The method of determining the onset of cavitation in control valves (options)

 

The invention relates to the field of hydraulics and are intended for use in the development of valves (RK) and hydraulic valves for various purposes, as well as in the study of hydraulic characteristics of different local resistance. The method of determining the onset of cavitation in the regulating valve is that the measured pressure at the inlet and outlet of the valve at a fixed position of the shutter and the change of pressure drop on it. The modes are held at constant values of pressure at the inlet of the RK with the determination of the vibrational characteristics of the valve as a function of differential pressure on it when hydrodynamic and cavitation modes. Coordinate deviations of the latter from each other is fixed corresponding to the critical pressure drop across the valve. The critical difference and the pressure values at the input of the Republic of Kazakhstan and the saturated vapor of the liquid at the temperature of the stream depends on the actual value of the coefficient cavitation. The second option, determine the actual values of the coefficient cavitation repeats all the steps of the first method, except that instead of the vibrational characteristics of the valve takes the acoustic noise level of the valve. Ukazannayav change of Reynolds number with the aim of developing cavitation free RK. 2 C. p. F.-ly, 3 ill.

The invention relates to the field of hydraulics and can be used in the development of control valves (RK) and hydraulic valves for various purposes, as well as the study of the hydraulic characteristics of different local resistance (hereinafter they are United by the term RK).

Criteria that define the boundaries of the onset of cavitation in the Republic of Kazakhstan, is one of the most important indicators of how the features of the current environment, and reliability characteristics of a particular RK, and therefore the numerical values entered in forms or passport of the finished product.

There is a method that eliminates cavitation in the contact area seat stem seat body dvuhsezonnogo valve implemented by RF patent 2056562, namely, that cavitation free regime, the saddle stem is provided by the reduction of pressure drop on their regulatory edges by triggering the most part differential on special chokes operating in the cavitation mode.

The disadvantages implemented in the valve of the method include: - the inability of the actual definition of cavitation free drop on the seat core chokes from the common peripads orifice, working in the cavitation mode, which will lead to instability of the hydraulic characteristics of the valve; - the presence of cavitation in non-profiled chokes, and therefore (in addition to their cavitation wear) inevitable vibration and acoustic noise of the valve as a whole; - the problem of maintaining health through periodic replacement of destroyed by cavitation of the elements of the valve - choke.

Known hydraulic ways to avoid cavitation process in the Republic of Kazakhstan and local hydraulic resistances, in ensuring the implementation of which requires that the flow parameters or changes in the Republic of Kazakhstan met the requirements of the specific numbers or coefficients cavitation.

The first of them (cavitation) is determined by the values of critical cavitation numbers (representing twice the value of Euler's criterion), which is a sharp increase in the values of drag coefficients RK, [1], Arzumanov E. C. Cavitation in local hydraulic resistances. M., "Energy", 1978, page 144, Fig.4-32.

The most common practical use and closest to the technical essence is a hydraulic method of determining coefficients cavitation KS RK, in which the fixed poliziotti(flow rate in function of the square root of the magnitude of the pressure drop) from the linear law are determined by the values of the above coefficients cavitation KS; [2] RTM 108.711.02-79. The energy valve. Methods of determining the bandwidth of the regulatory bodies and the choice of optimum discharge characteristics, 1979, page 33, damn.2.

The disadvantages of these methods is based on the premise that the appearance of cavitation and the corresponding vapor bubbles increases the hydraulic resistance of the RK with the corresponding violation of the linearity of the quadratic expenditure characteristics. However, the linearity of the dependenceobserved only at constant hydraulic resistance RK in cavitation free regime, i.e., the application of known methods is limited to the region of turbulent flow environment (at values of Reynolds numbers exceeding specific RK value).

In practice, there are various of Kazakhstan, including its relatively large size, in which the onset of cavitation process does not change the hydraulic resistance in the range, the value of which it is possible to measure even under laboratory conditions. In addition, znachitelbnogo mode), where even when cavitation free flow environment is not observed constancy of the values of the coefficients of hydraulic resistance when changing the differential pressure on the ROK, [3], Blagov E. E., ivnitskii B. Y. Throttle valves in the energy sector. M., "Energy", 1974, page 38.

I.e., the known hydraulic methods have very limited scope of their potential use. This is confirmed encountered in practice, the discrepancy obtained by the above methods, numbers and ratios actual cavitation processes in various local hydraulic resistances and RK. For example, in [3], page 41 States that: "often the working environment in the valve coincides with a transition region". In [1] , page 161 with reference to the work of several authors noted that "the beginning of the deviation of the discharge characteristics from the quadratic law corresponds to the harsh conditions of cavitation, in which there is significant vibration, noise and erosion destruction of the material. Thus, when designing RK for cavitation free operation to use the values of the coefficients cavitation COP with the determination of the critical differencepCAWviolation of lesto maximum destruction of material RK and connecting pipelines. The actual cavitation occurs when the costs and the pressure drops below the point of deviation from the linear law."

Object of the present invention is to provide a method for determining the coefficients of cavitation valves of different DN when working at any time (without limitation) the range of variation of Reynolds number with the aim of developing cavitation free RK.

It is known ([1], section 1.2) that the root cause cavitation in any local resistance and RC is the process of "collapsing" high speed gas-vapor bubbles formed in the zone of the flow part with minimal pressure, and release of a large amount of energy, which causes the vibration of the flowing parts and chassis RK. I.e., the most reliable way to determine both starting and changes in the cavitation process is the specification of the nature of the change in the vibration of the Republic of Kazakhstan.

Proceeding from the above, to solve this problem is proposed a method of determining the onset of cavitation in the Republic of Kazakhstan, which consists in the fact that the measured pressure at the inlet and outlet of the valve at a fixed position of shutter RK and the change of pressure drop on it, in which the modes are held at constant values of pressure medium is dinamicheskom and cavitation modes, coordinate deviations of which from each other is fixed corresponding to the critical pressure drop across the valve, and the pressure values at the input of the Republic of Kazakhstan and the saturated vapor of the liquid at the temperature of the stream depends on the actual value of the coefficient cavitation; the way in which the modes are held at constant values of pressure at the outlet RK with determination in addition to vibrating the dependency levels of acoustic noise of the valve as a function of differential pressure on it when hydrodynamic and cavitation modes, coordinate deviations of the levels of acoustic noise from each other is fixed corresponding to the critical pressure drop across the valve, which the pressure values at the input of the Republic of Kazakhstan and the saturated vapor of the liquid at the temperature of the stream depends on the actual value of the coefficient cavitation.

In the first of these modes at a certain constant pressure p2at the outlet of the Republic of Kazakhstan and the change of the differential pressure on it due to the increase in pressure before the RK - Fig.1 (p1in cavitation free modes, fixed relationship values of vibration levels LinRK as a function of pressure drop on it Lin= f(p). Analogchannel mode. The latter is provided by the corresponding decrease in the pressure after RK (p2).

Coordinate deviations specified dependency Lin= f(p) from each other is determined by the corresponding critical pressure drop on RKpCAW= (p1-p2). The values of the above parameters by the following formula (2) is calculated the value of the coefficient cavitation KSA RK for a given position of the shutter.

Thus, in the practical implementation of the proposed method in an arbitrarily chosen point of the shell RK are defined on a fixed frequency (for example, f=10 kHz) functions of the form: Lin= f(p), (1) where Linthe vibration levels in dB;p is the pressure drop in the Republic of Kazakhstan in kgf/cm2when: - hydrodynamic mode of the Republic of Kazakhstan without cavitation;
- cavitation (mode) for environments with a fixed value of pressure at the outlet RK (Fig.2),
coordinates reject them, as noted above, the fixed values of the critical differences and then to calculate from these data the coefficients cavitation KSA. The specified calculation is performed according to the formula, the structure of CII, defined the proposed vibration method;
p1and R2the absolute pressure on the inlet and outlet RK, kgf/cm2;
pCAWthe ultimate pressure drop in the Republic of Kazakhstan, which is cavitation free flow, kg/cm2;
pnthe vapor pressure of the liquid at the temperature of the flow, kg/cm2.

In Fig.2 shows the changes in the dependency Lin= f(p) one of RK (operating in the transition region of Reynolds numbers) when hydrodynamic (p230 kgf/cm2and cavitating flows environment (the latter obtained with fixed values of pressure at the outlet RK R2=5,0; 10 and 20 kgf/cm2).

Point on the curve Lin= f(p) characterizing the hydrodynamic flow regime in the Republic of Kazakhstan (described by a logarithmic equation), which begins the process of cavitation, are recorded aspCAWthe ultimate pressure drop in the Republic of Kazakhstan, which is cavitation free flow. On the obtained values of p1andpCAWby the formula (2) is determined by the vibratory cavitation coefficient KS is m for environment resulting vibration is the root cause acoustic (air) noise Landcreated RK, the value of which for specific provisions shutter RK and pressure drop on it can be fixed corresponding measuring microphone. An example of such a scheme for measuring the level of acoustic noise is shown in Fig.3-31, page 104 [1].

Taking into account the dependence of the levels of acoustic noise from the vibration levels of RK obtained in this case, the nature of the change dependencies La= f(p) is almost identical to the character discussed above vibration dependencies Lin= f(p). As an example in Fig.3 shows the results of determining the vibration and acoustic dependencies Lin= f(p) and La= f(p) PK kladovoi design, cylindrical "skirt" of the shutter which (in one plane) implemented nozzles 18 M4. In Fig.3 scale of values, levels of acoustic noise caused by the axis of ordinate on the left side, and vibration levels from the right. Experimentally obtained point dependenciesin= f(p) is applied in the form of dots and fine lines, and dependenciesa= f(p) in the form of triangles.

The analysis presents the La= f(ppo) when turbulent flow with sufficient practical accuracy describes logarithmic equations, on structure similar to that discussed above for vibration curves Lin= f(ppo- type equation (1).

2. The difference between dependency La= f(ppoand Lin= f(ppo) on the y-axis of Fig. 3 explains the structure of expressions, which are determined by the values of the levels of vibration and acoustic waves.

3. For fixed values of pressure p2with increasingppoon the PK with a certain difference, defined aspCAWappears cavitation, leading to increase not only vibration, but, respectively, and the level of acoustic noise, the magnitude of which increases with increasing pressure drop.

4. Given equidistantly dependencies La= f(ppoand Lin= f(ppo), and that in accordance with the above formula Stiles (2) the numerical values of the coefficients cavitation KSA determine the systematic noise La= f(ppo).
In the case of the data of Fig.3 is a critical differencepCAW= (p2-p1) = 13,12 kgf/cm2where respectively KSA=0,69.

Again, it must be noted, however, that the acoustic noise is a consequence of the vibration of the housing of the test device and its constituent elements, i.e., the secondary criterion, which may introduce some error in the results obtained. In addition, the correctness of the measurement of acoustic noise level is fraught with difficulties due to possible interference and interference of sound waves in the measuring room (which is excluded when measuring vibration characteristics). At the same time, the acoustic method for determining the numerical values of the coefficients cavitation KSA is quite simple to implement.


Claims

1. The method of determining the onset of cavitation in the regulating valve, which consists in the fact that the measured pressure at the inlet and outlet of the valve at a fixed position of the shutter and the change of pressure drop on it, characterized in that the modes are held at constant values of pressure at the inlet of the RK with the definition of vibrationmaddened deviation of which from each other is fixed corresponding to the critical pressure drop across the valve, which the pressure values at the input of the Republic of Kazakhstan and the saturated vapor of the liquid at the temperature of the stream depends on the actual value of the coefficient cavitation.

2. The method of determining the onset of cavitation in the regulating valve, which consists in the fact that the measured pressure at the inlet and outlet of the valve at a fixed position of the shutter and the change of pressure drop on it, characterized in that the modes are held at constant values of pressure at the inlet of the RK with the definition of the levels of acoustic noise of the valve as a function of pressure drop when hydraulic and cavitation modes, coordinate deviations of which from each other is fixed corresponding to the critical pressure drop across the valve, and the pressure values at the input of the Republic of Kazakhstan and the saturated vapor of the liquid at the temperature of the stream depends on the actual value of the coefficient cavitation.

 

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