A few years back, we posted a blog about one of the most common vibration signatures measured on pumps: turbulence.
Causes of Turbulence in Pump Systems
Several different things can cause turbulence in a pump system:
- Cavitation: when “cavities” of low-pressure vapor bubbles form in the suction side of the pump.
- Aeration or Air Entrapment: air trapped in the pump system or a low fluid level on the inlet side pulls air into the system.
- Recirculation: typically caused by a low flow rate.
- Turbulence: can occur from any obstruction, including piping design, constrictions, etc.
Vibration Signatures: What to Look For
From a vibration analysis standpoint, most of these look the same in a vibration signal. Cavitation and air entrapment are typically not common. However, recirculation (from lower than design flow due to VFD drives) and turbulence from piping design are quite common.
The Impact of Piping Design on Turbulence
Let’s look at one: piping design. Piping design recommends that the inlet pipe length be at least 5 to 10 times the diameter of the pump inlet. If I have a 6” inlet on the pump, I should have at least 30” of straight pipe run back to the first elbow. I have seen thousands of pumps – only a few had the correct pipe length.
Why? Here are a few reasons:
- Cost
- “Real estate” – just not enough room
- Lack of understanding
When the inlet piping length is not long enough, the flow through the pipe toward the pump is turbulent – with pressure and flow direction changes. Think of the fluid as “bouncing around” in the pipe. As this turbulent flow goes into the pump, it causes vibration. While turbulent flow is typically not damaging, it is present. When the inlet length is longer, this flow has enough distance to become laminar or smooth out and become uniform.
Design engineers will often use an inlet diffuser designed to smooth out the flow.
When Turbulence is Just a Nuisance
If you see random vibration in a pump signature, look for clues about why this random vibration may be occurring.
- Is the pipe inlet length correct?
- Is a suction diffuser being used?
- Is the pump on a VFD? If so, is the pump operating well below full speed? If so, the pump may be operating to the left of the Best Efficiency Point.
If these are present, the vibration in the pump caused by turbulence may only be a nuisance vibration. You should be able to trend it over time and see if it stays consistent. If it does, then it may be a non-issue.
Looking for alignment, bearing, and lubrication problems is much more important. These problems are much more likely to shorten the life of the pump.
