Pump curve

From Chemepedia
System Curve
Ideal vs real pump curve

A pump curve is a clever way to graph the characteristics of a single pump.

Depending on the manufacturer and how it they decide to show the information, will decide what is on the x-axis and y-axis.

Normally the flow rate of the fluid is displayed on the x-axis and the measurements of head are displayed on the y-axis.

Net Positive Suction Head required, NPSHr, can either be displayed on the x-axis, on the opposite side y-axis or in a separate graph in-line vertically to the Head vs. Flow graph.

Introduction

On the graph will normally be several lines with the size diameter of impeller and how much head is created at a certain flow rate.

The impeller sizes normally change by ¼ inch margins. This will be displayed as a curve and regardless of impeller size this curve will deviate downwards as the flow increases.

In between the lines will sometimes be dotted lines with percentages next to them. This will display an efficiency for a range of flow rates and head created.

Since no pump is ever 100% efficient, there will be a point on the curve called the best-efficiency-point, BEP. This is the maximum efficiency that the displayed pump can achieve. Depending on the application will decide how close to that BEP is needed.

Pumps will always run better the closer to the BEP it is functioning at, so it is best to pick a pump that will be operating as close to the BEP as possible.

The NPSHr axis displays the amount of NPSH required for a certain flow and head height. This is the minimum amount of NPSH, for the pump to operate correctly and the value isn't very dependent on the size of the impeller.

If the amount of suction available is below this NPSHr value, the desired flow will not be achieved and/or cavitation may occur inside the pump which is very damaging on the pump’s impeller.

How to read

Depending on what you know will decide how to read the graph. There are three scenarios that could happen:

  1. You know the desired flow rate
    • Once you know the desired flow rate, you can find that value on the x-axis and follow that line vertically until you reach the family of curves. It is important to have the pump operate as close to the BEP as possible so if there are no impeller design requirements, find the impeller size that gives the best efficiency rating, and go with that. If the impeller size is in between two sizes, go with the bigger diameter, and ask the manufacturer to shave the impeller to size. Once you have the flow rate and impeller diameter, you can follow that point horizontally to the y-axis and this will show the amount of head created by that pump, so depending on design requirements will determine if this pump is usable. Finally check how much NPSH is available in the system and compare to the pump curves NPSHr. If it is above the required, then everything will be fine.
  2. You know the desired head
    • If you know how much head is required in the system, the same process as the first scenario can be applied but in reverse. Follow the desired head to the pump curves, find an appropriate impeller diameter, and then obtain the flow rate required for that set of parameters. Finally check the pumps NPSHr.
  3. You know the desired head and flow rate
    • If this is the case, it is as simple as drawing a vertical line from the flow rate axis and a horizontal line from the head axis. Where these two lines meet will determine the required impeller diameter size. Next, look at where that set-up lies on the efficiency curves. If the efficiency achieved with that set-up is far from the BEP, it may be best to look for another curve that would allow for better efficiency at desired operating parameters.

How to utilize the pump curve

Depending on the information known, the pump curve can be used to calculate the ideal volumetric flow rate, shaft work or size the pump required for a given flow rate.

  1. Ideal volumetric flow rate
    • To calculate the ideal volumetric flow rate of a pump, the mechanical energy balance needs to be superimposed onto the pump curve. This could be done by first setting the velocity to zero and determining the static head. The mechanical energy can the be projected onto the pump curve using the x-axis at different volumetric flow rate to calculate the shaft work. The ideal volumetric flow rate will where the mechanical energy balance curve intersects with the given specifications of the curve that is being used. In addition, it is important to make sure that the pump is operating beyond the specified (NPSH)R to avoid cavitation and damage to the pump.[1]
  1. Fixed volumetric flow rate
    • If there is a fixed volumetric flow rate at which the system is operating, the pump curve could be used to find the size of the pump that can be used safely to pump the fluid. The pump size can be determined by ensuring that the pump operates to the right of the (NPSH)R line and will lie where the known volumetric flow rate and its corresponding shaft work pressure head intersects. From the intersection of the shaft work and the flow rate, the diameter of the impeller can be estimated by observing where the diameter of impeller curve intersects the operation point. The motor size will be the corresponding motor size to the right of the operation point and the efficiency of the pump will lie within the efficiency curves in the pump curve.[1]

Notes

  1. 1.0 1.1 [Satterfield, P. E., Zane. “Reading Centrifugal Pump Curves.” Nesc.wvu.edu, THE NATIONAL ENVIRONMENTAL SERVICES CENTER , 2013, www.nesc.wvu.edu/pdf/DW/publications/ontap/tech_brief/TB55_PumpCurves.pdf]