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Critical value of cavitation factor is given by

The Cavitation Number is useful when analyzing fluid flow dynamics problems where cavitation may occur. The Cavitation Number can be expressed as σ = (pr - pv) / (1/2 ρ v2) (1 Cavitation Bubble The fluid Surface Tension is a key factor in the energy that is Allowable Using Liquid Critical Pressure Ratio Factor Water at 70F P1= 134.7 psia 4 3208.2.3631 0.957 for a given set of conditions. Piping Geometry Factor The above equation allows the cavitation index for a particular device and application to be determined. The value of C i above is compared against acceptable values for particular equipment and applications to indicate the likelihood of cavitation occurring. For example in the case of a typical square-edged concentric orifice plate a C i of 2 or above would be unlikely to cause cavitation. 5.2 Coefficient of incipient cavitation x FZ and coefficient of constant cavitation K c 5.3 Piping geometry factor F P 5.4 Combined liquid pressure recovery factor and piping geometry factor of a control valve with attached fittings F LP 5.5 Liquid critical pressure ratio factor F F 5.6 Expansion factor Y and specific heat ratio factor

Cavitation Number - Engineering ToolBo

Cavitation in Restriction Orifices and Valves Neutriu

P c = pressure at thermodynamic critical point (bar abs) The liquid pressure recovery factor is given by the valve manufacturer ΔPs can be approximated as ΔPs=P1-Pv if Pv < 0.5*P1 The relations given below are valid only for a Newtonian liquid in turbulent flow Cavitation: The formation, growth, and collapse of vapor filled cavities or a bubble in a flowing liquid due to local fall in fluid pressure is called cavitation. The critical value of cavitation factor (σc) is given by. σc = ( Ha - Hv - Hs ) / H. Where, Ha = atmospheric pressure head in meters of water Due to cavitation, the metal of the runner vanes and the draft tube is gradually eaten away, which results in lowering the efficiency of the turbine. In order to determine whether cavitation will occur in any portion of the suction side of the pump, the critical value of Thoma's cavitation factor is calculated The mean cavitation pressure, p c a v *, is then obtained by averaging the individual cavitation pressure values. The mean cavitation time in the constant rate protocol follows as τ c a v * = ∫ 0 ∞ f (t) d t, which can be converted into the mean cavitation pressure as p c a v * = p ⋅ τ c a v *

Cavitation - Engineering ToolBo

  1. Pressure Recovery Factor - F. L. Liquid Pressure Recovery Factor F. L. The critical flow factor F. L. is a dimensionless expres-sion of the pressure recovery ratio in a control valve. F. L. is an ISA nomenclature. F. L = Valve critical flow factor. P. 1 = Inlet pressure PSIA P = Outlet pressure PSIA P. VC = Pressure at Vena Contracta. 10 20.
  2. The onset of rotating cavitation was assigned to the value / 2σ α (where α is the the incidence angle) of 2.5-2.8, and was mainly driven by an interaction of the cavity closure region and the following impeller blade
  3. In general, for dry gases, the C7+ mole fraction is a small value. Therefore, the weight of C7+ critical properties is also small in the summations given by Eqs. (3.17) and (3.18). Any C7+ critical properties correlations can therefore be used. Wichert and Aziz (1972) correlation can then be used to correct for nonhydrocarbon impurities
  4. Cavitation inception occurs when the local pressure falls sufficiently far below the saturated vapor pressure, a value given by the tensile strength of the liquid at a certain temperature. [2] In order for cavitation inception to occur, the cavitation bubbles generally need a surface on which they can nucleate
  5. Therefore, cavitation resistance is now seen as one of the major physiological factors driving reductions in forest productivity and drought-induced mortality in trees (Anderegg et al., 2012; Choat et al., 2012). It is expected that studies on cavitation resistance will show considerable developments in the near future as this trait start to be.
  6. Trailing edge cavitation Trailing edge of the Manual Control (guide vanes of Francis) blade Critical cavitation factor (3) is related to the specific speed of Francis turbine by the following relation Hydraulic turbine (Francis Turbine) Sc = 341.(Ns)2.10-8 Thoma‟s cavitation factor is based on the layout of turbine Dynamometer and ambient.
  7. Equation (5.12) yields two possible solutions for radius R: nuclei with a lower radius than the Blake radius are stable to small disturbances for p l values below vapour pressure p v, but above the critical value (p l,cr), whereas nuclei with R > R cr are unstable to small disturbances and grow explosively, thus giving rise to cavitation

Vena contracta is filled with vapor from cavitation or flashing. Fluid velocity at vena contracta reaches sonic. Vapor pressure: It is the pressure at which the given liquid will vaporize at the given temperature; Cf (Critical Flow Factor): The Cf factor is an indication of the valve's vena contracta pressure relative to the valve's outlet. Depending upon the geometry, a given flow has a critical value of C a, below, which the flow will begin to cavitate. The author's search for the critical cavitation number for water in published literatures continued from November 2003 to October 2004

For one given molecular weight, the pre-oriented iPP showed the smallest critical stress for whitening and failure stress along the pre-orientation direction (0°) while the samples displayed larger values for the same behaviors when stretched at 45° or 90° with respect to the pre-orientation direction Where: F L = Liquid pressure recovery factor F F = Liquid critical pressure ratio factor P V = Vapor pressure of the liquid at inlet temperature, psia P 1 = Upstream pressure, psia. See Table 3-I for F L factors for both full-open and partstroke values. F F can be estimated by the following relationship: PV FF = 0.96 - 0.28 (3.4) P critical value. [ ′krid·ə·kəl ′val·yü] (mathematics) The value of the dependent variable at a critical point of a function. (statistics) A number which causes rejection of the null hypothesis if a given test statistic is this number or more, and acceptance of the null hypothesis if the test statistic is smaller than this number U1, U and Uch are incipient, critical or choking cavitation velocity (to be determined) for a given butterfly valve. C1 is correction factor for the valve size used, Figure-8 Ui,. U, and Uch are the reference cavitation velocities (incipient, critical, or choking as applicable) for the bas A critical aspect of the cavitation wear process is surface destruction and material displacement caused by high relative motions between a surface and the exposed fluid. As a result of such motions, the local pressure of the fluid is reduced, which allows the temperature of the fluid to reach the boiling point and small vapor cavities to form

Cavitation is the formation and subsequent collapse of vapour bubbles in a flowing liquid and is often responsible for significant damage flow equipment such as pumps. This article will provide an overview of cavitation and provide insight into the identification and prevention of cavitation conditions through the calculation of Net Positive Suction Head (NPSH) Cavitation. In a control valve handling a pure liquid, cavitation may occur if the static pressure of the flowing liquid decreases to a value less than the fluid vapor pressure. At this point, continuity of flow is broken by the formation of vapor bubbles. Since all control valves exhibit some pressure recovery, the final downstream pressure is.

The nss value of 0.155 is much less than the critical value of 0.494, therefore, the pump is in the safe operating range. Problem 8.32 Two pumps having the performance curve shown are operated in series in the 18-in. diameter steel pipe. When both are operating, estimate the time to ll the tank from the 150-ft level to to the 200-ft level ΔP s = critical pressure drop = P 1 - (0.96-0.28*√(P v /P c))*P v P v = vapor pressure of the liquid at the temperature of the flow (bar abs) P c = pressure at thermodynamic critical point (bar abs) The liquid pressure recovery factor is given by the valve manufacturer; ΔP s can be approximated as ΔP s =P 1-P v if P v < 0.5*P

Guidelines for sizing of Restriction Orifice for single

Cv = Valve flow coefficient. Q = Fluid flow, (also given by Area of pipe x mean velocity) S = Specific gravity of fluid relative to water @ 60ºF. ∆P = Pressure drop (P1 - P2) across the control valve at maximum flow, psi. The equation shows that the flow rate varies as the square root of the differential pressure across the control valve This value is higher than the critical cavitation index of the cavitation start, and therefore, the risk of the phenomenon occurrence is minimal. A summary of some calculated profiles, such as flow depth, pezometeric level pressure and geometric characteristics of spillway profile are given in Table 1

Control Valve Sizing - Chemical plant design & operations

the incipient cavitation number by a factor of 2 to 3, in high-speed flows past hemispherically nosed bodies. in principle, to calculate the stress, given the history of the strain rate in a fluid element. It is rather obvious that none of the nucleus radius must be greater than critical value R, such that However, if the growth begins. which coincide with the values and as described by and respectively. Also, the formula for the number of the embryonic voids of critical radius produced per unit time per unit volume coincides with . The value at corresponds to the barrier-free cavitation that is not dependent on the fluid temperature (figure 2) For a given micropump deflection, the critical cavitation frequency is defined as the operating frequency at which the cavitation inception occurs. In order to identify the inception of cavitation, the diaphragm deflection was varied and the critical cavitation frequency was found out for each pump deflections

Control valve flow coefficient Cv Kv - Valve sizing

  1. ar or turbulent
  2. Calculation of expansion factor. The expansion factor Y, which allows for the change in the density of an ideal gas as it expands isentropically, is given by: [2] For values of β less than 0.25, β 4 approaches 0 and the last bracketed term in the above equation approaches 1. Thus, for the large majority of orifice plate installations
  3. In figure 9, the fixed sphere case given by the critical value in is shown with a continuous line, whereas a more sophisticated translating spherical cavitation bubble yields the dashed line, but which requires the solution of an ordinary differential equation, rather than the algebraic expression of
  4. Vapor Pressure and Cavitation . pressure is the pressure required to boil a liquid at a given temperature. Soda water is a good example of a high vapor pressure liquid. Even at room temperature the carbon dioxide entrained in the soda is released. In a It's critical to get the NPSH R value from the pump manufacturer AND to insure that.
  5. imum v *>0. This critical point is found from the two conditions ∂Ftot / ∂v =0 and ∂2Ftot / ∂v2 =0 that deter

Francis Turbine - Diagram, Working, Parts , Advantages

Cavitation is the formation of small vapor bubbles in a liquid which then collapse almost as quickly as they form. In a pump, cavitation occurs at the point of lowest pressure, which is at the inlet to the impeller vanes. When the vapor bubbles produced by cavitation collapse they generate a tremendous amount of pressure in a very small point So using the simple equation for water flow, Q = Cv*√(dP/SG) where SG is the specific gravity of water (1), dP is the pressure drop of 25psi, and with the given Cv of 20, this system will flow. cavitation model which can effectively correlate creep cavities with the remaining life of the components. Efforts have been made in the past, but till now, the methods already (which is the critical value of Put the values in the A-D relation given below: (D / D cr) * [1 - (1 - D / Dcr assume that the bubble reaches its critical size r∗ at mechanical equilibrium so that the experimental value of the vapor pressure (pv)ex ∗ satisfies Laplace's equation ( ) ∗ ∗ = + r p pL v ex 2σ , (5) the extremum condition given by eq. (2) then implies that the phenomenological correction to the free energy has to b Cavitation processes in water that are observed at positive pres- We will not discuss the pre-exponential factor in (3) in more de-Figure 2. Critical radius and critical nucleation work as functions of pres- we show in Fig. 2 the values of critical radius and critical nucleation work calculated according to Eqs. (5)

2. Cavitation is checked by using the FL value (Liquid Pressure Recovery Factor), and the expansion factor needed for compressible fluids is obtained using the Xt values (Pressure Drop Ratio Factor). Both values are found inthe Cv charts supplied on the following pages. 3. Select a nominal valve size from the Cv charts based on the calculated Cv The factors considered in these cavitation models were similar to the model suggested by Singhal et al. (2002). In these models, the microscopic factors that influence the nucleation and growth of vapor bubbles, for exam-ple, cluster surface tension and the Zeldovich factor, were not included in the cavitation model. Therefore, the wor the bubble volume is reduced by a factor of 17.7. The ablation pulse is best applied when the pre-pulse bubble is maximally expanded, but the timing is not very critical. PACS: 62.50. + p; 79.20.Ds; 87.00 In various fields of pulsed laser surgery, e.g., laser angio For uniform1 y rough surf aces the boundary cavitation index is given by Other roughness elements have simi lar expressions for the cavitation potential. - - 1 Figure 1. Incipient Cavitation Index of Into-the-f low Chamfers The boundary cavitation index has the same form as equation (1). Cavitation will occur when the flo upon cavitation for rubber particles of different radius subjected to a constant volumetric strain of 0.3%. Under the conditions of this example, cavitation is favorable for particles larger than 100 nm. Table S1 shows the parameters used to perform this calculation. Figure 14 in the main text includes two critical cavitation curves

[SOLVED] Cavitation in a pump is associated with

  1. Cavitation pressure as a function of temperature can be calculated based on classical nucleation theory (CNT, see theory section for a description of bubble nucleation by CNT) and is given as the.
  2. 2.2.3 Explicit creep cavity damage fracture model. The explicit creep fracture model can be derived with the given values of creep cavitation constants [ 12 ]. For the given values of α = 1 , β = 2 , and γ =1 for P91 [ 12 ], Eq. (5), w , is simplified as: w = π × 3 5 × 32 3 × U5 × A12 / 3A2 × t1 + γ E11
  3. cavitation decreases (see Figure 1). When a critical tension is reached where the bubble size is equal or smaller than the pore size, a bubble of water vapor could spontaneously form and grow without being suppressed by pore walls. For the spherical pore the critical tension (P t*) is given by jj¼P a P* t 2s r 1: ð4
  4. Thoma's cavitation number is also a notable parameter for cavitation performance. It is derived as the ratio of NPSH to pump (or impeller) head. H σ=NPSH (3) Like the specific speed, which is a parameter to show the type of a rotor, suction specific speed S can represent the cavitation performance of a pump. Its definition is given by.

Cavitation in lipid bilayers poses strict negative

  1. g average acoustic and thermal parameters for soft.
  2. Cavitation may appear, due to a decrease in flow pressure at the reference point or an increase in the Δp-value. Cavitation begins to occur for C v values below the threshold value, C v *. In most of studies it was observed that the cavities formation started at C v equal to 1
  3. tions in use, some where the cavitation intensity increases with the cavitation number (2, 3) and some where it decreases (4, 5). However, each definition includes the ratio of a factor tending to create cavitation, such as average flow velocity or pressure drop, to a factor tending to suppress it, such as downstream pressure
  4. Introduction. Since Milburn and Johnson found that cavitation in plants can be detected by acoustic methods, cavitation and embolism have been intensively studied (Tyree and Sperry 1989, Milburn 1993).Many studies have confirmed that cavitation in xylem conduits is induced by air seeding and the hypothesis has been tested by experiments (Lewis 1988, Salleo et al. 1992, Cochard et al. 1992)

If the cavitation number is higher than the critical cavitation number, the average value of the core length is 2.7 d. For lower cavitation numbers the average value of the core length is 1.8 d. The core length is generally depending on the internal velocity profile and turbulence inside the nozzle (Carlomagno and Ianiro 2014 ) Pumps and compressors are frequently characterized by complex geometries, motion and physical phenomena—such as cavitation, pressure pulsation and vibration, all of which affect the efficiency and reliability of the device. Taking these factors into account is critical to designing an optimal machine, and computational fluid dynamics (CFD) is a tool that can streamline the process @article{osti_5086928, title = {Cavitation in liquid cryogens. Pt. 4. Combined correlations for venturi, hydrofoil, ogives, and pumps}, author = {Hord, J}, abstractNote = {Previous volumes contain basic analyses and experimental data for a venturi, a hydrofoil, and 3 scaled ogives. Cross-correlation of the developed cavity data for these 5 hydrodynamic bodies is performed in this report Cavitation in such water will in principle be heterogeneous cavitation, but with gas molecules being ubiquitous it resembles homogeneous cavitation. Values of tensile strength of the order of 300 bar, measured by focused high-intensity ultrasonic wavepackets, are probably set by such nuclei (Sankin & Teslenko Reference Sankin and Teslenko 2003.

A review of cavitation in hydraulic machinery - ScienceDirec

a) Cavitation Due to a very high inlet pressure and almost saturated water at the outlet, a mu tistage valve 's required with a very high pressure recovery factor (F to avoid cavitation. Changes of direction within the valve also contribute to increase the F value. b) Pressure drop distribution The F of a multistage valve with n stages is given The variation amplitude of radial force at the critical cavitation conditions is much larger than that at the non-cavitation conditions. The maximum values and minimum values of radial force on the impeller are 62.2 and 10.8 N at the non-cavitation conditions, while 78.5 and 3.3 N at the critical cavitation conditions

NPSH refers to two quantities: NPSH available (NPSHa): a measure of how close the liquid at a given point is to boiling, and so to cavitation. NPSHa is usually calculated at the suction flange of the pump. NPSH required (NPSHr): the head value at a specific point (usually the inlet of a pump) required to keep the liquid from cavitation in a pump the pump. This is critical, of course, but acceleration head is another important factor to consider. Frequently, pump inlet conditions are not given proper consideration in system design. With positive displacement pumps, especially reciprocating types, it is crucial to include a complete design analysis of pump inlet conditions This model can also tell if noise is being produced in a valve just due to turbulent flow, or cavitation. For values of An, see ; it is normally -4.6 for global valves, -4.3 for butterfly valves, and -4.0 for expanders. This model was checked against three examples in ; they match to all given decimals F critical value: F statistic is a statistic that is determined by an ANOVA test. It determines the significance of the groups of variables. The F critical value is also known as the F -statistic. The F - statistic value is obtained from the F-distribution table.Decisions are made based on the F-critical value.The F-distribution is always a right-skewed distribution

According to a sweeping 2016 survey, there were, at that time, more guns in the hands of Americans than there are cars on the road.Of the nearly 250 million guns owned by Americans at that time, somewhere between 40 and 50 million of them were handguns. Of handgun owners alone, 63 percent owned these for the purpose of protecting themselves against other people, with another 20 percent owning. When finding the critical value, confidence level will be given to you. If you are creating a 90% confidence interval, then confidence level is 90%, for 95% confidence interval, the confidence level is 95%, and so on. Here are the steps for finding critical value: Step 1: First, find alpha (the level of significance). \alpha =1 - Confidence. Calculated values of F F can range between 0.68 and 0.96, depending on the ratio of vapor pressure to critical pressure. Lines 2a and 2b show calculation of the value of F F for this application, which is 0.86. Lines 3 and 3a calculate the value of F F P V to be used in the equation on Line 1 Fp = piping geometry factor (reducer correction) FF = liquid critical pressure factor = 0.96 - 0.28 pv pc FL = liquid pressure recovery factor for a valve FLP = combined pressure recovery and piping geometry factor for a valve with attached fittings Ki = velocity head factors for an inlet fitting, dimensionless pc = pressure at thermodynamic. 4.2. Results at Cavitation Condition. Cavitation usually has a great influence on performance of model pump, and Figure 6 presents total delivery head coefficient versus cavitation number from flow rate 0.9 to 1.4.At higher value of cavitation number, total delivery head of model pump almost remains substantially constant

cavitation model which can effectively correlate creep cavities with the remaining life of the components. Efforts have been made in the past, but till now, the methods already (which is the critical value of Put the values in the A-D relation given below: (D / D cr) * [1 - (1 - D / Dcr The crest factor of a sine wave is 1.414; i.e. the peak value is 1.414 times the RMS value. A typical vibration signal from a machine with a large imbalance will have a crest factor similar to this, but as the bearings begin to wear, and impacting begins to happen, the crest factor will become much greater than this For a given value of specific energy, the critical depth gives the greatest discharge in an open channel, or conversely, for a given discharge, the specific energy is a minimum for the critical depth. So at a control section, the discharge can be calculated once the depth is known e.g. The critical depth meter value of the blade tip clearance, very few flow instabilities have been detected on the inducer, including: steady asymmetric cavitation caused by the different extension of the cavitating regions on the blades; cavitation surge at a frequency equal to 0.16 times the inducer rotational frequency; a higher-order axia Also shown is the actual value of K for a nozzle outlet pressure of 6 MPa. In this example, cavitation occurs when the nozzle hole inlet pressure exceeds a relatively modest 45 MPa [2209]. Figure 13. Critical (K crit) and actual (K) cavitation numbers at different nozzle hole inlet pressures Nozzle outlet pressure of 6 MPa

Critical Pressure - an overview ScienceDirect Topic

Given large unit discharge and cavitation coefficient, running without cavitation requires a larger suction height. It is a failure to exclude blade channel vortex beneath 60% of Pr due to relatively less energy produced by unit water head Cavitation refers to the process of formation, growth, and collapse of vapor or gas cavities (cavitation bubbles) in the liquid where the local pressure drops below a critical value. During the collapse process of the cavitation bubble, high local temperature and pressure are formed, accompanied by a strong shock wave and high micro-jet velocity

Cavitation - Wikipedi

Design and analysis of control valve with a multi stage anti cavitation trim. Valves are the components in a fluid flow or pressure system that regulate. either the flow or the pressure of the fluid. This duty may involve stopping. and starting flow, controlling flow rate, diverting flow, preventing back For pumps, Thoma's cavitation factor is defined as or The cavitation will occur if the value of is less than the critical value, at which the cavitation just begin. Suction Specific speed Suction specific speed is another cavitation parameter Cavitation, the formation of vapor-filled bubbles in a liquid at low pressures, is a powerful phenomenon with important consequences in nature and technology. For instance, cavitation bubbles may interrupt water flow in plants under dry conditions or severely damage the metal surfaces of machines such as pumps and propellers. Using molecular simulations, we have studied cavitation in water at. At a given temperature, a liquid can coexist with its gas at a single value of the pressure. volume of the initially evacuated chamber is (a) less than the critical volume, (b) a factor of 10 larger than the critical volume, and (c) a factor Of 1.05 larger than the critical volume? This is the case because a gas and a liquid are in. It is the value of N.P.S.H.A., which includes pressure level and other factors, that will determine whether the pump will operate properly or not. The N.P.S.H.A. is compared to the N.P.S.H.R. (Net Positive Suction Head Required) which is given by the manufacturer for a specific pump at a specific speed and flow rate

Methods for measuring plant vulnerability to cavitation: a

Y is the expansion factor defined by: with choked flow situations exclusive to compressible fluid flows, thereby eliminating some of the inaccuracies of the other, derived methods. Typical values of XT for solenoid valves can range from 0.25 to 0.5, the lower value applying to valves with more tortuous flow paths Cn1 is the critical value of \(C^{\prime}_n\) at leading-edge separation for positive AoA and should be extracted from airfoil data at a given Reynolds number and Mach number; Cn1 can be calculated from the static value of Cn at either the break in the pitching moment or the loss of chord force at the onset of stall; Cn1 is close to the. uted to the onset of cavitation. At an NPSH value of 23 m the sudden drop in AE r.m.s. was at-tributed to the attenuation caused by bubble clouds. The loss in AE intensity due to the presence of cavitation was noted by Neill [7, 8] and Derakhshan [6]. This is not surprising if cognizanc

(PDF) EXPERIMENTAL EVALUATION OF A FRANCIS TURBINE eSAT

Generally, there exists a critical feed pressure Pc necessary to prevent cavitation. This critical pressure Pc depends, on one hand, on factors essentially tied to the nature of the fuel at the temperature prevailing at the inlet of the pump, and on the other hand, on a factor which, for a pump of a given type, depends only on the feed rate ⇒ As the velocity V and thus the Reynolds number of a flow past a sphere increases from very low value, the drag force for Re << 1 increases linearly with V. decreases linearly with V. decreases as V 2. none of these. ⇒ For a given Reynolds number, in a hydraulically smooth pipe, further smoothening _____ the friction factor In view of this evidence, it is recommended that, for patients who have cavitation on the initial chest radiograph and whose 2-month culture is positive, the minimum duration of treatment should be 9 months (a total of 84--273 doses depending on whether the drugs are given daily or intermittently) (Figure 1 and Table 2). The recommendation to.

By rearranging his equation, a transient factor (F T) can be defined where a value equal to or greater than 1.0 indicates an acceptable design. The equation is: The equation is: F T = (M / m. Fill-finish operations must be designed with an awareness of the innate properties of proteins and external factors that can affect a given protein's behavior and stability (8, 10). Special processes, procedures, and equipment must be in place to ensure product integrity during fill-finish manufacturing

Two pumps, one each for fuel and oxidizer were tested for obtaining the critical Net Positive Suction Head for ensuring the cavitation free operation of these pumps in flight. The experimentally obtained values of NPSH have been compared with the theoretical values obtained using the model developed by Stripling (1962) † Cavitation noted on either chest radiograph or computed tomography (CT). For cases, cavitation was noted in 18 (10%) and 40 (21.5%) of chest radiograph and CT exams, respectively. For non-cases, cavitation was noted in 8 (4%) and 14 (7.5%) of chest radiographs and CT scans, respectively Crane Engineering noted a value of 50% capacity at 82.3% travel which is denoted by the dashed line in the capacity curve. The tabulated coordinates for this performance curve have been defined in the FluidFlow control valve database. The control valve performance curve is automatically generated by the software and can be viewed in Figure 2 effects, most importantly cavitation at inlet. Others are centrifugal stresses (either at the impeller or in the driving turbine), limiting peripheral speeds for bearing and seals, and avoidance of critical speeds. Head rise is used commonly instead of pressure rise to express the performance of pumps The critical low pressure must depend on the size of the bubble and its gas content. The severity of cavitation must depend. on the number of nuclei present in a unit volume of the fluid. Since nuclei size and concentration are at prese,nt us­ ually unknown, true modelling of cavitation is difficult and inception poin