CN101750258B - Method and system for detecting cavitation of pump and frequency converter - Google Patents

Abstract

A method and a system for controlling a pump by means of a frequency converter. The method involves controlling a pump (4) with a frequency converter (2), where the frequency converter feeds a motor (3) connected to drive the pump. A torque estimate (Test) and/or a rotational speed estimate (nest) of the motor are provided from the frequency converter. Features (1, 2, 3, 4) indicating cavitation or likelihood (1, 2, 3, 4) of cavitation of the pump (4) and/or reverse flow of the pump (4) are formed using the provided estimates (Test, nest). The cavitation or likelihood of cavitation is detected from the formed features (1, 2, 3, 4). An estimated volumetric flow is estimated from direct components of the estimates (1, 2, 3, 4).

Description

For detection of method and system and the frequency converter of the cavitation of pump

Technical field

The present invention relates to a kind of method and system of cavitation of testing pump, relate in particular to a kind of method and system, utilize this method and system can in the situation that not adding measurement, detect the cavitation of the pump of controlling with frequency converter.

Background technology

The known problem relevant with pump is the trend that cavitation occurs for they.Cavitation refers to such situation: in this case, the suction of pump is depressed and dropped under the value that the liquid that will carry out pumping starts vaporization, drops under the vapour pressure of liquid.This phenomenon produces bubble, once these bubbles enter the region of higher pressure in pump, unexpected crumbling and fall will occur.The unexpected variation of getting back to liquid phase from gas phase causes unexpected pressure change, and this causes audible noise, and may damage the mechanical part of pump.

The importance relevant to pumping process to the detection of cavitation or cavitation possibility.If the risk of cavitation or cavitation can be detected, will greatly reduce so the mechanical wear of pump, and can in larger workspace, operate safely pump.

Pump (such as centrifugal pump) often utilizes the speed change driver with frequency converter to control, and described frequency converter provides controlled voltage to motor.The axle of described motor is connected to pump, thereby provides mechanical output for the running of pump.

Detect the known publication of cavitation and comprise United States Patent (USP) 6,757,665, wherein propose: the frequency spectrum that utilizes power pack independently to observe to make the stator current of the motor that pump rotates.The value of the measurement of the method based on such: to these values, some known features appear in pump when approaching cavitation or just cavitation occurring.The method is not considered working point or pumping process itself.

The scheme based on model for detection of the another kind of method of cavitation.In this method, from the electricity or mechanical parameter of motor and pump, for described system forms system model.Model be input as for example motor current, voltage and frequency.As at United States Patent (USP) 6,918, propose in 307, the pressure head (head) that described pump model can transmit produced volume flow rate (volumetric flow rate) and it is estimated.If measurement volumes flow rate and pressure differential (pressure head) simultaneously can be that these two amounts are determined error variance.Based on described error variance, can determine abnormal in pump behavior, and can diagnose possible fault.The shortcoming of the method is to have additional measurement, is used for producing described error variance.This measurement requires to have additional transformation device (transducer), and this is just because the cost of installing, safeguarding and connect up produces further cost.Described transducer is also potential danger for the reliability of whole system, because this transducer is mechanical part, can under state that may be harsher, work.The fault of a transducer can make cavitation to be detected.In addition, this transducer is difficult to change, and may cause the longer shut-down in pumping process.

United States Patent (USP) 6,663,349 disclose a kind of method of cavitation for detection of pump or cavitation possibility.In this method, determine the required pressure head (NPSH that only just sucting according to the value obtaining from sensor _r, net positive suction head required) and effectively only just sucting pressure head (NPSH _a, net positive suction head available).More describedly requiredly only just suct pressure head and effectively only just sucting pressure head, and relatively carrying out to determine the possibility of cavitation based on this.The problem relevant with the method is also that requirement has additional survey sensor or transducer.

Summary of the invention

The object of the invention is: provide a kind of method and for realizing described method so that the system addressing the above problem.Object of the present invention is by realizing with the method and system that is claimed as feature in independent claims.The preferred embodiments of the present invention are disclosed in the dependent claims.

The present invention is based on such thought: the estimated value based on directly obtaining from the frequency converter of driving pump forms one or more indexs, these indexs relate to the possibility that in pump, mercury vapour loses or flows backwards.Specifically, the estimation moment of torsion that these indexs are produced by motor and motor estimate that rotating speed forms.The detection of cavitation also requires some parameters relevant with the pump using to pumping process.

The advantage of method and apparatus of the present invention is: without any need for additional measurement in the situation that, can detect reliably cavitation, approach cavitation or refluence situation.Therefore, the present invention has eliminated the demand of the sensor to measured process variable.

According to a preferred embodiment of the invention, carry out the detection of cavitation or cavitation possibility by multiple indexs, these indexs are the estimated value based on from frequency converter all simultaneously.The use of more than one index makes described detection more reliable, and does not substantially have any extra cost.

The present invention also relates to a kind of frequency converter, it is used for carrying out method of the present invention.

Brief description of the drawings

By preferred embodiment, the present invention is described in further detail below with reference to the accompanying drawings, in the accompanying drawings,

Fig. 1 illustrates the pumping system that frequency converter drives;

Fig. 2 is block diagram of the present invention;

Fig. 3 a is the curve map relevant with the working point of determining pump with Fig. 3 b;

Figure 4 and 5 illustrate the measurement result relevant with the present invention;

Fig. 6 illustrates the pump relevant to process variable;

Fig. 7 illustrates moment of torsion and the AC RMS level of rotating speed estimation and the funtcional relationship of volume flow; And

Fig. 8 illustrates funtcional relationship measured and estimated pressure ratio and volume flow.

Embodiment

Fig. 1 illustrates the basic structure of the pump being driven by frequency converter.Frequency converter 2 is connected to supply network (supplying network) 1 via three-phase cable.Frequency converter is also connected to motor 3, and motor 3 then be mechanically connected to pump 4.Frequency converter is with the mode control motor of expectation and the rotation of pump.Frequency converter is also connected to automated system via interface 5.Automated system can be the controller of higher level, the process that control pump is associated.Therefore, automation interface provides the operational order of pump, is managed to realize the operation of pump by frequency converter.In Fig. 1, omit all measurements to system, in the mode without sensor, motor and pumping process are controlled.

Fig. 2 is the block diagram that presents the process of carrying out in the present invention.In the present invention, frequency converter provides moment of torsion to estimate T _estestimate n with rotating speed _est.Modern frequency converter is equipped with control system, and described control system is used electric notor model.Among other value, these motor models use and produce motor torsional moment and the rotating speed of estimating generation.Some control programs also, using moment of torsion as with reference to value, make it possible to carry out direct moment of torsion control.

According to the present invention, the frequency converter that pump is controlled provides the moment of torsion of motor to estimate T _estestimate n with rotating speed _est.Because motor is mechanically connected to pump, therefore described estimation is also described pump operated.As mentioned above, these values are ready-to-use in the control system of frequency converter.

In addition, in the method, form one or more features according to provided estimation, the refluence (reverse flow) of these feature instruction cavitations of pump or the possibility of cavitation and/or pump.The feature that can obtain from described estimation will be described in detail below.

In the time having formed one or more indicative character, detect cavitation or cavitation possibility with the pump of frequency converter control.In the situation that detecting cavitation or not detecting cavitation, also can detect refluence.In the decision box 21 of Fig. 2, carry out and detect.Decision box 21 receives moment of torsion based on estimating and one or more features of calculating of velocity gauge as input.In Fig. 2, the number of the feature of inputting is 4.

According to the embodiment of the present invention, compare to form the feature of indicating cavitation or cavitation possibility and/or refluence by the RMS value of alternating component (AC) and the nominal RMS value of the alternating component that described moment of torsion is estimated that described moment of torsion is estimated.Fig. 2 illustrates this indicative character, as feature 1.The calculating of this feature starts from frame 22 torque T of estimating _estcarry out bandpass filtering.The passband of described bandpass filter is for for example from 0 to 10Hz, as shown in Figure 2.Therefore, the value T after bandpass filtering _accomprise the low-frequency alternating component content of described estimation, but do not comprise DC component.Described AC component T _acfurther delivered to frame 23, frame 23 calculates effective value or the RMS value T of AC component _{ac, RMS}.

The AC component of the moment of torsion of estimating can be by described calculating below., also DC component is calculated, its use in described method will be elaborated below meanwhile.Can from the sample data of x (x is the general expression of any variable (such as moment of torsion)), remove DC component by the Hi-pass filter for example with extremely low cutoff frequency.Selectively, can be according to sample data [x ₀... x _n-1] calculating mean value, DC component value when this value correspondence sampling:

$x_{\mathrm{dc},\mathrm{est}}=\frac{1}{n}\underset{i=0}{\overset{n-1}{\mathrm{\Σ}}}{x}_i$

Then obtain the AC component of variable x:

x _ac＝[x ₀...x _n-1]-x _dc，est

The effective value of described AC component or RMS value can be calculated with the general formulae of RMS value (r.m.s.):

$x_{\mathrm{ac},\mathrm{RMS}}=\sqrt{\frac{1}{n}\underset{i=0}{\overset{n-1}{\mathrm{\Σ}}}{x}_{\mathrm{ac},i}^2}=\sqrt{\frac{x_{\mathrm{ac},0}^2+x_{\mathrm{ac},1}^2+...+x_{\mathrm{ac},n-1}^2}{n}}$

Once calculate the RMS value of AC component, by the nominal value T of the RMS value of itself and described AC component _{ac, N}compare.Can use nominal or the representative value that detect and store the RMS of AC component before the present invention, or, detect and store nominal or the representative value of the RMS of described AC component during can using this method in the situation that pump is operated in nominal operation point certainly.

Comparison between the RMS value of carrying out nominal RMS value and calculate in the frame 24 of Fig. 2, described comparison represents with the form of the ratio of surveyed RMS value and nominal RMS value.Fig. 4 illustrates measurement result, has indicated this ratio calculating along with effectively only just sucting pressure head and the required ratio (NPSH that is only just sucting pressure head _a/ NPSH _r) variation, wherein, NPSH in this manual _a/ NPSH _rbe known as pressure ratio (pressure ratio).

For fear of cavitation, described pressure ratio should at least be greater than 1, because NPSH _rrepresent this situation: the pressure head wherein being produced by pump has declined 3%.As shown in Figure 4, in the time that described pressure ratio declines, ratio T _{ac, RMS}/ T _{ac, N}start index formula increases.Utilize sensor to measure measured data point and described pressure ratio, so that the availability of this method to instruction cavitation to be shown.Carry out measurement data points with different volume flows, as shown in the legend in Fig. 4.In Fig. 4, also draw exponential fitting curve.

In the time that pump approaches cavitation, ratio T _{ac, RMS}/ T _{ac, N}increase, this be because: when cavitation starts or when generation of flowing backwards, the duty of pump becomes discontinuous.This point can see from the axle of pump, and moment of torsion ripple (AC component) has increased.In other words, the needed power of pump vibrates.Result is that, compared with nominal case, the RMS value of low frequency AC component increases.Therefore, can the feature 1 based on shown in Fig. 2 determine cavitation or refluence.

According to another embodiment of the invention, compare formation indicative character by the RMS value of alternating component and the nominal value of the alternating component that rotating speed is estimated that rotating speed is estimated.Described rotating speed is estimated as described moment of torsion is estimated, to be used for estimating in a similar manner cavitation or refluence.With reference to figure 2, estimating speed n _estbe given to bandpass filter 25.The AC component n estimating _acfurther delivered to RMS frame 26, this frame calculates the RMS value n of described alternating component _{ac, RMS}.In frame 27, by the nominal RMS value n of the AC component of the RMS value of described alternating component or effective value and rotating speed _{ac, N}compare.This comparative result is represented as feature 2 at Fig. 2, and this feature can be used for the refluence in cavitation or the pump of testing pump.

As seen from Figure 2, the rotating speed of estimating and the moment of torsion of estimation are carried out to similar processing.As mentioned above, in the time that the work of pump is undesired, torque ripple.Similarly, the described fluctuation of speed or vibration, this RMS value that can show as alternating component is higher.

For estimating moment of torsion and estimating speed, be similar for the mathematical computations that obtains the undesired pump work of instruction, so omitted the calculating for estimating speed.

The nominal operation point of having determined two nominal values of the alternating component of moment of torsion and rotating speed can be for example this situation: wherein, described pressure ratio is more than 1.5.In working point in pressure ratio more than 1.5, AC component is much smaller compared with approaching AC component in cavitation situation or refluence situation.Must determine described nominal value, because RMS level depends on application to a great extent, therefore, each pump and each installation have its oneself feature, and the RMS value of surveying is by comparison without any absolute limitations.

Fig. 5 illustrates the measurement result that the ratio between the RMS value of alternating component of the rotating speed of estimation and the nominal value of the alternating component of rotating speed changes with pressure ratio.Result in described measurement result and Fig. 4 is from same pump.As shown in Figure 5, along with described pressure ratio approaches 1, the ratio calculating increases.This means, in the time that pump approaches cavitation situation or refluence situation, rotating speed is with regard to starting oscillation.Therefore, feature 2 as shown in Figure 2 can be used for detecting cavitation or cavitation possibility.

Fig. 7 illustrates that moment of torsion is estimated and rotating speed estimates that both AC RMS levels are with the measurement result of volume rheology.Fig. 7 also has vertical line, and the minimum volume stream that the manufacturer of pump is recommended is shown, and curve, illustrates that the efficiency of pump is with the variation of volume flow.Having produced in the test of Fig. 7, reduce the stream of pump by being positioned at the valve of pressure end, thereby by the process guide situation that flows backwards.Visible, along with volume flow is reduced to minimum stream, the AC level of described estimation starts to increase.Meanwhile, the efficiency of pump also reduces.The cavitation that the AC level of the estimation producing as seen from Figure 7, can produce the refluence of institute's pump liquid provides clearly instruction.

According to the embodiment of the present invention, form indicative character by the estimation volume flow that utilizes pump model to calculate in pump according to the DC component that moment of torsion is estimated and rotating speed is estimated.Estimate after volume flow calculating, allow volume flow to compare itself and I.The result of this comparison is used as indicative character, for detection of the cavitation of institute's pump medium or the possibility of refluence.Particularly, this is relatively for determining the possibility flowing backwards.

In Fig. 2, the feature relevant to minimum stream is marked as feature 4.In frame 28 and 29, the moment of torsion being produced by frequency converter is estimated to T _estestimate n with rotating speed _estcarry out the DC component T that low-pass filtering is estimated to obtain described moment of torsion _{dc, est}dC component n with rotating speed estimation _{dc, est}.Therefore, described DC component (DC) refers to the value after low-pass filtering, refers to that moment of torsion is estimated and rotating speed is estimated the level having.Selectively, the DC value of described estimation can be by determining that its mean value calculates.

After calculating the DC value of described estimated value, by described DC value T _{dc, est}and n _{dc, est}give and deliver to the frame 30 that comprises pump model, described pump model calculates and estimates volume flow Q according to the estimation of input _est.Described pump model combination can be stored database of the data relevant to pump and so on.The data of storing comprise the Q-P figure of pump or the data point of selecting from described figure.The example of Q-P figure is shown in Fig. 3 b, wherein, has drawn the power (P) and volume flow (Q) of pump for the different-diameter of pump.Once it is known to send the power of pump to, the curve map that pump model comprises can be estimated volume flow.

Utilize following formula to calculate according to the estimation DC level of rotating speed and moment of torsion the estimating power P that sends pump to _{dc, est}:

$P_{\mathrm{dc},\mathrm{est}}=2\mathrm{\π}\frac{n_{\mathrm{dc},\mathrm{est}}}{60}\·T_{\mathrm{dc},\mathrm{est}}$

Because Q-P figure is only known to a rotating speed conventionally, so, must use scaling law (affinity laws) to change it, with corresponding with current rotating speed:

$Q=\frac{n_{\mathrm{dc},\mathrm{est}}}{n_{\mathrm{nom}}}\·Q_{\mathrm{nom}}$

$P={\left(\frac{n_{\mathrm{dc},\mathrm{est}}}{n_{\mathrm{nom}}}\right)}^3\·P_{\mathrm{nom}}$

Wherein, subscript _nomrefer to the corresponding datum speed of given figure.Selectively, scaling law can be applied to the power P being consumed by pump _{dc, est}thereby, the number of minimizing mathematical computations.

When estimating volume flow Q _estallow volume flow Q with the I being provided by the manufacturer of pump and be stored in pump model _minrelatively time, can easily determine whether pump is operated in its nominal operation district.I allows stream Q _mindepend on the rotating speed of pump.So, in the time comparing, should calculate I with scaling law and allow volume flow to consider operating rate.If volume flow, below the 30-70% of nominal volume stream, generally can flow backwards.The uncontinuity that refluence causes the pressure similar to cavitation to change and flow suddenly.Therefore, estimate that volume flow can be as the feature of the possibility of cavitation or refluence situation in instruction pump.

According to the embodiment of the present invention, according to the required pressure head (NPSH that only just sucting _r) and effectively only just sucting pressure head (NPSH _a) another feature of the possibility that relatively forms instruction cavitation or refluence situation, wherein, based on estimating moment of torsion, estimate that rotating speed and systematic parameter are calculated and describedly requiredly only just sucting pressure head and effectively only just sucting pressure head.Ratio between them is known as pressure ratio.

As shown in Figure 2, in frame 31 based on estimating volume flow Q _estcarry out the calculating of pressure ratio, estimate that for determining the process of volume flow is described above.In the time having determined estimation volume flow, can read NPSH from the curve map providing for considered pump _r.At this curve map shown in Fig. 3 a, wherein, draw NPSH _rwith the variation (Q-NPSH of volume flow _rcurve).As Fig. 3 b, provide the curve for different pump sizes.

Due to only for nominal speed provides described curve, so, must reuse scaling law to obtain the NPSH of the rotating speed of being considered _r:

${\mathrm{NPSH}}_R={\left(\frac{n_{\mathrm{dc},\mathrm{est}}}{n_{\mathrm{nom}}}\right)}^2{\mathrm{NPSH}}_{R,\mathrm{nom}}$

Compared with other similarity transformation time, the pressure head (suction head) that sucts being required by pump has minimum value, and the value of utilizing similarity transformation to obtain can not be lower than minimum value.If the volume flow being produced by pump is so low, to such an extent as to do not appear at the Q-NPSH of manufacturer _ron curve, must consider that this situation is this situation: the AC level of wherein said estimation may thereby increase due to the former of the cavitation forming by flowing backwards.Correspondingly, if the volume flow being produced by pump is so high, to such an extent as to it does not appear at the Q-NPSH of manufacturer _ron curve, may there is cavitation, the AC level of described estimation is increased.

In order to calculate described pressure ratio, determine and effectively only just suct pressure head.Can estimate NPSH with following formula _a:

$\mathrm{NPS}{H}_A=H_n+\frac{p_0-p_l\left(Q\right)-p_v\left(\mathrm{Temp}\right)}{\mathrm{\ρ}\left(\mathrm{Temp}\right)g}+\frac{v_0}{2g}$

Wherein, H _sfor the pressure head that sucts of pump, p ₀for environmental stress, p _vfor the evaporation of the liquid of institute's pumping is pressed, p _ffor the estimation of suction side pressure loss, v ₀for the flow rate of container top, g is universal gravitational constant.Temp is fluid temperature (F.T.), and ρ is fluid density.

Fig. 6 illustrates pump 61 and has the container 62 of wanting pumping liquid, and the liquid level of described liquid is being H apart from pump 61 _sat The Height.Can will all offer pump model by the needed all data of above-mentioned equation.Described data also should given possible variation range.The data that require due to described equation can not be very accurate, so, NPSH _acan estimate reliably with following formula:

${\mathrm{NPSH}}_A=H_s+\frac{p_0-{\mathrm{kQ}}^2-p_v}{\mathrm{\ρg}}$

Wherein, constant k represents the flow resistance of suction side, and variable provides by worst-case.So in order to calculate pressure ratio, need:

● from the rotating speed of frequency converter and the estimation n of moment of torsion _est, T _est

● represent the operation of institute's consideration pump and curve (Q-P and the Q-NPSH that pressure requires _r)

● for determining evaporation pressure p _vwith density p by the characteristic of pumping fluid

● pump suct pressure head H _s

● the environmental stress p of liquid in pressurizing vessel time ₀

● the estimation k of the flow losses of suction side

In listed parameter, statical head H _sit is most important one.

Once calculate described pressure ratio, just can indicate by cavitation possibility, cavitation or the refluence of the liquid of pumping as feature with it.As mentioned above, described pressure ratio should be at least more than 1.But the pressure ratio needing depends on used pump.The variable N that is called suction specific speed (suction specific speed) of the principle of work (radial-flow type or axial-flow type) that the pressure ratio needing can be based on pump or the type of reflected pump _ssdetermine, wherein said suction specific speed is defined as:

$N_{\mathrm{ss}}=\frac{N\·\sqrt{\text{Q}}}{{\left({\mathrm{NPSH}}_r\right)}^{0.75}}$

Wherein, N, Q are respectively the rotating speed [min of the pump at the best efficiency point place of pump characteristics ^-1] and volume flow [m ³/ h].If the nominal suction velocity of pump less (being generally radial-flow pump), can be used 1.5 pressure ratio.Relevant to suction specific speed (normally axial-flow pump), the value of described pressure ratio can, up to 4, this means, effectively sucting pressure head must be at least required 4 times of sucting pressure head.Once know the type of pump, estimate that pressure ratio just provides clearly instruction to the working point of pumping process.In Fig. 2, this estimation pressure ratio is illustrated as feature 3.

Fig. 8 illustrates and measures and the pressure ratio of estimation and the funtcional relationship of volume flow.Can see, although there is error in volume flow is estimated, the estimated value of pressure ratio is corresponding with its measured value.

In Fig. 2, four indicative characters are given to decision box 21.But described decision box can receive the indicative character of any number, comprise one, two, three or shown in four features.Frame 21 comprises one group of rule, fuzzy logic or for making decision and by the similar means of its output.Described judgement can have digital form, the cavitation of this numeral indicating liquid or the possibility of refluence or seriousness.For example, the output 33 of frame 21 can be from 1 to 10 integer, and wherein 1 in output is described and operated in nominal operation district, that is, all features that are input to decision box provide the instruction of working in nominal state.In the time that some instructions start to show that cavitation or cavitation possibility have little sign, the output 33 of frame 21 starts to increase, and when all instructions are during shown with the sign of cavitation or refluence, frame 21 exports 10.

Very clear for a person skilled in the art, described decision box can be worked in a different manner.The output of described decision box 33 can be directed to top control system further to operate, comprise and for example change duty or give the alarm.With reference to figure 1, can the output of described decision box 33 be directed to top control system via interface 5.

Decision box 21, pump model 30 and the parameter for operating of storing preferably realize at the frequency converter that pump is controlled.Therefore, method of the present invention is preferably for example carried out by software completely in frequency converter.Desired calculating and the data of storing also can be placed in the control system of described top, and described frequency converter only provides and estimates rotating speed and moment of torsion to top control system thus, may also have thrust of pump and flow rate.

To those skilled in the art, clearly, in the time of technical progress, concept of the present invention can realize in various manners.The present invention and embodiment are not limited to above-mentioned example, can in the scope of claims, change to some extent.

Claims (10)

1. the method for pump based on utilizing frequency converter control, is characterized in that, described method comprises step:

Utilize frequency converter (2) to control described pump (4), described frequency converter (2) feeds the motor (3) that is connected to drive described pump,

Provide the moment of torsion of described motor to estimate (T from described frequency converter _est) and/or rotating speed estimation (n _est),

Estimation (the T providing is provided _est, n _est) form the cavitation of the described pump of instruction (4) or one or more features of the refluence of cavitation possibility and/or described pump (4), and

From the cavitation of pump described in formed one or more feature detection or the refluence of cavitation possibility and/or described pump.

2. method according to claim 1, is characterized in that, by the RMS value (T of alternating component that described moment of torsion is estimated _{ac, RMS}) the nominal RMS value (T of the alternating component estimated with described moment of torsion _{ac, N}) compare to form indicative character.

3. method according to claim 1, is characterized in that, by the RMS value (n of alternating component that described rotating speed is estimated _{ac, RMS}) the nominal RMS value (n of the alternating component estimated with described rotating speed _{ac, N}) compare to form indicative character.

4. method according to claim 1, is characterized in that, forms indicative character by following step:

DC component (the T that uses pump model to estimate according to described moment of torsion _{dc, est}) and described rotating speed estimate DC component (n _{dc, est}) calculate and estimate volume flow (Q _est), and

By described estimation volume flow (Q _est) flow (Q with the allowed minimum volume that is transformed into current rotating speed _min) compare.

5. method according to claim 1, is characterized in that, forms indicative character by following step:

DC component (the T that uses pump model to estimate according to described moment of torsion _{dc, est}) and the DC component (n of described rotating speed estimated value _{dc, es}t) calculate the required pressure head (NPSH that only just sucting _r),

Calculate and effectively only just sucting pressure head (NPSH according to systematic parameter _a), and

To effectively only just suct pressure head (NPSH _a) and the required pressure head (NPSH that only just sucting _r) compare.

6. according to the method described in any in claim 1 to 5, it is characterized in that the RMS value (T of the alternating component that described moment of torsion is estimated _{ac, RMS}) and the RMS value (n of alternating component that estimates of described rotating speed _{ac, RMS}) calculating comprise step:

From described estimation, isolate low-frequency alternating component, to obtain the alternating component value (T of separation _ac, n _ac),

Calculate RMS value according to the alternating component value of described separation.

7. according to the method described in any in claim 1 to 5, it is characterized in that, the DC component that described moment of torsion and rotating speed are estimated by low-pass filtering or by calculating respectively, estimate and the mean value of rotating speed estimation is determined by described moment of torsion.

8. according to the method described in any in claim 1 to 5, it is characterized in that described estimation volume flow (Q _est) calculating comprise step:

DC component (the T estimating according to described moment of torsion _{dc, est}) and described rotating speed estimate DC component (n _{dc, est}) calculate the estimating power consumption (P of described pump _{est, dc}), and

Based on described estimating power consumption (P _{est, dc}) determine and estimate volume flow (Q according to given pump parameter _est).

9. according to the method described in any in claim 1 to 5, it is characterized in that the required pressure head (NPSH that only just sucting _r) calculating comprise step:

DC component (the T estimating according to described moment of torsion _{de, est}) and described rotating speed estimate DC component (n _{de, est}) calculate the estimating power consumption (P of described pump _{est, dc}),

Based on described estimating power consumption (P _{est, dc}) determine and estimate volume flow (Q according to given pump parameter _est), and

Based on described estimation volume flow (Q _est) according to given pump parameter determine estimate the required pressure head (NPSH that only just sucting _r).

10. the system of pump based on utilizing frequency converter control, is characterized in that, described system comprises:

The frequency converter (2) that described pump (4) is controlled, described frequency converter (2) feeds the motor (3) that is connected to drive described pump,

Estimate (T for the moment of torsion that described motor is provided from described frequency converter _est) and/or rotating speed estimation (n _est) device,

For provided estimation (T is provided _est, n _est) forming the device of one or more features, described feature is indicated the cavitation of described pump (4) or the refluence of cavitation possibility and/or described pump (4), and

For the device from the cavitation of pump described in formed one or more feature detection or the refluence of cavitation possibility and/or described pump.