Why finding the root cause matters
Lesson learnt
When a pump fails, always look for the root cause before simply replacing it — even if the replacement is “like for like”.
The call
I had an interesting job recently that turned into a valuable learning moment.
A client contacted us to say the new replacement pump we supplied was “making a funny noise”.
On paper, the job was simple.
It was a direct like-for-like replacement:
- Same brand
- Same model number
- Same 11 kW motor
Nothing unusual.
The original pump had reportedly run for many years without any issues, so at first glance there was no reason to suspect a problem.
But something didn’t sit right.
Digging a little deeper
When a pump that should be comfortable in its duty suddenly becomes noisy, it’s usually not the pump itself — it’s the system.
So I started asking more questions.
That’s when the missing piece of information surfaced.
The original pump had always been operated on a VFD, running at approximately 35 Hz.
However, the new pump was not.
Why?
Because the VFD had failed.
To keep the process running, the end-user’s electrician bypassed the VFD and wired the pump direct-on-line at 50 Hz.
No speed control.
No reduced load.
Just full speed and full power.
The problem revealed
At 35 Hz, the pump had been operating comfortably within its intended duty for years.
At 50 Hz, it wasn’t.
By running the pump at full speed, the operating point shifted well out toward the end of the pump curve. The pump was no longer operating where it had been selected to run.
The result?
- Increased noise
- Hydraulic instability
- Cavitation
The noise being reported wasn’t due to a faulty pump.
It was the sound of a pump being forced to operate outside its curve.
Looking back at the original failure
Once this information came to light, another question followed naturally:
What caused the original pump to fail in the first place?
In hindsight, it’s very likely the original pump failed for the same reason.
Once the VFD failed and the pump was unknowingly run direct-on-line at 50 Hz, it would have been exposed to higher flow, higher power demand, increased vibration, and cavitation. That final period of operation likely shortened its remaining life significantly — and ultimately finished it off.
The grey area: who’s responsible?
This is where things get interesting.
The client provided a specific request:
“Supply this exact pump — same model, 11 kW.”
And that’s exactly what was supplied.
But when things go wrong, it raises an important question:
Whose responsibility is it?
- The client’s, for not mentioning the VFD, reduced-speed operation, or that the system configuration had changed?
- Or the supplier’s, for not asking why the original pump failed — even though we didn’t supply the original system and had no visibility of the original duty?
There’s no clear-cut answer.
And that’s the reality of pumping systems.
The real takeaway
This story isn’t about blame.
It’s about understanding that:
- A model number alone doesn’t describe an application
- Pumps selected for variable-speed operation may not survive at fixed full speed
- A “like-for-like” replacement can still be wrong if the operating conditions have changed
Most importantly:
Replacing a pump without understanding why it failed is a risk.
You can supply exactly what’s asked for
and still end up with the wrong outcome.
That’s why asking the uncomfortable questions upfront often saves time, money, and equipment in the long run.
