ArticleDetermining Vibration Severity with An Excel Spreadsheet
by Robert X. Perez
This paper shows how a simple Excel spreadsheet can be used along with a new approach developed to access machine condition. The new approach is described in detail in an earlier paper and requires knowledge of a machine’s current and baseline vibration levels. A way to compensate for machine criticality is proposed and three examples are given.
“A previous article  described a general machine evaluation approach for casing vibration measurements based on accepted assessment methods. A general method was developed to enable users to make machine-specific decisions that account for machine construction and criticality. A dimensionless number termed the Severity Level S allows consistent comparisons of mechanical condition across diverse machine populations. The Severity Level regions are defined as:
S<1 = newly commissioned levels S>1 but <2 = watch for upward trends S>2 but <3 = plan for repair S>3 = failure is imminent, prepare to shut down
The meaning and importance of (a), a term used in the ISO Casing Vibration Severity guidelines , was also described.
The author has developed an Excel spreadsheet  that assesses severity levels based on input values and provides a recommended course of action. The spreadsheet contains a Calculator page and a Risk Matrix page. Examples are given to illustrate the proper use of the calculator.
A 20 HP centrifugal process pump had an initial vibration level of 0.15 IPSrms (a) at the inboard horizontal (PIBH) position. After two years of operation, the PIBH vibration level was 0.46 IPSrms (V). The pump is fully spared and does not represent a significant safety or environmental risk.
The nature of the operation and the risk matrix (Table 1) indicate that the pump has a low criticality (K= 2.51). This information is used in the Vibration Severity Calculator. Low is selected on the criticality pull down menu (Table 2). The severity level is 2.218, just inside the Plan to Repair region. There is plenty of time to plan a repair.
A 750 HP induction motor driving a centrifugal fan had an initial vibration level of 0.08 IPSrms (a) at the motor outboard vertical (MOBV) position. The MOBV vibration level was 0.30 IPSrms (V) after a year of operation. When the motor/fan train is down, the unit can operate at only 50% of the rated production. There is very little safety or environmental risk if a catastrophic failure occurs.
Table 1 indicates that this is a medium criticality motor (K = 2). The Vibration Severity Calculator (Table 3) and Medium on the criticality pull down menu indicate that the severity level is 2.907. This places the motor just outside the Prepare to Shutdown region. Management should be alerted that the motor is nearing the Prepare for Shutdown region and that plans should be made for an immediate shutdown.”
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