EP0228665A2 - Limiting surge regulation method for turbocompressors - Google Patents

Abstract

In a method for the surge limit control of turbocompressors, in which a relief valve is controlled as a function of the approach point of the operating point in the characteristic diagram to a blow-off line (A), the actual value of a further operating parameter is additionally monitored according to the invention and compared with the setpoint of this parameter assigned to the current operating point . A deviation from the actual and target value indicates that the variables used to calculate the map are no longer correct and the blow-off line defined in this map is no longer at the correct safety distance from the actual surge limit (P), which e.g. may be due to contamination or other changes in the geometry of the compressor. Depending on the actual setpoint comparison, a correction signal for controlling the relief valve and / or a warning signal is then generated.

Description

The invention relates to a method for the surge limit control of turbo compressors of the type specified in the preamble of claim 1. Such a method is known from "Nachrichten für den Maschinenbau" 5/82.

Pumping is an unstable condition of a turbocompressor, in which intermittent or periodic conveying gas flows back from the pressure to the suction side. Pumping occurs when the end pressure is too high and / or the throughput is too low. In the map determined by final pressure and throughput or coordinates derived therefrom, a line can therefore be clearly defined which separates the stable from the unstable area and is referred to as the surge limit. The surge limit control is intended to prevent the operating point of the compressor from reaching the surge limit and pumps thereby entering. For this purpose, a blow-off line is defined in the map at a safety distance from the surge limit. When the operating point exceeds the blow-off line, a relief valve branching off from the compressor outlet is opened more or less in order to blow off the conveyed medium or to blow it off to the suction side and thereby lower the final pressure or increase the throughput.

wobei P, der Saugdruck, P2 der Enddruck, ΔP der Wirkdruck an einer saugseitigen Drosselstelle und T, die Temperatur an der Saugseite sind und diese Werte als ständig überwachte Meßwerte vorliegen. R, ist die Gaskonstante und x der Isentropenexponent des jeweiligen Fördergases, während K eine von der Geometrie der Dosselstelle im Kompressoreintritt abhängige Konstante ist. Z ist ein konstanter Faktor (Realgasfaktor).The course of the surge limit and thus the blow-off line in the map is then clear and unchangeable and independent of the current operating state, if the adiabatic delivery head Δhad and the intake volume flow V are used as map coordinates. These coordinates can be calculated according to the formulas from the continuously measured operating variables of the compressor, in particular the suction and final pressure and the pressure difference at a throttle point on the suction side

where P, the suction pressure, P2 the final pressure, ΔP the differential pressure at a throttle point on the suction side and T, the temperature on the suction side and these values are available as constantly monitored measured values. R is the gas constant and x is the isentropic exponent of the respective feed gas, while K is a constant that depends on the geometry of the dosing point in the compressor inlet. Z is a constant factor (real gas factor).

In the map defined by Ah _ad and V, the position of the surge limit and thus also the blow-off line is independent of changes in the parameters contained in formulas (1) and (2). However, these map coordinates can only be calculated from the measured values of the pressures and temperatures if the values for R, x and K are known. Given the unchangeable compressor geometry and the unchanged composition of the conveying gas, the sizes R; x and K are measured once and then treated as constants. However, a change in the composition of the conveying gas can result in a change in the associated values for R and / or x. However, these changes cannot be measured directly. In such a case, the retention of the previous values for R and x would lead to an incorrect calculation of the map coordinates, so that the surge limit in the map thus calculated no longer has the correct course. The same applies if, for example, the effective compressor geometry changes due to contamination.
If the surge limit control refers to such an incorrect course of the surge limit and thus the blow-off line in the map, then this either means that pumping is not prevented with certainty or that the relief valve is already opened too far from the true surge limit is triggered, which can lead to undesirably high power losses of the compressor.

The object of the invention is to provide a method for the surge limit control of the type mentioned, which makes it possible to record effects on the position of the surge limit in the map used caused by changes in the gas composition and / or, for example, by pollution-related changes in the compressor geometry, and corresponding corrections in the surge limit control to make.

The solution to the problem is specified in claim 1. The subclaims relate to advantageous further refinements.

In the solution of the task according to the invention, it is assumed that for each operating point in the stable map area, a characteristic curve includes further unique parameters such as speed, blade position, power, etc., so that there is a clear connection between the map coordinates of the operating point and the parameters. From the map coordinates calculated according to the formulas (1) and (2) above, an associated target value can be calculated using a calculated or measured map, e.g. the speed n of the compressor can be determined. If the actually measured speed deviates from this nominal value, this means that the actual working point also deviates from the working point calculated according to formulas (1) and (2) because one or more of the variables R, x and K have changed. The deviation between the target value and the actual value of the speed or another characteristic parameter such as blade position or compressor output thus serves as a correction variable which indicates that the actual course of the surge limit in the characteristic diagram deviates from the required course. This deviation can, if it e.g. based on changed gas composition, be taken into account by appropriate correction within the calculation of the map coordinates according to formula (1) for the determination of the controlled variables or by directly applying a corresponding correction variable to the control. On the other hand, by operating the compressor with a standard gas with known values for R and x, it can be determined whether a deviation between the setpoint and actual value of the speed indicates contamination of the compressor system. In this case, an appropriate maintenance or shutdown of the system can be initiated by a warning signal.

• Fig. 1 die verallgemeinerte Darstellung des Kennfeldes eines Kompressors mit Pumpgrenze, Abblaselinie und Kennlinien konstanter Drehzahl;
• Fig. 2 eine schematische Darstellung des Verlaufs des rechnersichen Vergleichs;
• Fig. 3 eine komplette schematische Darstellung der Einrichtung zur Pumpgrenzregelung eines Kompressors.

The invention is explained in more detail below with reference to the drawings. It shows:

• Figure 1 shows the generalized representation of the map of a compressor with surge limit, blow-off line and characteristic curves of constant speed.
• 2 shows a schematic illustration of the course of the computational comparison;
• Fig. 3 is a complete schematic representation of the device for surge control of a compressor.

The surge limit P of a compressor is clearly defined in the map with the coordinates V and Δh _ad , as shown in FIG. 1. The location of the surge line is independent of changes in the parameters of suction pressure, final pressure, temperature, gas constant or isentropic exponent.

While quantities such as pressures and temperatures are easy to measure, gas constant R and isentropic exponent x cannot be measured directly, at least not quickly and economically enough. Gas analyzes often take considerable time, so that the measurement results are too late and are unusable for the control of the relief valve. The method according to the invention, which can detect and take changes in these quantities into account, presupposes that there is always a clear relationship between the isentropic exponent x and the gas constant R when the gas composition is varied.

This is e.g. then always guaranteed if the gas composition is changed by admixing a gas of constant composition or if several gases are admixed that have similar gas constants or isentropic exponents. In general, it can be said that the method can be used whenever there is a clear relationship between the gas constant R and the isentropic exponent x for all occurring gas compositions.

According to the invention, a standard composition of the gas is assumed, in which R and x have a predetermined known value. For this standard gas, regardless of the actual gas composition, the delivery head and the intake volume flow are calculated from the measured values as map coordinates. The formulas (1) and (2) give the calculated values Δh _adr and V r.

If gas with a different composition is used, the current values Δh _ada and V _a deviate from the calculated values Δh _adr and V.

wonach zu der wirklichen Förderhöhe Δh _ada und dem wirklichen Durchfluß V _a eine Drehzahl n_a gehört.According to FIG. 1, exactly one characteristic curve K, K 'with constant speed n ,, n2 etc. runs through each working point with the coordinates V and ΔH _ad . Thus, the arithmetic values Ahadr and V also include a clear arithmetical speed n,. Equation also applies

after which a speed n _a belongs to the actual delivery head Δh _ada and the actual flow rate V _a .

This speed is the actual actual speed, which can be measured very easily and very precisely by measuring the speed of the drive turbine.

• Aus den Größen P1, P2, T1 bestimmt der Rechner 1 unter Zuhilfenahme von R_r und x_r die theoretische Förderhöhe beim Normzustand (Ah_adr). Der Rechner 2 bestimmt aus Wirkdruck Ap, Saugdruck p1 und Saugtemperatur T1 sowie der Normgaskonstanten R_rden theoretischen Volumenstrom. Im Rechner 3 ist entweder der Verlauf der Kompressorkennlinien in Form mathematischer Gleichungen oder in Form einer Matrix mit der jeweiligen theoretischen Drehzahl n, als Inhalt der Matrix dargestellt. Der Rechner 3 berechnet entweder die rechnerische Drehzahl n, oder liest sie direkt aus dem Matrixspeicher ab.

Assuming further that the map of the compressor (Δh _ad over V) has been determined by calculation or experiment and is known, and assuming that the isentropic exponent x is a unique function of the gas constant R (x = F (R) ) all data are known to determine x. According to FIG. 2, this is done as follows:

• From the quantities P1, P2, T1, the computer 1 _uses R _r and x _{r to determine} the theoretical delivery head in the _normal state (Ah _adr ). The computer 2 determines the theoretical volume flow from the effective pressure Ap, suction pressure p1 and suction temperature T1 and the standard gas constant R _r . Computer 3 either shows the course of the compressor characteristic curves in the form of mathematical equations or in the form of a matrix with the respective theoretical speed n, as the content of the matrix. The computer 3 either calculates the calculated speed n or reads it directly from the matrix memory.

This computed speed is now compared in comparator 4 with the actually measured speed n _a . If the actual speed matches the calculated speed, the actual gas composition also matches the standard composition. However, if the measured speed deviates from the calculated speed, the gas composition is different. Then the relation of the blow-off line A specified in the characteristic diagram to the calculated working point does not match that to the actual working point, and the surge limit control works incorrectly. This must be corrected by taking into account the changed gas composition.

A feasible but very difficult way would be to use the formulas (1) and (2) and the known relationship between R and x to calculate the quantities R _a and x _a , i.e. the actual gas constant and the actual isentropic exponents. These values for R and x can then be used in the formulas for Ah _ad and V, and the actual delivery head and the actual flow rate result. These two quantities can be applied as the setpoint and actual value of a conventional surge limit control, which protects the compressor from pumping.

This regulation itself can work as follows, for example according to FIG. 1: According to the Δh _ad formula, a computer determines the actual delivery head of the compressor. The minimum permissible intake flow V target is determined from this by mirroring on the blow-off line A. This is compared with the actual flow V measured. The relief valve remains closed as long as the measured flow V is greater than the minimum permissible V target. The relief valve does not open until V falls below.

A simpler way of taking changes in the gas composition into account is to empirically deduce a shift in the surge limit from the deviation in the speeds and to automatically shift the blow-off line accordingly. Such a method will be described in more detail below.

If, instead of the correct data for x and R, only the data of the standard state are given in a surge limit control, as described above, this leads to an error if the gas composition deviates from the standard state. An incorrect delivery head and a flat flow rate are calculated. Pumping of the compressor will occur at a different location than on the surge limit P defined in FIG. 1. The actual surge limit shifts depending on the difference between the current gas state and the standard state. This shift clearly depends on the variation in the gas composition. Since x is a unique function of R, as assumed at the beginning, this shift is also unique. Since it was also found that the speed deviation between the calculated speed n _r and the actual speed n _a depends exclusively on the gas composition, the speed deviation is also a clear measure of the shift in the surge limit.

This influence is usually non-linear, so that it makes sense to switch the speed deviation to a function generator and to have the displacement of the blow-off line controlled by the output of the function generator. The most practical way of realizing this is to determine the theoretical course of the surge limit for different gas compositions and plot it graphically. The speed deviation is also determined and a function generator is set to this relationship. 3 shows the schematic of such a surge limit control.

A compressor 10 is driven by a turbine 11 or by another variable speed drive. In the suction line 13, the pressure difference (differential pressure) at a throttle point 17 is measured with a measuring transducer 15 and further the suction pressure with a pressure sensor 19 and the temperature on the suction side with a temperature sensor 21. The arithmetic intake flow V r is determined from these variables in computer 1 (cf. FIG. 2) using the gas constants R _r for the normal composition of the gas. At the compressor outlet 23 with a pressure sensor 25 Final pressure is determined and from this and from the suction-side measured variables, the delivery head _{Δh adr is} determined in the second computer 2 using the values R, and x _r for the normal composition of the conveying _gas . In a computer or matrix memory 3, the computed speed n _r belonging to the values V and Δh _{adr is} determined with the normal _{composition of} the gas. This is compared with the actual speed n _a measured on the shaft of the turbine 11 by means of a speed sensor 27 in a differential element 29.

The values V _r and Δh _adr calculated by the computers 1 and 2 also serve as control variables for the control of a relief valve 31 branching off from the compressor outlet _23. The value of the delivery head _{Δh adr} is fed to a function generator 33 in which the course of the blow-off line is stored. The function generator 33 generates, for each value of Δh _adr, the associated setpoint V ′ target of the intake _flow, which is determined by the blow-off line A (cf. FIG. 1). This output V should of the function generator 32 is compared in a differential element 35 with the actual value V, and a control difference is formed therefrom, which is fed as an input signal to a controller 37, the output signal of which opens the relief valve 31 when the blow-off line A is exceeded in the characteristic diagram, so that pumping is prevented by lowering the final pressure and / or increasing the flow through the compressor.

The method according to the invention can also be used in other surge limit controls in which other characteristic diagrams are used or other criteria are decisive for triggering the relief valve.

The output signal of the differential element 29 is fed to a function generator 39 which, on the basis of the deviation of the calculated speed n from the actual speed n _a, generates a fixed correction quantity which reflects the non-linear relationship between the speed deviation and the required correction of the surge limit or blow-off line in the characteristic diagram Fig. 1 considered. The correction variable generated by the function generator 39 is added by a summing element 41 to the setpoint value V soll generated by the function generator 33, so that regulation of the relief valve is adapted to the changed gas composition.

Changes and configurations of the described embodiments are possible within the scope of the invention. The correction variable generated by the function generator 39 can also be added to the actual value V generated by the computer 1 or the control difference generated by the differential element 35. It is also possible to add the correction variable not simply additively, but multiplicatively or at the same time additively and multiplicatively or the control variable. Additive addition means a parallel shift, multiplicative addition means a rotation of the surge line P or blow-off line A in the map according to FIG. 1.

If the control is carried out on a multi-stage compressor system, it is possible to use the described method not over all stages, but only over one or more stages.

Instead of the speed, other parameters can be used that clearly define a characteristic curve running through the respective working point. Such a parameter is e.g. the guide vane position, especially in compressors that are operated at constant speed and controlled by changing the vane position. It is also possible to use the drive power of the compressor instead of the speed.

As mentioned at the beginning, not only changes in the gas composition, but also e.g. pollution-related changes in the compressor geometry are recorded and taken into account. For this purpose, the compressor is operated with a conveying gas with a standard composition, the values for R and x are known and are identical to the data used in computers 1 and 2. In this case, the setpoint n, and actual value n, the speed should be the same, so that no output signal occurs at the differentiator 29. If a signal from the differentiator 29 occurs anyway, it can be concluded that the geometry of the compressor e.g. has changed due to pollution. In this case, the signal generated by the differentiator 29 can be used to control a warning signal generator 43, which provides an indication that the compressor has to be serviced or even stopped in the event of danger.

If a gas with a standard composition (R _r , x _r ) is not available, this check can also be made with another gas with known values for R and x. In this case, even with a clean compressor, a deviation between n _a and n will occur in the differentiator 29.

In a separate calculation process outside of the arrangement shown in FIG. 3, this deviation must be determined for a clean compressor using the method described above. A comparison of this arithmetically determined deviation with the output signal of the differentiator 29 shows whether there is contamination or any other change in the compressor geometry.

Claims (8)

1. Method for the surge limit control of turbocompressors, in which the map coordinates of the current operating point of the compressor are calculated from continuously measured values of suction and outlet-side pressures and temperatures, and the opening as a function of the distance of the operating point from a surge limit line and / or blow-off line specified in the map or closing a relief valve branching from the compressor outlet is controlled,
characterized in that the actual value of an operating parameter independent of the suction and pressure-side measured values, which defines a family of characteristic curves in the characteristic diagram, is continuously measured as a monitoring parameter, that the setpoint value of the monitoring parameter which is assigned to the characteristic curve passing through the operating point is determined from the characteristic diagram coordinates and compared with the measured actual value of the monitoring parameter, and that if the actual value deviates from the target value of the monitoring parameter, a correction signal influencing the control of the relief valve and / or a warning signal is generated. 2. The method according to claim 1, characterized in that the adiabatic delivery head (Ah _ad ) and the intake volume flow (V) are calculated from the measured values as the map coordinates. 3. The method according to claim 1, characterized in that the speed (s) of the compressor is determined as the monitoring parameter. 4. The method according to claim 1, characterized in that before or at intervals during the operation of the compressor with the respective delivery gas, the compressor is operated with a standard gas of known composition and that in this operation with standard gas in the event of a deviation of the actual value from the target value of the monitoring parameter a warning signal indicating the contamination of the compressor is generated. 5. The method according to claim 1, characterized in that with each change in the composition of the feed gas supplied to the compressor, the monitoring parameters are measured and, if its actual value deviates from the target value, the correction signal is generated. 6. The method according to claim 1, in which the actual value (v) ist) of a map coordinate (v̇) as a controlled variable is compared with a setpoint value (V soll) obtained by comparing the other map coordinate (Δh ad) with the blow-off line and the control difference thus obtained is compared Regulator for the relief valve is supplied as an input variable, characterized in that the correction variable obtained by comparing the actual value (n _a ) and target value (n,) of the monitoring parameter is the actual value - (v̇; _st ) or the target value (V soll) of the controlled variable or whose control difference is given additively and / or multiplicatively. 7. The method according to claim 1, characterized in that on the basis of the map coordinates of the operating point the associated setpoint (n,) of the monitoring parameter is determined arithmetically on the basis of predetermined formulas which take into account the gas composition or is stored in a matrix memory and called up. 8. The method according to claim 1, for the surge limit control of a multi-stage compressor system, characterized in that it is used only for one or a part of the compressor stages.

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