ArticleGeneralized Approach to Casing Vibration Assessments
by Robert X. Perez
This paper presents a new method of accessing machine condition using both current and baseline overall vibration levels. The ratio of current to baseline levels is used in conjunction with an equation to determine the machine condition. A new dimensionless severity number (S) is introduced that correlates with the four general ISO condition levels. Assuming the ISO Newly Commissioned Machinery level (a) by machine class to be the baseline level, each increasing level of deterioration from the ISO table was used to form the ratio V/a until points on a curve were plotted S. vs. V/a. Then, curve fitting was performed on the data to arrive at the general severity equation (S). The author provides examples and proposes a way to compensate for machine criticality.
“A generalized machine evaluation approach for casing vibration measurements based on accepted assessment methods is presented. Various assessment guides were surveyed to develop a general method that enables users to make better machine-specific decisions that account for machine construction and criticality. A new dimensionless number termed the severity level, S, is introduced that allows consistent comparisons across diverse machine populations. The meaning and importance of newly commissioned machinery are also described for vibration criteria used both in the ISO Vibration Severity Tables and in the new methodology presented here.
Traditionally, the commonly used vibration alarm level for velocity was 0.3 IPSrms with a danger level of 0.5 IPSrms. It was applied unconditionally to all general-purpose machines and was an empirically-derived convenient rule of thumb that was easy to remember and seemed to fit most situations. A technical basis for this assessment was not available, but it seemed to work.
Improvements to this rule of thumb involved the concept that, if there is a reliable baseline vibration reading, the alarm should be set at two or three times the baseline; shutdown should be set at four times the baseline. This methodology accounts for the fact that baseline amplitudes vary greatly due to size, construction, load, and speed of the machine.
It is known that casing vibration is a ratio of the internal shaking force to machine dynamic stiffness and that vibration is an indication of machine condition but is not an absolute indicator. This article is an attempt to assemble all the relevant factors required to make logical machinery assessments and to develop a new methodology for casingbased evaluations.
General Vibration Guidelines
A survey of the array of machine casing vibration reference tables and graphs used by vibration professionals provides a perplexing picture. Is there a way to generalize the approach to vibration severity? The various guidelines for casing vibration, such as those offered by ISO (Table 1), DLI, and Rockwell Automation, follow a similar mathematical pattern.
This article is an attempt to develop a generalized approach for most common classes of machines. First, every major vibration assessment level was assigned an integer severity value and listed in a table next to the corresponding acceptable ISO vibration amplitude (Table 2). Next the vibration Severity Level (S) was plotted against a Vibration Ratio (R). R is the ratio of ISO vibration limit (V) for the three (3) ranges (i.e., Newly Commissioned Machinery, Unrestricted Operation, and Restricted Operation given in Table 1) to the vibration level expected for a Newly Commissioned Machine (a). A logarithmic curve fit was performed on the data (Figure 1). The data in Table 2 closely fit the following equation.”
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