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Common Causes of Split Case Pump Vibration

Categories:Technology Service Author: Origin:Origin Time of issue:2023-03-04
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During the operation of split case pumps, unacceptable vibrations are not desired, as vibrations not only waste resources and energy, but also generate unnecessary noise, and even damage the pump, which can lead to serious accidents and damage. Common vibrations are caused by the following reasons.


1. Cavitation

Cavitation typically produces random high frequency broadband energy, sometimes superimposed with blade pass frequency harmonics (multiples). Cavitation is a symptom of insufficient net positive suction head (NPSH). When the pumped liquid flows through some local areas of the flow parts for some reason, the absolute pressure of the liquid decreases to the saturated vapor pressure (vaporization pressure) of the liquid at the pumping temperature, the liquid vaporizes here, generating steam, Bubbles are formed; at the same time, the gas dissolved in the liquid will also be precipitated in the form of bubbles, forming a two-phase flow in a local area. When the bubble moves to the high-pressure area, the high-pressure liquid around the bubble will quickly condense, shrink and burst the bubble. At the moment when the bubble condenses, shrinks, and bursts, the liquid around the bubble will fill the cavity (formed by the condensation and rupture) at high speed, generating a strong shock wave. This process of generating bubbles and bursting of bubbles to damage the flow-passing parts is the cavitation process of the pump. The collapse of steam bubbles can be very destructive and can damage the pump and impeller. When cavitation occurs in a split case pump, it sounds like "marbles" or "gravel" are passing through the pump. Only when the required NPSH of the pump (NPSHR) is lower than the NPSH of the device (NPSHA) can cavitation be avoided.

2. Pump flow pulsation

Pump pulsation is a condition that occurs when a pump is operating near its closing head. The vibrations in the time waveform will be sinusoidal. Also, the spectrum will still be dominated by 1X RPM and blade pass frequencies. However, these peaks will be erratic, increasing and decreasing as flow pulsations occur. The pressure gauge on the pump outlet pipe will fluctuate up and down. If thesplit case pumpoutlet has a swing check valve, the valve arm and counterweight will bounce back and forth, indicating unstable flow.

3. The pump shaft is bent

The bent shaft problem causes high axial vibration, with axial phase differences tending to 180° on the same rotor. If the bend is near the center of the shaft, the dominant vibration typically occurs at 1X RPM; but if the bend is near the coupling, the dominant vibration occurs at 2X RPM. It is more common for the pump shaft to bend at or near the coupling. A dial gauge can be used to confirm shaft deflection.

4. Unbalanced pump impeller

Split case pump impellers should be precisely balanced at the original pump manufacturer. This is especially important because the forces caused by the imbalance can greatly affect the life of the pump bearings (bearing life is inversely proportional to the cube of the applied dynamic load). Pumps may have center hung or cantilevered impellers. If the impeller is center-hung, the force imbalance usually exceeds the couple imbalance. In this case, the highest vibrations are usually in the radial (horizontal and vertical) direction. The highest amplitude will be at the operating speed of the pump (1X RPM). In the case of a force imbalance, the horizontal lateral and medial phases will be approximately the same (+/- 30°) as the vertical phases. Additionally, the horizontal and vertical phases of each pump bearing typically differ by about 90° (+/- 30°). By its design, a center-suspended impeller has balanced axial forces on the inboard and outboard bearings. Elevated axial vibration is a strong indication that the pump impeller is blocked by foreign matter, causing axial vibration to generally increase at operating speeds. If the pump has a cantilevered impeller, this usually results in an excessively high axial and radial 1X RPM. Axial readings tend to be in-phase and stable, while cantilevered rotors with radial phase readings that may be unstable have both force and couple imbalances, each of which may require correction. Therefore, adjustment weights usually have to be placed on 2 planes to counteract forces and couple imbalances. In this case it is usually necessary to remove the pump rotor and place it on a balancing machine to balance it to sufficient accuracy as 2 planes are usually not accessible at the user site.

5. Pump shaft misalignment

Shaft misalignment is a condition in a direct drive pump where the centerlines of two connected shafts do not coincide. Parallel misalignment is the case where the centerlines of the shafts are parallel but offset from each other. The vibration spectrum will usually show 1X, 2X, 3X... high, and in severe cases, higher frequency harmonics will appear. In the radial direction, the coupling phase The difference is 180°. Angular misalignment will show high axial 1X, some 2X and 3X, 180° phase out of phase at both ends of the coupling.

6. Pump bearing problem

Peaks at non-synchronous frequencies (including harmonics) are symptoms of rolling bearing wear. Short bearing life in split case pumps is often the result of poor bearing selection for the application, such as excessive loads, poor lubrication or high temperatures. If the bearing type and manufacturer are known, the specific frequency of failure of the outer ring, inner ring, rolling elements and cage can be determined. These failure frequencies for this type of bearing can be found in tables in most predictive maintenance (PdM) software today.

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