US3240422A - Method of and apparatus for the prevention of surging with axial compressors - Google Patents

Description

March 15, 9 J. PETTERSEN ETAL 3,240,422

METHOD OF AND APPARATUS FOR THE PREVENTION OF SURGING WITH AXIAL COMPRESSOHS Filed March 27, 1965 2 Sheets-Sheet 1 f Z gzg I h GOA/$77 INVENTORS Johan Pe'kt ersen Hans Baumcmh /iw, JWN Y ANA ATTORNEY S March 15, 1966 .1. PETTERSEN ETAL 3,240,422

METHOD OF AND APPARATUS FOR THE PREVENTION OF SURGING WITH AXIAL COMPRESSORS Filed March 27, 1963 2 Sheets-Sheet 2 3 69 INVENTOR S Jul 1cm Peitersen Hans Baumam n ATTORNEYS United States Patent 3,240,422 METHOD OF AND APPARATUS FOR THE PREVENTION OF SURGING WITH AXIAL COMPRESSORS Johan Pettersen, Baden, and Hans Baumann, Nussbaumen, Aargau, Switzerland, assignors to Aktiengesellschaft Brown, Boveri & Cie, Baden, Switzerland, a joint-stock company Filed Mar. 27, 1963, Ser. No. 268,306 Claims priority, application Switzerland, Apr. 3, 1962,

4,048/62 Claims. (Cl. 230-415) The present invention relates to an improved method for the prevention of surging with single or multi-stage axial compressors and to an apparatus for putting this method into practice.

Should the back pressure of an axial compressor be gradually increased then the output quantity decreases in accordance with a certain law. The reference point of the operating condition sochanging describes a curve on the pV diagram, the so-called characteristic of the compressor at the speed concerned. On reaching a certain pressure the compressor, which has up till now been running smoothly, now begins to operate in an irregular manner and considerable pulses and oscillations occur in the flow. This unstable manner of working of a compressor is termed surging? and the collection of all points at which the phenomenon appears is described as the surge limit.

In some cases of operation, even on approaching the surge limit there arises a disturbance in the flow, this being designated as rotating stall. In all cases, however,

violent aperiodic shocks occur just before reaching the surge limit as well as in the surge zone itself. These impulse disturbances stimulate oscillations in the blading concerned, the former being capable of assuming such proportions that danger of breakage arises.

To prevent surging and the blade vibrations caused by it, it. is a known procedure to employ a surge prevention regulator which opens a blow-off valve as seen as the operating point approaches the critical zone. By this means the output quantity is increased and the compressor can once more operate in a quiet and stable fashion. With the present-day surge prevention regulators-one of this type is indicated in FIG. 3 with 16 and described in the accompanying text-a function is formulated of the suction volume and the end pressure or pressure at an intermediate stage with the aid of a cam, this then. acting as input for the regulator. Nonetheless, these regultors have a considerable disadvantage:

On the commissioning of a new compressor, the surge limit a (FIG. 1) is fixed by experiment. A line b is now chosen at an appropriate distance from this limit, this acting as the limit of approach for the surge-prevention regulator. As soon as the fluctuating operating point P reaches this limit, the regulator comes into action. In this way it is prevented that the operating point goes as far as the surge limit whereby quiet operation of the compressor should be guaranteed under all operating conditions. Now, however, surge limit a can displace itself in the direction of the limit of approach b of the regulator, e.g. due to contamination of the blading or change in the gas inlet temeprature, so that the safety factor becomes too small. As the limit of approach is de- "ice termined on the basis of measurements at commissioning and thereafter remains constant, it can happen that the compressor begins to surge despite perfect functioning of the regulation.

This disadvantage of known regulation apparatus is avoided according to the invention by measuring the vibrations of the guide and/ or rotor blades and using the greatest determined amplitude of vibration as the parameter of regulation for the purpose of preventing surgmg.

In FIGS. 2 and 3 are represented schematically two constructional examples of the invention, whereby like components are denoted by the same reference symbols. Detail views of them are given in FIGS. 4 and 5.

According to FIG. 2, the axial compressor 1 isdriven by motor 2, draws gas in from connection line 3 and passes it thru outlet connection line 4. Vibration feelers 6 are mounted on the guide blades 5, the impulses from the former being transmitted as an input parameter to regulator 7 via an amplifier 8. As soon as the amplitude of the blade vibrations exceeds a maximal permissable valuewhich is declared by the proximity of the surge limit--the regulator is acutated. It opens an outflow 9 through which control oil runs out of the pressure system 10, the feed 11 to which is adjusted by orifice 12.

As soon as the oil pressure in system 10 sinks, control piston 13 of the servo-motor 14 opens the blow-off valve 15 from output line 4 and thus makes an additional outlet available to the compressed gases. The output volume becomes greater once more, the operating point of the compressor falls back into the region of stable flow because of thisto the right in FIG. 1-and the Surging is prevented. If the blade vibrations fade away again, regulator 7 closes off the outflow 9, the pressure in system 10 increases and piston 13 of servo-motor 14 shuts the blow-oil valve 15.

FIG. 3 shows another possible application of the apparatus as according to the invention. Axial compressor 1, motor 2, line connections 3 and 4, outflow 9, pressureoil system 10, oil feed 11, orifice 12, control piston 13 of the servo-motor 14 and the blow-off valve 15 have the same function as in FIG. 2. The outflow 9 together with the blow-off valve 15 are now, however, actuated by a surge-prevention regulator 16 which is already given in the introduction to the description as being known. It is commented on only briefly here.

The surge prevention regulator is provided with a pressure difference measuring apparatus 17 on the membrane 18 of which acts the pressure difierence at the throughput measuring point 19 in the suction connection 3. Should the throughput quantity or, respectively the pressure difference at the measuring point 19 decrease to a certain minimum value, then the force exerted upwards by the membrane 18 falls below the pretension of spring 20 which now moves the oil blow-off valve 21 to the open position. As soon as oil runs out through the outflow 9, the pressure falls in the hydraulic system 10 which then causes the opening to blow-01f valve 15.

The characteristic value of the regulator 16, ie the minimum pressure. difference at measuring point 19 at which the membrane 18 opens the oil blow-0E valve 21, is thus determined by the pretension in the spring 20. This pretension can be altered by displacing cam 22 which is set during the commissioning and which can be corrected by hand with the aid of adjustment spindle 23 in later service. During operation, however, this results by means of the bellows element 24 which makes the setting as a function of the pressure in pressure line 4. For example, should the output pressure increase, then the bellows is pressed together against the force of the adjustment spring 25 thereby displacing the cam to the left. The form of the latter is chosen so that on displacement to the left, the pretension of spring 20 is raised. This means, however, that the oil blow-off valve 21 now opens at a greater pressure difference on the membrane 18. Otherwise expressed: the greater the output pressure is in line 4, the greater too is the minimum suction quantity at which the regulator is actuated. The form of the cam 22 permits the limit of approach b of the regulator to be adapted to the surge limit a of the compressor with any desired exactness (see FIG. 1).

The surge-prevention regulator 16 and that part of the installation as given in FIG. 3 and as described earlier are known and are frequently employed in this arrangement. It will now be shown that the apparatus as according to the invention may also be applied with advantage for this purpose.

As in FIG. 2, vibration feelers 6 are also attached to the guide blades 5, these feelers serving to transmit the blade vibrations as impulses to the regulator 7 via amplifier 8. Here, however, the regulator does not act directly on the blow-off valve 15 any more. When the vibrations of the blades become too great and thus when the operating point is drawing too close to the surge limit without the surge-prevention regulator operating, then regulator 7 acts on the adjustment motor 26 which actuates the adjustment spindle 23. Cam 22 thus travels to the left and opens the oil blow-off valve 2 1, this then causing an opening of blow-off valve 15, as is described above. The pressure in the output line 4 and at the throughout measuring point 18 remain at the same time, or leastwise at first, unchanged. By changing the position of the cam 22, the function between output pressure and suction quantity was thus altered. This means that the regulator 7 has displaced the limit of approach b (FIG. 1) of the surgeprevention regulator 16 and thereby adapted it to the new, actual surge limit.

In the constructional examples in FIGS. 2 and 3, vibration feelers are mounted on each row of guide blades but in fact the moving blades could equally as well be called upon for this. Admittedly, there arise certain difiiculties in leading out the connecting cables from the rotor with this arrangement. In principle it is suflicient merely to mount a vibration feeler on only one guide or moving blade. Naturally, one would choose for that purpose the row of blades at which surging first occurs as given by calculation or experience. But as this can be various rows of blades depending on the changing operating conditions, it is usual to attach a vibration feeler to one blade of each endangered row. Nonetheless, the amplifier accepts only the greatest amplitude of vibration at any one time and passes this on to the regulator which comes into action as a limit regulator only from a predetermined minimum value onwards.

As vibration feeler there are various known types of equipment at disposal. In FIG. 4 is shown the application of a strain gauge 27 of which one is attached to moving blade 28 and another to guide blade 5. Preferably, the strain gauges are fixed to the feet of the blades as the greatest material stresses occur there due to the blade vibrations and are thus the easiest to measure. The connecting'cables 29 lead to amplifier 8 which is not drawn in here.

FIG. shows the application of vibration guages 30. For this there may be considered such appliances which measure the amplitude, the velocity or the acceleration of the blade vibrations. These types of apparatus are best built into the blade tips as the blade amplitude is greatest theme and the measurement most effective.

4 Should it be feared that disturbances may arise in the flow due .to the apparatus itself or due to the connecting cables 29, then the vibration gauges may also be mounted on the blade foot; in this case, however, more sensitive apparatus and a higher amplification of the impulses are then required.

As was said above, the vibration feelers can be mounted on either one or on several rows, and both on the guide and moving blades, as is represented in FIGS. 4 and 5, or only on one of the two. If it is to be avoided that the connection cables be led out of the rotor, whereas, on the other hand it is a row of moving blades which stands in the greatest danger, then a possible advantageous method of measurement is presented if the natural frequency of the moving blade to be measured coincides with the natural frequency of the neighboring guide blade. In this case the vibrations of the moving blade are transferred synchronously on to the guide blade and can be measured there more simply.

The regulator 7 can act in various ways on the compressor. Arrangements are thus known with which the regulator actuates a component other than the blow-off valve or the cam of the surge-prevention regulator. It may clearly be understood that corresponding arrangements can also be effected with a regulation as according to the present invention.

Furthermore, the apparatus can also serve only for surveillance so that, should the amplitude of the blade vibrations exceed an allowable value, the impulse from the vibration feeler releases an acoustic or visual signal whereupon the necessary control measures may result from hand.

The apparatus can, of course, be employed to check blade vibrations of turbines, e.g. at start up whereby the regulator operates naturally on another control organ, preferably the inlet valve.

By means of the described method and the appertaining apparatus, surging of axial compressors is avoided. The regulation does not take effect on reaching some fixed line in the pV-diagram, but rather on approaching the effective surge limit so that any displacement of the latter is taken into consideration without correction by hand being necessary. The amplifier can be so set that the regulator comes into action even before the occurrence of those amplitudes of vibration which would cause damage of the blades.

We claim.

1. In an axial flow compressor having at least one stage comprising a row of stationary guide blading and an adjacent row of blading mounted on the compressor rotor, the method of preventing said compressor from surging which comprises the step of measuring vibrations induced in the blading as a result of fluid flow therethrough and utilizing only such measured vibrations as reach a predetermined amplitude for regulating fluid flow through the compressor in such sense as to reduce its tendency to surge.

2. Apparatus for preventing surging in an axial flow compressor having at least one stage comprising a row of stationary blading and an adjacent row of blading mounted on the compressor rotor comprising, a feeler element in direct association with said blading, said feeler element producing a signal proportional to the amplitude at which said blading vibrates, and means responsive only when said signal reaches a predetermined level for regulating fluid flow through said compressor in such sense as to reduce its tendency to surge.

3. Apparatus as defined in claim 2 for preventing compressor surge wherein said feeler element is applied to said guide blading and the natural frequency of said guide blading matches that of said movable blading.

4. Apparatus as defined in claim 2 for preventing compressor surge wherein said compressor has a plurality of stages and a vibration feeler element is provided for the blading of several stages, the signals produced by said feeler elements being transmitted to the input of an amplifier and said amplifier producing at its output a control signal regulating fluid flow through said compressor whenever the signal produced by any one of said feeler elements reaches said predetermined level.

5. Apparatus for preventing surging in an axial flow compressor having at least one stage comprising a row of stationary blading and an adjacent row of blading mounted on the compressor rotor, means responsive to a predetermined decrease in pressure obtained across a measuring point of said compressor for regulating fluid flow through said compressor in such sense as to reduce its tendency to surge, means for adjusting the pressure at which said regulating means becomes operative, a feeler element in direct association with said blading, said feeler element producing a signal proportional to the References Cited by the Examiner UNITED STATES PATENTS 2,395,995 3/1946 Dewey 23017 2,424,137 7/1947 Ball 230115 2,455,292 11/1948 Borden 230-1 15 2,696,345 12/1954 Hopper 2301 14 3,058,339 10/1962 Shapiro 737 1.4

15 LAURENCE V. EFNER, Primary Examiner.

Claims (1)

1. IN AN AXIAL FLOW COMPRESSOR HAVING AT LEAST ONE STAGE COMPRISING A ROW OF STATIONARY GUIDE BLADING AND AN ADJACENT ROW OF BLADING MOUNTED ON THE COMPRESSOR ROTOR, THE METHOD OF PREVENTING SAID COMPRESSOR FROM SURGING WHICH COMPRISES THE STEP OF MEASURING VIBRATIONS INDUCED IN THE BLADING AS A RESULT OF FLUID FLOW THERETHROUGH AND UTILIZING ONLY SUCH MEASURED VIBRATIONS AS REACH A PREDETERMINED AMPLITUDE FOR REGULATING FLUID FLOW THROUGH THE COMPRESSOR IN SUCH SENSE AS TO REDUCE ITS TENDENCY TO SURGE.

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