Recently, our company's after-sales department received a report from a customer that the multistage boiler circulating feed water pump purchased from us was experiencing excessive vibration. After a thorough understanding of the situation, our company promptly arranged for after-sales technicians to go to the customer's site for handling. After on-site investigation by the technicians from multiple aspects, it was finally determined that the uneven concrete foundation of the pump was the cause of the excessive vibration of the multi-stage boiler circulating feed water pump.
Changsha Zoomlian Pump Industry hereby reminds all users that an uneven foundation is one of the common causes of vibration in high-pressure multistage centrifugal pumps. In this article, the centrifugal pump manufacturer, Changsha Zoomlian Pump Industry, will specifically introduce the impact of an uneven foundation on multi-stage centrifugal pumps and the solutions, hoping that you will pay attention to the flatness of the foundation when installing and using multi-stage centrifugal pumps.
1. The Influence Mechanism of Uneven Foundation on Vibration
Insufficient rigidity causes resonance.
When the foundation stiffness is insufficient or the secondary grouting is poor, the vibration generated by the pump set during operation cannot be effectively absorbed or isolated, resulting in the formation of an elastic vibration system between the foundation and the pump set. When the vibration frequency of the pump is close to the natural frequency of the foundation, resonance occurs and the amplitude increases sharply.
2. Loosening of anchor bolts leads to failure of constraints.
Uneven foundation can cause uneven pre-tightening force of anchor bolts. After long-term operation, the bolts become loose, resulting in a decrease in the constraint stiffness between the pump set and the foundation, which intensifies the vibration transmission.
3. Thermal expansion stress concentration
If the foundation of the boiler feed water pump for transporting high-temperature water is uneven, the pump body cannot uniformly release stress when it expands due to heat, resulting in changes in the stiffness of the pump shaft support system and inducing vibration.
II. Typical Characteristics of Vibration Caused by Uneven Foundation
1.Vibration directivity
Excessive vibration in both the horizontal and vertical directions is out of sync.
Axial vibration is relatively small (unless accompanied by coupling alignment issues)
2. Vibration Trend
The vibration is relatively mild when the load is zero, and gradually intensifies as the load increases.
The vibration amplitude shows a slow upward trend after long-term operation.
3. Accompanying Phenomena
There is a significant gap between the base and the pump body contact surface.Loose or rust marks are visible on the anchor bolts.
III. Solutions and Preventive Measures
1. Foundation construction stage
Weight requirement: The basic weight should be no less than three times the total weight of the pump set to ensure stability.
Grouting process: Use non-shrinking grouting material and pour in two stages (first fill the bottom of the shim, then do the overall grouting) to avoid hollowing.
2. Installation and commissioning stage
Shim plate arrangement: The contact area of each shim plate should be ≥ 70%. The gap should be checked with a 0.05mm feeler gauge (insertion depth ≤ 10mm).
Bolt tightening: Tighten the anchor bolts in a diagonal sequence in three steps. The final torque value must meet the design requirements.
3. Operation and Maintenance Phase
Regular inspection: Use a laser alignment instrument to check the foundation settlement every month. The allowable deviation is ≤ 0.05mm/m.
Vibration reduction measures: Install rubber vibration isolation pads (recommended Shore hardness 60-70) or spring vibration isolators between the pump and the foundation to reduce the vibration transmission rate.
IV. Typical Case References
1.A case study from a certain power plant shows that the horizontal vibration value of a multi-stage boiler feedwater pump gradually increased from 4.5 mm/s to 11.2 mm/s due to the poor secondary grouting of the foundation. The treatment measures included:Remove the existing grouting layer and carry out high-strength non-shrink grouting again.
2. Add 8 sets of inclined shims and adjust the levelness to 0.02mm/m.
3. Use corrugated compensators instead at the connection between the pump body and the pipeline.
After rectification, the vibration value was reduced to 2.8 mm/s, meeting the A-level standard of GB/T 29531-2013 "Measurement and Evaluation Method for Vibration of Pumps".
V. Suggestions for Investigating Other Related Factors
1.If the vibration still exceeds the standard after adjusting the foundation, a simultaneous inspection is required:Axial alignment: The deviation of the coupling concentricity should be ≤ 0.05mm.
2. Rotor dynamic balance: The residual unbalance must be ≤ G2.5 grade.
3. Cavitation Risk: Ensure that the inlet pressure is greater than the NPSHr (Net Positive Suction Head Required) by a safety margin of 1.5 meters.