Process and apparatus for prevention of surging in turbocompressors

A process and apparatus for operating a turbocompressor having an output at an actual value of flow and pressure and a blow-off valve for regulating the output to prevent surging of the output above a surge limit and to prevent a reduction of the output below a minimum value, with a comparing unit for measuring the actual value, and a controller unit for controlling the blow-off valve in accordance with the output of the comparing unit comprising, forming a first difference value between the theoretical value and the actual value, forming a delayed value from the difference value and thereafter forming a second difference value between the delayed value and the first difference value to control the blow-off valve. The apparatus for effecting the process comprises summing units for forming the various difference values, a delay unit for delaying the first difference value and, in one embodiment, a limiter unit for limiting the second difference value which is thereafter added to the theoretical value before the resulting value is applied to the controller unit for controlling the blow-off valve.

FIELD AND BACKGROUND OF THE INVENTION 
The present invention relates to a new and useful apparatus and process for 
the operation of turbocompressors in which the throughput or a signal 
derived from the throughput and the discharge pressure or the pressure 
ratio, are continuously monitored and compared with permissible values. 
Measures are taken, according to the invention, to prevent surging. Upon 
reaching a blow-off line of the compressor characteristic curve which runs 
parallel to the surge limit, one or a plurality of blow-off valves are 
opened, for example, so that the compressor throughput does not fall below 
a minimum value which depends on the pressure ratio. 
Surge limit control for compressors have already been undertaken with the 
use of mechanical/hydraulic controls. Despite high instrument and 
engineering expenditures, however, it is not possible in known controls to 
exactly adjust the blow-off line in order to reliably prevent surging. A 
further disadvantage lies in the high maintenance expense and the 
considerable susceptibility of these mechanical hydraulic control devices 
to trouble. 
It is also known that an electronic surge limit control may be used for 
compressors (see Mitteilung 542 der Warmestelle des Vereins deutscher 
Eisenhutterleute, Report 542 of the Thermal Section of the German 
Ironworkers' Association). The surge limit control on compressors with 
guide vane adjustments is, in these units, designed similar to that of 
compressors with throttle adjustment, while, of course, there is the 
difference that because of the non-linear surge limit curve, a function 
generator is provided for the formation of the command variables of the 
surge limit control. 
It has proven to be a disadvantage in these known control devices, that 
under certain operating conditions, that is, in the case of manual control 
intervention and with strong pressure fluctuations, surging of the 
compressor cannot readily be prevented. 
U.S. Pat. No. 4,139,328 to Kuper et al (German Offenlegungsschrift 
2,623,899), is incorporated here by reference. This patent teaches that an 
electronic surge limit controller may be used to control the blow-off 
valves, in which the control difference for the controller, which depends 
upon the actual pressure and throughput values, is amplified in a 
non-linear manner such that the amplification is increased when the 
control difference becomes negative, that is, when the operating point of 
the compressor moves into the impermissible range on the other side of the 
blowoff line of the compressor characteristic. Moreover, in this unit, an 
extreme value selector is imposed on the control, which selector takes 
into consideration the greatest control deviation, namely, the actual 
control deviation or the difference between control output and manual 
control command. 
In this functional control device, it is, to be sure, a disadvantage that 
the rate of variation of the actual value cannot be taken into 
consideration or that allowance cannot be made for whether the deviation 
from actual value becomes greater or smaller. In practice, this means 
that, regardless of operating status, the blow-off line remains constantly 
in service. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to obtain improved prevention of 
surging in turbocompressors, while the blow-off line or value level, for 
reasons of efficiency, lies as close as possible to the surge limit line 
and its position is displaceable corresponding to the variations of the 
actual value. The expenditure for the control device should be as low as 
possible, and it should be capable of being assembled largely from 
commercially available circuit elements. 
This object is accomplished pursuant to the invention by a process and 
apparatus of the type mentioned above which is characterized in that 
outside the controller the difference is formed from the theoretical value 
and the actual value. This is supplied, once directly and secondly delayed 
by way of a delay member, to a subtracting station and finally added to 
the theoretical value. The signal may advantageously be supplied to a 
limiter and carried out further in such fashion that the control 
difference is formed from a theoretical value and an actual value outside 
the controller, supplied, once multiplied by the factor of 2 and secondly 
delayed by way of a delay member, to a subtracting station and then 
imposed on the controller. 
A further object of the present invention is to provide an apparatus for 
effecting the process which is simple in design, rugged in construction 
and economical to manufacture. 
For an understanding of the principles of the invention, reference is made 
to the following description of typical embodiments thereof as illustrated 
in the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Turning first to FIG. 1, in the known control device, generally designated 
40, the difference between a theoretical value at 10 for a particular 
selected parameter of compressor operation which is generated by a 
function generator, and an actual value (control deviation) at 12 of the 
same parameter, is formed in a unit 16 outside the controller 14. 
For manual adjustment of the blow-off valve 22 by the controller 14, a 
second fictitious "control deviation" is formed as a difference between a 
manual control signal from a manual control signal unit 18 and the control 
output at 20. These two signals are imposed on the controller 14 via a 
maximum selector 24. 
The maximum selector 24, in each instance, selects the most positive 
control deviation and imposes it on the controller 14. The selected 
control deviation thus is the one which opens the valve 22 the most 
rapidly or closes it the most slowly. The valve 22 remains open until the 
most positive signal is zero. It is, by this means possible, therefore, to 
open the valve further by a positive control deviation than is prescribed 
by the theoretical value. 
In contrast to this known system, the control device according to the 
invention, in FIGS. 2 and 3, has additional members for taking a time 
component into consideration, which operates as follows: 
Upon a shift of the working point of a compressor which is controlled by a 
valve 23 connected to the inventive control system 50, toward the blow-off 
line in the compressor characteristic, the actual value at 13 decreases. 
The difference potential x.sub.d changes with a positive stop. This value 
is the output of difference unit 17. This magnitude signal x.sub.d is 
multiplied by a fixed magnitude in multiplier 27, preferably by 2, and 
supplied directly to a summing station 31. The signal x.sub.d is also 
delayed through a PT.sub.1 -member 33, and then also supplied to the 
summing station 31. The output of the summing station 31 thereby becomes 
more positive than the signal x.sub.d. This is equivalent to a shift of 
the blow-off line toward the working point. The faster the working point 
moves toward the blow-off line, the greater the shift. 
When the working point moves away from the blow-off line, the blow-off line 
shifts toward the surge limit. This is, in most cases, without influence 
on the function of the control system. Where this is not desirable, this 
effect may be prevented by a parallel imposition of magnitude signal 
x.sub.d (dotted circuit completion in FIG. 2) on the maximum selector 25 
which is connected to the controller 15. 
In FIG. 3, like elements, as those found in FIG. 2, are designated with 
like numbers with the addition of a prime. 
In control systems in which the control deviation cannot be externally 
influenced, the system may be designed according to FIG. 3. The limiter 41 
causes the expansion to be effective in only one direction. 
As shown in FIG. 3, a function generator provides the theoretical value 
signal. A first summing unit 17' receives both the theoretical value 
signal and a signal representative of the actual value of the operating 
point and produces a difference signal which is fed directly to one input 
of a second summing unit and through a delay circuit to the other input of 
the second summing unit. The difference signal produced by the second 
summing unit is fed through the limiter 41 to one input of a third summing 
unit, the third summing unit, which also receives the theoretical value 
signal, produces a difference signal which is fed to one input of the 
controller 15', the other input of the controller receives the signal 
representative of the actual value of the operating point of the 
compressor (13'). The purpose of the limiter circuit 41 is to make any 
increase in deviation effective in only one sense. 
As is evident, the invention has for the first time, made it possible for 
the rate of a variation in actual value to be taken into consideration and 
thereby to ensure reliable control, particularly in the case of strong 
disturbances, that is, even then reliably to prevent surging. Since only 
known commercially available electronic components are used and the 
control device is accordingly economically feasible and easily applied by 
those skilled in the art, the invention may be regarded as an ideal 
solution to the problems which arise. 
According to FIG. 2, the comparing signal is amplified, for example, twice 
(X2). This amplification may, of course, alternatively be replaced by, for 
example, having the signal x.sub.d undergo a corresponding weakening in 
the parallel branch. Essential, in this connection, is only the proportion 
of the signal being added. 
While specific embodiments of the invention have been shown and described 
in detail to illustrate the application of the principles of the 
invention, it will be understood that the invention may be embodied 
otherwise without departing from such principles.