Complete explanation from our technicians on how a centrifugal pump works

In the vast universe of fluid engineering, few pieces of equipment are as ubiquitous and crucial as the centrifugal pump. From water recirculation in cooling systems to the transfer of aggressive chemicals or the conveyance of liquid foods, these machines are the beating heart of countless industrial processes.

However, their apparent mechanical simplicity often hides a complex physics that, if not properly understood, can lead to inefficient selections, high energy consumption, and costly plant shutdowns.

At InoxMIM, with over 25 years of experience in the development and manufacture of industrial machinery, we understand that a pump is much more than a motor and a hydraulic body.

Our global experience, serving markets as diverse as European, Latin American, and Asian, has taught us that the key to operational success lies in correctly adapting the equipment to the unique specifications of each fluid and process.

This technical guide aims to break down, from an applied engineering perspective, the operating principle of the centrifugal pump, its critical components, and the dynamics that govern its operation, so that you can make informed decisions about the selection and maintenance of these vital assets.

Table of contents

Anatomy of a centrifugal pump: Breakdown of critical components

To truly understand the operating principle and, above all, to perform an adequate selection or effective maintenance, it is essential to “dissect” the pump. Although the market offers infinite variations, at InoxMIM we standardize our equipment under criteria of maximum robustness and hygiene.

Below, we analyze the four components that define the performance of the pump:

The impeller (Rotor): The heart of the system

It is the rotating element responsible for imparting energy to the fluid. Its design determines not only the flow rate and the manometric height, but also the capacity of the pump to handle solids or delicate products.

Types of design

  • Semi-open impeller: Used in models such as the FL20C or the FL50CI. This design is versatile and efficient, allowing the passage of clean fluids or with small particles, also facilitating cleaning in CIP processes.

  • Helical impeller: Present in the FL–CH pump. Its special screw-shaped geometry is specifically designed for the delicate transfer of solids in suspension (such as grapes or olives in the food industry), minimizing damage to the product.

  • High-performance impeller: In the hygienic range FLUID, we use impellers optimized by computational design to reduce the required NPSH and maximize hydraulic efficiency up to 100 m³/hour.

Balancing

All impellers must be dynamically balanced to avoid vibrations that would destroy the motor bearings and the mechanical seal at high revolutions (2900 rpm).

The volute (Casing): Where speed becomes pressure

The casing is not only the container of the liquid; it is an active hydrodynamic component. Its spiral design (volute) progressively increases the flow area towards the outlet, which decelerates the fluid in a controlled manner to transform kinetic energy into pressure.

  • Materials: In industrial and sanitary applications, resistance to corrosion is vital. Therefore, our pump bodies are manufactured in AISI 316 Stainless Steel, guaranteeing durability against chemical and food agents.

  • Surface finish: In sanitary pumps such as the FL50CI or FLUID, the design of the casing avoids dead zones and offers polished finishes that comply with hygienic regulations such as EHEDG, allowing complete cleaning and sterilization without disassembly.

  • Connections: Depending on the sector, the casing integrates industrial connections type GAS or Flange (FL30CI models), or sanitary connections DIN 11851 and CLAMP (FL50CI/FLUID models) for quick disassembly.

The shaft and the transmission system

The shaft transmits the motor torque to the impeller. Being the element that supports the radial and axial loads, its rigidity is fundamental to maintain the alignment.

Regarding the materials, we manufacture the shafts in AISI 316 stainless steel to ensure that the part in contact with the fluid does not suffer corrosion or premature wear, even in demanding working conditions.

The sealing system: mechanical seal

It is the most critical component and, often, the most misunderstood. Its function is to seal the passage of the shaft through the casing, preventing fluid leaks to the outside. In modern industry, the mechanical seal has almost completely replaced the old packing due to its reliability and tightness.

  • Composition: It consists of a fixed and a rotating part, whose rubbing faces are kept together by the pressure of springs and the fluid itself.

  • Materials of the rubbing faces: The selection depends on the abrasiveness of the product. We use combinations of g raphite, tungsten carbide and silicon carbide.

  • Secondary seals: To ensure chemical and thermal compatibility, the seals are equipped with NBR (Nitrile), Viton or EPDM seals.

  • Special configurations: For critical applications, such as sticky or high-temperature fluids, we have double mechanical seals cooled by thermosiphon that create a barrier of clean and fresh fluid to protect the rubbing faces.

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How does it really work? The step-by-step process

Although the operation of a centrifugal pump seems continuous and simple, from the point of view of fluid physics, it is a sequential process of energy transformation. Understanding what exactly happens inside the volute is the key to diagnosing problems and optimizing performance.

The pumping cycle can be broken down into three critical phases that occur in milliseconds:

It all starts at the suction flange. For the fluid to enter the pump, the pressure in the eye of the rotor (the geometric center of the impeller) must be lower than the pressure in the source tank or the inlet pipe. When rotating, the blades of the rotor expel the liquid that was already inside towards the periphery. This displacement creates an area of low pressure (partial vacuum) in the center.

  • The natural flow: Thanks to this pressure difference, the “new” fluid is pushed (either by atmospheric pressure or the pressure of an elevated tank) towards the eye of the rotor to fill the generated vacuum.
  • Technical note: If the pressure at this point drops below the vapor pressure of the liquid, cavitation will occur, a destructive phenomenon that we will address later.

Once the fluid comes into contact with the blades of the rotating rotor, it is trapped and forced to rotate with them. Here, the transfer of mechanical energy (from the motor) to the fluid occurs. The centrifugal force projects the liquid particles radially outwards at high speed.

  • In this journey from the center to the outer edge of the impeller, the fluid gains enormous kinetic energy (speed).
  • The speed of the fluid at the tip of the blade can reach very high magnitudes (for example, in a 2900 rpm motor), which represents the maximum kinetic energy of the cycle.

The fluid is ejected from the impeller at high speed and enters the volute or pump body. This is where the final transformation that makes the pump useful occurs. The volute has a spiral shape that widens progressively towards the discharge port. By increasing the cross-sectional area through which the liquid circulates, its speed necessarily decreases.

  • According to the Bernoulli’s Theorem, in an ideal fluid flow, the total energy is conserved. Therefore, if the speed (kinetic energy) decreases, the pressure (potential energy) must increase to compensate.
  • The volute acts, therefore, as an intelligent “hydrodynamic brake”, converting that unrestrained speed into the pressure necessary to raise the liquid or overcome the resistances of the discharge pipe.

One of the most common mistakes when selecting a pump is to confuse these two terms or think that they are independent. In a centrifugal pump, they are inversely related according to their Characteristic Curve:

  • Flow rate (Q): It is the volume of liquid moved per unit of time (m³/h or l/min).
  • Manometric Height (H): It is the energy per unit of weight that the pump delivers to the fluid, usually expressed in meters of liquid column (m.w.c.). It represents the maximum pressure that the pump can generate.

How do they interact? A centrifugal pump does not deliver a fixed flow rate. It adapts to the system:

  • If the back pressure of the system is low (open valve, low height), the pump will deliver its maximum flow rate.
  • If the back pressure increases (closed valve, long or narrow pipe), the pump will generate more pressure to overcome that resistance, but the flow rate will decrease.

The best efficiency point (BEP): Every pump has an optimal point on its curve where the energy conversion is maximum and the wear (vibrations, load on bearings) is minimum. Operating the pump too far from this point (either at very low or very high flow rates) drastically shortens its lifespan.

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Technical selection criteria: What should I consider before choosing?

Selecting a centrifugal pump based solely on the maximum flow rate or the motor power is a common mistake that usually leads to energy inefficiencies or premature mechanical failures. To guarantee the operability of the process, the selection must be based on four fundamental technical pillars:

1. The viscosity and density of the fluid

The centrifugal pump is the queen of low viscosity fluids (water, milk, wine, solvents). However, its hydrodynamics has clear physical limits.

  • Viscosity: As the viscosity increases (above 150-300 cP), the efficiency of the impeller drops drastically due to internal friction. For viscous products (such as dense creams, concentrated syrups or pastes), the centrifuge loses effectiveness and positive displacement technologies (lobe or progressive cavity) should be chosen.

  • Density: The power absorbed by the motor is directly proportional to the density of the fluid. A pump sized for water (density 1) will overload if it pumps a liquid fertilizer or a dense syrup (density > 1.2) if the motor power is not adjusted.

2. The working point and the Q-H curve

A pump should never be selected to work at the extremes of its curve. The objective is that the required working point (Flow rate and Pressure of the installation) coincides as much as possible with the Best Efficiency Point (BEP) of the pump.

  • Operating very to the left of the curve (minimum flow rate) causes internal recirculation, heating and vibrations.

  • Operating very to the right (maximum flow rate, low pressure) can cause cavitation and motor overload.

3. NPSH and suction conditions

It is the most critical hydraulic calculation. It must be verified that the available NPSH (the absolute pressure at the pump inlet minus the vapor pressure of the liquid) is always higher than the NPSH required by the manufacturer.

  • Temperature: If you pump hot liquids (as in CIP or cooking processes), the vapor pressure increases, reducing the safety margin and increasing the risk of cavitation.

  • Design: In critical applications with low available NPSH, pumps with specific impeller designs (such as inductors) or self-priming pumps are required if there is a risk of air entering.

4. Compatibility of materials and sealing

Chemical and thermal resistance determines the lifespan of the equipment.

  • Metallurgy: For most industrial and sanitary applications, AISI 316L Stainless Steel is the standard due to its resistance to corrosion and zero porosity.

  • Mechanical Seal: The choice of rubbing faces is vital.

    • Silicon Carbide/Tungsten: For abrasive fluids or those that can crystallize.
    • Graphite: For clean and lubricating fluids.
    • Elastomers: EPDM seals for standard sanitary applications, or Viton (FKM) for oils and high temperatures.

Comparative table: Types of centrifugal pumps according to application

Although the physical principle is the same, the construction of the pump varies radically according to its purpose. This table summarizes the technical differences between the most common types in the industry:

Pump Type Main Application Solids Handling Hygiene and Cleaning Max. Recommended Viscosity
Industrial Centrifuge Transfer of water, chemicals, glycols, and auxiliary processes. Low. Requires relatively clean fluids. Standard. Functional design, not suitable for critical sterile processes. Low (< 150 cP)
Sanitary Centrifuge Food industry (milk, juices), cosmetics, and pharmaceuticals. Medium. Allows small particles in suspension. Very High. Drainable design, suitable for CIP/SIP cleaning and EHEDG regulations. Low / Medium (< 300 cP)
Self-priming Unloading of tanks, lines with air or occluded gas (CIP return). Low/Medium. Able to evacuate air from the line. High. Available in sanitary finishes to avoid contamination. Low
Helicoidal Turbine Delicate processes (e.g., wine pumping, oils with solids). High. Specific design to avoid damaging solids in suspension. High. Open design to facilitate the passage of solids without obstruction. Medium (< 500 cP)

Frequently asked questions about the operation of centrifugal pumps

This is the most common question. The centrifugal pump generates pressure energy (Manometric Height). The flow rate is the consequence of how that pressure interacts with the resistance of your pipe. If you close a valve (increases resistance), the pump maintains its maximum pressure, but the flow rate drops to zero. Unlike positive displacement pumps, here pressure and flow are inversely proportional according to the performance curve.

Dry running is destructive because the mechanical seal (the component that seals the shaft) depends on the pumped fluid itself for lubrication and cooling. Without liquid, friction raises the temperature of the rubbing faces instantaneously, causing breakage and immediate leaks.

The NPSH (Net Positive Suction Head) is the minimum pressure required at the pump inlet to prevent the fluid from boiling. If the NPSH available in your installation is less than that required by the pump, vapor bubbles form that violently implode (cavitation), eroding the impeller and causing severe vibrations.

Centrifugal pumps are optimized for low viscosity fluids such as water, wine, or solvents. From 150-300 cP, their efficiency plummets and electrical consumption skyrockets. For viscous products such as creams, gels, or molasses, it is necessary to change technology and use positive displacement pumps, such as lobe or progressive cavity.

You should choose a self-priming pump (such as the models in the FL–CAI) when the pump is installed above the liquid level (negative suction) or when the suction pipe may contain air or gas. A standard centrifugal pump cannot purge that air and would stop pumping, while the self-priming pump can evacuate it and prime itself.

The mechanical seal is the wearing part par excellence. Its maintenance is mainly preventive: periodically verify that there are no leaks, ensure that it does not run dry and, in the case of refrigerated double seals (used in demanding applications), check the level and circulation of the coolant in the thermosiphon.

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Understanding the physics behind your pump is the first step towards efficient operation without unexpected stops. Selecting the equipment with the appropriate curve, impeller, and sealing is the best investment for your plant.

Do you have questions about which pump is best suited to your process? Fill out the following form and our engineering team will analyze your flow, pressure, and product requirements to offer you the most cost-effective and durable technical solution.

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