Costly Mistakes to Avoid in Pump Sizing

Getting pump sizing right means reliable performance, energy efficiency, and long equipment life. Getting it wrong leads to costly downtime, poor flow control, and constant maintenance issues.

In this article, we’ll go over common mistakes engineers make when sizing pumps — and, more importantly, how to avoid them.

1. Using Resistance Coefficients Without Adding a Buffer

Engineers fuse standard resistance coefficients (K-values) from charts or manufacturer datasheets to estimate pressure drops through fittings and valves. Relying only on textbook K-values without a safety margin added to the total dynmaic head calculated can lead to underestimating pressure losses. This can cause pumps to operate off their curve.

What can go wrong:

• K-values from standard charts may not accurately represent the specific fittings used in your system.
• Field installations often differ from design due to 3D model inaccuracies or last-minute changes — extra fittings may be added, and pipe roughness may vary.
• Scale or fouling may build up over time, increasing the friction factor and head loss.
• Future expansions or modifications are often overlooked. In practice, systems are frequently retrofitted rather than rebuilt from scratch.

How to avoid it:

• Adding a 10–20% buffer to your total dynamic head (TDH) is typically fine. But don't oversize the pump too much or you might create another problem.  

2. Relying on a Generic Excel Template

Pre-made pump sizing templates seem convenient, but they often come with fixed assumptions that don’t match real-world systems.

Why this causes problems:

• Templates are often rigid — they might not account for two suction sources, bypass lines, or recycle loops.
• You might end up forcing your system to fit the template or, worse, accept inaccurate results without realizing it.

Better approach:

• Build your own Excel model tailored to your specific system.
• This helps you understand the physics, and gives full control over assumptions, units, and layout.
• Your model doesn’t need to be fancy — even a basic, customizable spreadsheet can outperform a rigid template.

3. Choosing a Pump with a Maxed-Out Impeller

When selecting a centrifugal pump, impeller trim (diameter) matters. A maxed-out impeller means the largest size is already installed.

Why that’s a problem:

• If system conditions change later, you won’t be able to increase flow or head without replacing the entire pump.
• Starting with a slightly smaller (trimmed) impeller gives you the option to upgrade later.

Best practice:

• Choose a pump with an impeller size that allows future adjustment.
• Pump sizing should account for possible changes — not just today’s conditions. Don’t limit your system’s flexibility.

4. Final Thoughts

From experience, the best pump sizing results come when you:

• Add a smart buffer to head loss calculations,
• Create your own flexible sizing models, and
• Choose pumps that allow for future adjustment — not just in valves or pipe sizes, but in impeller trims too.

These small design choices can prevent costly redesigns and system failures for years to come.

Want to take your pump sizing skills further? Visit the Pump Sizing & Modeling Piping Systems For Liquids Course and learn how to build your own Excel pump sizing and piping model from scratch with real-world examples and practical tips.