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API RP 11S8-2012 pdf free download

API RP 11S8-2012 pdf free 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. In general, whenever the amplitude of axial vibration is greater than 50 % of the highest radial vibration, then misalignment or a bent shaft should be suspected. 4.3.4 Flow Induced Pump vibration can occasionally be caused by flow through the system. The amplitude usually depends upon where the pump is operated on the head-capacity curve. This normally causes a vibration due to turbulence. In diffuser-type pumps, 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 to cavitation problems, testing indicates that when the pump is operated within its recommended operating range, the impact of turbulence is minimal. Nonsymmetrical fluid passages in a pump can induce hydraulic imbalance that may be 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 less than 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 rotating frequency. 4.3.7 Mechanical Rub Contact between the rotating and stationary surfaces results in a vibration at a frequency normally 1 / 3 to 1 / 2 the operating 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 external sources, and reduction of the response. 4.4.2 Reduction at the Source Methods of vibration control in this category include the following.
4.4.3 Isolation 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 frequency by approximately 25 % above or below. Isolation of equipment being tested is the responsibility of the tester. Isolation of equipment in service is the responsibility 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—If 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 the frequency of the excitation is substantially constant, it often is possible to alleviate the vibration by changing the natural 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) all connected by a small-diameter, high-strength, coupled shaft. 4.5.2 Vibration Modes Vibration modes can be axial, lateral (transverse), torsional, or combinations of all three. Torsional vibration is known to be a potential problem, particularly when starting and when changing speeds. Axial and transverse vibrations on shaft seals and thrust bearings may be important under certain circumstances. 4.5.3 Critical Speeds Torsional and lateral critical speeds exist in ESP systems. If possible, operation of the ESP near a critical speed for an extended period of time should be avoided. When this problem is identified over specific, planned rotating frequencies, alteration of the response may be in order and should be addressed. This problem may be particularly acute when the ESP is operated over a wide speed range or during start-up.

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