The Untold Story of Pump Performance Curves & Reliability

As we covered in a previous article, reading a pump performance curve is not always as easy as it seems, but once you understand the basics, it appears to be pretty straight forward. A pump performance curve provides us with all sorts of useful information, including pump model, size, speed, flow rate (GPM), efficiency, etc. What they do not explain however, is the very important relationship between operating point and equipment reliability. Failing to understand this correlation is oftentimes the difference between equipment longevity and frequent, costly pump repairs. 

Before we dive further into this, let's make sure we're on the same page. It all starts with the Best Efficiency Point, or BEP. In basic terms, BEP identifies an operating point or area along the pump performance curve where the highest efficiency is achieved for a given impeller diameter. Pumps would never operate outside of the BEP in a perfect world. However, variables such as fluctuating demand and system upsets are common and can cause the operating point to move.

1. Operating away from BEP: Operating at flows greater than or less than BEP 
2. Operating to the right of the BEP: Operating at a higher flow rate than the BEP 
3. Operating to the left of the BEP: Operating at lower flow than the BEP 

So what? A couple of efficiency points are gained or lost depending on the operating point, which will have some impact on energy costs. This won't be significant unless the pump runs all the time or has a large horsepower motor.

This is correct, but there's more to the story. Think of the BEP as the pump’s mechanical and hydraulic sweet spot. Operating within +/- 10% of this sweet spot will greatly extend mean time between failures (MTBF) and reduce costs assuming all other factors, such as materials of construction, NPSH, alignment, and etc. have not been overlooked. See Figure 1. 

Understand that operating the pump too far to the left or right of the BEP for an extended period of time can result in the problems below: 

• Cavitation
• Excessive vibration occurs when the pump operates too far to the left or right of BEP resulting in the accelerated wear of seals, shafts, impellers, and bearings.
• Temperature elevation, which is caused by suction or discharge recirculation, vibration, or lubricant breakdown.
• Reduced seal and bearing life resulting from any of the above that will require frequent repair or replacement.