Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)

Vibration and Noise in Pumps 457
Acceptable Unbalance Levels
Various engineering organizations have set forth criteria for allowable
residual unbalance. The Acoustical Society of America defines balance
quality grades for various types of rotors as described in Table 18-3.
Pumps may have a range of balance quality grade from G2.5 to G6.3
depending upon size. The ASA Standard 2-1975 defines maximum residual
unbalance [34] that is dependent upon speed as shown in Figure 18-
22. For example, a 3,600 rpm pump with a rotor weight of 1,000 Ibs for
balance grade G2.5 would have an allowable residual unbalance of 4.5
in.-oz.
The revised API-610 (seventh edition, 1989) specifies an allowable residual
unbalance for centrifugal pumps. The maximum allowable residual
unbalance per plane (journal) may be calculated by the following formula:
where
U b = allowable unbalance, inch-ounces
W = journal static weight load, Ibs
N mc = maximum continuous speed, rpm
The total allowable unbalance (two planes near the journals) for a
3,600 rpm pump with a 1,000 Ib rotor would be 1.1 in.-oz. This calculated
residual unbalance from the current edition of the API code is significantly
less than the allowable unbalance from earlier editions of the
API codes for speeds less than 10,000 rpm. The previous edition of API-
610 simply specified dynamic balance for all major rotating components
with no specific value for an allowable unbalance. The various balance
criteria are compared in Figure 18-23 for a 1,000 Ib rotor.
Allowable Vibration Criteria
It is difficult to define the absolute maximum vibration level that can be
tolerated without damage to the rotor. Some allowable vibration criteria
are based on bearing housing vibrations. With rolling element bearings,
the ratio of shaft vibrations to case vibrations is close to unity. The API-
610 [7] specifies that the unfiltered vibration measured on the tearing
housing should not exceed a velocity of 0.30 ips (inches per second) nor
exceed a displacement of 2.5 mils peak-peak at rated speed and capacity
±10%.