How to Read a Pump Curve for Centrifugal Pumps

Posted June 10, 2021 by DXP Marketing - filed under Rotating Equipment.

Reading pump performance curves is an essential skill when sizing pumps for efficient and reliable operation in various applications. It is also useful when troubleshooting or verifying that the pump performance matches desired application requirements. In addition to exploring their other benefits, this article seeks to broaden your understanding of centrifugal pump curves.

Understanding Centrifugal Pump Curves

A centrifugal pump curve gives a clear picture of the pump’s performance regarding its pressure head and flow rate. It is a plot of the pump curve head vs flow. Various factors are considered when selecting a pump for a new application. One of the most critical factors is its Best Efficiency Point (BEP). This is the point on the curve, at any given impeller diameter, where the pump efficiency is at its highest value and where axial and radial loading is minimized to the greatest extent. While it would be great to be able to size pumps to operate at BEP in all cases, this is not practical based on application conditions. Operating pumps too far away from this point may cause premature failure. Some common challenges this can cause are:

Excessive vibration
Cavitation
Premature wear and tear of the pump parts
Waste of money and time

Reading pump curves can be a confusing and intimidating task for beginners, as different pump types have corresponding different types of curves. However, this article borders on how to read pump curves for centrifugal pumps.

What is a Centrifugal Pump?

A centrifugal pump is a pressure-producing device that produces the pressure required to transport liquids from one point to another for various industrial and market applications. It converts velocity energy to pressure, and the resultant flow is dependent on characteristics of the system. Centrifugal pumps are used in a wide range of industrial applications, including:

The Elements of a Pump Curve

Getting acquainted with the key elements will help you understand how to read a pump curve better. They include:

Title Box
Flow
Head
Efficiency
Impeller Trim
Horsepower
NPSH_R
Minimum Flow

Title Box

If you need to find out information on the pump’s model, size, speed, and other unique specifications, you can refer to the title box. This element is useful in matching an existing pump to its performance information.

Flow

Before you can select an appropriate pump for a specific application, you must know the flow required. Flow is indicated on the horizontal axis of the pump performance curve and is commonly measured in gallons per minute (GPM).

Head

This part of the curve shows the total developed head (TDH) the pump must generate at any given flow based again on the specific application requirements. Head is normally expressed in feet (FT), and it is located on the vertical axis, and its intersection with the required flow on the horizontal axis represents the pump’s performance point.

Efficiency

Efficiency is a very important factor to consider in selecting a pump. Being a ratio of the energy delivered by the pump to that supplied to the pump, it is clearly indicated on the performance curve for any flow and head, or the operating point. Highly efficient pumps require less energy to operate. An efficiency range of 60-80% is typical, but lower efficiencies are also typical when operating far to the left of BEP.

Impeller Trim

Since the maximum impeller diameter is typically too high to meet the desired operating point, impellers can be trimmed to meet this design point. All pumps have an acceptable range of impeller diameters, expressed in inches, ranging from the maximum impeller diameter to a minimum impeller diameter. These diameters are typically shown in one-inch increments on the performance curve. Many times, the required impeller diameter is determined by interpolating between two impeller diameter curves.

Horsepower

You can also read the corresponding required horsepower (HP) for the given flow and head since these HP lines are clearly shown on the performance curve. Actual required HP is also calculated per the following formula: GPM X TDH X S.G. divided by 3960 X Efficiency (expressed as a decimal).

NPSH_R

Net Positive Suction Head Required for centrifugal pumps (NPSH_R) generally means the minimum amount of positive absolute pressure needed at the pump suction to keep the fluid in a liquid state and avoid cavitation. There are NPSHR curves clearly identifiable on every pump performance curve, and since NPSHR is a pressure, it is represented in feet (FT). Every pump system has a certain positive pressure available to it, known as net positive suction head available (NPSHA). NPSHA must always be a greater value than NPSHR for a pump to operate normally and reliably.

Minimum Flow

This is typically represented by a thick vertical line on the pump performance curve. It specifies the lowest amount of continuous flow required to allow the pump to dissipate heat efficiently (thus minimizing flashing or cavitation) and also operate with acceptable radial and axial loads.

The Importance of Reading Pump Curves

The ability to read pump curves is beneficial in the following ways:

Aiding the selection of pumps that meet performance specifications for new applications.
Helping to troubleshoot existing pumps
Giving engineers and operators the ability to evaluate the pump conditions and solve performance problems
Ensuring optimal operation
Preventing pump cavitation

Contact DXP For Centrifugal Pumps Today

At DXP, we are committed to providing world-class centrifugal pumps of all types and classes to suit your business needs. We represent competent suppliers that can supply you with several alternatives for you to choose from. If you are looking to access effective and efficient centrifugal pumps, DXP is the right choice.