Urgent help needed with a special pump issue
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The oil storage and transportation depot where I work has recently encountered some very tricky problems, and I am hoping to get the help of experts. Here’s the situation: We have replaced the pump in our warehouse with a new horizontal self-priming centrifugal pump (of the same model as the previous one), and we have installed a metal flexible connection with a reducer flange that is 300 mm long between the suction line and the pump – one end of this connection being DN125 and the other end being DN100, corresponding to the pump’s suction inlet. The purpose of this is to reduce noise and eliminate any displacement issues. A flanged metal flexible connection (DN100) with a length of 300 mm is installed at the head end. During testing after installation, it was observed that when the lift ball valve was turned to 45° (i.e., about half way), a sharp humming sound emanated from the pump chamber; the louder the sound as the valve was opened further, while it became quieter when the valve was turned back to 45°. We contacted the manufacturer, who believed that cavitation was occurring. We checked all the pipeline fittings and found no abnormalities; with the manufacturer’s permission, we disassembled the pump completely yet still detected no faults. Reinstalling and testing it still results in the same issue. I don’t know what the problem is now, so I’d like to seek help from some experts in this field.
What’s wrong with the flexible connection at the suction pump inlet?
Previously, it was a flanged reducer for carbon steel seamless pipes with a length of 150 mm; to prevent displacement of the pump, it was changed to a metal flexible connection. I’m not sure if it’s due to the change
Now the head of the mechanical pump is at its maximum; the noise is high, but it can still be used. The vibration is very low; the bearings of the pump experienced a temperature rise of around 59°C after approximately 1.5 hours of operation in the experiment, while there was no temperature rise in the pump chamber and no surging in the pipelines.
Analyze the pressure changes; if the valve is opened too wide, vaporization occurs, so it should be opened a bit less, or another method should be used.
Elimination method: If the pump is an original-model pump of the same type. So, the problem with the pump can basically be ruled out. The problem lies in the area that was newly renovated this time; consider restoring it to its original state step by step.
It seems the problem still lies with the flexible metal hose; excessive pressure causes humming to occur through it
Try using a cold water hose on the inlet to see if that works
First, it is necessary to clarify whether a \"head ball valve\" refers to the valve located before the centrifugal pump or after it. If it is a centrifugal pump, keeping the inlet fully open is correct; adjusting the flow rate is done using the pump’s outlet valve. Moreover, the opening degree of the pump’s outlet valve must be determined based on the flow rate and the motor current readings. It is not necessarily necessary to use full valve opening as a reference.
The outlet pipe is too large, so throttling can only be achieved through valves; increasing the flow rate will inevitably lead to cavitation.
The main process problems associated with pumps are cavitation and gas entrapment; how can these be prevented? Import pipelines should use steel pipes (elbows can be used for diameter changes); metal flexible connections should not be employed to minimize system issues.