How does flow rate affect pump head?

Increasing flow rate introduces friction into the system as the liquid travels along the pipes from the suction tank to the pump and from the pump into the discharge pipe. This friction reduces the amount of total head that the pump can produce.

Why does the pump flow rate decrease as the system pressure increases?

More pressure changes the velocity of the fluid, but it also decreases the flow or output. The cause of the flow decrease is due to two factors: volumetric efficiency of the pump and reduced motor speed. So, when pressure in the pump causes more load on the motor, it slows down.

What do the pump curve the system curve and the efficiency curves represent?

A pump curve denotes flow on the x-axis (horizontal) and head pressure on the y-axis (vertical). The system curve represents the system head in your specific application at various flow rates and is calculated by determining the system’s static head and friction loss.

What happens when a pump runs off the curve?

If a pump is running out on the end of the curve it can cause cavitation inside the pump and result in premature wear of the impeller and bearings of the motor. It can also cause the motor to over amp and prematurely fail.

Does the flow rate increase or decrease with head rise?

An increase in total head on the pump curve will result in a decrease in flow (see Figure 1). Figure 1. Centrifugal pump system with valve partially closed.

What happens to pressure when flow decreases?

Bernoulli’s equation states mathematically that if a fluid is flowing through a tube and the tube diameter decreases, then the velocity of the fluid increases, the pressure decreases, and the mass flow (and therefore volumetric flow) remains constant so long as the air density is constant.

Why does pump efficiency increase with flow rate?

The numerator in the efficiency equation is the product of head and flow rate primarily. Though flow rate continues to increase, the rate that output power increases is stunted by the loss in head. Peak efficiency occurs in a pump when the ratio of head and flow compared to the power into the system is maximized.

What is system head curve?

►System Head Curves. System curves are performance curves of piping systems. They are graphical representations of the energy required to move a given flow rate through a piping system and are used to identify some of the necessary characteristics of the system’s pump.

How does Suction head affect pump performance?

Net Positive Suction Head Required/Available You always want NPSHa>NPSHr. NPSHa, with “a” standing for available, is determined by the process piping. You always want NPSHa to be greater that NPSHr. Without enough net positive suction, the pump will cavitate, which affects performance and pump life.

How does the pump move along the system curve?

In practice the combined head and volume flow moves along the system curve as indicated from 1 to 3. In practice, if one of the pumps in parallel or series stops, the operation point moves along the system resistance curve from point 3 to point 1 – the head and flow rate are decreased.

How are pump head requirements calculated for a system curve?

At each flow rate, the pump head that is required to overcome the system resistance and elevation change at that flow rate is calculated. After the model is run several times over a range of flow rates, it is the resulting pump head requirements at each given flow rate that will be plotted in order to generate the system curve.

What is the purpose of a pressure curve in a pump?

Pump curves are useful because they show pump performance metrics based on head (pressure) produced by the pump and water-flow through the pump. Flow rates depend on pump speed, impeller diameter, and head.

What happens to the pump efficiency when the head is worn?

Most people understand the pump efficiency will drop due to wear, causing the power to increase, but not all users realize that the head and flow will also deteriorate. Note the revised pump curve still intersects the system curve, but the meeting point is at a lower flow rate and a lower head.