Pump selection: all the parameters

As pump manufacturers, we daily face with a lot of inquiries that are often not complete because of the lack of data that are essential to let us quote the correct pump solution. In order to help our customer to get their way out of the jungle of the parameters, we have defined a table of the main variables that should be considered during the pump selection. This article would like to be a light vademecum to simplify the communication requesting for a quotation.

Variables of the pump selection

The following table includes the indispensable variables that our technicians have to know during the selection. We strongly recommend our customers to communicate the following data so that we can proceed with the selection.

pump selection

Thanks to these references, our technical department will be able to select the best solution for every inquiry.

Moreover, once the selection is done, our technical department will provide the customers with some helpful derived parameters that are necessary for the plants.

Parameters confirmed by the pump technicians

Indeed, after the selection, our technicians, first of all, confirm the actual head and flow that pump can reach and the diameter of the impeller. Then they give the customers the data shown in the table below. This information let our customer be able to technically evaluate our proposal according to the construction features of the plant.

after the pump selection


Is it more clear now? Should you need any further information, please contact us at info@gemmecotti.com. We will be glad to dissipate all your doubts.

Don’t forget to save this post in your bookmarks, in order to easily recover it! And to select the right material for the pump, take a look also at our chemical compatibility guide.




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Pressure vs head, what is the difference?

The pump selection of a pump is driven by the following main parameters:
– Head (H)
– Flow rate (Q)
– Fluid characteristics (ρ, γ, T ….)
Sometimes head can be confused with pressure during pump choice. As a matter of fact there is a strict relation between them which is defined by the fluid specific gravity, so the relation is fluid dependent. So what is the difference between pressure and head?

Definition of head and pressure

Head is the height given by the pump to the fluid and it is measured in meters of liquid column [m.l.c.] or simply indicated in meters [m]. The given head is fluid independent: different fluids
with different specific gravities are all lifted at the same height.
Pressure, instead, is fluid dependent and it is affected by the liquid density. In fact the force of a fixed height liquid column over a unitary area will change with different specific gravities. So in this case the same head generates different pressures.

Measurements: Pressure or Head?

Head is not directly measured. Manometers on the pump suction and delivery line give the measure of the pressure. Measurements given by manometers indicate differential pressures imposed by the pump between suction and discharge. These measures are read in [bar] [atm] [psi] [ft H2O] etc.. Specific gravity γ has to be considered to evaluate the correspondent head.

Conversions & Practical Example

As stated before, the same pump at the same working point will give always the same head with different pressures in accordance with the density γ of the working fluid.
For example, a mag drive centrifugal pump HTM 10 working at a given point Q= 7.5 m3/h H = 10 mlc operating with water and concentrated H2SO4 gives the same head (H=10m) to water ( γ= 1kg/dm3) and to sulfuric acid (γ = 1.8kg/dm3), while measurements of differential pressures between suction and delivery will be:

equazione pressione 12

The mathematical relationship is reported in the following equation:

equazione pressione 1
Also power consumption is influenced by the previous relationship of pressure, as:

equazione pressione 2curve pressure

Notes for technicians

– Previous relations are valid for low viscosity fluids (water equivalent), along with increasing viscosity, pump performances have to be reduced using pump derating rules.
– At a fixed rotational speed a centrifugal pump generates head related to the flow rate following its characteristic curve.
– The computation of the needed head that should be delivered by the pump, is not so straight as the evaluation of the desired height difference. Needed Head is composed by the following terms:

  • Geodetic head: difference between suction and delivery height expressed in meters of liquid column

equazione pressione 3

  • Difference between absolute pressure of the delivery and suction reservoir

equazione pressione 4

  • Distributed and concentrated friction losses also expressed in m.l.c.
    equazione pressione 5
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How to read a pump curve

If you are new in the pump field, reading a pump performance curve (usually just called “pump curve”) can be difficult and sometimes confusing. That’s why we want to help you by offering a simple guide for a first approach to a pump curve.

What is a pump performance curve and why do you need to know how to read it?

A pump curve is a graphical representation of the performance of a pump based on the tests of the pump manufacturer. Every pump has its own curve and it varies very widely from pump to pump. The variation depends on many factors such as the kind of pump (centrifugal pump, turbine pump, vane pump etc.), size and shape of the impeller. Pump curves can be used to provide many information of pump performance and absorbed power which are important for a user to determine the working point and motor power and size.

What information can I find on a pump curve?

1- In the simplest and widely used pump curve (commercial multi-pump curve) you can see two vital pump performance factors: flow and head.

The flow or capacity (Q) is measured in m3/second according to the international standards but usually you can find it expressed in m3/h, l/min or gpm (in the US). It is the volume of liquid moved in an amount of time. On the curve below (Picture 1) you can see the flow marked in red on the horizontal axis.

The head (H) is the height at which a pump can raise a liquid up. It is measured in meters (m or mlc meter of liquid column) or feet and you can see it marked in blue on the vertical axis on the picture below.

Pump performance curve with flow and head


How can you read this pump curve?

To select the right pump model you should, first of all, identify the capacity and the head needed for  your system.

If you need for example a flow of 15 m3/h at 20 m you can find the right pump curve and consequently the right pump in the intersection of the two red arrows in the chart. In the example below the pump suitable is magnetic drive centrifugal pump model HTM 31.



The curve enables you to see how the pump will perform at any given point within its performance range. For example, the same pump model HTM 31 at 15 m3/h will produce a head of 20 m, or at 24 m of head the pump will generate a flow of 8 m3/h.

Once you have chosen the right pump type whether centrifugal, turbine, vane or any other, you can study in details the specific curve of the selected pump model with other technical information.

2– In picture 2 you can see an example of a centrifugal pump curve (HTM 31 PP/PVDF) with additional details than curve in picture 1 such as for example the impeller diameter (curve A circled in red), the NPSHr (curve B) and the absorbed power (curve C). In some case you can also find the pump efficiency.  This kind of curve is usually used by pump manufacturers to select the correct pump model among their set of performance curves.

Performance curve centrifugal pump


How can you read this pump curve?

In the title box at the top you can find the pump model, the number of poles of the motor, the frequency, the RPM and the pump material (selected according to the liquid to pump).

In curve A you can see the flow and head as described in the paragraph above but there is also a reference of the impeller diameter. For this pump model the available impeller diameters vary from a  minimum of   110mm to a maximum of 134 mm. The impeller would be trimmed by the manufacturer to whatever diameter needed to meet your conditions of service. If the impeller selection is 122 mm at a flow of 10 m3/h the head is about 19 mlc. The manufacturer would determine the proper impeller diameter for your conditions and trim it to the correct diameter.

Curve A - centrifugal pump

In curve B you can see the NPSHr of the pump measured in meters or feet in accordance with the capacity required. This is the minimum head at the suction of the pump that allows the pump to work properly. If sufficient NPSH available is not supplied by the plant (NPSHa) the pump will cavitate and this will affect both the performance and the pump lifetime. In case of 10m3/h capacity you have to find the intersection with the curve of the selected impeller diameter and read the value on the left.

Curve B - centrifugal pump

In curve C you can find  the absorbed power required for pumping a liquid with a SG of 1. Once determined the impeller diameter and the flow you can find the intersection where you can read the absorbed power, which is necessary to determine the relevant motor power.

Curve C - centrifugal pump

The correct selection of a pump using a pump curve is essential to permit a proper working of your system. A working point too far out on the curve, or too far back, can cause damage to the pump, excessive energy consumption, poor performance and pump failure.

For further information don’t hesitate to contact us www.gemmecotti.com



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