Pdf part 3 absolute machine specific standards

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Part 3 ? Absolute, Machine Specific Standards

Barry T. Cease Cease Industrial Consulting

September 9th, 2011

1) ABSOLUTE, GENERAL (OK) 2) ABSOLUTE, MACHINE SPECIFIC (GOOD) 3) COMPARATIVE (BETTER) 4) HISTORICAL (BEST)

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Machine Vibration Standards: Ok, Good, Better & Best

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These machine-specific standards improve in relevance versus general standards for most real problems as they are adjusted to best fit the unique design and operation of specific types of machinery. They are typically based on real historical data from equipment fitting the description involved. Some examples are as follows: 1) Technical Associates Standards 2) Sohre-Erskine R/C Standards (shaft vibration, fluid film bearings) 3) ISO 7919 (shaft vibration, fluid film bearings) 4) OEM Specifications

PROS: a) Can be applied to plant equipment from the beginning of a condition monitoring program.

No prior machine history is necessary to make a basic assessment of a machine's condition. b) b) Takes into account the basic differences between different types of machinery & base types

(ie: pump versus fan, rigid versus isolated base, etc).

CONS:

Your plant's machinery, process, loading, speed, mounting, etc is no doubt unique in some ways that can make your final vibration levels end up on the high or low side of these standards without anything being wrong with the equipment or in some cases with a whole lot wrong with the equipment.

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Machine Vibration Standards: Ok, Good, Better & Best

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? These standards account for both the machine type & base type of rotating equipment.

? In the opinion of the author, these standards represent an excellent starting point for overall vibration alarm levels on machinery.

? In addition to these overall standards, recommendations for the levels of common parameters such as 1x rpm, 2x rpm, vanepass frequencies, bearing frequencies, etc are made.

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Machine Vibration Standards: Ok, Good, Better & Best

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Machine Condition

Normal Surveillance Shut down at next convenient time Shut down immediately

Allowable R/C

3,600 rpm

10,000 rpm

0.3

0.2

0.3 ? 0.5

0.2 ? 0.4

0.5

0.4

0.7

0.6

R = Shaft Vibration (pk-pk) C = Diametral Bearing Clearance

R/C Method (fluid-film bearings) Erskine & Sohre have suggested the use of relative shaft vibration (R) and bearing clearance (C) for the evaluation of the condition of machines with fluid film bearings. The state of the bearing is judged by the ratio R/C and rotor speed. This provides a basis that is directly applicable to the specific machine in question. Erskine divided his results into two speed categories -- turbine generators (3,600 RPM) and centrifugal compressors (10,000 RPM). These could also be applied to other machines such as motors & pumps with similar speeds. The work of Erskine was refined by Eshleman and Jackson.

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Machine Vibration Standards: Ok, Good, Better & Best

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? Chart at right is from ISO 7919 and relates relative shaft vibration severity to shaft speed.

? All vibration is relative to bearing (ie: from proximity probes ).

? Shaft vibration is expressed in displacement, micrometers pk-pk

? 100 micrometers ~ 4 thous of an inch.

Zone Descriptions :

Zone A ? Newly commissioned machinery.

Zone B ? Acceptable for unrestricted, long-term operation.

Zone C ? Unsatisfactory for long-term operation.

Zone D ? Damage likely occurring to machine.

A complete copy of this vibration standard is available from the ANSI website at the following:

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Machine Vibration Standards: Ok, Good, Better & Best

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