Vibration Standards

ISO 2372 (10816) Standards provide guidance for evaluating vibration severity in machines operating in the 10 to 200Hz (600 to 12,000 RPM) frequency range. Examples of these types of machines are small, direct-coupled, electric motors and pumps, production motors, medium motors, generators, steam and gas turbines, turbo-compressors, turbo-pumps and fans. Some of these machines can be coupled rigidly or flexibly, or connected though gears. The axis of the rotating shaft may be horizontal, vertical or inclined at any angle. Use the chart below combined with additional factors described in this manual to judge the overall vibration severity of your equipment.

Class l

Individual parts of engines and machines, integrally connected with the complete machine in its normal operating condition. (Production electrical motors of up to 20HP (15 kW) are typical examples of machines in this category.)

Class ll

Medium-sized machines typically, electric motors with 20 to 75 HP (15-75 kW) without special foundations, rigidly mounted engines, or machines on special foundations up to 400 HP (300 kW).

Class lll

Large prime movers and other large machines with rotating masses mounted on rigid and heavy foundations which are relatively stiff in the direction of vibration measurement.

Class lV

Large prime movers and other large machines with rotating masses mounted on foundations which are relatively soft in the direction of vibration measurement (for example, turbo-generator sets, especially those with lightweight structures).

Note: These ISO 2372 (10816) Standard classes do not apply to prime movers or driven equipment in which the major working components have a reciprocating motion.

 VIBRATION LIMITS TO PUMPS

Various industry organizations, such as the Hydraulic Institute (Table 1, per ANSI/HI 9.6.4), and the American Petroleum Institute (API-610) have set vibration limits to help guide users to avoid excessive levels of vibration in centrifugal pumps.  However, since neither of these standards is intended for moderate-speed positive displacement pumps, one can often rely on other, experience-based values.  As an example, the bearing life vs. vibration approximation for general-purpose machinery, Fig. 1 is of interest here.

Electric Motor Severity Standard

FAN VIBRATION

RESONANCE

Natural frequencies of the completely assembled fan unit shall not be excited at the operating speed. (Running speed should be at least 25% removed from a natural frequency of the system.)

LIMITS

New and Rebuilt/Repaired Fans shall conform to the vibration limits specified when operating at specified system CFM and Fan Static Pressure.

The frequency range for fan certification shall be from Fmin = 0.3 X Running Speed of Fan to 60,000 cpm for velocity and to 120,000 cpm for acceleration.

For fan speeds up to 3600 RPM, the maximum velocity amplitude (inch/sec-Peak) of vibration at bearing locations in any direction shall not exceed the Line Amplitude Band Limit values specified and graphed. When determined in accordance using the frequency range defined.

For fan speeds up to 3600 RPM, the Band-Limited Overall vibration level of acceleration (g's Peak) at bearing locations in any direction (as defined in Section 4) shall not exceed the Band-Limited Overall Amplitude Acceptance Limit values specified and graphed, when determined in accordance using the frequency range defined.

Acceptance limits for fans running over 3600 RPM shall be specified by the purchaser.

SENSOR PLACEMENT FOR FANS 

Accelerometers are typically placed at key locations on the motor and fan bearings.

Since the bearings are the load carrying part of the mechanical drive train, accelerometers should be placed on the input and output bearing housings to measure the vibration levels.

Vibration sensors should be placed in the radial (vertical & horizontal) and axial locations on the motor and fan bearings.

This will provide the best detection of all vibration components including:

• bearing vibration
• unbalance
• aerodynamic disturbances)
• belt frequencies
• misalignment
• electrical faults
• blade pass