The mechanical power requirement of a centrifugal pump (shaft power) is directly proportional to the density of the pumped medium.
P 2 ~ ρP 2 = shaft power
ρ = density of the pumped mediumIn contrast, the throttle curve of the pump H (Q) and the efficiency η (Q) are independent of the density.These relationships are automatically taken into account by the pump selection software.
In particular, the following criteria must be taken into account:(1) General suitability of the pump type
In principle, centrifugal pumps are only suitable up to a certain viscosity, which depends on the size. A positive displacement pump should be selected for very high viscosities.(2) Pump characteristics
The pump characteristics are usually measured with water under standard conditions. If the density and viscosity are different, the characteristics must be converted accordingly. This is usually done automatically by the pump selection software if it is a Newtonian fluid.(3) Material selection
The selection of suitable materials is extremely important, especially if the pumped medium contains abrasive or aggressive components. Many manufacturers offer media resistance information for this. For such use, however, direct contact with the manufacturer is definitely recommended.(4) Seal selection
In addition to the media-material resistance, the characteristics of the pumped medium must be taken into account when selecting a suitable seal. In the case of dangerous substances (e.g. toxic or explosive media) or particularly valuable fluids, it is usually necessary to use a leak-free solution. Pumps without a shaft seal with a canned motor or magnetic coupling are ideal here.(5) Solid or gas fractions
If the pumped medium contains solid and / or gas components, special pump designs that are intended for this must be selected. For example, suitable impellers prevent the pump from blocking when there are solid or fibrous components. Gas proportions affect the suction behavior of centrifugal pumps. A higher gas content leads to an increase in the required NPSH value and thus to the earlier occurrence of cavitation.
Due to historical developments, a mains frequency of 50 Hz is used in Europe, Asia, Australia, most of Africa and parts of South America, while a mains frequency of 60 Hz has become established for the public power grids in North America.The specification of the mains frequency is necessary for the selection of the electric drive. For pumps that are operated with single-phase or three-phase motors without a frequency converter, the synchronous motor speed changes in the same ratio as the frequencies, i.e. from 50 to 60 Hz by a factor of 1.2. In accordance with the affinity laws, this also changes the pump’s performance data.A frequency converter is often used to adapt the performance data to the operating point. The electrical energy drawn at the mains frequency is converted into a voltage with a different frequency in order to change the motor speed.
It consists basically of two parts:
- The pressure difference between discharge side and suction side tank. It is zero for open tanks and closed circulation systems.
- The height difference between the liquid levels of discharge side and suction side tanks respective the system inlet and outlet. It is zero for closed circulation systems.
This means for circulation systems the static head is always zero,p
in, p
out = pressures on suction respectively discharge liquid levels
ρ = fluid density
g = gravity (9.81 m/s
2)
H
geo = static height difference between suction and discharge liquid levels
It should be noted that the
inner pipe diameter can differ considerably from the
nominal diameter depending on the material and the standard.A
minimum diameter is often required for solids-laden pumping media to prevent clogging or blockage of the pipeline. For example, a minimum diameter of DN 80 is required for sewage containing faeces in accordance with EN 12056 or DWA guidelines, unless a pump with a shredding device is used.Another important parameter is the mean
flow velocity . In doing so, contradicting requirements have to be reconciled:
- In the case of media laden with solids, a minimum flow rate is required in order to avoid deposits. In wastewater technology, 0.7 m / s is required for this.
- A small pipe diameter leads to high flow speeds and thus to high pressure losses. This increases the operating costs of the system. In addition, the risk of flap blows and pressure surges is greater at high flow speeds.
- High flow velocities can also lead to noise pollution. This is particularly important for installations inside buildings.
- In contrast, a large pipeline diameter results in higher investment costs due to the higher material requirements.
For many areas of application there are regulations or recommendations for the dimensioning of pipelines in national or international standards.The following values can provide an initial reference point for dimensioning the pipeline for some applications:
- Wastewater / waste water – 0.7 … 2.3 m / s
- Installation in the building – 0.7… 1.2 m / s
- Irrigation – 0.5 … 1.5 m / s
- Heating – 0.3 … 1.5 m / s
- Recommended – 0.5 … 0.8 m / s
- Main water pipes – 1… 2 m / s
- Water pipelines – up to 3 m / s
- Drinking water / service water
- Pressure lines – 1… 2 m / s
- Suction lines – 0.5… 1 m / s
