{"id":16052,"date":"2024-07-05T15:56:04","date_gmt":"2024-07-05T18:56:04","guid":{"rendered":"https:\/\/omel.com.br\/introducao-ao-fenomeno-da-cavitacao\/"},"modified":"2026-10-13T16:25:39","modified_gmt":"2026-10-13T19:25:39","slug":"phenomenon-of-cavitation","status":"publish","type":"post","link":"https:\/\/omel.com.br\/en\/escola-de-bombas-en\/phenomenon-of-cavitation\/","title":{"rendered":"Introduction to the Phenomenon of Cavitation"},"content":{"rendered":"\n

1. DEFINITION AND GENERAL CONCEPTS OF CAVITATION<\/strong><\/h2>\n\n\n\n

(Bibliographic References: [1], [2], and [5])<\/p>\n\n\n\n

1.1. Definition<\/strong><\/h4>\n\n\n\n

The phenomenon of cavitation occurs when vapor bubbles or pockets form in a flowing liquid as a result of localized pressure reductions (for example, in certain regions of the blades of hydraulic pumps or turbines).<\/p>\n\n\n\n

The pressure at the point where cavitation begins may be equal to or lower than the liquid’s vapor pressure, depending on the number and distribution of particles in the liquid, to which small bubbles of undissolved gas or air may attach.<\/p>\n\n\n\n

These particles act as nucleation sites that initiate vaporization. The growth and subsequent collapse or implosion of these vapor bubbles near solid surfaces can cause severe erosion damage to the material surfaces.<\/p>\n\n\n\n

<\/div>\n\n\n\n
\"CAVITATION\"
Figure 1 \u2013 Nucleation and implosion of a vapor bubble.
<\/figcaption><\/figure>\n\n\n\n
<\/div>\n\n\n\n

1.1. Vapor Pressure<\/h4>\n\n\n\n

The vapor pressure of a liquid is the absolute pressure at which the liquid vaporizes or changes into a gas at a specific temperature, as shown in Graph 1.<\/p>\n\n\n\n

<\/div>\n\n\n\n
\"\"
Graph 1 \u2013 Vapor temperature vs. pressure<\/figcaption><\/figure>\n\n\n\n

Vapor pressure is typically expressed in bar(a) or kPa. The vapor pressure of a liquid increases with its temperature. Therefore, vapor pressure must always be specified together with the liquid temperature.<\/p>\n\n\n\n

As an example, let us examine the following condition:<\/p>\n\n\n\n

At sea level, water boils at 100\u00b0C under an atmospheric pressure of 1.013 bar. If we wish to increase this temperature, a boiler or pressure vessel designed to withstand pressures above 1.013 bar is required.<\/p>\n\n\n\n

In fact, in the Andes, at an altitude of 4,600 m above sea level, the local atmospheric pressure is 0.573 bar instead of 1.013 bar. It is known that water boils at 84.4\u00b0C under these conditions. Therefore, at 84.4\u00b0C, we can state that the vapor pressure of water is approximately 0.573 bar.<\/p>\n\n\n\n

<\/div>\n\n\n\n

1.3. The effects of cavitation<\/h4>\n\n\n\n

In liquid flow through metal pipes or centrifugal pumps, vapor bubbles may form in low-pressure regions and subsequently collapse in areas where the pressure is higher.<\/p>\n\n\n\n

When these bubbles collapse, the surrounding liquid impacts the metal surfaces at a velocity approaching the speed of sound in the fluid. This produces a characteristic noise resembling marbles or stones striking metal components.<\/p>\n\n\n\n

Sound travels at a speed of 1,463 m\/s in water. The velocity head, representing kinetic energy per unit weight, associated with a bubble imploding due to cavitation can be calculated as follows:<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n
<\/div>\n\n\n\n

Converting this energy into pressure energy, we obtain:<\/p>\n\n\n\n

<\/div>\n\n\n\n
\"\"<\/figure>\n\n\n\n
<\/div>\n\n\n\n


Therefore, based on the velocity head equation, a cavitation bubble, upon implosion, exerts a sudden pressure of 10,702 bar on the metal surfaces of pumps or pipes.<\/p>\n\n\n\n

Experiments conducted in hydraulic laboratories, using more accurate calculations and measurement techniques, have recorded pressures of approximately 1 gigapascal (GPa), equivalent to 10,000 bar.<\/p>\n\n\n\n

This explains the appearance of cavitation damage on pipes and internal pump components. The affected surfaces exhibit deep indentations resembling marks made by ball-peen hammers on the impeller or casing, potentially resulting in material removal from the metal surfaces of these components, as shown in Figure 2.<\/p>\n\n\n\n

<\/div>\n\n\n\n
\"\"
Figure 2 \u2013 Effects of cavitation on a centrifugal pump impeller<\/figcaption><\/figure>\n\n\n\n
<\/div>\n\n\n\n

2. CAVITATION IN CENTRIFUGAL PUMPS<\/strong><\/strong><\/h2>\n\n\n\n

(Bibliographic Reference: [3])<\/p>\n\n\n\n

The following section describes cavitation in centrifugal pumps, its consequences, and methods for preventing it.<\/p>\n\n\n\n

2.1. Description of the phenomenon in centrifugal pumps<\/strong><\/h4>\n\n\n\n

As a general rule, inside a pump, pressure decreases at the impeller eye due to an increase in fluid velocity. For this reason, the fluid may partially vaporize at the pumping temperature. For example, if the pressure in this region drops to just 0.07 bar(a), water may vaporize at a temperature as low as 38\u00b0C.<\/p>\n\n\n\n

When cavitation occurs in a pump, its efficiency decreases. Sudden disturbances in fluid flow and a pressure drop at the discharge nozzle may also occur.<\/p>\n\n\n\n

At this point, it is important to mention two parameters that are essential for preventing cavitation in centrifugal pumps: NPSHa (Net Positive Suction Head Available), calculated by the user, and NPSHr (Net Positive Suction Head Required), determined by the pump manufacturer. Both will be discussed in greater detail throughout this article.<\/p>\n\n\n\n

As briefly mentioned in previous sections, the noticeable effects of cavitation include noise and vibration originating from the pump casing. If the pump operates under cavitation conditions for a certain period, the following damage may occur:<\/p>\n\n\n\n

    \n
  • Erosion marks (pitting) on the impeller blades and the internal surfaces of the pump volute. See Figure 2.<\/li>\n\n\n\n
  • Premature mechanical failure of the bearings.<\/li>\n\n\n\n
  • Premature mechanical seal failure.<\/li>\n\n\n\n
  • Shaft breakage and fatigue-related failures in other metallic pump components.<\/li>\n<\/ul>\n\n\n\n
    <\/div>\n\n\n\n

    2.3. Causes of cavitation in centrifugal pumps<\/strong><\/h4>\n\n\n\n

    Cavitation problems in centrifugal pumps generally originate from:<\/p>\n\n\n\n

      \n
    • A reduction in pressure at the suction nozzle.<\/li>\n\n\n\n
    • An increase in the temperature of the pumped fluid.<\/li>\n\n\n\n
    • An increase in fluid inlet velocity at the suction nozzle.<\/li>\n\n\n\n
    • Flow separation or reduced flow due to changes in the viscosity of the pumped fluid.<\/li>\n\n\n\n
    • Unsatisfactory suction piping conditions caused by obstructions or components that generate excessive pressure losses, such as the use of sharp elbows instead of long-radius bends.<\/li>\n\n\n\n
    • A pump that is unsuitable for the system or application.<\/li>\n<\/ul>\n\n\n\n
      <\/div>\n\n\n\n

      2.4. Suggested solutions to cavitation problems<\/strong><\/h4>\n\n\n\n

      Most cavitation problems can be resolved or minimized through the following measures, whenever feasible:<\/p>\n\n\n\n

        \n
      • Increasing the pressure at the pump suction.<\/li>\n\n\n\n
      • Reducing pressure losses in the suction piping, particularly those near the pump inlet nozzle (friction losses).<\/li>\n\n\n\n
      • Using a larger pump.<\/li>\n\n\n\n
      • Lowering the liquid’s vapor pressure, for example, by reducing its temperature.<\/li>\n\n\n\n
      • Venting air or vapor bubbles carried by the suction fluid to the atmosphere.<\/li>\n\n\n\n
      • Selecting harder materials for critical pump components. Although this does not eliminate cavitation, it helps reduce erosion.<\/li>\n<\/ul>\n\n\n\n
        <\/div>\n\n\n\n

        2.5. Practical example involving cavitation<\/strong><\/h4>\n\n\n\n

        To illustrate the phenomenon of cavitation in practice, consider the following real-world problem:<\/p>\n\n\n\n

        An industrial facility needs to pump 3 m\u00b3\/h of water from a well where the groundwater level is 12 meters below the surface. Is it possible to perform this operation using a conventional centrifugal pump?<\/p>\n\n\n\n

        Answer: No conventional centrifugal pump can perform this operation under these conditions, because the water would vaporize before reaching the pump’s suction flange. Any vapor bubbles entering the pump would cause severe cavitation.<\/p>\n\n\n\n

        Explanation: The suction pressure available to the pump is determined by the atmospheric pressure acting on the groundwater surface in the open well. Therefore:<\/p>\n\n\n\n

        \"\"<\/figure>\n\n\n\n

        But, on the other hand:<\/p>\n\n\n\n

        \"\"<\/figure>\n\n\n\n

        In other words, the maximum theoretical suction lift that a centrifugal pump can achieve is 10.33 m. However, the actual value is considerably lower, depending on numerous factors related to the type of pump.<\/p>\n\n\n\n

        <\/div>\n\n\n\n

        2.6. Types of cavitation in centrifugal pumps<\/strong><\/h4>\n\n\n\n

        There are five recognized types of cavitation in centrifugal pumps:<\/p>\n\n\n\n

          \n
        • Vaporization cavitation, also known as cavitation caused by insufficient NPSHa.<\/li>\n\n\n\n
        • Internal recirculation cavitation.<\/li>\n\n\n\n
        • Cavitation caused by insufficient clearance between the impeller blade tips and the volute tongue.<\/li>\n\n\n\n
        • Air ingestion cavitation.<\/li>\n\n\n\n
        • Cavitation caused by turbulent flow at the pump suction nozzle.<\/li>\n<\/ul>\n\n\n\n
          <\/div>\n\n\n\n

          In this article, we will analyze only the first two types, as they are the most common. An analysis of the remaining types can be found in the existing technical literature.<\/p>\n\n\n\n

          <\/div>\n\n\n\n

          2.6.1. Vaporization cavitation<\/strong><\/h4>\n\n\n\n

          Vaporization cavitation accounts for approximately 70% of cavitation cases in centrifugal pumps. It is also known as classical cavitation.<\/p>\n\n\n\n

          To make this concept easier to understand, it is important to remember that water boils when the temperature is sufficiently high or when the pressure is sufficiently low.<\/p>\n\n\n\n

          According to Bernoulli’s equation, as fluid velocity increases, pressure decreases. In an operating centrifugal pump, the liquid accelerates toward the impeller eye until it reaches a certain inlet velocity. Under specific conditions, the liquid may boil or vaporize at the impeller eye. When this occurs, the pump experiences vaporization cavitation.<\/p>\n\n\n\n

          This type of cavitation indicates insufficient NPSHa in the system, meaning that the available NPSH (NPSHa) is lower than the NPSH required by the pump (NPSHr).<\/p>\n\n\n\n

          The general recommendation for preventing this type of cavitation is expressed by the following formula:<\/p>\n\n\n\n

          <\/div>\n\n\n\n
          \"\"<\/figure>\n\n\n\n

          As previously mentioned, this is a general recommendation, since in certain cases a safety margin of 1 meter of liquid column may be insufficient and must be increased.<\/p>\n\n\n\n

          At this point, it is important to present the NPSHa formula, which must be applied to the system. NPSHa must always be calculated in meters of the pumped liquid column (mlc).<\/p>\n\n\n\n

          \"\"<\/figure>\n\n\n\n

          Where:<\/p>\n\n\n\n

            \n
          • Ha = Local atmospheric pressure, expressed in meters of liquid column (mLC). At sea level, it is equivalent to 10.33 meters of water column (mWC) and must therefore be converted to mLC.<\/li>\n\n\n\n
          • Hs = Static suction head, representing the height of the liquid above the pump, measured relative to the pump centerline and expressed in mLC.<\/li>\n\n\n\n
          • Hvp = Vapor pressure of the liquid at the pumping temperature. As a valid approximation, this value can be obtained from the liquid’s saturated vapor pressure tables. It must be converted to mLC.<\/li>\n\n\n\n
          • Hf = Friction head loss in the suction piping up to the pump inlet flange, expressed in mLC.<\/li>\n\n\n\n
          • Hi = Friction head loss from the pump inlet flange to the impeller eye, expressed in mLC. This value may be estimated at 0.6 mLC, although it depends on the pump design.<\/li>\n<\/ul>\n\n\n\n

            In this case, when the pump is disassembled for inspection in the workshop, damage caused by vaporization cavitation is generally observed on the back surfaces of the impeller blades, toward the impeller eye, as schematically illustrated in Figure 3.<\/p>\n\n\n\n

            <\/div>\n\n\n\n
            \"\"
            Figure 3 \u2013 Vaporization cavitation in the impeller (counterclockwise rotation)<\/figcaption><\/figure>\n\n\n\n
            <\/div>\n\n\n\n

            To resolve and prevent these problems, several measures can be implemented whenever possible:<\/p>\n\n\n\n

              \n
            • Reduce the fluid temperature, thereby decreasing Hvp.<\/li>\n\n\n\n
            • Increase the liquid level above the pump, thereby increasing Hs.<\/li>\n\n\n\n
            • Reduce the pump rotational speed, indirectly reducing Hf.<\/li>\n\n\n\n
            • Increase the diameter of the impeller eye, thereby reducing Hf and Hi.<\/li>\n\n\n\n
            • Install an inducer at the impeller inlet, thereby reducing Hi and increasing the available suction pressure.<\/li>\n\n\n\n
            • Install two smaller pumps in parallel, thereby reducing Hf and Hi.<\/li>\n\n\n\n
            • Install a booster pump to feed the main pump, thereby increasing the available suction pressure.<\/li>\n<\/ul>\n\n\n\n
              <\/div>\n\n\n\n

              2.6.2. Internal recirculation cavitation<\/strong><\/h4>\n\n\n\n

              This is an extreme operating condition that occurs when the pump operates well below the minimum flow rate recommended by the manufacturer. It occurs when the discharge flow rate is significantly reduced, preventing the fluid from flowing out of the pump properly. As a result, the liquid is forced to recirculate through the impeller, moving from high-pressure regions to low-pressure regions.<\/p>\n\n\n\n

              Cavitation may then occur due to two fundamental factors. First, the liquid circulates inside the volute at a velocity associated with the impeller rotation and rapidly overheats. Second, the liquid is forced to pass through narrow clearances at high velocity, such as in open-impeller pumps, between the impeller blade tips and the internal wall of the volute.<\/p>\n\n\n\n

              High temperatures and high fluid velocities contribute to cavitation in the pumped liquid.<\/p>\n\n\n\n

              If the pump operates at flow rates close to the minimum recommended by the manufacturer, cavitation itself may be only incipient. In such cases, noise and vibration may result from less severe internal recirculation without actual cavitation occurring. Although this condition can still be highly detrimental to pump components, such as bearings and mechanical seals, the surface damage associated with this type of recirculation is entirely different from that caused by vaporization cavitation.<\/p>\n\n\n\n

              Open-impeller pumps that experience actual cavitation due to internal recirculation generally exhibit damage along the leading edges of the impeller blades, toward the impeller eye, as well as at the blade tips, as schematically illustrated in Figure 4.<\/p>\n\n\n\n

              \"\"
              Figure 4 \u2013 Internal recirculation cavitation in the impeller (counterclockwise rotation)<\/figcaption><\/figure>\n\n\n\n

              To mitigate internal recirculation problems in open-impeller pumps operating near the minimum flow rate, measures can be taken directly in the discharge line to reduce pressure losses downstream of the pump. Alternatively, an automatic recirculation valve can be installed to return the liquid to the suction vessel, or a bypass line equipped with an orifice plate can be used for the same purpose.<\/p>\n\n\n\n

              <\/div>\n\n\n\n

              2.6.3. AThe issue of pump NPSHr<\/strong><\/h4>\n\n\n\n

              Generally, the NPSHr of a pump is not calculated, although formulas can be found in technical literature. These formulas provide only approximate results.<\/p>\n\n\n\n

              The correct approach is to obtain the NPSHr directly from the pump performance curves provided by the manufacturer. NPSHr is a function of flow rate: the higher the flow rate, the higher the NPSHr value.<\/p>\n\n\n\n

              It is important to remember that NPSHa is a characteristic of the system, while NPSHr is a characteristic of the pump.<\/p>\n\n\n\n

              However, it is possible to reduce the pump’s NPSHr by adopting the following measures, whenever feasible:<\/p>\n\n\n\n

                \n
              • Use a pump with a larger suction nozzle diameter. For example, replace a 3\u00d74\u00d710 pump with a 4\u00d76\u00d710 pump. By increasing the suction flange diameter from 4 inches to 6 inches, the inlet friction losses (Hi) will decrease. Since the pump head is approximately proportional to the square of the impeller diameter (D\u00b2), the same head required by the system can be maintained.<\/li>\n\n\n\n
              • Machine and improve the surface finish of the pump inlet nozzle, as this is generally one of the roughest areas of the cast pump casing. Center the component on a lathe and perform precise alignment to reduce Hi.<\/li>\n\n\n\n
              • Use a larger pump operating at a lower rotational speed. This will also reduce Hi.<\/li>\n<\/ul>\n\n\n\n
                <\/div>\n\n\n\n

                3. CAVITATION IN LIQUID RING VACUUM PUMPS<\/strong><\/strong><\/h2>\n\n\n\n

                (Bibliographic References: [2], [4], and [6])<\/p>\n\n\n\n

                The following section provides an overview of the problems commonly encountered in liquid ring vacuum pumps due to cavitation, including their causes, consequences, and possible solutions.<\/p>\n\n\n\n

                3.1. Definition<\/strong><\/h4>\n\n\n\n

                Cavitation in liquid ring vacuum pumps results from vapor bubbles formed under vacuum conditions that are subjected to an increase in pressure at the pump discharge port, causing them to implode or collapse.<\/p>\n\n\n\n

                If a liquid ring vacuum pump produces abnormal noises resembling marbles or small stones moving inside the pump, it is very likely that the pump is experiencing cavitation.<\/p>\n\n\n\n

                3.2. General concepts \u2013 condensation and cavitation<\/strong><\/h4>\n\n\n\n

                As is well known, cavitation on the suction side occurs when the vacuum pump operates at a suction pressure close to the vapor pressure of its sealing liquid.<\/p>\n\n\n\n

                Based on the above, it is advisable to maintain the suction pressure of a liquid ring vacuum pump at more than twice the saturation vapor pressure of the sealing liquid at a given temperature, thereby ensuring safe operation.<\/p>\n\n\n\n

                For more critical operating conditions, a minimum suction pressure limit may be established at least 16 mbar above the vapor pressure of the sealing liquid at its operating temperature.<\/p>\n\n\n\n

                If the temperature of the sealing liquid increases, its vapor pressure also increases, consequently increasing the risk of cavitation.<\/p>\n\n\n\n

                Figure 5 illustrates the effects of cavitation in a liquid ring vacuum pump, while Figure 6 shows these effects on the suction and discharge port plate of the same type of pump.<\/p>\n\n\n\n

                \"\"
                Figure 5 \u2013 Damage caused by cavitation in a vacuum pump impeller<\/figcaption><\/figure>\n\n\n\n
                \"\"
                Figure 6 \u2013 Damage caused by cavitation on the port plate of a vacuum pump<\/figcaption><\/figure>\n\n\n\n

                3.3. Key considerations regarding cavitation in liquid ring vacuum pumps<\/strong><\/h4>\n\n\n\n

                Cavitation in liquid ring vacuum pumps is a phenomenon that occurs in the pump’s sealing liquid and is an important operating condition to consider when evaluating pump performance.<\/p>\n\n\n\n

                The phenomenon begins with the formation of vapor bubbles in the sealing water under specific pressure and temperature conditions. For example, at an absolute pressure of 52 Torr (52 mmHg), water will boil at 39\u00b0C. If the absolute pressure is increased, the water will remain in its liquid state, moving out of the cavitation zone.<\/p>\n\n\n\n

                As previously mentioned, vapor pressure tables indicate the critical pressure-temperature relationship for the sealing liquid, in this case water, under which this phenomenon occurs.<\/p>\n\n\n\n

                When examining a pump that has experienced severe cavitation, typical cavitation damage can be observed on its internal components. This damage resembles hammer impacts or crater-like indentations uniformly distributed across the impellers, as shown in Figures 7, 7.1, 8, and 8.1.<\/p>\n\n\n\n

                In two-stage pumps, these marks may appear in both stages, depending on the specific operating conditions.<\/p>\n\n\n\n

                Another common location for cavitation damage is the suction and discharge ports, as illustrated in Figure 6.<\/p>\n\n\n\n

                \n
                \n
                \"\"
                Figure 7 \u2013 Cavitation damage to the first-stage impeller<\/figcaption><\/figure>\n<\/div>\n\n\n\n
                \n
                \"\"
                Figure 7.1 \u2013 Enlarged view of the cavitation area<\/figcaption><\/figure>\n<\/div>\n<\/div>\n\n\n\n
                \n
                \n
                \"\"
                Figure 8 \u2013 Cavitation damage to the first-stage impeller<\/figcaption><\/figure>\n<\/div>\n\n\n\n
                \n
                \"\"
                Figure 8.1 \u2013 Enlarged view of the cavitation area<\/figcaption><\/figure>\n<\/div>\n<\/div>\n\n\n\n
                <\/div>\n\n\n\n

                Generally, to control or prevent cavitation, the sealing liquid must be capable of withstanding the expected vacuum level in the medium being evacuated without vaporizing or boiling. Further details will be discussed in Section 3.4.<\/p>\n\n\n\n

                The vaporization point of the sealing liquid determines the minimum absolute suction pressure at which the pump can operate without cavitation. At pressures below this limit, cavitation will occur. However, what actually causes cavitation damage is the implosion of the bubbles, not their formation.<\/p>\n\n\n\n

                When the bubbles collapse, high-velocity microjets of water strike the metallic components of the pump. For vapor bubbles to collapse, there must be an increase in pressure or a decrease in temperature at a specific location within the pump.<\/p>\n\n\n\n

                An increase in pressure occurs as the gas is compressed through the pump toward its discharge port. Therefore, to keep the pump free from cavitation, the sealing water temperature must remain below the saturation temperature corresponding to the operating vacuum pressure.<\/p>\n\n\n\n

                The sealing liquid must remain sufficiently cool to prevent vaporization under any operating condition. Once vaporization occurs inside the pump, forming bubbles due to an absolute pressure lower than the minimum specified in the design, bubble collapse becomes inevitable as the gas is compressed to a higher pressure toward the pump discharge port.<\/p>\n\n\n\n

                The pump capacity (suction volume) is also affected by the sealing water temperature. Capacity depends on the suction pressure (vacuum), which in turn is influenced by the sealing water temperature. As the temperature increases, only higher absolute suction pressures and lower pump capacities can be achieved.<\/p>\n\n\n\n

                The following example illustrates this relationship: if the discharge water temperature, before entering the separator vessel, remains below 22\u00b0C, an absolute suction pressure of 18 Torr can be achieved, which is close to the vapor pressure of water at this temperature.<\/p>\n\n\n\n

                However, if the discharge water temperature increases to 27\u00b0C and the desired suction pressure is 25 Torr, the partial pressure of water vapor at 27\u00b0C is 26.3 Torr. Therefore, the pump would experience cavitation when operating at 25 Torr.<\/p>\n\n\n\n

                <\/div>\n\n\n\n

                3.4. Measures to prevent cavitation<\/strong><\/h4>\n\n\n\n

                Cavitation in vacuum pumps can often be attributed to changes in the original operating conditions for which the pump was selected. The temperature that must be maintained for the sealing water is also an important selection criterion, as it may depend on environmental conditions, such as the cooling water tower.<\/p>\n\n\n\n

                Therefore, if a pump has been operating normally and suddenly begins to cavitate, this may indicate a sudden increase in the sealing water temperature.<\/p>\n\n\n\n

                The location of cavitation damage inside the pump may vary, as can be observed when pumps are disassembled in the workshop for internal inspection. It is important to emphasize that this damage may occur in any part of the pump. In two-stage pumps, cavitation may even affect components outside the first stage, where the lowest pressures occur.<\/p>\n\n\n\n

                To prevent cavitation in liquid ring pumps, the causes of temperature increases in the sealing water must be identified.<\/p>\n\n\n\n

                Although the operation of a liquid ring pump is considered approximately isothermal, this is not entirely true in practice. The sealing water temperature may increase for several reasons.<\/p>\n\n\n\n

                For example, a temperature increase occurs due to heat transfer resulting from the gas compression work performed by the liquid ring. Vapors present in the suction gas condense and release heat during the phase change.<\/p>\n\n\n\n

                Non-condensable gases in the suction mixture also reach thermal equilibrium with the liquid ring, transferring heat to the water. All these energy sources contribute to an increase in the sealing water temperature.<\/p>\n\n\n\n

                Therefore, the amount of cold make-up water supplied through the separator tank is important to maintain the discharge water temperature below the saturation temperature corresponding to a given suction pressure.<\/p>\n\n\n\n

                Cavitation can be prevented by adopting certain measures in pump design. The use of abrasion-resistant materials can reduce the effects of cavitation on specific pump components; however, these materials cannot prevent cavitation itself.<\/p>\n\n\n\n

                To prevent cavitation, the flow rate of non-condensable gases must comply with the pump performance curve at its minimum allowable absolute suction pressure, which must always be maintained.<\/p>\n\n\n\n

                Alternatively, to prevent pressures from falling below the minimum allowable value, the pump can be connected to an inert gas line running from the top of the liquid separator vessel, normally installed at the pump discharge, back to the pump suction through a control valve.<\/p>\n\n\n\n

                Other devices may also be used, including manual valves or automatic valves, such as adjustable vacuum relief valves, which allow atmospheric air to enter directly into the pump suction side.<\/p>\n\n\n\n

                The following is a summary checklist of inspections intended to prevent possible cavitation in liquid ring vacuum pumps:<\/p>\n\n\n\n

                  \n
                • Verify that the circulating liquid ring temperature is sufficiently low to maintain the desired vacuum level.<\/li>\n\n\n\n
                • Check whether the water temperature at the discharge to the separator tank is excessively high, as this may indicate internal cavitation.<\/li>\n\n\n\n
                • Check the make-up water entering the separator tank, including its flow rate and temperature. Calculate whether the supply is sufficient to compensate for sealing liquid losses due to evaporation.<\/li>\n\n\n\n
                • Check whether the recirculated water heat exchanger is obstructed by deposits or fouling. See the considerations in Section 3.5.<\/li>\n\n\n\n
                • Check the cooling water pressure, temperature, and flow rate in the heat exchanger. Based on the heat balance, these parameters must provide adequate cooling for the recirculated water entering the pump.<\/li>\n\n\n\n
                • Verify the set point of the vacuum relief valve. This valve must allow sufficient atmospheric air to enter when the absolute suction pressure falls below the design limit.<\/li>\n<\/ul>\n\n\n\n
                  <\/div>\n\n\n\n

                  3.5. The sealing water heat exchanger<\/strong><\/h4>\n\n\n\n

                  One of the main pieces of equipment requiring attention when a liquid ring vacuum pump is not operating satisfactorily is the sealing water heat exchanger.<\/p>\n\n\n\n

                  This heat exchanger, generally a plate-type unit, is installed in the recirculation circuit when the sealing liquid from the separator tank is reused.<\/p>\n\n\n\n

                  The sealing liquid must be cooled to a lower temperature before re-entering the pump. Its inlet temperature and flow rate must be suitable for achieving the required vacuum level and volumetric capacity of the pump.<\/p>\n\n\n\n

                  It is often observed that during maintenance of a vacuum system, this equipment is overlooked and does not receive the attention it deserves.<\/p>\n\n\n\n

                  We know that the desired vacuum level (suction pressure) can only be maintained when it is compatible with the vapor pressure of the sealing liquid. Otherwise, the pump may experience cavitation due to vaporization of the sealing liquid.<\/p>\n\n\n\n

                  Therefore, if satisfactory heat transfer does not occur in the heat exchanger, the pump will lose vacuum performance and capacity. In other words, proper maintenance of this equipment is of fundamental importance in a vacuum system. To achieve this objective, several important points must be considered, as described below:<\/p>\n\n\n\n

                    \n
                  • The heat exchanger must be kept clean to ensure effective heat transfer performance.<\/li>\n\n\n\n
                  • With the pump operating, it is advisable to measure the inlet and outlet temperatures of both the cooling water and the sealing water. These values should be compared with the original design specifications.<\/li>\n\n\n\n
                  • Measure the cooling water flow rate and its temperature increase. If the flow rate is correct but there is no significant temperature rise, this indicates insufficient heat transfer and may suggest fouling inside the heat exchanger. Consequently, the liquid ring water will enter the pump at a higher temperature, compromising the vacuum level and pump capacity.<\/li>\n\n\n\n
                  • It is also useful to investigate whether air is entering the system and accumulating inside the heat exchanger. This is particularly relevant for shell-and-tube heat exchangers, where trapped air can reduce heat transfer efficiency.<\/li>\n\n\n\n
                  • It is common for cooling systems to operate below their expected performance. The heat exchanger may be undersized, or the original design conditions may differ from the current operating conditions. Improper startup preparation is another possibility, such as piping that was not adequately flushed before startup, allowing contaminants to accumulate inside the heat exchanger.<\/li>\n<\/ul>\n\n\n\n

                    Therefore, regardless of its size or type, the heat exchanger must be inspected during maintenance and monitored throughout operation.<\/p>\n\n\n\n

                    <\/div>\n\n\n\n

                    4. BIBLIOGRAPHY<\/strong><\/strong><\/h3>\n\n\n\n

                    The following publications were consulted in the preparation of this technical paper:
                    [1] Introduction to Fluid Mechanics \u2013 8th Edition \u2013 Fox \/ McDonald \/ Pritchard
                    [2] Fundamentals of Thermodynamics \u2013 Translation of the 8th American Edition \u2013 Borgnakke and Sonntag
                    [3] Know and Understand Centrifugal Pumps \u2013 1st Edition \u2013 Larry Bachus and Angel Custodio
                    [4] Liquid Ring Vacuum Pumps, Compressors and Systems \u2013 1st Edition \u2013 Helmuth Bannwarth
                    [5] Vacuum Technology in the Chemical Industry \u2013 1st Edition \u2013 Wolfgang Jorisch
                    [6] Cavitation in Liquid Ring Vacuum Pumps Used in Condenser Venting Service \u2013 Graham Corporation<\/p>\n\n\n\n

                    <\/p>\n","protected":false},"excerpt":{"rendered":"

                    1. DEFINITION AND GENERAL CONCEPTS OF CAVITATION (Bibliographic References: [1], [2], and [5]) 1.1. Definition The phenomenon of cavitation occurs when vapor bubbles or pockets form in a flowing liquid as a result of localized pressure reductions (for example, in certain regions of the blades of hydraulic pumps or turbines). The pressure at the point […]<\/p>\n","protected":false},"author":3,"featured_media":8435,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[171],"tags":[],"class_list":["post-16052","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-escola-de-bombas-en"],"_links":{"self":[{"href":"https:\/\/omel.com.br\/en\/wp-json\/wp\/v2\/posts\/16052","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/omel.com.br\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/omel.com.br\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/omel.com.br\/en\/wp-json\/wp\/v2\/users\/3"}],"replies":[{"embeddable":true,"href":"https:\/\/omel.com.br\/en\/wp-json\/wp\/v2\/comments?post=16052"}],"version-history":[{"count":2,"href":"https:\/\/omel.com.br\/en\/wp-json\/wp\/v2\/posts\/16052\/revisions"}],"predecessor-version":[{"id":16071,"href":"https:\/\/omel.com.br\/en\/wp-json\/wp\/v2\/posts\/16052\/revisions\/16071"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/omel.com.br\/en\/wp-json\/wp\/v2\/media\/8435"}],"wp:attachment":[{"href":"https:\/\/omel.com.br\/en\/wp-json\/wp\/v2\/media?parent=16052"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/omel.com.br\/en\/wp-json\/wp\/v2\/categories?post=16052"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/omel.com.br\/en\/wp-json\/wp\/v2\/tags?post=16052"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}