Tech Talk: How to Read a Centrifugal Pump Curve
If you are new to pumps and fluid processing, reading a pump curve can be a daunting and confusing task. Then just when you think you understand curves, you realize that different types of pumps (centrifugal, positive displacement, air operated diaphragm…) have different types of curves as well. In this post, we’ll break down the anatomy of a centrifugal pump curve.
Pump Curve Basics
A centrifugal pump imparts energy on a liquid, and based on the system it is installed on, has flow and head characteristics. The amount of pressure the pump is required to 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. Below are descriptions of the basic parts of a performance curve with 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 its associated curve.
2 – FLOW
To start your selection, identify the amount of flow you require from the pump. For this example, we have chosen 300 gpm. Flow is indicated across the bottom horizontal axis of the curve.
3 – HEAD
You will also need to know the total head the pump is required to overcome at the specified flow. For this example, we will use 100 ft. Head is indicated in increments along the vertical axis. Follow 100ft across the curve intersects your flow line which indicates your performance point.
4 – IMPELLER TRIM
To accommodate different performance points, centrifugal pumps have the capability of trimming impellers. By reducing impeller size, the pump can be limited to your specific performance requirement. The impeller diameters are listed on the left side of the curve and the performance for each trim is shown 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
Now that you have your performance point, we can determine the amount of horsepower required. 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 is important for proper pump operation. This number represents the minimum amount of pressure on the suction side of the pump needed to overcome pump entrance losses. If sufficient NPSH is not met the pump will cavitate, negatively affecting pump performance and pump life.
7 – EFFICIENCY
When selecting the best pump for an application, efficiency many times is an important factor. The higher the efficiency, the less energy required to operate for a specific performance point.
8 – MINIMUM FLOW
A centrifugal pump requires a minimum amount flow to be moving through the pump to dissipate 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.
For assistance in identifying the correct performance characteristics for your pump application, contact the pump experts at Ryan Herco Flow Solutions at 800-848-1141.