UNIVERSIT `
A DEGLI STUDI DI TRIESTE
XXXI ciclo di Dottorato di Ricerca in Ingegneria e
Architettura
Ph.D. THESIS
funded by The Research Hub
TMby Electrolux Professional
Characterization of centrifugal
pumps used for professional
equipment, development of control
strategies to prevent cavitation
Settore scientifico-disciplinare: ING-INF/04
Ph.D. Candidate
Ing. Valentino Cucit
Supervisor
Prof. Paolo Gallina
Co-Supervisor
Prof. Felice Andrea Pellegrino
Company Scientific Referee
Ph.D. Ing. Riccardo Furlanetto
Company Technical Coordinator
Ph.D. Ing. Michele Simonato
Ph.D. Coordinator
Prof. Diego Micheli
Acknowledgements
First, I would like to thank my Co-Supervisor, Prof. Eng. Felice Andrea Pellegrino, for his guidance and support during my research that has re-sulted with this doctoral thesis. I am deeply grateful to my Supervisor, Prof. Eng. Paolo Gallina. I would like to thank Prof. Eng. Gianfranco Fenu for numerous theoretical advice and for numerous experimental sug-gestions. The research presented in this thesis was funded by Research HubTM by the Electrolux Professional; this help is gratefully appreciated.
Abstract
The field of household appliances, especially professional equipment used in the catering sector, must always present new developments and con-trols to improve their operating cycles and life cycles. The work refers to an industrial research aimed to investigate the effect of cavitation on cen-trifugal pumps. The need to undertake this study arises from the demand to solve a problem related to a machine used for catering. It is therefore understandable the interest shown by Electrolux Professional Spa for the development of new solutions and control strategies. Therefore the pur-pose of this research is the study of cavitation on centrifugal pumps. Cavitation is a well-known phenomenon that may occur, among other turbo-machines, in centrifugal pumps and can result in severe damage of both the pump and the whole hydraulic system.
Contents
I
1
1 Washing devices 3 1.1 Washing machines . . . 4 1.1.1 Cycle type . . . 5 1.2 Dishwashing machines . . . 5 1.2.1 Cycle type . . . 6 1.3 Ovens . . . 7 1.3.1 Cycle phase . . . 91.4 The Sinner circle . . . 9
1.5 Optimization of the washing cycle . . . 12
II
13
2 Test benches 15 2.1 Test rig . . . 172.1.1 Layout . . . 17
2.1.2 Acquisition . . . 19
2.1.3 Sensor and location . . . 20
2.1.4 Instrumentation . . . 22
2.2 Other Test rig . . . 23
2.2.1 Acquisition . . . 23
2.2.2 Sensor and location . . . 24
2.3 Function test rig . . . 25
3 Effect of washing 29 3.1 Effects of the Sinner circle . . . 30
3.1.1 Polymers effects . . . 30
3.2 Temperature and detergent effects . . . 33
3.2.1 Temperature effects . . . 34
3.2.2 Detergent effects . . . 36
3.3 Life test . . . 41
3.3.1 Test carried out . . . 42
3.3.2 Results . . . 43 4 Cavitation phenomenon 47 4.1 Effects of cavitation . . . 48 4.2 Study of cavitation . . . 49 4.3 Incipient Cavitation. . . 52 4.3.1 Numerical simulations . . . 52 4.3.2 Experimental tests . . . 53
4.3.2.1 Noises and vibrations . . . 53
List of Figures
1.1 Domestic washing machines . . . 4
1.2 Dishwashing machines . . . 6
1.3 Washing system . . . 8
1.4 Sinner circle . . . 10
1.5 Sinner circle working principles . . . 11
2.1 Centrifugal Pump . . . 16
2.2 Test rig . . . 17
2.3 Accelerometers position . . . 22
2.4 Schema Acquisition . . . 22
2.5 Other Test rig . . . 24
2.6 NPSH curve . . . 27
3.1 Test rig . . . 32
3.2 Friction factor for dilute aqueous solutions . . . 32
3.3 Comparison at different solution . . . 33
3.4 Total head at different temperature . . . 35
3.5 Power supply at different temperature . . . 35
3.6 Efficiency at different temperature. . . 36
3.7 Total head (Pump A) . . . 37
3.8 Power supply (Pump A) . . . 37
3.9 Performance (Pump A). . . 38
3.10 Total head (Pump B) . . . 38
3.11 Power supply (Pump B) . . . 39
3.12 Performance (Pump B). . . 39
3.13 Comparison between pump A and pump B . . . 40
3.14 Detail of the leakage by gravity . . . 44
3.15 Details of the leakage from the seal-gasket coupling system . . . 44
3.17 Details of the pump. . . 45 4.1 Cavitation erosion. . . 48 4.2 Impeller cavitation . . . 49 4.3 NPSH trend . . . 51 4.4 Signal analysis. . . 57 4.5 Acceleration signals . . . 58
4.6 Time interval of a signal . . . 59
4.7 Power spectral density of the signal Z axis . . . 62
4.8 Power spectral density of the signal Y axis . . . 63
4.9 Cavitation detection . . . 65 4.10 PDF axis X . . . 68 4.11 PDF axis Y . . . 68 4.12 PDF axis Z . . . 69 4.13 Kurtosis axis X . . . 69 4.14 Kurtosis axis Y . . . 70 4.15 Kurtosis axis Z . . . 70 4.16 RMS axis X . . . 71 4.17 RMS axis Y . . . 71 4.18 RMS axis Z . . . 72 4.19 CF axis X . . . 72 4.20 CF axis Y . . . 73 4.21 CF axis Z . . . 73 4.22 PEAK axis X . . . 74 4.23 PEAK axis Y . . . 74 4.24 PEAK axis Z . . . 75 4.25 Energy axis X . . . 75 4.26 Energy axis Y . . . 76 4.27 Energy axis Z . . . 76
4.28 3D graph representing the N P SH trend and the total head at different frequencies. . . 78
4.29 3D graph representing the N P SH trend and the total head at different rpm. . . 79
5.1 Functional control scheme . . . 85
6.2 Test 1 ( PI control). A test carried out using PI control. The plant is brought to a decrease of total head of 3%. . . 90
6.3 Test 1 ( PI control). A test carried out using PI control. In this curve the plant is brought to a decrease of total head of 5%. . . 91
6.4 Test 1 ( PI control). A test carried out using PI control. In this curve the plant is brought to a decrease of total head of 3%. . . 91
7.1 Non linear Control scheme . . . 94
7.2 Test 1 (Non linear-control). Energy of axis Z and RP M for a test carried out using non-linear control. . . 95
7.3 Test 2 (Non-linear control). A test carried out with non-linear control where the detector’s response, may be observed. . . 96
7.4 Test 1 (Non-linear control). A test carried out using non-linear control. In this 3D curve the plant is brought to a decrease of total head of 0.5%. 97
7.5 Test 1 (Non-linear control). A test carried out using non-linear control. In this 3D curve the plant is brought to a decrease of total head of 1%. 97
7.6 Test 1 (Non-linear control). A test carried out using non-linear control. In this 3D curve the plant is brought to a decrease of total head of 2%. 98
Chapter 1
Washing devices
The world of professional equipment is vast, many devices were developed to improve and to simplify people’s lives. In everyday life, people continuously use both domestic appliances and professional equipment. The first devices are used in homes: washing machines, dryers, dishwashers, ovens, microwave ovens and many others. The second class of devices are used in professional and in semi-professional fields, such as restau-rants, bed and breakfasts, hotels, hospitals, schools, universities and other places. In larger towns or in metropolitan cities, where the rhythm of life is very fast and chaotic, people continuously use services, such as laundromats, restaurants or hotels. Therefore, to guaranteed a correct functionality and a fast use of these machines it is crucial to improve and optimize machines and their cycles.
This work of research is focused on professional equipment, aims to increase perfor-mance and increase the operational efficiency of professional machines. The main topic of the doctorate is the study of cavitation conditions in centrifugal pumps. In particular, in this thesis a control methodology able to free pumps from conditions of cavitation has been developed.
1.1
Washing machines
Washing machines are fundamental both in the domestic and in the professional fields. Washing machines are a consumer good to be considered within the group of commodities, i.e. something of not absolute necessity, but that is strongly useful and considered of common use. This device proves useful to wash clothes, to remove filth, dyeing residues or supporting yarns used during industrial processing.
To perform a washing cycle, water combined with detergents is used as a main means. Other devices that do not use detergent and water, but perform dry cleaning using organic solvents, exist and are usually utilized in particular lines of business. These devices can be used in domestics and in professional fields. In the first field the machines are used only to wash clothes, while in the second the purpose is not just to wash clothes but also to work on un-worked textile fibers, yarns and fabric patches. Currently there are two types of washing machines:
• Top-loading, where the loading door is placed on the top of the machine. They are usually smaller in size than front-loading washers and machine loading and unloading operations are simplified. Shown in Fig. 1.1a
• Front-loading washing machines, instead, have a round door on the front of the machine. The advantage of these models is that they allow an overlap with a tumble dryer. Shown in Fig. 1.1b
(a) Top loading washing machines. (b) Front loading washing machines
1.1.1
Cycle type
In recent years washing machines able to manage washing efficiency, guaranteeing the best results and respecting the environment have been developed. Several washing cycles have also been developed, from the most economical and standard cycles to the most modern and sophisticated ones. The latest modern cycles allow the adjustment of program duration based on the amount of the load, thus reducing the consump-tion of water, energy and time, and introducing an innovative steam loop capable of stretching fabrics and avoiding the formation of folds; they also allow a more gentle washing by combining the detergent and softener mixture with water before it comes into contact with the fabrics.
1.2
Dishwashing machines
Dishwashers are devices used to wash tableware, pots, pottery and other household utensils. In recent decades, dishwashers have became a common household appliance, but also are used in many locations where food is cooked or where food and beverages are served and consumed, such as restaurants and canteens. These devices can also be used in domestic and in professional fields. (e.g. in Fig. 1.2)
Figure 1.2: Professional dishwashing machines [1]
1.2.1
Cycle type
The dishwashing cycle phases of a dishwasher are different in domestic and in indus-trial machines: in a domestic dishwashing machine, there are the following phases:
• in the first phase, after a possible pre-wash (optional), hot water is sprayed; this water is heated by a resistance at a temperature between 45 ◦ and 75 ◦C, depending on the selected wash, and mixed with detergents with an emulsifying function.
• In the second phase, the dishes are rinsed with cold water from detergent residue by adding a dose of “rinse aid” at the temperature of roughly 65 ◦C, which decreases the surface tension of the water and facilitates the drying of the load and the removal of the remaining residue of filth and of detergent that may be present.
• In the last phase (present in top range machines), the dishes are dried by sucking out the steam created in the washing tank. This operation can be carried out naturally, by means of a convective current created with the aid of a pipe lateral to the tank, or forcefully by means of a dedicated fan.
• During the washing phase, usually preceded by a pre-wash phase, the same solution of water and detergent is continuously recirculated, at a specific con-centration and at a temperature of roughly 60◦C. The solution is sprayed by a very powerful pump through fixed or rotating sprayers on the load to be washed.
• The rinsing phase is carried out through a real “shower” of clean water mixed with rinse aid at a temperature of 90 ◦C. All these phases, which make up a complete wash, are performed within 2-3 min.
1.3
Ovens
Also in the professional oven developed by Electrolux professional s.p.a. and used in the catering sector, there is a washing circuit. These systems can have an open or a closed hydraulic circuit.
With open circuits it is possible to convey network water or washing solutions, taken by a pump from a dedicated external tank. The hydraulic circuit consists of a hollow arm keyed into a cavity, which, by presenting appropriately arranged holes, is placed in rotation during the passage of liquid. The result is that the working fluid, passing through this arm, is uniformly dispersed on all cavity surfaces. On the bot-tom of the cavity there is a hole through which liquids used during washing can drain off to the drain. This circuit, with the following characteristics, is called “open”. The washing starts by loading the detergent (or the rinse aid) into the hydraulic circuit and spraying it through the arm in the cavity by introducing pressurized network water, always in the hydraulic circuit; this water pushes the detergent (or the rinse aid) out. Once the detergent has been sprayed, the cavity’s temperature is measured for the necessary time to guarantee a good chemical action. At this point, by once again spraying abundant water through the hydraulic circuit and through the arm, the cavity is rinsed.
the other hand, it centrally sucks the work fluid that falls from the top of the cavity and sprays it radially. Air flows in the cavity are then sufficient to make sure that the detergent touches all the internal surfaces to the cavity. Electric heaters surrounding the fan, which are used to maintain the cavity’s temperature, can be used to heat the working fluid; it is sufficient to activate them during the re-circulation of the detergent to make it so the latter, by touching the cavity’s surface, is heated. [2]
(a) Fan is not active (b) Fan is Active
1.3.1
Cycle phase
In the most modern professional ovens, various washing cycles have been developed, specifically according to the individual needs of the customers.
Typically in professional oven the used detergent is of a basic type, with a pH gener-ally included between 13 and 13.9.
In order to make the detergent act efficiently and specifically, during the washing operation various phases have been developed. The complete cleaning of the inside surfaces of the oven can be carried out through four washing cycles: “extra strong”, “strong”, “medium” and “soft”. These cycles are performed with different temper-atures and different times. The importance of re-circulation is due to the slow and continuous action of the detergent on dirty surfaces. Therefore, the washing cycle is divided into four main phases: pre-washing, washing, rinsing and drying.
1.4
The Sinner circle
The professional machines used in food processing are constantly exposed to harsh environments, resulting from a mix of chemical, thermal, and mechanical stress. The appliances used can have different problems related to the materials and to the envi-ronment they are exposed to. Therefore the washing cycle plays a fundamental role, necessary to preserve the safety and the performance of the devices.
show that the reduction of one factor can be compensated for by any of the three other factors. This simple circle-shaped scheme the four main factors that determine the success of washing are shown in Fig. 1.4.
Figure 1.4: General representation of the Sinner circle [4]
It is necessary to keep all factors in mind to always carry out a quality cleaning without damaging any surface of any given appliance or any object contained in any given washing appliance [4]:
• Mechanical action is the physical fact of removing filth. It is represented by the force and friction generated during the washing process. This action can be manual (e.g. the hand movement of using a cloth to remove dust) or mechanical (e.g. when using a machine to execute the activity). The combined work involving mechanical action and chemical action is fundamental to reach a good dispersion of detergent in the solution, enabling it to reach all the areas in need of cleaning.
• Temperature is directly linked to the thermal energy transferred through water. The temperature at which the washing happens plays a crucial role in favor-ing the action of the chemicals present within detergents; heat promotes an increasing reaction rates due to the higher kinetic energy of surfactant ions and the higher sorption velocity of the same. As a result, the effective removal of filth is much easier. As the temperature increases the efficiency of the chemical action also increases. However, there is a maximum temperature at which the chemical properties decay, leading to a bad washing performance. It is there-fore advisable to choose the temperature of maximum effect in relation to the specific detergent in order to obtain the desired cleaning performance.
• Time is the only factor strictly correlated to temperature and in reverse pro-portion with mechanical action and detergent concentration. The contact time between substrate filth and washing agents must be balanced to allow the chem-ical reactions and interactions to take place. The general rule is the following: if time increases, cleaning efficiency increases. Also in this case there is a technical limit regarding the chemical effect’s decay in relation to time.
The Sinner Circle is a useful tool in predicting the impact on the overall cleaning process of the main four dependent factors. A change in one of the Sinner Circle factors must be compensated by changes in the other parameters [5]. These four factors will combine differently depending on the filth present, on the surface to be cleaned and on the cleaning means at one’s disposal. When using mechanical power, for instance, the mechanical factor will be larger and the other three smaller. This is shown in Fig.1.5.
1.5
Optimization of the washing cycle
Professional equipment’s washing cycles have a fundamental rule both for washing devices and for dishwashing devices. Washing devices (e.g. washing machines) are used to wash clothes with different materials and characteristics. These devices must guarantee excellent washing capabilities and a good repeatability.
Dishwashing devices (e.g. dishwashers) are used to wash dishes such as plates, glasses and cups. In fact Electrolux’s dishwashing equipment provides washing and rinsing options. This diverse product range offers glass-washers, under-counters and other devices able to meet specific needs. Washing cycles have in recent years grown of importance in relation to cooking devices. These devices have developed the constant need to have clean surfaces and to be sterilized. Examples of machinery with washing cycles are professional ovens and professional braising pans. The hydraulic layout has therefore undergone various changes. It went from an open circuit to a re-circulation circuit. In all these devices it is possible to optimize the washing cycle. For this reason it is necessary to improve the washing cycle by combining the four effects of the Sinner Circle.
Chapter 2
Test benches
In washing and in dishwashing machines, used in professional equipment for catering, different centrifugal pumps with different sizes and different performances (electrical and hydraulic) are present. All pumps used are electrical pumps with electrical motors (three phase or mono phase power supply). The centrifugal pumps used in the field of catering can be divided into three different classes, based on electrical power supply and size. This distinction also depends on the size of the equipment and the working conditions of the machine.
Generally speaking, a centrifugal pump is a machine that uses the centrifugal effect of its impeller to move liquid, transforming the mechanical energy of the engine first into kinetic energy and then into potential energy. The rotation of the impeller creates a centrifugal force that acts by pushing the fluid, included between the impeller blades, outwards. This movement causes a low-pressure area in the suction section. The fluid after being sucked passes through the impeller blades. The characteristic curved shape of the blades pushes the fluid both radially outwards and tangentially. Through the volute, which has a characteristic shape with an increasing area, the kinetic energy (expressed by the velocity of the fluid) is transformed into potential energy. Finally, the fluid further decelerates into the exhaust nozzle with a relative increase in pressure. The centrifugal pump is by far the most commonly used pump, both in the civil and in the industrial field.
In this work cavitation acting on a centrifugal pump is studied. The Fig. 2.1 shows the cross section of a typical centrifugal pump, in which the following hydraulic parts can be observed: the pump impeller, the vanes, the suction and delivery sections and the volute.
Figure 2.1: Centrifugal pump (two parts of the pump)
In this research several centrifugal pumps were used in many professional equip-ment. These pumps have different sizes and different hydraulic characteristics. For a correct characterization of all types of centrifugal pumps, two test benches were built. The first test rig allows to test the medium - large size centrifugal pumps, while the second test rig allows to test the smaller centrifugal pumps. The following paragraphs show the characteristics of the two test benches and of the employed measurement sensors.
In the larger test rig, cavitation tests were carried out on a centrifugal pump powered by a three-phase motor. The cavitation phenomenon was characterized by the vibra-tion analysis of the pump. On the other hand, with the smaller test rig, tests were carried out demonstrating the effects of temperature and detergent on centrifugal pumps.
2.1
Test rig
Research begins with the study and building of a test rig (see Fig.2.2a).
The project is composed of a mechanical system and an electrical one. The mechanical part, in particular the length and the size of the ducts, was designed according to the International Standard 9906:2012 [7], to reduce the hydraulic losses and to make a correct acquisition. The electrical part is Instead composed of the drivers, with relay, switches and controls.
(a) Figure Test Rig (b) Scheme Test Rig
Figure 2.2: Test rig
2.1.1
Layout
The test rig is a close circuit where it is possible to test the centrifugal pump. It is op-portunely designed to measure and collect the most significant parameters indicating cavitation and allows the measuring of the characteristic curves and of the N P SHr (Net Positive Suction Head request) of different pumps. In the experimental tests it is possible to change the barometric load of the free surface level of the tank and the temperature of the fluid.
All tests focused on a centrifugal pump, used in professional dishwashers for catering; the pump has the electrical and hydraulic characteristics shown in following Table
2.1.
Table 2.1: Centrifugal pump characteristics Electrical quantities Power supply 400V Power rate 2.2kW Current 5.4A Speed 2800rpm Frequency 50Hz Hydraulic quantities Flow rate 0 m3/s to 0.0158 m3/s Total head 0 m to 16 m
the lower and upper parts of the tank. With the upper valve it is possible to fill the tank and connect the vacuum pump in order to obtained a pressure decrease, while the lower valve allows the emptying of the tank. Finally a solenoid valve was installed in the upper part of the tank, to bring back the pressure at the initial value.
Connected to the tank, both upstream and downstream, two manually regulated bulb valves are installed. Through the upper valve (valve number 2 in Fig. 2.2b) the flow rate regulation is obtained. Moreover, inside the tank a spray nozzle device and a vacuum inlet are installed to control the pressure in the free surface level inside the tank. For a correct measurement it is necessary to install the instruments in the right position. Two relative pressure transducers are positioned at a distance of two diameters from suction and outlet sections of the pump.
The flow meter measurements’ accuracy is reached positioning the flow meter ten di-ameters from the pipe curve and at five didi-ameters from the deaeration nozzle branch. To reach a uniform distribution of velocity and pressure a transparent Plexiglas pipe with length of twelve diameters is installed in the suction side of the pump.
Another transparent Plexiglas pipe is positioned at the delivery section of the pump. These transparent pipes are used to visualize the fluid in cavitation condition. In the system the electrical circuits used to supply electrical power to the instruments and the electrical load and to control the drives are present.
Four main loads are present in the system: the pump being tested, the vacuum pump, the solenoid valve and two electric heaters (one of 17 kW and the other one of 9 kW ). All the loads are connected with the drivers, opportunely wired in a dedicated elec-trical cabinet. Upstream of the elecelec-trical circuit there are protection devices against overloads and short-circuits, i.e. magneto-thermal protection devices and differential protection devices.
switch, that interrupts the load if pressed and a Klixon, a normally closed contact (bimetallic) that opens the contact if the temperature reaches T-max.
The whole system of electric drives composed by the vacuum pump, the test pump, the heating elements and the solenoid valves are controlled through a PLC (pro-grammable logic controller). The PLC used is a smart compact module from the Crouzet automation model number 88974051, easy to program and to implement. This device is composed of 8 relays (8 A), 6 digital inputs and 6 analog inputs. The device is programmed by M3 Soft, using the FBD (Function Bloc Diagram) and SFC (Sequential Function Chart) languages. Thanks to the buttons present in the PLC it is possible to control the whole test rig.
An inverter used to regulate the pump’s rotational speed has also been installed in the system. The use of this device increases the cost of the system by an amount that corresponds roughly to the cost of the pump alone. The inverter model is the Com-bivert F5 Basic by KEB, with the following characteristics: maximum power of 4 kW , maximum current of 15 A and a frequency range between 0 and 400 Hz. Through the inverter feedback it is possible to acquire the number of pump revolutions per time unit. The analogue output of the inverter in voltage, 0− 10 V , is converted first into frequency and then into the rpm (revolutions per minute) of the centrifugal pump.
2.1.2
Acquisition
Since the test rig is electrically and mechanically complex, it was important that the data acquisition system used be portable in order to easily gather data. The tests in-cluded acquiring data simultaneously from multiple type sensors installed at different locations on the test facility. All the hardware components are described in detail in this part.
The electrical signals have been acquired through a power meter model Yokogawa WT330E. This instrument measures three-phase electrical quantities, three electric voltages, three electric currents and the total electrical power supply.
All the other signals have been acquired through NI (National Instruments) compo-nents. The hardware device is the chassis c-DAQ 9174 (compact Data Acquisition) with a USB connection. This device can be used with a combination of different analog or digital modules I/O (Input / Output). The modules used are the following:
input channels simultaneously acquire signals at rates up to 51.2 kS/s. Each channel has built-in anti-aliasing filters that automatically adjust the sampling rate.
• NI 9205 measures input voltage signals. Models also offer isolation and over current protection for high-voltage applications. Each channel has a programmable input range of ±0.2 V , ±1 V , ± 5V , and ± 10V .
This module also includes a channel-to-earth-ground isolation barrier for safety, noise immunity and high common-module voltage range.
• NI 9485 provides an output signal from electromechanical and SSR (solid state relays). This module has eight SSR digital sourcing output channels. Each channel provides access to an SSR for voltage switching up to 60 V dc. The NI 9485 features 60 V dc continuous channel-to-channel isolation. It is designed to directly connect to a wide array of industrial devices.
• NI 9263 generates voltage signals. The module has four analog output channels with a voltage range of 10 V dc. The module has overvoltage protection, short-circuit protection and high relative accuracy.
The software used to acquire the signals is LabVIEW R(abbreviation of Laboratory
Virtual Instrument Engineering Workbench), a visual programming language by NI. It is a systems engineering software needed to measure data and to perform debugging. LabVIEW Roffers simple graphical programming to integrate hardware measurement
and complex logical diagrams, develop data analysis and design user interfaces.
2.1.3
Sensor and location
Different sensors were used to easily test the effects of the centrifugal pumps. To detect the cavitation phenomena it is required to measure hydraulic, mechanical and electrical quantities. Hydraulic quantities include: the upstream and downstream pressure of the pump, the tank pressure, the flow rate and the fluid temperature. The mechanical quantities measured include: pump vibrations along orthogonal directions and the pump’s rotational speed. Finally, to measure the electrical quantities in all the different conditions a power meter was used. In the following all the instruments used in the test rig to perform the tests are described.
range is between−1 and 1 bar for the pressure transducers on the suction side, and between−1 and 3 bar on the delivery side. These instruments are provided by Trafag and need a power supply of 24 V dc and provide a signal output of 4− 20 mA. The precision output on full scale is 0.15%.
• The tank pressure is measured through a pressure transducer. The transducer has the following characteristics: measurement range from 0 to−1bar, voltage output between 1 and 5 V dc, error at full scale of 2%.
• The flow meter is positioned after the outlet section of the pump at a distance of 10 diameters from the pipe curve. The outlet section of the flow meter is positioned 5 diameters from the branch connecting the spray nozzle used for deaeration purposes. Such distances are needed in order to perform an accurate measurement of the fluid flow. The flow meter is provided by Endress Hauser, it requires a power supply of 230 V ac, delivers an output signal between 4− 20 mA and has a fluid flow measurement range between 0 m3/s and 0.0158 m3/s.
Its error is 0.5% on the read value. In order to reach a uniform velocity and pressure distribution at the suction of the pump, 12 diameters are guaranteed in length from the pump suction.
• To detect the temperature of the fluid, necessary to obtain the vapor pressure, a thermal-resistance was installed in the tank. The Pt100 used has an accuracy of 0.01% and a measuring range between −50 and 600◦C.
• All the accelerometer sensors were installed at positions close to and around the impeller so that they could measure the pulses produced in the water flow due to cavitation, with a high degree of response. The vibrations were acquired by three piezoelectric ICB accelerometers, installed on the pump volute (see Fig. 2.3) in the following position: X axis in the direction of the rotation axis of the pump, Y and Z axes in the radial directions of the volute. These three accelerometers having the following characteristics: sensitivity of 10 mV /g and a range of−500 to 500 g.
Figure 2.3: Accelerometers position
2.1.4
Instrumentation
The instrument used to measure the related signals during the operation of cavitation in the test-rig is shown in the scheme in Fig. 2.4. In the following block diagram the connection, the power supply and the characteristics of physical channel (Current or Voltage) of each sensor with the single NI module are shown. The signal outputs from the sensors have been divided and connected in the different NI modules acquired through c-DAQ 9274.
Figure 2.4: Schema Acquisition
frequency ( 10 Hz ), while the accelerometers at high sample frequency. In the initial phase of the experimental campaign the sample frequency was set at 12800 Hz, while in the second phase it was shifted to 25600 Hz.
2.2
Other Test rig
Another test rig was carried out (see Fig.2.5a). This device allows to test other cen-trifugal pumps, with different sections, both to characterize the cavitation phenomena and to find their characteristic curve. These curves are essential to characterize the correct point of work of the centrifugal pump. In washing and dishwashing machines it is necessary to know the correct flow rate to optimize the washing performance and the power consumption. All machines in fact present different hydraulic layouts that cause different hydraulic losses, therefore each washing and dishwashing machine has a specific work point with a certain flow rate and a certain total head. Another im-portant test that was carried out with this test rig is the life test cycle. This cycle is necessary to find the end life of pumps. This test is carried out at the same conditions of centrifugal pump work, in different appliances, selecting temperature and chemical concentration. The stress effect caused by the temperature and chemical concentra-tions can be dangerous for centrifugal pumps, and they could cause the breaking of the hydraulic circuit pump. Therefore this test rig has been development to test small and medium sized centrifugal pumps, in relation to the field of pumps typical used in professional appliances. The specific sizing of the pipes and the geometric layout are designed to minimize hydraulic losses. This test rig, similar to the larger one described above, has the same hydraulic and electric layout, but has different types of instruments installed, as described in the following section.
2.2.1
Acquisition
The acquisition system used in this new device is the same as the larger test rig present in the last paragraph. The differences are mainly related to the change used in the input/output instruments. In fact different accelerometers, flow meter and pressure transducers have been used. Multiple acquisition programs have been developed using Labview R. With the first of these it is possible to find the characteristic curve and
the N P SH curve. Other programs, developed in Labview R, for this test rig concern
at the sampling frequency of 25.6 kHz. While the last program acquires the signals every 5 minutes for a duration of 1 min. These programs allow to detect, with an experimental approach, the failure point of the centrifugal pump. The programs developed, furthermore, are equipped with an automatic load activation, directly on the user interface on the front panel. The last program developed acquires signals of the characteristics of acquisition of the centrifugal pump every 5 minutes for a duration of 1 minute, at the sampling frequency of 25.6 kHz
2.2.2
Sensor and location
The instruments necessary to measure all the parameters characterizing a mono-phase, smaller centrifugal pump are connected to the test rig. Three accelerometers (ICB piezoelectric) with a range of ±50 g with an accuracy of 10 mV/g are used in this system, positioned on the pump volute and another accelerometer with a ±500 g range, with an accuracy of 100 mV /g is positioned on top of the motor. The following Fig. 2.5b shows the pump and the sensors.
(a) Test rig layout (b) Accelerometers position
Figure 2.5: Other Test rig
test rig is the Promag 50E25 model supplied by Endress Hauser; it requires a 230 V ac power supply, has an output between 4− 20 mA and a range of flow included between 0 and 0.0042 m3/s. The fluid temperature was acquired through a platinum resistance. The Pt100, installed in the tank has an accuracy of 0.01% and a measuring range between −50 and 600 ◦C. Finally, a power meter was used to acquire all the electrical parameters of the single-phase pump. The instrument used is the Yokogawa model WT333E, with an accuracy of 0.1% on the measured value.
2.3
Function test rig
This simple machine allows to test the performance of the centrifugal pump. In order to characterize the pump, many tests were carried out under both nominal condi-tions, varying the pump flow rate (from 0 m3/s to 0.0158 m3/s), and the cavitation condition, varying the tank pressure load. It is necessary to acquire knowledge of the characteristics of the centrifugal pump to describe the characteristic curve, which shows the relationship between the flow rate Q and the total head HT , the efficiency
curve and the N P SH (Net Positive Suction Head) curve in function of the total head HT, used to identify cavition. The test rig allows to test the various pump
configura-tions in different operating condiconfigura-tions, obtained by varying the fluid temperature and it properties, e.g. water-polymers solution and detergent at different concentrations. In the test rig it is possible to change the flow rate of the pumps, through a manual ball valve, the barometric load of the free surface level, through a vacuum pump, and the temperature of the fluid, through two electrical heaters.
The test rig allows the measurement of the characteristic curves and the N P SHr of
different pumps, with various fluids. The tests to find the characteristics curve have been performed at nine flow rate values, from 0 m3/s to 0.0158 m3/s with a step of
0.00166 m3/s. This curve represents the variation of the total head H
T as a function
of the flow Q. The total head of the pump is traced experimentally by points and at a constant number of pump revolutions; It is calculated with the following Eq.2.1:
HT = 4 p
ρ g +
(u2d− u2s)
2g +4z (2.1)
Where 4p represents the pressure difference between the pump delivery and suction sections, ud and us are respectively the flow velocity of the delivery and the flow
height difference between the two pressure transducers. To find the total efficiency curve of the pump it is necessary to compare the useful power Wu, Eq. 2.2, and the
absorbed power Wa, Eq.2.3. At the point of maximum efficiency (Eq.2.4) the pump’s
operation is optimal, this point’s name is BEP (best efficiency point).
Wu = ρgQHT (2.2) Wa = √ 3V Icosφ (2.3) η = Wu Wa (2.4)
Where V and I are the voltage and the intensity of current measured within the pump’s electrical motor, and cosφ represents the power factor. In the N P SHr test
the fluid flow is fixed and the N P SHa is progressively reduced. The tests have been
performed at a fixed rotation speed of the pump (2900 rpm).
The required net positive suction head, N P SHr, is defined by the following Eq.2.5:
N P SHr = ps ρg + u2 s 2g + pv ρg (2.5)
Where (ps) is the pressure at the suction section of the pump and (pv) is the vapour
pressure at the test temperature. In a typical test, the rotating speed of the pump and the flow rate are maintained constant during signal process. The test is performed with this sample procedure: lowering the tank pressure by means of a vacuum pump, getting the total head to decrease than 3% compared to the total head detected under normal conditions, and analyzing the characteristics of the fluid flow [8], [11], [12]. In the Fig. 2.6 an example of the N P SHr curve at the flow rate of 0.0083 m3/s and
at a speed of 2800 rpm is shown. In this figure also the absorbed power of the pump during the test is shown. At the decreasing of the total head there is a corresponding decrease also of absorbed power.
decrease in the total head and the absorbed power of the centrifugal pump. At the instant t = 138 s there is a decrease in total head of 3% with respect to the initial total head. This indicates the cavitation. During the whole experimental cavitation characterization phase, all the hydraulic, electrical and mechanical indicators of the centrifugal pump will be monitored and acquired during the cavitating transient. In particular, the study of cavitation will be carried out by analyzing the vibrations of the centrifugal pump acquired with three mono axial accelerometers.
Chapter 3
Effect of washing
In the present chapter the effect of washing in a centrifugal pump are presented. During normal work conditions the pump can be subject to different condition. In a dishwashing and in a washing cycle, in order to obtain a good washing result, the wa-ter presents a quantity of chemical concentration and a different range of temperature, depending on the load and of the wash cycle. Therefore it can be possible that the centrifugal pump present in the hydraulic system works in different conditions. Other particular conditions, typically present in the dishwashing machines is the presence of residue of food in the solution of water and detergent. This last condition also causes damage to the washing machine and causes the washing performance to decrease. In the test rig it is possible to carry out tests by changing the fluid conditions. Many tests have been carried out by changing the temperature and detergent concentration. In the test rig, the same operating conditions of the centrifugal pump must be repro-duced. Therefore, it was decided to act on the temperature and concentration of the detergent.
3.1
Effects of the Sinner circle
The effects of the Sinner circle have been analyzed, through experimental tests on the centrifugal pump presented in Chapter 2. The characteristics of the centrifugal pump are shown in detail in Table 2.1.
The effects caused by temperature and chemical concentration have been studied through the characteristic curve of the pump. This curve indicates the hydraulic conditions of a pump and represents the variation of the total head H according to the flow rate Q. In general, in order to have a more detailed characterization of a pump, in this graph the flow rate Q is shown in the abscissa, while in the ordinates the different variables depend on the flow rate: total head H, efficiency η and absorbed power P . The comparison of different operating conditions, discussed in the following paragraphs, causes a significant variation in pump performance.
3.1.1
Polymers effects
In professional dishwashing machines, the working conditions of the pump are in-fluenced by the properties of the fluid. The operating fluid is not pure water, but contains detergents made of components and additives such as polymers and sur-factants, which can influence pump performance, both under normal conditions and under cavitation conditions. Washing products such as detergents are characterized by different components such as surfactants, alkalis, acids, and other substances that influence the properties of the fluid in which they are present. In particular, polymers influence the effect of cavitation.
In this study, various experimental tests have been carried out to understand the effect of detergent concentration on the centrifugal pump. The properties of the fluid have been modified by adding a particular polymer in the water Polyox WSR301. This section presents the results of the experimental tests performed with various polymer concentrations. Each solution was then compared with the performances obtained with pure water.
3.1.1.1 Polymers results
The tests carried out refer to four different polymer concentrations Polyox WSR301. This polymer is part of a class of substances that are usually found in detergent com-position.
the number of monomers present in a macromolecule [13]. Polymers normally form random coils. A chain of polymers can be deformed by an external stress such as an applied force. Depending on the concentration of polymers in a solvent, two solution regimes can be defined: diluted and concentrated. In a dilute polymeric solution, intramolecular interactions control the conformation of the individual chains while the interactions between the coils are negligible, i.e. there is no overlap between the chains. Instead, in a concentrated regime, each chain affects the nearest ones, so there is a phenomenon superimposed between the polymer coils.
By doubling the concentration in a diluted solution viscosity increases its value in a rather proportional way, instead, at concentrated regimes, viscosity increases by ten or more times.
The characterization work carried out on the test rig [14], aims to identify the values at which the solution of Polyox WSR301 is passed from the diluted to the concen-trated regime. An analysis of fluid aerology, through an experimental study, was carried out on a re-metric platform.
The solutions were obtained respectively with the following concentrations: 20000, 10000, 5000, 2500, 1250, 625 and 312 ppm. Measurements taken with a rheometer indicate the conditions of the fluid in laminar conditions. Considering the constant viscosity, the fluid is presented as a Newtonian fluid. The difference between diluted regimen and concentrated regimen in the solution containing the polymer Polyox WSR301 is better represented by the Eq. 3.1 indicating the specific viscosity, in which η0 is the viscosity of the polymeric solution while ηs is the viscosity of the pure
solvent (water in this case).
ηsp,0=
(η0− ηs)
ηs
(3.1)
The Fig.3.1a shows the trend of η0, while the Fig. 3.1b shows the trend of ηsp,0 with
increasing concentration values. The value indicated with c∗ represents the overlap value, defined as the upper limit between diluted and concentrated regimens. There is a law of proportionality that defines the diluted regime as ηsp,0 ∝ c1.3, while the
concentrated regime is represented as ηsp,0 ∝ c5.1.
(a) Trend of the viscosity solution.
(b) Trend of the specific viscosity in depen-dency of the solute concentration.
Figure 3.1: Trends of viscosity [14]
Figure 3.2: Friction factor for dilute aqueous solutions of poly(ethylene oxide) [14]
In this work the polymer Polyox WSR301 was used at the concentration of 100, 200, 400 and 800 ppm. The following figures show the performance differences ob-tained in the centrifugal pump between the 800 ppm concentration and the absence of concentration. The head, power consumption, efficiency and N P SHr of the pump are shown in these Figs.3.3a,3.3b,3.3c,3.3d.
(a) Total head (b) Power supply
(c) Efficiency (d) N P SHr
Figure 3.3: Comparison between the performances obtained with water (blue) and solution at 800 ppm (red).
The results of experimental analysis show that at low concentrations, the presence of polymers leads to a relatively large improvement in efficiency, especially at high flow rates, and to a slight increase, less evident than the total head.
3.2
Temperature and detergent effects
of the detergent nor its name will be indicated. In the following pages, this detergent will be simply indicated with the name “detergent”. Therefore several tests have been carried out, with or without detergent, and by varying the operating temperature, in order to detect pump performances.
To understand the performance under different operating conditions, several tests have been carried out on a centrifugal pumps. For reasons of company confiden-tiality, the pumps will not be indicated with their trade name but will be indicated simply with Pump A and Pump B. The hydraulic and electrical characteristics of these pumps are described in the following tables( Table 3.1 and Table3.2).
Table 3.1: Pump A Electrical quantities Power supply 230V Power rate 150W Current 0.7A Speed 2600rpm Frequency 50Hz Hydraulic quantities Flow rate 0 m3/s to 0.0015 m3/s Total head 0m to 7m Table 3.2: Pump B Electrical quantities Power supply 230V Power rate 120W Current 0.5A Speed 2700rpm Frequency 50Hz Hydraulic quantities Flow rate 0 m3/s to 0.0013 m3/s Total head 0m to 5m
3.2.1
Temperature effects
In these centrifugal pumps (3.1 and 3.2), tests have been carried out to characterize different operating conditions. First of all, Pump A with water, without detergent, was characterized at different temperatures: 15, 40, 50, 60 and 70 ◦C. Figs. 3.4, 3.5,
Figure 3.4: Total head at different temperature (Pump A)
Figure 3.5: Power supply at different temperature (Pump A)
the head H, of the absorbed power P and of the efficiency η, according to the flow rate expressed in liters per minute.
Figure 3.6: Efficiency at different temperature (Pump A)
Fig. 3.4 the increase in temperature leads to an increase in total head in all flow rates. It is possible to observe an increase in total head in each test performed at the test temperature different from the temperature of 15 ◦C (temperature at the outlet of the aqueduct during the specific test), considered ambient temperature of the water. In the Fig. 3.5 the different power absorbed during the different temper-ature conditions is illustrated. The increase in tempertemper-ature, in general, produces a lowering of the absorbed power. As a consequence of the decrease in absorbed power, a remarkable increase in efficiency η can be seen in Fig.3.6. At the point of maximum efficiency BEP or “Best Efficiency Point”, it is possible to observe, by comparing the temperature of 15 ◦C with the curve of 60 ◦C, an increase in efficiency of 4%.
3.2.2
Detergent effects
Figure 3.7: Total head (Pump A)
Figure 3.9: Performance (Pump A).
Figure 3.11: Power supply (Pump B)
The comparison between the different conditions is presented in the following graph showing the trends of pump A and pump B in two different operating conditions. It is possible to observe a general increase in the performance of the machine as the temperature increases, while maintaining the same concentration of solute for solution (see Fig. 3.13).
3.3
Life test
Another type of test performed on the centrifugal pump is the life cycle of the pump. This test, carried out in a continuous time, has as its objective the determination of potential faults and the useful life of the component. This test defined as “Accelerated life testing” is the process used to test a product by subjecting it to particular con-ditions (such as stress, voltage, temperature and pressure), in order to detect defects and potential failure modes [15]. By analyzing the product’s response to these tests it is possible to define, by means of forecasts, the useful life span and the maintenance interventions on the product itself.
This experimental test methodology is often used in the field of chemical engineering and materials, to test the behavior of materials subjected to critical conditions. For example, in polymers, the test can be carried out at high temperatures to achieve the result in shorter time than what could be produced at environment temperature. To speed up the tests, stress factors must be significantly increased. In order to have a correct answer, with a high degree of precision, the same conditions are usually reproduced by means of several repeated tests. If you perform accelerated tests e.g. on particular materials the number of samples is increased, while if it refers to more complex objects it is necessary to increase the number of tests.
3.3.1
Test carried out
The life test allows to reproduce accelerated conditions of operation of the professional equipment, acting on the temperature and concentration of the detergent. This par-ticular test is designed to bring the components to the end of life.
A centrifugal pump installed in a professional equipment has the following operating conditions dependent on the selected cycle: operation with “duty cycle” (ratio be-tween the time that an entity passes into an active state in proportion to the total time considered) generally of 50% , maximum operating temperature of 70 ◦C and maximum concentration of 32.5 g/l. Therefore, the accelerated life cycle will be car-ried out under conditions of maximum stress; continuous cycle, temperature of 70
◦C and maximum detergent concentration, 32.5 g/l. During the test, the solution of
water and detergent, at the temperature of 70◦C is circulated to the whole of the test rig. This test, carried out in a continuous manner allows to perform an accelerated life test on the centrifugal pump.
This test was performed on several pumps made up of different materials. The fol-lowing Table 3.3 shows the pumps, operating conditions, to which the tests were performed, and the operating time. Also in this case for reasons of corporate confi-dentiality, it is not possible to indicate either the model of the pumps or the supplier, so they will be indicated simply with with a numeric subscript.
The pumps shown in the table 3.3 have the hydraulic and electrical characteristics presented in Tables 3.1 and 3.2, respectively. The characteristic curves of both cen-trifugal pumps (pump A and pump B) are shown in figure 3.13. Despite having a different characteristic curve, the two centrifugal pumps in question have similar ge-ometry. The life test performed consists in verifying the compatibility of the materials with the chemical substance.
Table 3.3: Operating conditions
Pump Temperature [◦C] Chemical [g/l] Time [h] Analysis
Type A N.1 70 32.5 192 break
Type A N.2 70 32.5 89 slight loss Type A N.3 70 32.5 72 white solid crusting Type B N.1 70 32.5 432 no loss Type B N.2 70 32.5 168 no loss Type B N.3 70 32.5 68 no loss
3.3.2
Results
The result of this accelerated test is shown in pump Type A N.2. The pump per-formances are identical to pump A presented in the previous paragraph, that is: total head between 0 and 7 m, flow rate between 0 and 90 l/min and single-phase power supply voltage of 230 V ac. This pump is composed of the following flange parts, pump body and impeller made of glass fiber reinforced polypropylene, C45 shaft and gasket aluminum/NBR (acronym of Nitrile Butadiene Rubber) gasket and seal NBR/PTFE(polytetrafluoroethylene). This pump was carried out a preliminary analysis to determine the possible causes of leakage of the seal-gasket coupling sys-tem of the seal to the outside of the flange. The accelerated life test was interrupted after 7 days of continuous work due to heavy leaks with percolation of the liquid and formation of white encrustation on the external side of the volute, on the motor side. Already after 3 days of continuous operation a leak was discovered that was not considered so important to stop the test.
From the visual and stereo microscopic analysis the following results emerged here de-scribed. The gravity loss on the external side of the flange before being disassembled is the volute shown in the Fig.3.14.
It is possible to observe the formation of the white encrustation coming from the sealing gasket. The losses of the seal-gasket coupling system are even more visible in the Figs. 3.15a and 3.15bin which the disassembled outer side is indicated.
Figure 3.14: Detail of the leakage by gravity on the external side of the flange
(a) (b)
Figure 3.15: Details of the leakage from the seal-gasket coupling system
Figure 3.16: Detail of the dismounted external side of the flange
(a) (b)
Chapter 4
Cavitation phenomenon
This chapter represents the heart of this research. Through the theoretical study of cavitation an experimental approach has been found which is able to detect the characteristics of a pump in normal operating conditions and in anomalous conditions such as those taking place due to cavitation. In this part of this work the following aspects will be presented: the damaging effects of cavitation on the structural parts of a pump, such as the breakage of the rotor and the impeller bearings, the theoret-ical approaches performed with CFD and numertheoret-ical simulations to detect cavitation formation and the classical cavitation detection techniques. Through experimental tests, it is possible to derive the indicators of cavitation from hydraulic measurements. Another experimental, in-depth and developed approach consists in the detection of cavitation by measuring the vibrations and the noise of a pump. Moreover, the curves are presented through 2D graphs, which characterize the normal operation and cav-itation of centrifugal pumps, such as the characteristic curve, cavcav-itation curves at different flow rates and the vibration trends obtained during the transition opera-tions of the pump from normal operating condiopera-tions to cavitation condiopera-tions.
4.1
Effects of cavitation
Cavitation is a harmful phenomenon for turbo-machinery, occurring in both pumps and turbines. The most serious problem is the damage that occurs when bubbles present inside a fluid, collapse and explode near a solid surface. This phenomenon is very complex and complicated to study because it involves, within the turbo-machine impeller, turbulent phenomena combined with the reaction of the material the solid surface is made of. In centrifugal pumps, cavitation occurs in the eye of the pinwheel. Due to the pressure of the liquid a sudden lowering of pressure occurs. If pressure is below the vapor pressure of the liquid, it vaporises. With the presence of steam, pressure and flow become unstable, and if the pump has been installed at a lower pressure or presents a lower pressure, it is no longer self-priming. It is therefore necessary to assure a sufficient pressure supply in order to guarantee that pressure doesn’t fall below steam pressure, at operation temperature, during the normal functionality of the pump. During the damaging phenomenon of cavitation internal fluid circulation vortices form both in the suction section and in the delivery section.
The cavitation erosion mechanism is due to two main phases. During the implosion of the vapor bubble a high energy is associated that is transmitted to the surrounding walls and which can cause the removal of surface material. During the collapse of the bubble there is a deformation of the bubble itself and the subsequent appearance, a moment before the collapse of the bubble, of liquid jets at high speed that go to collide on the surrounding walls exerting an erosive action (see Fig. 4.1).
Figure 4.1: Cavitation erosion
surface tensions. The continuous repetition of this phenomenon causes damage due to fatigue on the surface and the subsequent separation or flaking of pieces of the material. Surface detachment occurs in particular on the blades of the impeller. In Fig.4.2we can observe the complete erosion of the impeller caused by the continuous effect of cavitation for a long time.
Figure 4.2: Example of damage caused by cavitation
The damage caused by cavitation, caused by shock waves or micro-jets, has been discussed and studied for many years. The main effects are the breakage of the pump impeller, the breaking of the axis and of the bearing.
4.2
Study of cavitation
of the pump and expresses the energy load of the fluid required in order to cross the portion of the pump between the suction flange and the first impeller, Eq. 4.2. While, the term N P SHa (Net Positive Suction Head available) is a characteristic of
the plant, Eq. 4.1.
N P SHa = ( pa γ + c2 a 2g)− ps γ (4.1) N P SHr = ( c21 2g + ∆pc+ λ w12 2g) (4.2)
Where ca represents the velocity of the fluid in the intake section; pa is the fluid
pressure in the intake section; ps is the saturation pressure of the fluid at the
oper-ating temperature; ∆pc are the pressure drops distributed along the section of the
pipes; λ is the loss coefficient; w1 represents the relative velocity of the fluid at the
impeller inlet.
The trend of the N P SHr can be obtained in an experimental way. It consists in
determining each point of volumetric flow by progressively lowering the pressure level of the tank until the situation in which the symptoms of cavitation arise can be identified, i.e. loss of efficiency, cavitation noise and flow rate irregularity. The ex-perimental part was carried out on a centrifugal pump, whose hydraulic and electrical characteristics were presented in table number 2.1.
In the following Fig. 4.3 the trend of the N P SH for the centrifugal pump sidered for the analysis is shown. This curve has been obtained under different con-ditions: from the minimum flow rate of 0 m3/s to the maximum flow rate of 0, 015
m3/s, with a 0, 0016 m3/s, and with a calculated pump pressure drop of 3%.
Another method necessary for the determination of the N P SHr curve, in the
ab-sence of experimental data, can be obtained in good approximation by the following equation, through n, the speed of the pump. The specific velocity of the pump ns
is a dimensionless parameter, expressed in the following way as a function of the ω angular velocity, the volumetric flow rate Q and the total head H, Eq. 4.3:
ns= n
Q1/2
H3/4 (4.3)
Figure 4.3: Example of NPSH trend (calculated from the experimental tests)
The value of N P SHr is then obtained with the following Eq. 4.4:
N P SHr ={
(n× Q)1/2
nsc } 4/3
(4.4)
To ensure operation in the absence of cavitation it is necessary to compare the value of the N P SHr with the value of the N P SHa. The condition to be respected to not
provoke the phenomenon of cavitation is the following Eq. 4.5 . Furthermore a limit value is defined to guarantee a safety condition relatively to cavitation, Eq. 4.6.
N P SHa> N P SHr (4.5)
N P SHa≥ α × NP SHr (4.6)
In which the value of α varies:
• α=1.15 for low density fluids and high temperature water.
Another dimensionless parameter, used to define the cavitation condition, is the fol-lowing Eq. 4.7:
σ = N P SHa
H (4.7)
it compares the value of the N P SHa with the total head value H of the pump.
Below the critical value σ there is the cavitation condition, while above sigma pump operations are regular. These parameter reference values, in general, are provided by the pump manufacturer.
4.3
Incipient Cavitation
The cavitation start point is called incipient cavitation. This point is conventionally defined as the one in which a pump head fall of 3% occurs. The point of incipient cavitation is usually found through the detection of the curve of the N P SH.
Cavitation forms at the upper point of the impeller corresponding to the upper point of the volute. In this point cavitation forms, which extends across the entire surface of the impeller blading. The initial cavitation formation is called “incipient cavita-tion”. A good quantity of research indicates the incipient cavitation point and how to identify it. The identification can be carried out through numerical simulations CFD (Computational Fluid Dynamics) or through experimental tests.
4.3.1
Numerical simulations
Regarding the first case stated in literature there are several studies, related to numer-ical simulations indicating the operating status of the pump in abnormal conditions and in conditions of cavitation. Computational fluid dynamics simulations are widely used to solve very complex problems such as the effect of cavitation on centrifugal pumps. These simulations are necessary in order to reproduce the same pump geom-etry and the same physical conditions of the fluid and the material in a 3D model. In recent years, many authors have studied the phenomenon using fluid dynamic simu-lations.
developed cavitation conditions. Characteristic trends of the pump at different flow rates from the design one and in different cavitation conditions are presented.
H. Ding [20] presents a CFD simulation simulating cavitation in an axial flow hydraulic pump. The simulation results were compared with the experimental results in different pump operating conditions (flow variation). Furthermore, the cavitation model was compared to the video images recorded during the experiments carried out. Milan Sedlar [21] uses the ANSYS RCFX package to observe the phenomenon of
cavitation, at 70% of nominal flow rate, using the decrease in total head of 3% as an indicator.
4.3.2
Experimental tests
Another experimental and practical approach, on the other hand, is present in the research articles discussed below. Baldassarre [22] presents a method to detect the incipient cavitation throught image analysis: the aim is to identify in real time the presence of cavitation (analysing images of the impeller blade) and to use an acoustic signal in case of incipient cavitation.
Other experimental studies detect the incipient cavitation through the acquisition of acoustic emissions. L.Alfayez [23] presents a method to detect the incipient cavita-tion and the best efficacy point BEP of a 60kW centrifugal pump, through acoustic emissions. The same author in another research [24] highlights the method for de-tecting the incipient cavitation in centrifugal pumps of different cross-sections (60kW , 127kW and 2.2M W ). This method highlights significant results.
GD Neill [25], through an experimental apparatus, detects the incipient cavitation point of a 75kW centrifugal pump thanks to acoustic emissions. The test consists in reducing the total head of the pump by recording the acoustic noise. This method allows the incipient cavitation to be detected and prevented before a significant re-duction in the pump head is present.
M.Cdina [26], instead, detects and prevents the phenomenon of cavitation instability by measuring acoustic noise. In experimental tests, a discrete frequency tone was used within the audible noise spectrum at 147Hz. This frequency is strongly dependent on the cavitation process and is therefore used to detect the incipient cavitation.
4.3.2.1 Noises and vibrations
and vibrations of the pump increase. Pump vibrations are easier to acquire than sound. In fact, the acquisition of sound involves, for a correct measurement, particu-lar locations. A useful location is, for example, an anechoic chamber, which presents special wall surfaces able to reduce or eliminate the reflections of acoustic signals on the walls. Therefore, for ease of acquisition even in particularly noisy places, such as in industrial environments, it is possible to use and acquire the vibrations of the centrifugal pump.
Noise and vibration in the pumps are generated by various mechanisms. In gen-eral, vibration measurements in pumps are used to monitor and detect imbalances, misalignments, defective bearings and resonances [27]. In centrifugal pumps the vi-brations are due to:
• Hydraulic forces
• Turbulence, cavitation and recirculation
Vibrations due to hydraulic forces are related to the pressure pulsation and are generated when the impeller blade passes on a stationary diffuser or on the tongue of the volute. Hydraulic forces are caused by all pumping events in each spin of the impeller. They are calculated considering the number of blades, components of the impeller, the speed of the pump. It is possible to calculate the passage frequency of the blade and the relative harmonics by means of the following Eq. 4.8:
fp = BEF =
(n× rpm × Z)
60 (4.8)
With BEF (Blade Passing Frequency) the passage frequency of the blade is indicated; n indicates the number of harmonics; rpm indicates the rotation speed of the pump shaft expressed in rpm; Z indicates the number of events of pumping per revolution corresponding to the number of blades in the impeller. In centrifugal pumps, the hydraulic pulsations are reduced considerably if the impeller is centrally aligned with the diffuser, and there is sufficient space between the impeller blades and the diffuser.
