Cavitation: One of the most severe failures for centrifugal pumps

Cavitation: One of the most severe failures for centrifugal pumps

The cavitation It is one of the most insidious and common problems in industrial pumping systems, and if neglected, it can significantly compromise the performance and service life of centrifugal pumps. In highly specialized systems where aggressive fluids, high pressures, and intensive cycles I am a constant in daily work; cavitation is a lurking danger that, if neglected or not discovered in time, can lead to disastrous problems for the entire system, such as: loss of hydraulic efficiency, increased consumption, noise, vibrations, and irreversible damage to the pump's mechanical components.

Always keeping in mind the Typical failures that process pumps are subject to, the range of pumps designed by CDR (including solutions with magnetic drive and mechanical seal) is distinguished by its robustness and ability to operate in extreme chemical and mechanical conditions, even for 24/7 solutions. Thanks to a consulting approach developed over decades of experience in the sector, in both plant selection and maintenance, for CDR Is it possible to effectively intervene in cavitation prevention, ensuring the continuity of the system.

What is cavitation and how does it occur?

Cavitation is a phenomenon that occurs when the local pressure of a fluid drops below its vapor pressure and then rises again in a higher pressure zone. During this time, vapor bubbles are generated which, upon collapse, cause mechanical damage.

Inside a centrifugal pump, especially in the impeller area where the fluid is accelerated and the pressure can change rapidly, favorable conditions for bubble formation are created. These bubbles, once transported by the flow to a higher pressure zone, implode, producing localized shock waves, mechanical vibrations, surface erosion (pitting), and anomalous noise.

From an operational standpoint, cavitation reduces hydraulic efficiency why part of the fluid “works” as vapor, increase energy consumption and shortens the lifespan of the pump casing, impeller and seals. In many cases, cavitation can become the trigger for other serious failures in centrifugal pumps, leading to unpredictable and often very costly downtime.

It is therefore fundamental that operators, designers, and maintenance facility managers clearly understand the phenomenon of cavitation to prevent and manage it, correctly choosing the pump, materials, suction line, and plant configuration. In Scheduled system downtime, In addition, CDR Pompe offers predictive maintenance services to prevent any future problems or malfunctions.

There isn't a single type of cavitation

Cavitation is a broad and complex issue that can affect various parts of the pump. Indeed, there are different manifestations of this failure, each linked to specific operating or design conditions. Recognizing the pattern that occurs in the plant means being able to intervene with targeted and timely countermeasures.

• Suction cavitationThis occurs when the NPSHa (Net Positive Suction Head available) is less than the NPSHr (Net Positive Suction Head required) specified by the manufacturer. This is the most common and dangerous form for a pump, as it quickly reduces the impeller's service life.

• Internal recirculation cavitationThis occurs when the pump operates outside its designed operating point, for example, with flow rates that are too low or too high compared to what is expected. In these cases, recirculation zones form within the impeller, which promote bubble formation and cavitation.

• Turbulence cavitationis generated by obstacles in the flow, such as undersized valves, overly open bends, sudden changes in section in the intake piping, or excessively long intake ducts. These conditions induce local depressions and cavitation bubble formation.

Design implications

Cavitation prevention does not depend solely on the correct operation of the pump, but above all on a accurate system design.

Every stage, from calculating suction parameters to selecting the impeller, to configuring the suction line, directly influences the possibility of this phenomenon occurring.

Analyze precisely operating conditions and NPSH values allows for the design of more reliable, efficient, and durable systems.

Cavitation and materials

The choice of materials used in the construction of the pump and its components is a decisive factor in managing cavitation.

For example, Coated plastic materials (such as PP, PFA, PVDF, ETFE) They offer excellent chemical resistance, which is particularly useful in aggressive environments, but are less suited to withstanding the mechanical erosion caused by bubble implosions. In the presence of cavitation, coated surfaces may wear out more quickly than sturdier metallic materials.

On the other hand, metallic materials such as stainless steel (and.g., AISI) They offer greater resistance to the mechanical impact caused by cavitation, but are susceptible to phenomena such as pitting or cavitation cracks if the fluid places severe stress on the surfaces. It is therefore essential to carefully evaluate the trade-off between strength chemistry and cavitation resistance in the selection of materials for impellers, linings, and internal components.

In summary, in systems where the risk of cavitation is high—for example, due to high velocities, unstable fluids, or difficult suction conditions—designing with materials that have high mechanical strength, combined with an appropriate system configuration, makes all the difference.

NPSH: a fundamental parameter

The term NPSH (Net Positive Suction Head) Indicates the amount of energy, expressed as the height of a fluid column, available at the pump’s suction side to keep the fluid in a liquid state and prevent vapor formation. In other words, it represents a measure of the effective pressure at which the fluid enters the pump relative to its vapor pressure: a critical parameter for preventing cavitation.

In particular, it is helpful to distinguish between:

• NPSHa (available)the total head net suction head of the pump, that is, the total pressure at the pump inlet minus the vapor pressure of the fluid.

• NPSHr (required): the minimum net total head required for pump suction to ensure proper operation without cavitation. This value is indicated by the pump manufacturer.

To prevent cavitation, it is essential that NPSHa > NPSHr under all operating conditions. A safety margin should be considered in the plant design to account for variations in temperature, altitude, and pressure drops.

NPSH Calculation and Management

Given the sensitivity and importance of this parameter, it is necessary to perform an’Careful analysis of pressure drops in the suction line, taking into account the length of the ducts, the inner diameter, the bends, the fittings, and the visibility of the supply tank.

It is also important to consider the fluid temperature (which directly affects vapor pressure) and the weather conditions or altitude which reduce the absolute pressure available at the intake. All of this helps to determine the actual NPSHa that the system provides to the pump. Using fluid dynamics simulation software and relying on specialized technical expertise is essential to prevent cavitation from becoming a constant threat.

Impeller Selection

Design considerations that prevent or promote cavitation should also include thechoice of the centrifugal pump impeller. The impeller geometry is indeed a key element in avoiding cavitation: An impeller designed with consideration for the pump's characteristic curve, flow rates, heads, and the fluid involved allows for the reduction of low-pressure zones that promote bubble formation.

Advanced, technically sophisticated solutions, such as those offered by CDR Pompe, they foresee impellers with optimized configurations for extreme conditions and aggressive fluids, contributing to containing the risk of cavitation in the long term.

Intake line configuration

The configuration of the suction piping is as critical as the NPSH calculation and the selection of a suitable impeller. To minimize the risk of cavitation, the following are recommended:

• Short, straight pipes, with an adequate diameter and without sudden variations in cross-section;

• valves positioned so as not to generate low-pressure zones or turbulence before the pump inlet;

• reduction of open curves and connections which can generate local pressure drops and turbulence.
  A well-designed suction line promotes an increase in NPSHa and reduces the likelihood of the fluid entering low-pressure conditions, effectively preventing cavitation.

Cavitation can be detected by

In addition to accurate design and the right choice of pump components, promptly recognizing the signs of cavitation is one of the fundamental aspects that allow for preserving the useful life of the system.

Cavitation can indeed be detected by vibrational analysis, which intercepts the typical frequencies generated by bubble implosions, or the acoustic monitoring, which allows the characteristic noise (similar to the passage of gravel or «grating») generated by micro-jets to be detected.

Beyond these necessary analyses, visual inspection of impellers and internal components is essential hammered surfaces, erosion craters, pitting, and geometric modulations on the impeller blades These are clear signs of cavitation occurring. A regular monitoring and inspection program, supported by technical expertise and advanced diagnostic tools provided by CDR Pompe, allows for prompt intervention and prevents cavitation from progressing to irreversible failures or costly production downtime.

Prevention strategies

As we have seen, the prevention of cavitation involves a series of integrated and consistent actions relating to plant design, pump selection, operational management, and maintenance.

First and foremost, it is essential accurate design of the pumping system, which includes analysis of suction conditions, available NPSH, piping configuration, and selection of appropriate materials.

Secondly, it is necessary to use correctly sized pumps for the systemOversizing and undersizing relative to the optimal operating point both increase the risk of cavitation.

Even with suitable design and optimal product size, the constant monitoring of operating temperature and pressure remains primary: elevated temperatures increase the fluid's vapor pressure and thus facilitate cavitation, while insufficient suction pressure can cause it rapidly.

Fourth element is the Preventative maintenanceClogged filters, sediment in the tank, damaged piping, or malfunctioning valves increase pressure drop and consequently promote cavitation.

Finally, Technical training of personnel It's a strategic factor: recognizing the signs of cavitation in a timely manner allows for intervention before the phenomenon turns into a failure.

At CDR, we work with a focus on prevention.

By relying on CDR Pompe’s expertise, from the initial consultation to the’After-sales support, from the kick-off meeting to predictive maintenance programmed, operators can obtain pumps designed to withstand the harshest conditions, effectively monitor the system, and prevent cavitation durably. With an integrated strategy, cavitation need not be considered an unavoidable risk, but a manageable and controllable element.

Contact us to learn about our products and our range of new items and services, both in Italy and abroad.

Frequently Asked Questions

What is cavitation in a centrifugal pump and what damage does it cause?

Cavitation occurs when the local pressure of the fluid falls below its vapor pressure and then rises back to a higher pressure zone. The steam bubbles implode and produce shock waves, vibrations, noise, pitting, loss of hydraulic efficiency, increased consumption and wear impeller, pump casing and sealing.

What are the types of cavitation that can occur in a process pump?

There are three main forms. Suction cavitation occurs when the NPSHa is lower than the NPSHr and is the most common and harmful one impeller. Internal recirculation cavitation occurs when the flow rates are too low or too high compared to the working point. Turbulence cavitation results from poorly dimensioned valves, narrow curves, section changes, or too long suction lines.

What is the difference between NPSHa and NPSHr, and what condition prevents cavitation?

The NPSHa is the total head net available for aspiration, that is, the total pressure at the inlet minus the vapor pressure of the fluid. The NPSHr is the minimum required by the pump, indicated by the manufacturer. To avoid cavitation, the NPSHa must be greater than the NPSHr under all operating conditions, with a safety margin for temperature, altitude, and load losses.

How is the suction line configured to reduce the risk of cavitation?

The suction line must have short, straight pipes with an appropriate diameter and without sudden changes in section. The valves should be positioned so as not to create depressions or turbulence before entering the pump, and tight curves and connections should be avoided. This reduces load losses and increases NPSHa.

Which materials are better at resisting cavitation: coated with PFA and PVDF or stainless steel?

It depends on the balance between chemical and mechanical resistance. PP, PFA, PVDF and ETFE coatings are well-suited to aggressive fluids but wear out more quickly due to the erosion caused by the implosion of bubbles. AISI stainless steel is more resistant to impact, but can suffer pitting or cavitation cracks under severe conditions.

How is cavitation detected in an operating centrifugal pump?

Cavitation is detected by vibrational analysis, which detects the frequencies of implosions, and by acoustic monitoring, which picks up a gravel-like noise in motion or a creaking sound. Visual inspection impeller confirms the problem if it shows martindamaged surfaces, erosion craters, pitting or deformed blades.