API RP 11S8 pdf download

API RP 11S8 pdf download

API RP 11S8 pdf download Recommended Practice on Electric Submersible System Vibrations
A characteristic of misalignment and bent shafts is that vibration will occur in both radial and axial directions. Ingeneral, whenever the amplitude of axial vibration is greater than 50 % of the highest radial vibration, thenmisalignment or a bent shaft should be suspected.
4.3.4 Flow lnduced
Pump vibration can occasionally be caused by flow through the system.The amplitude usually depends upon wherethe pump is operated on the head-capacity curve.This normally causes a vibration due to turbulence. In diffuser-typepumps, certain combination of impeller blades and diffuser vanes are more likely to produce vibration than others.Although this phenomenon can produce vibration amplitudes that are unacceptable, especially at rates conducive tocavitation problems, testing indicates that when the pump is operated within its recommended operating range, theimpact of turbulence is minimal. Nonsymmetrical fluid passages in a pump can induce hydraulic imbalance that maybe seen as a once per revolution vibration.Multiphase flow can also induce vibration.
4.3.5 Journal Bearing Oil Whirl
A condition caused by hydrodynamic forces in lightly loaded journal bearings that results in a vibration at slightly lessthan one-half (42 % to 48 %) the rotating frequency.
4.3.6 Bearing Rotation
Journal bearings that are not properly secured can rotate with the shaft and produce vibration at one-half rotatingfrequency.
4.3.7 Mechanical Rub
Contact between the rotating and stationary surfaces results in a vibration at a frequency normally 1/s to 1/2 theoperating speed.Natural frequencies may be excited.
4.4 Control of Vibration
4.4.1 General
Methods of vibration control may be grouped into three broad categories: reduction at the source, isolation of externalsources, and reduction of the response.
4.4.2 Reduction at the Source
Methods of vibration control in this category include the following.
a) Balancing of rotating masses—Where vibration results from the unbalance of rotating components, the magnitude of the vibratory forces, and hence the vibration amplitude, can often be reduced by balancing.
b)Balancing of magnetic forces—Vibratory forces arising in magnetic effects of electrical machinery are minimized by proper design and fabrication of the stator and rotor, details of which are beyond the scope of this RP.
c) Control of clearances—Vibration can result when ESP system components and parts, operating within the clearances that exist between them, strike each other or otherwise come into impact-type contact duringoperation. Vibrations from this source can be minimized by avoiding excessive bearing clearances and byensuring that dimensions of manufactured parts are within acceptable tolerances.
d)Straightness of rotating shaft—Rotating shafts should be as straight as practical since lack of shaft straightness will have a large effect on system vibrations.
4.4.3 lsolation of External Sources
Other machines or equipment, unless properly isolated, may transmit vibration to an ESP under test or in operation.For example, a horizontal pump delivering high-pressure water may experience vibration interference from
neighboring pumps and drivers through the foundation.Accepted practice is to avoid the structure’s natural frequencyby approximately 25 % above or below.
lsolation of equipment being tested is the responsibility of the tester. lsolation of equipment in service is theresponsibility of the user.
4.4.4 Reduction of the Response
Methods of vibration control in this category include the following.
a) Alteration of natural frequency—lf a natural frequency of the system coincides with the frequency of the excitation,the vibration condition may be made much worse as a result of resonance. Under such circumstances, if thefrequency of the excitation is substantially constant, it often is possible to alleviate the vibration by changing thenatural frequency of such system.This generally involves modifying mass and/or stiffness of the system.
b) Operating at nonresonant frequencies—Sometimes ESPs are operated with variable speed drives. Operation at a frequency corresponding to a critical speed should be avoided to minimize damage to the system.
c)Additional damping—The vibration response of a system operating at resonance is strongly related to the amount of damping present. Techniques are available to increase the amount of damping.The addition of damping decreases unit efficiency.
4.5 Vibration in ESP Systems
4.5.1 General
The potential for vibrational problems is inherent with any rotating equipment having an extreme shaft length-to-diameter ratio such as an ESP system,consisting of a motor, seal chamber section, gas separator, and pump(s) allconnected by a small-diameter, high-strength, coupled shaft.Recognizing that all ESP machinery operates in somestate of unbalance, a reasonable displacement amplitude for new equipment should be established to allow a marginfor deterioration in service.Guidelines are set forth in the following.
4.5.2 Vibration Modes
Vibration modes can be axial, lateral (transverse), torsional, or combinations of all three.Torsional vibration is knownto be a potential problem , particularly when starting and when changing speeds. Axial and transverse vibrations onshaft seals and thrust bearings may be important under certain circumstances.

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