EP0554640A1 - Method and device for the determination of the erosion caused by cavitation of fluid-traversed components - Google Patents

Abstract

Cavitation-caused erosion in fluid-traversed components such as a pump (1) can be derived by detecting the vibration of the pump casing. The cavitation-caused fluid-propagated sound is transmitted to the pump casing and is detected by means of a sensor (7b) of a solid-propagated sound measuring device (7a), is amplified in a signal processing unit (10), is filtered and digitised and transmitted to a computer (11). Further measured-data registration appliances such as the outlet pressure measuring appliance (6), the suction pressure measuring appliance (8), the fluid temperature measuring appliance (9) or the rotational speed measuring appliance (4) make it possible for the computer (11) to calculate, for example, the flow rate or the relationship between the flow rate and the erosion rate. The values determined can be displayed via the output unit (13), the output unit (13) activating an alarm, for example, when a limit value which can be preset via the input unit (12) is exceeded, for example. <IMAGE>

Description

The invention relates to a method for determining the cavitation-related erosion in fluid-flowed components according to the preamble of claim 1 and an apparatus for performing the method.

With pumps and other components through which fluid flows, cavitation occurs in the fluid conveyed under certain conditions. For example, at the leading edges of the impeller blades of a centrifugal pump, zones with negative pressure occur due to local overspeeds, which lead to the formation of vapor bubbles when the local pressure falls below the vapor pressure of the fluid being pumped. The vapor bubbles are flushed with the flow in zones of higher pressure, where they implode. This creates a very high pressure peak or a very high cavitation intensity locally, which can cause material corrosion or cavitation damage.

It is known to continuously estimate the hydrodynamic cavitation intensity or the cavitation-related erosion rate in components through which fluid flows, such as pumps, turbines, control or shut-off valves, by measuring the liquid sound. An empirically determined relationship between liquid noise and erosion rate allows the potential risk of cavitation damage to be assessed quantitatively.

A corresponding method with a device for determining the erosion rate can be found, for example, in "Guidelines for Prevention of Cavitation in Centrifugal Feed Pumps, EPRI GS-6398, Project 1884-10, Final Report, Electric Power Research Institute, Palo Alto, California, 1989" page 2 -24 and page B-9 are known.

The measurement of the liquid sound requires that a corresponding pressure measuring device must be introduced into the component through which the fluid flows and has direct contact with the fluid. The creation of a corresponding opening in the outer wall of the component through which the fluid flows is extremely problematic, particularly in existing plants, for example in the nuclear field, for safety reasons. For this reason, the known method for determining the erosion rate is unsuitable, for example, for short-term checks on existing systems. A further disadvantage of the known method is that the pressure measuring device has to be fixed in terms of flow technology within the component through which the fluid flows, in order to avoid measurement errors due to accumulating air bubbles.

It is therefore an object of the present invention to determine the erosion rate due to cavitation without the need for a sensor within the component through which the fluid flows.

This object is achieved according to the invention in that a structure-borne noise measuring device detects the structure-borne noise or the vibration of the outer wall of the fluid-flowed component and forwards it to a signal processing unit, and that a computer calculates the liquid noise from the processed signal, determines the erosion rate and uses it transmitted to an output unit.

The cavitation-related erosion in components through which fluid flows, such as a pump, can be derived by detecting the vibration of the pump housing. The cavitation-related liquid sound is transmitted to the pump housing and is detected by a sensor of a structure-borne sound measuring device, amplified, filtered and digitized in a signal processing unit and transmitted to a computer. Additional measurement data acquisition devices such as the outlet pressure measurement device, the suction pressure measurement device, the fluid temperature measurement device or the speed measurement device enable the computer to calculate, for example, the flow rate or the relationship between flow rate and erosion rate. The determined values can be displayed via the output unit, the output unit activating a warning, for example, when a limit value that can be specified via the input unit is exceeded.

The advantages of the invention can be seen in the fact that the determination of the erosion rate without intervention in the component through which the fluid flows and also also temporarily and temporarily, e.g. for test or control purposes.

The imploding vapor bubbles cause pressure waves which can be measured as liquid sound with a pressure measuring device. The pressure waves also set the outer wall of the component through which the fluid flows, for example a pump housing, in vibration, which manifests itself as structure-borne noise and can be measured, for example, by means of an accelerometer which is attached to the outside of the pump housing. Both the liquid sound and the structure-borne sound are dependent on the hydrodynamic cavitation intensity, and are therefore potential parameters for estimating the erosion caused by cavitation.

• a) Dampfblasen entstehen in der fliessenden Flüssigkeit
• b) die Dampfblasen gelangen in Zonen, in denen der lokale Druck den Dampfdruck übersteigt
• c) die Dampfblasen implodieren nahe einer festen Oberfläche
• d) die hydrodynamische Kavitationsintensität übersteigt den Kavitationswiderstand des Materials

Sobald implodierende Dampfblasen auftreten, können Druckwellen im Frequenzbereich von typischerweise über 10 kHz gemessen werden. Es ist daher eindeutig feststellbar ob die Bedingungen a) und b) erfüllt sind. Es ist bis heute jedoch keine Möglichkeit bekannt, mittels der Messung von Schallwellen festzustellen, ob Dampfblasen in der Nähe der Oberfläche implodieren und somit potentiell schädlich sind, oder ob die Dampfblasen innerhalb der Flüssigkeit implodieren, wo sie nicht schädlich sind. Die Unfähigkeit festzustellen, ob die Bedingung c) erfüllt ist, ist die gravierendste Einschränkung, wenn mittels der Messung von Schallwellen eine Kavitationsdiagnose durchgeführt wird.Cavitation damage only occurs if the following four conditions are met:

• a) Vapor bubbles form in the flowing liquid
• b) the vapor bubbles reach zones in which the local pressure exceeds the vapor pressure
• c) the vapor bubbles implode near a solid surface
• d) the hydrodynamic cavitation intensity exceeds the cavitation resistance of the material

As soon as imploding vapor bubbles occur, pressure waves in the frequency range typically above 10 kHz can be measured. It can therefore be clearly determined whether conditions a) and b) are met. To date, however, there is no known method of measuring sound waves to determine whether vapor bubbles implode near the surface and are therefore potentially harmful, or whether the vapor bubbles implode within the liquid where they are not harmful. The inability to determine whether condition c) is fulfilled is the most serious limitation when a cavitation diagnosis is carried out by measuring sound waves.

Nevertheless, a diagnostic system enables you to determine whether the hydrodynamic cavitation intensity exceeds the cavitation resistance of the material, and thereby to quantify possible damage.

Empirical studies have shown that there is a correlation between metal loss due to cavitation-related erosion and liquid noise.

wobei E_R der Erosionsrate (in mm/h) und CNL (Cavitation Noise Level) dem kavitationsbedingten Flüssigkeitsschallpegel entspricht.The known relationship is:

${\text{E}}_{\text{R}}\text{= f (CNL) (1)}$

where E _R corresponds to the erosion rate (in mm / h) and CNL (Cavitation Noise Level) to the cavitation-related liquid sound level.

In contrast, no correlation was found between the casing vibration CV (casing vibrations) and the erosion rate E _R.

beschreibt den Zusammenhang der spezifischen Erosionsleistung P_ER als Produkt der maximalen lokalen Erosionsrate E_R mit der werkstoffabhängigen Materialkonstante U_R. Empirische Untersuchungen ergaben eine Korrelation zwischen der spezifischen Erosionsleistung P_ER sowie der akustischen Flüssigkeitsschallintensität I_ac (Gleichung 3).

${\text{P}}_{\text{ER}}{\text{= C}}_{\text{N}}{\text{· F}}_{\text{cor}}{\text{/F}}_{\text{Mat}}{\text{· (I}}_{\text{ac}}{\text{/I}}_{\text{R}}{\text{)}}^{\text{x}}\text{ (3)}$

mit den empirischen Konstanten C_N und x.

F_cor:

Korrosionsfaktor

F_Mat:

Faktor für metallurgische Struktur

I_ac :

akustische Intensität

I_R :

Referenzgrösse

Die gemessene Beschleunigung der Aussenwand einer fluiddurchströmten Komponente, wie z.B. eines Pumpengehäuses, ist ein Mass für die Druckschwankungen des Fluids innerhalb der Komponente. Mit akustischen Methoden wie z.B. der statistischen Energieanalyse oder durch Bestimmen der Transferfunktion ist eine Ermittlung des Flüssigkeitsschalles NL auf Grund der Gehäusevibration CV möglich (Gleichung 4).

$\text{NL = f(CV) (4}$

)

Aus der Gehäusevibration CV lässt sich somit die akustische Intensität I_ac des Flüssigkeitsschalles bestimmen und daraus die entsprechende Erosionsleistung P_ER. Die Erosionsleistung P_ER und damit die Erosionsrate E_R lässt sich daher aus der Gehäusevibration CV berechnen.the equation

${\text{P}}_{\text{HE}}{\text{= UR · E}}_{\text{R}}\text{(2)}$

describes the relationship between the specific erosion performance P _ER as the product of the maximum local erosion rate E _R and the material-dependent material constant U _R. Empirical investigations showed a correlation between the specific erosion power P _ER and the acoustic liquid sound intensity I _ac (equation 3).

${\text{P}}_{\text{HE}}{\text{= C}}_{\text{N}}{\text{· F}}_{\text{cor}}{\text{/ F}}_{\text{Mat}}{\text{· (I}}_{\text{ac}}{\text{/ I}}_{\text{R}}{\text{)}}^{\text{x}}\text{(3)}$

with the empirical constants C _N and x.

F _cor :

Corrosion factor

F _Mat :

Factor for metallurgical structure

I _ac :

acoustic intensity

I _R :

Reference size

The measured acceleration of the outer wall of a component through which fluid flows, such as a pump housing, is a measure of the pressure fluctuations of the fluid within the component. With acoustic methods such as statistical energy analysis or by determining the transfer function, it is possible to determine the liquid noise NL based on the housing vibration CV (equation 4).

$\text{NL = f (CV) (4th}$

)

The acoustic intensity I _{ac of} the liquid sound and thus the corresponding erosion power P _ER can thus be determined from the housing vibration CV. The erosion performance P _ER and thus the erosion rate E _R can therefore be calculated from the housing vibration CV.

Fig. 1

eine schematische Anordnung einer Vorrichtung zur Bestimmung der kavitationsbedingten Erosion in einer Pumpe;

Fig. 2

eine schematische Anordnung einer Vorrichtung zur Bestimmung der kavitationsbedingten Erosion sowie der Durchflussmenge;

Fig. 3

eine schematische Anordnung einer Vorrichtung zur Beeinflussung der kavitationsbedingten Erosion einer Pumpe.

The invention is described below with reference to exemplary embodiments and application examples. It shows:

Fig. 1

a schematic arrangement of a device for determining the cavitation-related erosion in a pump;

Fig. 2

a schematic arrangement of a device for determining the cavitation-related erosion and the flow rate;

Fig. 3

a schematic arrangement of a device for influencing the cavitation-related erosion of a pump.

FIG. 1 shows a pump 1 which is driven by a motor 2 via a rotating shaft 3. The pump conveys a fluid from the suction line 5b to the pressure line 5a. A structure-borne noise measuring device 7 detects the vibrations of the outer wall of the pump 1, wherein the structure-borne noise measuring device 7 can be firmly connected to the outer wall or, for example, senses the vibrations of the outer wall without contact. The structure-borne noise measuring device 7 can rest on the outer wall or, for example, be inserted more or less deeply into the outer wall through a bore. The signal conditioning unit 10 processes the vibration signal CV, for example by amplifying, filtering and digitizing it. The computer 11, to which values can be predefined by the input unit 12 and which outputs the calculated values on the output unit 13, calculates the liquid sound NL from the processed vibration signal and, using the empirical correlation according to equation 3, the erosion power P _ER and using equation 2 the erosion rate E _R. The output unit 13 can also display, for example, the cumulative erosion, or when a predefinable threshold is exceeded, for Example of the erosion rate E _R , trigger a signal such as an alarm.

• spezifische Haltedruckenergie (NPSH, net positiv suction head) der Pumpe 1
• Förderstrom
• Flüssigkeitsschall
• aktuelle Erosionsrate
• Lasthistogramm
• Histogrammes mit Last und Erosionsrate
• kumulierte Erosion

Figur 3 zeigt die gleichen Messvorrichtungen wie Fig. 2. Die Drehzahl des Motors 2 ist über eine Stelleinrichtung 14 steuerbar, welche ihrerseits durch die Ausgabeeinheit 13 des Rechners 11 gesteuert wird. Dieser geschlossene Regelkreis erlaubt zum Beispiel, den Förderstrom der Pumpe 1 abhängig von der Erosionsrate zu bestimmen.
Ein geschlossener Regelkreis dieser Art erlaubt es zum Beispiel den Arbeitspunkt der Pumpe durch Verstellen der Motorendrehzahl oder zum Beispiel durch Verstellen der Stellung eines Ventils so zu verändern, dass die Pumpe in einem Bereich mit geringerer Kavitationsgefährdung arbeitet.FIG. 2 has additional measuring devices compared to FIG. 1, such as an outlet pressure measuring device 6, a suction pressure measuring device 8, a fluid temperature measuring device 9 and a speed measuring device 4. The additional measuring devices compared to FIG. 1 allow the computer 11 to calculate the current delivery flow of the pump , the calculation of the operating point of the pump, and the conversion of the delivery pressure to a reference pressure with a reference speed of the shaft 3. The evaluation of the data provided by the signal processing device 10 allows the computer 11 to calculate the following variables, for example:

• specific holding pressure energy (NPSH, net positive suction head) of pump 1
• Flow rate
• Liquid sound
• current erosion rate
• Load histogram
• Histogram with load and erosion rate
• accumulated erosion

FIG. 3 shows the same measuring devices as in FIG. 2. The speed of the motor 2 can be controlled via an adjusting device 14, which in turn is controlled by the output unit 13 of the computer 11. This closed control loop allows, for example, the flow rate of pump 1 to be determined depending on the erosion rate.
A closed control loop of this type allows, for example, the operating point of the pump to be changed by adjusting the motor speed or, for example, by adjusting the position of a valve, so that the pump operates in an area with a lower risk of cavitation.

The exemplary embodiments and application examples described can of course also be implemented if the liquid noise NL is directly available via a pressure measuring device.

Claims (12)

Method for determining the erosion caused by cavitation in components through which fluid flows, such as pumps (1) or fittings, characterized in that
that an empirically determined relation between liquid sound and erosion rate is used,
that the vibration or structure-borne noise of the outer wall is detected at at least one point with a measuring sensor (7b) which is located outside the component (1) through which fluid flows, and
that the liquid sound and the erosion rate are determined from this measurement signal. Method according to claim 1, characterized in that the liquid sound is determined from the vibration or structure-borne sound signal using the method of statistical energy analysis known from acoustics. Method according to Claim 1, characterized in that the liquid sound is determined from the vibration or structure-borne sound signal by means of a transfer function. Device for determining the erosion caused by cavitation in components through which fluid flows, such as a fluid-conveying pump (1) which is driven by a motor (2) via a shaft (3), characterized in that a measuring device (7a) with a measuring sensor (7b) detects the Structure-borne noise or the vibration outside the fluid is recorded and forwarded to a signal processing unit (10), and that a computer (11) uses numerical methods to calculate a quantity representing the liquid sound from the processed signal, from this the erosion rate determined and transmitted to an output unit (13). Device according to claim 4, characterized in that the sensor (7b) of the measuring device (7a) is coupled to the component (1) through which fluid flows within the outer wall. Device according to claim 4, characterized in that the sensor (7b) of the measuring device (7a) is coupled to the outer wall of the component (1) through which fluid flows. Device according to claim 4, characterized in that the sensor (7b) of the measuring device (7a) measures the vibration or structure-borne noise of the outer wall of the component (1) through which fluid flows. Device according to one of claims 4 to 7, characterized in that the signals conditioning unit (10) additionally detects, amplifies and digitizes the signals of an outlet pressure measuring device (6), a suction pressure measuring device (8) and a fluid temperature measuring device (9) be such that the flow rate can be determined with the aid of the computer (11). Device according to one of claims 4 to 8, characterized in that a speed measuring device (4) passes the speed of the shaft (3) via the signal conditioning unit (10) to the computer (11). Device according to one of claims 4 to 9, characterized in that when the erosion rate exceeds a threshold value which can be predetermined by the computer (11), the output unit (13) draws attention to this. Device according to one of claims 4 to 10, characterized in that when a predeterminable erosion rate is exceeded, the output unit (13) changes the delivery flow through the component (1) through which fluid flows via an adjusting device (14). Fluid pump monitors, controls or regulates with a method according to one of claims 1 to 3 or with a device according to one of claims 4 to 11.

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