's work, although his curves were approximated to straight lines and the maximum rotational speed extended to 12,000 rpm.

The directive places no restriction on the type of vibration measured (e.g., displacement, velocity, or acceleration), but suggests that where several harmonic components are present an rms measurement of the complex signal should be used for com­parison with the given criteria.

S.C.U.K.L., like many other companies, have produced their own guidelines for acceptable vibration levels (Fig. 2) by modi­fying criteria such as those aforementioned to suit their own particular machinery in the light of their own experience.

The significant factor regarding all these criteria is, however, their derivation from subjective opinions with little regard to the dynamic systems to which they are applied. Rathbone recog­nized this factor to some extent when he suggested that resonance of the structure could serve to allow increased tolerances and that machines particularly sensitive to unbalance vibrations might require criteria based on residual unbalance.

The problem can be appreciated better from an examination of a mathematical model. Consider, for example, a relatively simple model of a single machine, Fig. 3.

Here vertical vibrations are considered only, coupling between vertical and horizontal vibrations are ignored, the oil film is represented by a linear spring, and casing modes of vibration are ignored, i.e., coupling between bearing housings is neglected. The system has thus been reduced to seven degrees of freedom if we ignore rotational inertias and consider only linear motion of the masses. To assess the vibratory loads and stresses in such a system by measurements at two points only (i.e., the bearing supports) is therefore meaningless unless additional information is available to supplement the measurements.

Ideally, if the stiffness, damping, and mass parameters for the system were known it would be possible to calculate the loads