How To Read A Centrifugal Pump Curve

The ability to read pump curves is essential to long-term pump performance. In new applications, they aid in the selection of a pump that meets performance requirements. In troubleshooting applications, they help engineers and operators evaluate conditions and solve performance problems.

If you are new to pumps and fluid processing, reading a pump curve can be a daunting, confusing task. Just when you think you understand curves, you realize that different types of pumps (centrifugal, positive displacement, air-operated diaphragm, etc.) have different types of curves as well. In this post, we'll break down the anatomy of a centrifugal pump curve.

Acentrifugal pump imparts energy on a liquid, and based on the system, has flow and head characteristics. The amount of required pressure the pump must overcome dictates where the performance point will be on the curve and how much flow is produced. As pressure increases, the flow decreases moving your performance point to the left of the curve. As pressure decreases, the performance point runs out to the right of the curve and flow increases. 

When looking at curves for a new application, there are many factors to consider. But in most cases, a selection should be made as close to the Best Efficiency Point, or BEP, as possible. The BEP is an operating point along the performance curve that shows the highest efficiency point for the impeller diameter of the pump. 

Ideally, pumps run at this point for their entire lives. But in real-world applications, system upsets and fluctuating demand cause pumps to operate outside ideal conditions. Here are some common terms you might hear when describing a pump running off its best efficiency point:

1. Operating away from BEP - the pump is operating at flows greater than or less than the best efficiency point
2. Operating to the right of BEP - operating at a higher flow rate than the best efficiency point
3. Operating to the left of BEP - the pump is operating at flows less than the best efficiency point

Why does it matter? Running a pump off its best efficiency point, one way or another can add, energy costs, but the biggest consequence of running off the BEP is an increased chance for failure.

Operating the pump to the left or right of BEP means the door is open to challenges like:

• Cavitation
• Excessive vibration, leading to premature wear of seals, shafts, impellers, and bearings
• Increased operating temperature caused by suction or discharge recirculation, vibration, or breakdown of lubricants
• Repeated failures that cost money and time to repair

Let’s start by identifying and describing the information you’ll find on a centrifugal pump’s performance curve. See examples as they relate to the performance curve provided below.

1. TITLE BOX

The title box provides information about the pump model, size, speed, and other identifying criteria specific to the pump. If checking the performance of an existing pump, confirm that you are matching the pump to the associated curve.

2. FLOW

To start your selection, identify the amount of flow required from the pump. For this example, we chose 300 gallons per minute. The horizontal axis of the curve indicates flow.

3. HEAD

You will also need to know the total head the pump needs to overcome at the specified flow. For this example, we will use 100 ft. The vertical axis indicates the head. Follow 100ft across the curve intersects your flow line which indicates your performance point.

4. IMPELLER TRIM

Sometimes, performance points cannot be met with maximum impeller sizes. To accommodate for those performance points, centrifugal pumps allow for trimmed impellers.

Reducing impeller size limits the pump to your specific performance requirement. The curve lists the impeller diameters on the left side of the curve and the performance for each trim across as a bold line. Our selection is between 10” and 11” so a trim of 10.5” is appropriate.
Centrifugal pumps can also be limited by variable speed, which is the ideal means of control when several performance points are required by a single pump and not achieved by a single trim without system modification. Variable speed curves will be covered in a later post.

5. HORSEPOWER

Horsepower is indicated across the curve as a dotted line in this case at a downward angle. Our performance point is between the 10hp and 15 hp lines, we estimate this selection to require 12 hp.

6. NPSHR

Net Positive Suction Head Required (NPSH_R) is important for proper pump operation. This is the minimum amount of pressure on the suction side of the pump to overcome pump entrance losses. 

The pump curve identifies the NPSH_R, not Net Positive Suction Head Available (NPSH_A). NPSH_A is a calculation of the head the system is able to deliver. If sufficient NPSH is not met, the pump will cavitate, affecting performance and pump life.

More information about how system changes affect NPSH can be found in this blog post: “9 System Changes that Screw with NPSH”.

7. EFFICIENCY

When selecting the best pump for an application, efficiency is an important factor. On the pump curve, efficiency is the ratio of energy delivered by the pump to the energy supplied to the pump. The higher the efficiency, the less energy required to operate for a specific performance point. Pump efficiency numbers of 60-80% are normal.

8. MINIMUM FLOW

A centrifugal pump requires a minimum amount of flow to be moving through the pump to dissipate the heat created. On the left side of the curve, minimum flow is indicated by a vertical bold line; operation to the left of this line is not recommended and can significantly decrease the life of the pump.

Knowing how to read a centrifugal pump curve is essential to the health of your system. Running too far out on the curve, or too far back, can cause damage to the pump, excessive energy consumption, and overall poor performance.

Here’s an additional video with illustrations that shows how to read a pump curve using the example above.

Need further assistance with your pump curve?Ask us about it! We are happy to provide technical assistance to businesses in Wisconsin, Minnesota, Iowa and Upper Michigan.