Net Positive Suction Head Research Articles

An improved cavitation model with thermodynamic effect and multiple cavitation regimes

Mechanical feed pumps in organic Rankine cycle (ORC) power plants can suffer from cavitation to lose their normal feeding performance or even damage. Cavitation models for organic fluids in ORC systems are lacking presently. Hence, a new cavitation model with thermodynamic effect was proposed. Surface tension-controlled, inertia-controlled, intermediate and heat transfer-controlled cavitation regimes, and two key elements: vapour bubble growth rate and vapour bubble number density are included in the model. A known air or non-condensable gas concentration in the liquid was employed to determine cavitation nuclei number density. The model was coded in ANSYS CFX as user defined model and validated with cavitating flows of organic fluid R114 in a venturi, liquid nitrogen and liquid hydrogen on a tapered hydrofoil and warm water around a hydrofoil NACA 0015 in cavitation tunnels based on visualised cavity length. Two model constants, temperature depression, and minimal cavitation number were correlated to bulk liquid temperature, Reynolds number, and Jakob number. The temperature and pressure profiles of liquid nitrogen and hydrogen on hydrofoil surface were examined against the experimental data. The model was applied to simulate unsteady cavitating flows of organic fluid R245fa in a diaphragm pump. It was shown that the temperature depression and minimal cavitation number cannot be correlated to bulk liquid temperature, Reynolds number and Jakob number. Two model constants can be correlated fairly to Reynolds number. The model underestimates the thermodynamic effect by 43% for R114, 18.6% for liquid nitrogen and 32.6% for liquid hydrogen based on temperature depression. The predicted temperature and pressure profiles on hydrofoil surface agree with the experimental data for liquid nitrogen. The model can produce an expected curve of mean pump flow rate against net positive suction head available.

Axial-Flow Pump with Enhanced Cavitation Erosion Resistance

Axial-flow pumps, in addition to providing high anti-cavitation properties, must have high anti-erosion properties to ensure the required lifetime of the pump. Erosion damage of surfaces occurs when the net positive suction head (NPSH) significantly exceeds its critical value. The object of the study in this article is the axial-flow pump with a specific speed of 600 in two alternatives: № 1 and № 2. By analysis of the flow in the impeller blade systems, the ratio value between the NPSH, which ensures the absence of erosion, and the NPSH3, at which pump operational failure occurs, was determined. Impeller variant № 1 did not provide the required ratio. Impeller variant № 2 had higher cavitation qualities, and the required ratio was achieved for it. Energy, cavitation, and erosion characteristics of the axial-flow pump with impeller № 2 in rotational frequency n = 2000 rpm were investigated. Easily breakable paint coatings were used for the accelerated study of cavitation erosion. The experiment was carried out at three different flow rates and confirmed the assumptions made—the pump with impeller № 2 was not affected by cavitation erosion at the optimum flow rate. Patterns of erosion zones were accompanied by calculations of vapor zones in the impeller. At flow rates less than the optimum, cavitation disruptions occurred and appeared behind the vapor region. As a result, the condition of ensuring erosion-free flow in the impeller of an axial pump with a specific speed of 600 was obtained, ensuring the ratio NPSH/NPSH3 > 2.5. Recommendations on designing of erosion-free flow part of the axial pump impeller were also obtained.

Experimental investigation of cavitation-induced noise and vibration in a centrifugal pump

Cavitation has become a serious concern threatening operational stability as the need for high-speed machinery has grown. In this study, noise (sound pressure level) and vibrations (structural) were measured in a radial flow centrifugal pump for design and off-design conditions under cavitation. Because the structure of a particular noise/vibration spectrum (as well as its total level) is affected by the operating conditions and stage of cavitation within the pump, a detailed analysis the noise/vibration spectra during the pumping process was carried out to detect of the cavitation phenomenon's progression. We aim to identify the association between noise/vibration and the corresponding net positive suction head value at different flow rates. The results showed that the cavitation structure affects the vibration/noise behavior of the pump. Moreover, total noise level measurements at partial flow rates cannot be used to diagnose full cavitation breakdown, whereas total noise level measurements can be used to detect cavitation intensity at higher flow rates than the optimum operating point.

Investigation of the mechanism of unsteady flow induced by cavitation at the tongue of a centrifugal pump based on the proper orthogonal decomposition method

To investigate the unsteady evolution characteristics and mechanism of cavitation at the tongue and its influence on the flow field in a centrifugal pump, numerical calculations were performed to research the occurrence and development of cavitation at the tongue under large flow rate (1.3Qd) conditions. The numerical calculation results were verified by experiments. The proper orthogonal decomposition (POD) method was used to decompose and reconstruct the flow field at the tongue of the centrifugal pump. The relationship between the typical frequency of pressure pulsation and the mainstream structure of the flow field was established. The POD method could accurately extract the coherent structure caused by cavitation and capture the unstable flow structure produced by vortex shedding. Under different available net positive suction head conditions, the first mode was the mainstream structural mode in the original flow field, with low-frequency and single-frequency components. The second, third, and fourth modes were the main oscillation modes of the original flow field and showed the characteristics of high frequency and multiple frequencies. High-order modes could capture the complex flow field structure caused by the change in the large-scale vortex structure induced by the unsteady pulsation of cavitation.

Hydraulic Optimization and Experimental Measurement of Low Specific Speed Centrifugal Fire Pump

As one of the core components of the fire-fighting water-supply system, the performance of a fire pump directly determines the extent of damage caused by the fire. Compared with conventional pumps, the design requirements of fire pumps not only need to ensure that the head of the pump is at 0Qd, 1.0Qd, and 1.5Qd and its efficiency is at 1.0Qd but also consider the cavitation performance at each flow rate, which presents a greater challenge for the design of high-performance fire pumps. By optimizing the design of a centrifugal fire pump with a specific speed of 24.7, numerical calculations were performed to obtain the best optimized scheme Y4. The results show that at the design flow rate the best optimized scheme improves the efficiency by 9.17% compared with the original scheme, and the head meets the design requirements of the fire pump while avoiding the hump phenomenon. Through a comparative analysis, it was found that the optimized scheme Y4 can reduce the pressure pulsations at the outlet of the pump and improve the cavitation performance at each flow rate. The experiment verifies that the head of the best optimized scheme at the design flow rate is 74.43m, the pump efficiency is 40.22%, and there is no hump in the head curve, which can meet the design and use requirements of the fire pump. The maximum reduction in the outlet pressure pulsations coefficient in the best optimized scheme was 47.12% on average. Compared with the original scheme, the critical net positive suction head (NPSHr) of the best optimized scheme was reduced by 21.5%, 17.6%, 15.7%, and 16.8%, respectively.

Experimental and Numerical Study on the Dynamic and Flow Characteristics of a Reciprocating Pump Valve

The structure and dynamics of a reciprocating pump liquid end affect the volumetric efficiency and net positive suction head. To match the kinematics with theoretical parameters, reciprocating pump valve motion and flow visualization tests and computational fluid dynamics (CFD) analyses were performed on a wing-guided bevel discharge valve in a horizontal quintuple single-acting reciprocating pump. The valve motion test results showed that the maximum pump valve displacement and the pump valve opening and closing durations were approximately 8.3 mm, 29 ms, and 38 ms, respectively. The corresponding flow visualization test results were 11.4 mm, 9.5 ms, and 35.5 ms. The valve closing durations obtained from the valve motion and flow visualization tests are approximately twice as high as the U-Adolph prediction. The maximum displacement obtained from the valve motion test is consistent with the U-Adolph prediction. Three-dimensional CFD analyses were performed to investigate the flow states, pressure, and velocity characteristics of the discharge valve opening. Finally, the proposed method was applied to develop a new horizontal quintuple single-acting reciprocating pump with a rated flow rate of 1250 L/min and pressure of 40 MPa. This developed pump exhibited good performance and excellent reliability.

Cavitation Characteristics during Startup Process of a Condensate Pump with Splitter Blades

The present study aims to describe characteristics of cavitation during the startup process of a condensate pump. The pump is featured by an impeller equipped with five splitter blades. A computational fluid dynamics (CFD) work was conducted to plumb the evolution of cavitation in the pump. Effect of the volumetric flow rate on instantaneous cavitation patterns as the rotational speed of the pump increased was analyzed. The results show that high resistance to cavitation of the pump depends greatly on large area of the impeller eye, which is related to the deployment of the splitter blades. The splitter blades are insignificantly affected by cavitation. During the startup process, both the pump head and the pump efficiency vary drastically, which is insensitive to the flow rate. At a net positive suction head (NPSH) of 2.0 m, high flow rates are responsible for intensified cavitation. High volume fraction of cavitation arises near the inlet of long blades. As the rotational speed increases, the evolution of cavitation is featured by intermittency and diversified cavity patterns. Furthermore, the sum of the volume fraction of cavitation fluctuates with continuously increasing rotational speed.

Study on Setting Elevation of Double-Suction Centrifugal Pump Based on Cavitation Characteristics

Setting elevation is an important factor affecting the safe operation of pumps, and the empirical calculation method in engineering cannot fully reflect the cavitation characteristics. In this paper, numerical simulation of cavitation in double-suction centrifugal pump with clearances is carried out. The cavitation characteristics under different flow rates was analyzed based on the setting elevation from the empirical method. It was found that there was cavitation inception under both large flow rate and small flow rate. The cavitation characteristics of the pump with different net positive suction head available (NPSHa) is analyzed. It is found that there is no obvious change in the pump characteristics from cavitation inception to critical cavitation, but after the point of the critical cavitation is reached, a large number of cavities plug the blade passage, and the efficiency decreases rapidly. The results show that in engineer although critical net positive suction head (NPSHc) is considered and the safety margin is increased, it is difficult to ensure that there is no cavitation inception happened in the pump. The cavitation characteristics analyzed by numerical simulation in this paper can provide guidance for the selection of suitable setting elevation.

Local Entropy Generation Analysis for Cavitation Flow Within a Centrifugal Pump

Abstract Local entropy generation is derived from the second law of thermodynamics, which can directly identify the source of irreversible dissipation and learn more about the physical mechanism of performance degradation. Zwart cavitation model is modified to clarify the rapid head drop of a cavitation flow within a test centrifugal pump, and it is used to simulate the cavitation flow. Local entropy generation method is used to analyze the cavitation flow with varying net positive suction head. The magnitude of entropy generation in different types and regions is compared. Spatial distribution of turbulent and near-wall entropy generation are analyzed. Results show that the modified Zwart cavitation model can better capture the cavity morphology and its evolution. Turbulent and near-wall entropy generations, which mainly concentrate on the impeller, have a large proportion of total entropy generation. The rapid increase in fluctuating velocity entropy generation near the impeller blade pressure side and near-wall entropy generation near the trailing edge of the impeller blade are the main reasons for the rapid head drop within a test centrifugal pump. The sources of turbulent and near-wall entropy generation are vortex and shear stress, respectively.

Research of the Flow Attenuation Mechanism of a Double-Suction Pump

As an important machine for energy conversion and fluid transmission, centrifugal pump has the advantages of high efficiency, reliable performance, and easy adjustment. In order to improve the stability of the pump, this article adopts the method of experimental and numerical simulation in the pump and studies the variation of the vibration characteristics in the operation process. Based on the numerical simulation results, the intake condition and the cavitation inside the centrifugal pump are analyzed and summarized, which reveals the flow attenuation mechanism of a pumping station. The main results of the work reported in this article are as follows: 1) Various experimental schemes to monitor the vibration of centrifugal pumps in real time have been designed. The vibration characteristics of the pumps have been significantly improved by adding special trash barriers. 2) The gas entering the pump was not enough to affect the normal operation of the pump, but cavitation did occur in the actual operation process. 3) The critical net positive suction head under the three conditions is calculated to be 8.9, 10.9, and 12.9 m, respectively.

Numerical Investigation on Blockage-Related Cavitation Surge and Pressure Gain of a Mixed-Flow Pump With Influence of Blade Leading Edge Shape on Suction Performance

Abstract The blade leading edge is a design variable that can affect the local flow patterns and pressure peaks, implying a direct effect on the cavitation performance. This study was conducted to analyze the effect of the blade leading edge shape on the cavitation and noncavitation states. A total of four sets, including the square shape, were selected under the definition of ellipse ratio, and the main focus was on the cavitation state rather than the noncavitation state. In the noncavitation state, the square set denoted a remarkable negative influence, while the other three sets obtained almost the same performance despite different ellipse ratios. In the cavitation state, the square set obtained a relatively low net positive suction head required, related to the inlet flow pattern with the cloud cavity. The other three sets contained the sheet cavity, and their suction performance tended to improve as the cavity blockage decreased. As a parallel focus, an in-depth analysis of cavitation surge and pressure gain was presented with the head drop slope for the other three ellipse sets. The numerical results included the off-design flow rate points and were validated through an experimental test.

Experiment on Influence of Blade Angle on Hydraulic Characteristics of the Shaft Tubular Pumping Device

In order to understand the influence of blade angle on the hydraulic characteristics of a shaft tubular pumping device, the energy characteristics, cavitation characteristics, runaway characteristics, and pressure pulsation of the tubular pumping device under different blade angles were studied by a model test. Comparing the efficiency of the best efficiency point of the pumping device under different blade angles, it can be seen that when the blade angle is +4°, the efficiency of the best efficiency point of the pumping device is the lowest, 74.10%. When the blade angle is −4°, the efficiency of the best efficiency point of the pumping device is the highest, 79.75%. Comparing the cavitation characteristics of the pumping device under different blade angles, it can be seen that when the blade angle is −4°, the overall cavitation performance of the pumping device is the best. When the design head point is deviated, the NPSHav (Net positive suction head available) of the pump will increase. At the same blade angle, the runaway speed increases with the increase in head. The runaway speed of the prototype pump decreases with the increase in blade angle. When the blade angle is −6°, the runaway speed of the prototype pump is the largest, which is not conducive to the safe operation of the pumping station. When the blade angle is −4°, the variation law of pressure pulsation is small, and there is no abnormal pulsation. The tubular pumping device has satisfactory hydraulic performance, high efficiency, cavitation, and runaway characteristics. Therefore, the shaft tubular pumping device has wide application prospects under ultra-low head, and we recommend prioritizing it in low-head pumping stations.

Effect of helical inducer usage on both performance and Npsh For A centrifugal pump

ABSTRACT Centrifugal pumps consume the equivalent of one fifth of the total energy consumption around the world. Also, increasing the requirement for fresh water negatively affects water levels from its various sources such as rivers, canals, and groundwater. As normal, this reduces the possibility of normal operation of some pump stations, and usually, this occurs as a result of the low suction level for these stations. Moreover, some other pump stations are affected by its operation efficiency i.e. decreases performance quality, each according to the value of the suction level decreasing. Therefore, the most effect parameter on the pump performance at the suction side of the centrifugal pump is defined as the Net Positive Suction Head (NPSH). However, using inducer at the suction side of the operating pump can improve its performance. Thus, the behavior of NPSH and the total performance of a centrifugal pump are experimentally and numerically studded. In addition to, the NPSH performance is determined using the NPSH test and improved by 3% at 2900 rpm with using the inducer at the operating point, and the performance of the pump is monitored at the low and high discharges value.

Experimental Study on the Effect of Gas Volume Fraction on the Cavitation Performance of a Low-Specific-Speed Centrifugal Pump

In order to study the cavitation performance of centrifugal pumps with low specific speeds under the condition of gas–liquid two-phase flow, a cavitation test rig for pumping gas–liquid two-phase flow was set up. The cavitation performance of the pump with a specific speed of 32 was studied. The variation of the head, pressure pulsation intensity, and vibration intensity with the cavitation allowance NPSHa (Net Positive Suction Head available) of the centrifugal pump were obtained at different inlet gas volume fraction (IGVF) conditions of 0, 1%, 2%, and 3%. The results show that the cavitation performance of a low-specific-speed centrifugal pump can be improved obviously in a certain liquid flow range when the IGVF is 1%, especially at a low liquid flow rate. When cavitation did not occur or the degree of cavitation was low, a lower IGVF can reduce pressure pulsation intensity at the pump outlet and the vibration intensity at the pump inlet under design flow rate and high flow rate conditions. Additionally, all performances of the low-specific-speed pump are more sensitive to gas when the liquid flow rate is low. The results can provide a reference for improving the cavitation performance of low-specific-speed pumps for transporting gas–liquid two-phase flow and single-phase liquids.

A new generation of centrifugal pumps for high conversion efficiency

Centrifugal pumps are fundamental machines in our daily life. Indeed, they can be found everywhere, for instance, in domestic, civil, industrial, and agricultural fields. In the last years, the pump market has newly gained a boost coherently with the world demographic growth. Pumps are among the main responsible for electric energy consumption, therefore even a slight efficiency improvement (+1%) can be effective in contrasting the growth of the global Greenhouse Gas Emissions (at least −570 tCO2/day in Europe). For this reason, a new generation of centrifugal pumps is here proposed. In particular, the impeller has been redesigned improving the flow guidance inside the vanes and simultaneously reducing the number of blades, with the aim to increase the blade loading, reducing manufacturing costs, and enhancing the conversion efficiency with respect to conventional configurations. Furthermore, the new impeller design respects all the geometrical constraints of the corresponding conventional configuration, allowing its retrofit with a boost of performance. To reach these goals, a 1D in-house code has been developed implementing a Harmony Search Algorithm (HSA) for the selection of the most suitable design point (H and Q), and introducing two parameters for the blade loading evaluation, namely, the blade pressure coefficient (Cp,blade), computed locally along the impeller blade walls, and the blade lift coefficient (CL), achieving an optimal vane arrangement. Starting from a baseline centrifugal pump, the novel impeller has been designed, manufactured, and finally experimentally tested. The resulting new generation of centrifugal pumps actually proves to have a higher performance (maximum efficiency at BEP +1%), a flatter efficiency curve, and a monotone bending characteristic curve, while keeping unchanged the required Net Positive Suction Head (NPSH3%).

Optimization of the Structural Parameters of a Plastic Centrifugal Pump

The structural design parameters of a plastic centrifugal pump were calculated and modeled, and flow field simulation analysis of the model was performed using CFD, in the framework of an orthogonal design method (or experiment). The inlet mounting angle β1, outlet mounting angle β2, wrap angle ϕ, and impeller inlet diameter D1 of the pump impeller were the four factors assumed for the application of the orthogonal experiment, using the efficiency and Net Positive Suction Head (NPSH) as evaluation indices. Moreover, taking the maximum efficiency and minimum NPSH of the plastic centrifugal pump as the evaluation factors, the parameters of the pump impeller were re-optimized through the Taguchi algorithm (leading to the following optimal combination: inlet diameter 35 mm, inlet angle 26°, outlet angle 27°, and wrap angle 110°). The minimum NPSH and the maximum efficiency have been found to be 0.957% and 61.5%, respectively.

Fluid dynamics of dredging

Centrifugal pumps are used for pumping slurry from the dredge cutter head or drag head located in front of the suction pipe inlet to the final placement site or dredge hopper. Cavitation is an important phenomenon that must be avoided on the suction side of the pump that is quantified by the net positive suction head. The settling velocity is important for determining whether the sediment settles to the bottom of the pipeline. There is a critical velocity for the slurry in the pipeline that must be maintained such that the dredged sediment remains in suspension and minimizes contact with the pipe wall to keep friction losses at a minimum. Friction losses and minor losses are calculated and input to the energy equation to determine the system head curve over a range of flow rates. The system head curve and pump head characteristic curve are used to evaluate the dredge pump head and flow requirements needed to satisfactorily transport the dredged sediment to the placement location. A submerged ladder pump is installed on the ladder or drag arm to avoid cavitation, and booster pumps are added along the discharge pipe to ensure the transport of slurry over long distances greater than a mile or 1.6 kilometers.

Methodology to determine net positive suction head for turbine driven feed pumps of advanced power plant

Methodology to determine net positive suction head for turbine driven feed pumps of advanced power plant

Assessment of Cavitation Erosion in a Water-Jet Pump Based on the Erosive Power Method.

Cavitation affects the performance of water-jet pumps. Cavitation erosion will appear on the surface of the blade under long-duration cavitation conditions. The cavitation evolution under specific working conditions was simulated and analyzed. The erosive power method based on the theory of macroscopic cavitation was used to predict cavitation erosion. The result shows that the head of the water-jet pump calculated using the DCM-SST turbulence model is 12.48 m. The simulation error of the rated head is 3.8%. The cavitation structure of tip leakage vortex was better captured. With the decrease of the net positive suction head, the position where the severe cavitation appears in the impeller domain gradually moves from the tip to the root. The erosion region obtained by the cavitation simulation based on the erosive power method is similar to the practical erosion profile in engineering. As the net positive suction head decreases, the erodible area becomes larger, and the erosion intensity increases.

A deeper understanding of the qualitative consequences of food pumping: A case study of wine

The consequences of pumping on food quality are poorly understood, and wine is a good example. Therefore, the present paper examines the use of four pumps and the application of inertization on wine quality. We develop a novel, physical qualitative index (PQI), which is the ratio between pumping specific energy and the net positive suction head, and examine the correlation with wine quality characteristics. Four pumps (a flexible impeller pump, a lobe pump, a peristaltic pump, and a rotary piston pump) that are widely-used in wine cellars were compared over a wide array of chemical-physical variables, during conventional racking, with and without inertization. The analysis found a correlation between the PQI and some quality variables, and this finding was confirmed by a conventional statistical analysis (ANOVA-GLM). Overall, the flexible impeller pump performed slightly differently compared to the other pumps with respect to some color-related parameters. All pumps behave differently as a function of inertization (or not). Inertization was effective in minimizing volatile losses.