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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