Vibration measurement of Centrifugal pumps and analysis of Typical faults
Understand the vibration measurement of centrifugal pumps, common faults and their causes
Preface
The pumping system is a key piece of equipment in industrial processes, and there are different types of pumps to meet various production demands. Among them, the centrifugal pump is the most widely used industrial pump. Centrifugal pumps are classified as power pumps and can be further subdivided into axial flow pumps and radial flow pumps. They possess various features, such as single-stage or multi-stage, vertical or horizontal, as well as open, semi-open or closed impellers, etc.
A centrifugal pump is a type of rotating power hydraulic machinery. It converts the mechanical energy of the impeller into kinetic energy or pressure energy by transferring it to an incompressible fluid. The fluid enters the center of the impeller through the suction pipe. The impeller has a series of blades and uses centrifugal force to push the fluid towards the discharge pipe. During this process, the fluid will pass through the volute or the pump casing of the pump, or in the case of a multistage pump, through other impellers.
Main components of centrifugal pumps
The main components that make up a centrifugal pump are:
1. Suction pipe
2. Impeller
3. Axis
4. Shell or volute
5. Bearing
6. Bearing box
7. Mechanical seal
8. Discharge pipe
Figure 1: Main components of the centrifugal pump
Figure 2: Main components of the centrifugal pump
Vibration measurement point
In a centrifugal pump, the vibration measurement points must correspond to the centerline of the shaft on the bearing box (perpendicular to the centerline of the shaft). Ensure that the sensor is firmly installed on A solid component and measure as much as possible in the horizontal (H), vertical (V), and axial (A) directions of each bearing to obtain accurate vibration data.
Figure 3: Vibration measurement points
Safety is the primary consideration when choosing vibration monitoring points. On the coupling side, measurements should not be carried out in the axial direction unless there are adequate safety guarantees. Some components of the pump, such as mechanical seals and their pipelines, are usually hot, so direct contact with these components must be avoided. The cables of measuring instruments should also be kept away from hot pipelines to prevent combustion accidents.
Figure 4: Vibration measurement points
For small pumps, some analysts measure the condition of the pump bearings at a single measurement point.
Typical failure modes of centrifugal pumps
1. Imbalance
In centrifugal pumps, imbalance is usually caused by one of the following reasons:
Uneven wear of the impeller (such as cavitation) or blade breakage
2) The coupling is in poor condition, with wear or deformation
3) The rotor winding of the motor is faulty
4) There is an error in the balancing procedure of the workshop
5) Inappropriate standards or inappropriate balance masses were used
When imbalance is found in the centrifugal pump, the following measures are recommended:
1) Check the wear condition of the impeller and analyze the causes of the wear
2) Check the runout of the coupling and its overall condition
3) Check the balance procedures in the workshop and their balance quality grades
Figure 5: Worn pump impeller
2. Misalignment
In centrifugal pumps, misalignment is usually caused by one of the following reasons:
1) Improper installation or incorrect alignment procedures
2) Pipeline stress
3) Soft feet
4) Thermal expansion of the pump itself or its pipelines
5) Lack of training for employees
6) The measuring instruments are inappropriate or have not been calibrated
When a malfunction of the centrifugal pump is detected, the following measures are recommended:
1)Verify the standards of the calibration procedures and applications used
2) Verify whether there is pipe stress or soft feet in the pump and motor
3) If safety conditions permit, measure the alignment when the machine has just stopped or is heating up
4) Record the centering displacement (i.e., thermal expansion) during the machine's temperature rise/heating process
Figure 6: Centering inspection of the centrifugal pump
3. Bearing issues
In centrifugal pumps, bearing problems are usually caused by one of the following reasons:
1) Improper installation
2) Poor lubrication
3) The grease or lubricant is contaminated by particulate matter
4) Excessively high temperature
5) Unbalanced/or unbalanced
6) Improper selection of bearings
When bearing problems are found in a centrifugal pump, the following measures are recommended:
Replace the bearings and conduct a root cause analysis
2) Check the lubricating grease condition of the bearings
3) Review the installation process of the bearings
4) Evaluate the lubrication method of the bearings
5) Confirm the alignment and balance of the pump
6) Check whether the operating conditions are suitable for the use of the bearings
Figure 7: Remove the defective bearing
4. Leakage
In most cases, leakage in centrifugal pumps occurs at mechanical seals. The possible reasons for the damage to the seal are:
1)High vibration caused by misalignment or imbalance
2) Improper installation
3) The seal overheats during no-load or dry operation
4) Improper sealing selection
When sealing problems are found in the centrifugal pump, the following measures are recommended:
1) Check the alignment and balance status of the pump
2) Ensure the correct installation of the mechanical seal
3) Avoid the pump being in a dry operating state
4) Verify whether the operating conditions meet the requirements of the mechanical seal
5. Rotational loosening
In centrifugal pumps, rotational loosening is usually caused by one of the following reasons:
1) Excessive wear of the bearing
2) Poor installation
3) Improper selection of bearings
4) Improper fit of the bearing housing or excessive manufacturing tolerance
When a loose rotational clearance is found in the centrifugal pump, the following measures are recommended:
1) Check the condition of the bearing
2) Check whether the bearing housing is worn or deformed
3) Check whether the selection and installation of the bearings comply with the specifications
Figure 8: Checking the clearance (wear condition) between the moving and static components of the multistage pump
6. Structural issues
In centrifugal pumps, structural loosening is usually caused by one of the following reasons:
Poor foundation
2) The base is deformed or twisted
3) The pump support or silent block is worn
4) Soft feet caused by loose bolts
When a loose structure of the centrifugal pump is found, the following measures are recommended:
Reinforce the structure that supports the centrifugal pump
2) Repair the foundation/base of the centrifugal pump
3) Replace the support, insulation materials or silent blocks
4) Use a torque wrench to tighten the bolts of the centrifugal pump
Figure 9: The foundation of the centrifugal pump in the best condition
7. Fluid dynamics problems
Hydraulic problems of centrifugal pumps are diverse and are usually caused by one of the following reasons:
1) Cavitation
2) Recirculation (i.e., internal reflux)
3) Overload
4) The inlet flow state is unstable
5) The operation of the pump exceeds the design requirements
When hydraulic problems are found in the centrifugal pump, the following measures are recommended:
1) Check the suction conditions of the centrifugal pump
2) Check whether the impeller and the pump casing are damaged
3) Confirm whether the working conditions (flow rate and pressure) meet the design requirements of the centrifugal pump
Figure 10: Some hydraulic problems of the centrifugal pump can be detected by checking the operating conditions, conducting visual inspections and reading the pressure gauge readings
8. Other prediction techniques
The inspection of the centrifugal pump must be comprehensive, including dynamic, hot state and operational behavior. The following technologies are also applicable to centrifugal pumps:
Norms and standards
The allowable vibration limits of centrifugal pumps are presented in the API 610 standard or ISO 10816-7.