5th Congress of Alps-Adria Acoustics Association 1-14 September 01, Petrčane, Croatia CAVITATION NOISE PHENOMENA IN CENTRIFUGA PUMPS Jan Černetič, Mirko Čudina University of jubljana, Faculty of Mechanical Engineering, jan.cernetic@fs.uni-lj.si Abstract: Today, a cavitation phenomenon is well known weakness of many hydraulic machines and devices. Absence of cavitation is crucial for safe and reliable operation of water pumps and turbines. In this paper, some important phenomena regarding cavitation noise in centrifugal pumps in audible frequency range are examined closely. When the cavitation is increasing, the flowing conditions are changing, which leads to increase of emitted noise in the surroundings. Sound pressure level increases, but when cavitation reaches its critical point, it starts to decrease rapidly. This phenomenon depends on the frequency range and on the type of the pump. The flow rate is also important here since it has influence on the point of cavitation inception. Due to higher flow velocity at higher flow rate, the cavitation will start to develop sooner. Knowledge of these cavitation noise characteristics can be successfully used for detecting onset and development of cavitation in centrifugal pumps. Keywords: cavitation, centrifugal pump, noise, measurement, detection 1. INTRODUCTION Cavitation phenomenon became important when it was found to be the cause of pump failure represented by pump performance deterioration and poor efficiency. Cavitation also causes physical damage and increased vibrations and noise [1]. It occurs when static pressure at some point within a pump falls below vapour pressure of the fluid at the prevailing temperature. The fluid then starts to vaporize and many gaseous bubbles arise. When the cavitation is increasing, the number and the size of the bubbles are also increasing. Bubbles travel in the fluid through the pump and at the end they reach an area with higher pressure, where they rapidly collapse. Forces, generated during the collapse, can have very destructive consequences. If the implosion of the bubble occurs near a solid wall, shock wave reaches the surface and erodes it. Therefore the pump surfaces are damaged which causes disturbances of the flow and after a certain time the pump could become useless. It is obvious that the cavitation phenomenon should be detected and prevented as soon as possible. There are a few different methods for cavitation monitoring that can be used to prevent destructive consequences of the cavitation in the pump and pumping systems []: determination of the net positive suction head (NPSH) at a constant speed and flow rate, visualization of the inlet flow at the impeller, paint erosion on impeller blades, static pressure measurement in the flow, vibration measurement of the pump structure and acoustic measurement in the pump. Acoustic measurement is well known by measuring ultrasound inside the pump, but not so well by measuring audible noise in proximity of the pump Rather new cavitation detection method was proposed before, which is based upon measurement of noise in audible frequency range [1, 3]. The method proved to be useful for on-site measurement and also reliable despite many effects on measurement uncertainty [4].. EFFECTS OF CAVITATION While operating, a pump has to overcome a pressure difference, which arises by pipe resistance, suction and delivery head between the pump and fluid levels in containers and pressure above the fluid levels [1]. In some circumstances the given pressure difference is too high for the pump to be capable of pumping the fluid, which means that the NPSH value is insufficient. As a result, the static pressure on the suction side becomes too low and it can reach the saturated vapour pressure of the liquid. This happens first at the nozzle of the impeller, where the pressure is the lowest. The higher the flow rate, the sooner the cavitation arises, because the flow velocity is higher and consequently the static pressure is lower. NOI-01 Page 1 of 6
When the pressure in the fluid drops to the saturated vapour pressure of the fluid and lower, the bubbles filled with dissolved air and/or vapour start to rise. There are two bubble generating mechanisms [5]. First, there is always some dissolved gas in the fluid. When the static pressure drops, operating conditions are changed and dissolved gas comes out of the fluid as a gas bubble. This is called gaseous cavitation. And second, when the static pressure drops below the vapour pressure, vaporization occurs and bubbles start to grow. This is called vaporous cavitation. Fluid volume is increased significantly when vaporized. In case of a fully developed cavitation, the water at 1 C, for example, increases in volume about 54.000 times [6]. Theoretically, it can be calculated the size of particular bubble by using of the generalized Rayleigh-Plesset equation for bubble dynamics [7], through the pump, they reach a higher pressure area and implode rapidly. It is well known that noise is created when bubbles implode due to a sudden increase of the pressure [8, 9]. This effect causes relaxation of very huge amount of concentrated energy, which can be very destructive. If a bubble implodes close to the solid surface (on impeller blade or volute casing) a shock wave appears, which hits the surface and damages it. This continuous process leads to the material erosion of the pump, which results in increased surface roughness and eventually perforated impeller and/or volute casing at the semi-open impeller pumps. This causes improper flowing conditions and worse efficiency, and also increases vibrations and noise. Therefore the cavitation phenomenon in a pump is unacceptable and should not be tolerated. p B ( t) p ( t) = ρ d R 3 dr 4ν R + + dt dt R dr + dt S ρ R where ρ is the fluid density, R is the bubble radius, t is the time, ν is the kinematic viscosity of the fluid and S is the surface tension. Values of the pressure in the fluid p (t) and in the bubble p B (t) are known. It was shown by Blake (1949) and Neppiras and Noltingk (1951) that critical radius exists [7]. The bubble is stable if his radius is smaller than critical radius, and it will grow fast, if radius is bigger. This is often referred to as Blake critical radius, (1) 3. CAVITATION DEVEOPMENT AT DIFFERENT FOW RATES Cavitation development is strongly dependent on the flow rate. In general, the higher the flow rate, the sooner cavitation arises. Fig. 1 shows NPSH available value of a pump at different flow rates as cavitation develops. During the cavitation development, the NPSH value is decreasing and at some point it reaches a value that is too low to prevent the cavitation. Black dots represent the critical point of the cavitation process, which means that the delivery head of the pump drops for 3% from the starting value. Higher flow rate therefore means lower delivery head and also higher NPSH value, which corresponds to the fact that the pump is more vulnerable to cavitation. R C 9kmGTB K = 8πS G 1 () where k is the polytropic constant, m is the mass of gas in a bubble, T is the bubble temperature and B G K is the G gas constant. Critical pressure derived from the critical radius is known as Blake threshold pressure, p c = p V 4S 8π S (3) 3 9kmGTB K G where p is the vapour pressure. This means that V pressure below the critical pressure will cause the bubble to grow fast. If the pressure is reduced further, the bubbles are growing. Because the bubbles are travelling with the fluid 1 Fig.1. NPSH available values of a centrifugal pump at different flow rates In case of higher flow rates, flow velocity is higher, therefore pressure at the inlet of the pump is lower. The most critical location is the pump nozzle, i.e. before impeller inlet (Fig. ), because there is the highest pressure drop due to lifting a fluid and overcoming all the pressure losses in the suction side of the piping system. On the other hand, at this point an impeller starts to NOI-01 Page of 6
accelerate the fluid, which leads to the effect that tends to tear the fluid into two parts (pressure and suction part). Fig.. Pressure distribution in the centrifugal pump [10] When the critical point is reached (3% drop of delivery head) and the pressure drops even more, the fluid soon contains a big amount of vapour bubbles causing twophase flow. If the pressure reduction continues, the fluid becomes even weaker and the delivery head drops dramatically (see Fig. 1). The two-phase flow becomes too weak to resist impeller forces and in the end the flow stops. This is an extreme case of cavitation development, which can have very dramatic consequences for the system, especially in case of bigger cooling systems. 4. CAVITATION NOISE MEASUREMENT Measuring the cavitation noise in order to detect the onset and progress of cavitation phenomenon is well known for a long time [5, 11-15], but almost each of these studies is limited on investigation of high frequencies, above 0 khz (i.e. in a frequency range of ultrasound). Measurements are often performed inside the pump or water turbine, at places where the cavitation is more likely to occur (for example, at the impeller eye), using a hydrophone or an Acoustic Emission sensor (AE sensor). ower frequency noise (below 0 khz) is usually filtered out because it contains too much noise in correlation with rotation (blade passing frequency, rotating frequency and their harmonics). Characteristically, at a constant flow rate, noise is increased when a delivery head (H) drops [3]. It is also known that a correlation exists between the pressure level amplitude and a cavitation erosion intensity, which increases by increasing noise [1]. Method for cavitation detection by using of noise in audible frequency range (0 Hz to 0 khz) is still in development phase and not many studies cover this field [1-4], although, in comparison to the other methods, it offers some important advantages. For cavitation detection also vibration signal can be used in order to get more reliable results [1]. Both, noise and vibration signals have very similar characteristics. A decision, which method to use in a particular case, should be done according to the requirements of the environment, in which the measurement have to be performed. In noisy surroundings, where noise from the neighbouring machines is too high to measure pump noise, vibration measurement is more convenient. Also it is suitable to use vibrations if other circumstances are preventing usage of a microphone. Strong air stream has important influence on the sound pressure measurement error, especially for unsteady streams. In areas with sprinkling water, oil or other liquid, the use of a standard measuring microphone is unacceptable. It is also important to take into consideration that the microphone needs some space to be out of the near field or sufficient distance from the reflective surfaces, where sound field can propagate with minimum disturbances. For accurate pump noise measurement, acoustical surrounding should be more or less stable. If not, cavitation detection algorithms have to be adapted each time the change occurs, or arrange an acoustical shield around microphone to minimize the effect of the background noise. Measuring with a microphone is useful mainly because of its simplicity; it is easy to place the microphone nearby the pump and there is no need to carefully choose the appropriate mounting surface on the pump (as it is in case of an accelerometer). Therefore the distance between the microphone and the appropriate pump surface is easily set. Where broadband or random vibrations of a pumping system are present, or where accelerometer placement is not reliable or is hard to mount it, use of noise signal is especially convenient. It is also good to consider that the cable of the microphone is less sensitive to long distances and mechanical deformations than the cable of an accelerometer. 5. CAVITATION NOISE DEVEOPMENT A typical cavitation noise development curve is presented in Fig. 3. This particular measurement was performed on the centrifugal pump with semi-opened impeller at the discrete frequency of 147 Hz, but similar result can be achieved at other frequencies or bands. Fig.3. Noise signal of the pump when decreasing NPSH value NOI-01 Page 3 of 6
While NPSH available value is decreasing, the delivery head (H) is quite stable and noise level ( p ) varies a little. When NPSH available value decreases further, cavitation inception appears and develops. With further development of the cavitation, noise level starts to grow rapidly and also the delivery head starts to decrease soon. The noise level typically reaches the highest value somewhere in the vicinity of the critical NPSH value, i.e. at the 3% drop of the delivery head (a big dot on the NPSH curve, Fig. 3). After that, both the noise level and the delivery head drops rapidly. As can be seen from Fig. 4, this process is similar for all flow rates. This Fig. is represented at discrete frequency 147 Hz, where the amplitude difference is the highest, but similar characteristic can be achieved also at other frequency ranges. decrease rapidly. Because of high density of gaseous bubbles, the cushion effect prevails over all others. Gülich [16] stated two main mechanisms for the intensive drop of noise level: 1) If big cavitation bubbles are present at the inlet of the impeller, a part of noise is absorbed in two-phase areas. Bubbles can implode also inside passages of the impeller blades, which serves as a shield for noise distribution in the surroundings. This is important especially in case of noise measurement inside of the pump, where sound transducer is sheltered from direct sound. ) At lower pressures at the inlet of the pump, air separates from the liquid. Because of that, bubbles are formed in the fluid, which reduces sound transmission. Two-phase flow alters acoustic properties of the liquid, especially sound speed. Due to compressibility of the bubbles and impedance change on the bubbles walls, the sound speed is reduced significantly comparing the sound speed in pure liquid. If the NPSH value is reduced further from the critical value, the flow rate will soon start to decrease until the whole pipe cross section is blocked with the cavitation bubbles. This process also takes part in further noise level reduction. 6. SOUND INSUATION OF PUMP WAS Fig.4. Relative noise level curves at 147 Hz versus NPSH value for different flow rates as parameter The explanation for drop of noise level can be found in the two-phase flow, which is developing in the pump at that time. While cavitation is developing, tension in the fluid is increasing due to lower pressure at the inlet of the pump. Due to the formation of gaseous bubbles, the fluid is transforming into the two-phase flow. During pressure drop, the bubbles are growing, therefore the fluid volume is increasing. Because of that, the flow speed increases, which also helps to reduce pressure even more. By inception and growth of the vapour bubbles the noise level starts to increase because the flow is becoming more and more turbulent; bubble clouds act, namely, as obstacles in the flow path. Implosions of cavitation bubbles and bombardment of the surrounding surfaces cause intensive vibration and noise. In addition, this process of implosion can be very violent and destructive for the pump material. At the same time bubble clouds forming two phase flow act as a cushion and absorber for cavitation noise. It is known that two-phase flow significantly reduces the speed of sound through the fluid [16]. It is interesting that in bubbly liquid, consisted of air and water, the speed of sound as low as 0 m/s can occur [7] This phenomenon becomes more important after a critical number or density of the bubbles in the flowing liquid, usually after 3% drop of total delivery head, when the noise level reaches its maximum value and it starts to Noise, produced inside the pump, is partially transferred outside to the surroundings. On the way through the pump s casing, it is subject of strong modifications. Only a part of the noise energy comes out of the pump while a big amount of it is either transferred to the other parts of the piping system or simply attenuated. Process of attenuation involves noise reduction in casing itself and also in bubbles, which arise due to cavitation. Fig.5. Experimental loop for measuring sound insulation of the pump s casing To clarify this phenomenon the measurement was performed with two microphones and one noise source. One microphone (M1) was installed inside the pump NOI-01 Page 4 of 6
(with impeller removed), made from HDPE material, and the other one (M) was installed outside in the vicinity of the pump (see Fig. 5). The background noise was recorded in the pump. The sound from the external noise source is transferred into the pump and recorded as a transferred noise. Fig. 6 shows the noise reduction through the plastic casing of the particular centrifugal pump. 7. MEASUREMENT UNCERTAINTY For each measurement, a measurement uncertainty has to be estimated. Measurement uncertainty estimation for measurement of cavitation noise and vibration was performed in details earlier [4]. The estimation was performed on the same pump that is the case in this paper. The results are presented in Fig. 7 [4]. Here, all possible influences on measurement uncertainty are included, such as repeatability, intermediate precision and also an assembly error, which arises after the pump is put together after disassembly. It means that the results show the highest possible measurement uncertainty that can happen during operation. Fig.6. The amount of sound, transferred through the pump s casing One can see that the transferred noise is mainly in the frequency range above 700 Hz. This result is in accordance with the fact that the cavitation noise is usually recorded mainly as noise of higher frequency range. It is connected with implosions of many bubbles, that produces pitting noise and also with very turbulent flow caused by presence of numerous bubbles. This finding is somehow contradictious because it is known that cavitation cause noise also in lower frequency range. It shows that cavitation produces not only noise, but also strong vibration of the pumping system at lower frequency range, which excites air in surroundings and is also heard as noise. These vibrations can be measured as noise during the operation of the pump. When measuring only transferred noise through the casing, absence of vibration causes the signal, free from noise in lower frequency range. When static pressure at the inlet of the pump is decreasing, the flow is constantly adapting to the new conditions. Therefore areas of different pressures are formed which cause local vortices and backflow. This process leads to unbalance of the impeller and therefore to excessive vibration. These vibration forces are usually transformed into noise in lower frequency range, under 1000 Hz. If this fact is taken into consideration, the Fig. 6, which shows the sound insulation of pump s casing, is more clear. This measurement results do not contain any low frequency content, measured in the pump, because they are caused by other mechanisms (vibration), which can not be included in the sound insulation measurement. During the cavitation process, strong low-frequency noise comes from the vibration and not from the cavitation noise itself. Fig.7. Measurement uncertainty of noise measurement at different frequencies, compared to the normally measured values of noise amplitude between cavitating and noncavitating regime of pump operation [4] It is presented for two different frequency ranges; one for broadband audible range (from 0 Hz to 0 khz) and the other for the discrete frequency of 196 Hz. At broadband, normal cavitation noise measurement gives amplitude differences of about 5 db from zero cavitation to fully developed cavitation at 3% drop of delivery head. The measurement uncertainty for this case is estimated to around ±0.7 db. At discrete frequency, the difference is much higher, about 15 db and the corresponding measurement uncertainty is estimated to around ±.5 db. In both cases, the measurement uncertainty represents just a small part of the actual amplitude difference, which shows that the measurements are reliable even in the worst-case scenario. 8. CONCUSIONS Some cavitation phenomena in a centrifugal pump were analyzed in this paper. During cavitation development, noise is changing according to the different circumstances in the centrifugal pump. Before inception of cavitation in a pump the noise is stable and is the result of normal flowing conditions at different load of pump. When the pressure in the pump decreases to the level that causes the growth of vapour bubbles and consequently delivery head to fall, other noise generating mechanisms start to prevail. NOI-01 Page 5 of 6
The flow becomes more turbulent because cavitation bubbles generate two-phase flow, which starts to spread all over the nozzle cross section. Also vortices at partial flow rate formed at the inlet of the pump are becoming large. With progress of cavitation, the noise is also increasing. Another source of noise comes from the vibration caused by unsteady cavitation flow through the pump. At some point, usually at about 3% drop in total delivery head, cavitation noise starts to decrease rapidly due to changes of fluid properties. The fluid becomes filled with vapour phase, which has completely different acoustical properties than pure liquid. The two-phase flow in combination with pump casing prevents the sound to pass freely to the outer side of the pump. Only a part of the cavitation noise can be heard outside of the pump, but in a much modified form. This noise contains also sound, generated by vibrations of the pump and its parts. This paper illuminates some interesting cavitation phenomena which are significant for centrifugal pumps. To make an efficient and reliable cavitation detection technique with audible sound, the awareness of those phenomena is very important. REFERENCES [1] J. Černetič: Use of noise and vibration signals for detection of cavitation in kinetic pumps, Proc. IMechE, Part C: J. Mech. Eng. Sci., 3 (C7), 009, 1645 1655 [] M. Čudina: Noise as an indicator of cavitation in a centrifugal pump, Acoust. phys., 49, 003, 463-474 [3] M. Cudina: Detection of cavitation phenomenon in a centrifugal pump using audible sound, Mech. Syst. Signal Process., 17, 003, 1335 1347 [4] J. Černetič, M. Čudina: Estimating uncertainty of measurements for cavitation detection in a centrifugal pump, Measurement, 44, 011, 193-199 [5] S. C. i: Cavitation of hydraulic machinery, Imperial College press, ondon, 000 [6] abour Pumps: Entrained air in centrifugal pumps, Peerless Pump Company, Alabama, 005 [7] C. E. Brennen: Cavitation and Bubble Dynamics, Oxford University Press, Oxford, 1995 [8] F. R. Young: Cavitation, McGraw-Hill, ondon, 1989 [9] J-K. Choi, G.. Chahine: Modelling of bubble generated noise in tip vortex cavitation inception, Acta acustica united with acustica, 93, 007, 555-565 [10] M. Čudina: Pumps and pumping system noise and vibration prediction and control, in: M. J. Crocker: Handbook of noise and vibration control, John Wiley & Sons, New Jersey, 007 [11] P. J. McNulty, I. S. Pearsall: Cavitation inception in pumps, Journal of fluids engineering, 104, 198, 99-104 [1] S. Gopalakrishnan: Modern cavitation criteria for centrifugal pumps, Proc. of the Second Int. Pump Symp., Turbomachinery aboratory, Texas A&M University, College Station, 1985, 3-10 [13]. Nelik: Centrifugal and rotary pumps. Fundamentals with applications, CRC Press, Boca Raton, 1999 [14] X. Escaler, E. Egusquiza, M. Farhat, F. Avellan, M. Coussirat: Detection of cavitation in hydraulic turbines, Mechanical systems and signal processing, 0, 006, 983-1007 [15] S.-Y. iu, S.-Q. Wang: Cavitations monitoring and diagnosis of hydropower turbine on line based on vibration and ultrasound acoustic, Proceedings of the Sixth International Conference on Machine earning and Cybernetics, Hong Kong, 5, 007, 976-981 [16] J. F. Gülich: Centrifugal pumps, Springer, Berlin, 008 NOI-01 Page 6 of 6