Centrifugal Pumps: The Bigger, The Better?

At a paper mill in northern Wisconsin, a maintenance manager was having a serious problem. No matter how many times he replaced the mechanical seal on one of his Gorman-Rupp T-Series, it leaked within an hour. Not to mention three of the four Gorman-Rupp T-Series pumps, working to pump wastewater from a pit, were running obnoxiously loud and had obvious excessive vibration. Was it faulty seals that caused the leak? Was there something wrong with the pumps? Closer examination of application determined they were just oversized (read the entire story here). 

When sizing a new pump during the design phase, it’s common that an engineer may over size the pump. The logic is, “if it’s too big, we’ll just add a throttling valve to dial it back.” But what they may not realize, is that they’ve just added to the pump’s total cost of ownership, not to mention paying for more pump than what’s needed!

MORE IS BETTER, RIGHT?

A centrifugal pump’s reliability largely hinges upon operating at the Best Efficiency Point (BEP). In an oversized pump situation, the pump is running to the right of BEP on the performance curve. Operating the pump’s motor faster, or slower than its sweet spot can bring on a myriad of ailments, including:

• Increased vibration
• Excessive noise
• Shaft deflection
• Premature seal failure
• Cavitation

Not only this, but talk about a waste of energy!

PUMP SIZING/SELECTION RESOURCES

If you’re sizing centrifugal pumps, here’s a couple of resources that can help you.

•  “How to Read a Centrifugal Pump Curve”: We wrote this post a while back. As the title suggests, it’s all about how to read a centrifugal pump curve.
• Gorman-Rupp Pumps Selection & Application of Self-Priming Centrifugal Pumps: This PDF from Gorman-Rupp is a fantastic downloadable resource, for sizing and selecting self-priming pumps.

DON’T GO IT ALONE

If all else fails, call up a local pump distributor to help. Note only are they well-versed in pump selection, but they’re probably familiar with applications like yours, so you’re certain to get the right pump.