Process for protecting a turbocompressor from operation in the unstable working range by means of fittings with two different regulating speeds

A process and to a device for protecting a turbocompressor from operating in the unstable working range by means of a blow-off device, wherein a control parameter is determined from measured values for the compressor flow, the compressor final pressure and the temperature measurement, as well as from preset or presettable desired values, and controlled opening of the blow-off fittings is performed by a pump surge limiter by means of a pneumatic actuating device each on the basis of the said control parameter. The measured values for the temperature, flow and pressure entered into the pump surge limiter are coupled with a dynamic blow-off line. The blow-off fittings are operated in sequence, and the control signals for the quick opening of the blow-off fittings are first sent to a small blow-off fitting, and then to a large blow-off fitting after the opening of the small blow-off fitting. In cooperation with the dynamic blow-off line, the pump surge limiter provides for continuous control, i.e., a continuous adjustment of the blow-off fittings as a function of the location of the working point in the characteristic diagram of the turbocompressor.

FIELD OF THE INVENTION 
The present invention pertains to a process for protecting a 
turbocompressor from operating in the unstable working range by means of a 
blow-off device, wherein a control parameter is determined from measured 
values for at least the compressor flow and the compressor final pressure 
as well as from preset or presettable desired values, and a controlled 
opening of the blow-off device is performed by a pump surge limiter on the 
basis of that parameter, as well as to a device which has a pump surge 
limiter for adjusting the blow-off device by means of an actuating device 
via a pressurized medium, as well as control lines for actuating the 
blow-off device in the opening or closing direction. 
BACKGROUND OF THE INVENTION 
Quickly responding blow-off or blow-by fittings are needed for the optimal 
protection of turbocompressors from operating in the unstable working 
range. 
Axial compressors are especially at risk during the pumping of the 
compressor. Hydraulic blow-off or blow-by fittings are preferably used in 
axial compressors, because the required adjusting times of 1 to 2 sec for 
an opening movement are guaranteed only by these fittings. These adjusting 
times can be reached with hydraulic fittings only in the case of quick 
opening (a solenoid valve in the pressurized oil circuit is actuated and 
it gradually shuts off the pressurized oil, and the fitting opens at 
maximum regulating speed) and in the controlled case (the pump surge 
limiter moves the blow-off fitting into an intermediate position in order 
to blow off only as much pumping medium as is absolutely necessary) alike. 
A machine protection control (pump surge limiter) presets a continuous 
control signal, and the blow-off fitting must assume this preset position 
as quickly as possible. 
A positioning device, which compares the currently measured position with 
the desired position and generates control/correction interventions in the 
case of a deviation is necessary in practice. This provides that the 
desired position of the fitting will be assumed, It has been known that 
short adjusting times can be achieved with a considerably greater 
difficulty with a positioning unit than by means of controlled solenoid 
valves. The valves release a gradual shut-off cross section whose size can 
be selected, in principle, as desired. The positioning unit must therefore 
be optimized such that a stable position must be maintained even over a 
longer period of time. A change in the actual position of the blow-off 
fitting as a consequence of a change in the correcting variable must take 
place such that a stable swinging into the new position is guaranteed. 
Hydraulically actuated fittings have been known to have the advantage over 
pneumatically actuated fittings that they permit markedly shorter 
adjusting times. However, they have the drawback of being markedly more 
expensive, especially in light of the need for an oil supply unit. 
The advantage of hydraulics lies in the incompressibility of the hydraulic 
oil. Compressed air is compressible and therefore considerably less suited 
for use as a control medium for quick-acting fittings. While adjusting 
times of 1 to 2 sec can be reached without problems with hydraulically 
actuated fittings even in the controlled case, i.e., in the case of an 
adjustment via the position regulator, this is nearly impossible with 
pneumatically actuated fittings. The adjusting times that can be 
realistically reached with pneumatic systems are 2 sec for quick opening 
and 6 sec for controlled valve movement. 
A compressor with a slowly opening fitting (6 sec) can therefore be 
protected only poorly, a level of protection which is considerably less 
than a compressor with a quickly opening, hydraulic fitting (1-2 sec). 
Another, essential cost factor of the use of blow-off and blow-by fittings 
for compressors is the design of the fitting. Valves are most suitable, 
because their characteristics (pressure/flow characteristics) can be 
adapted to the actual conditions by appropriately designing the valve 
seats. The valve characteristic can be adapted to the actual conditions of 
use by design measures. 
SUMMARY AND OBJECTS OF THE INVENTION 
The primary object of the present invention is therefore to develop a 
system including a process and a device which guarantee a similarly good 
opening and closing behavior for protecting a turbocompressor from 
operating in the unstable working range as in the case of the use of 
hydraulically driven blow-off fittings. 
According to the invention, a process is provided for protecting a 
turbocompressor from operating in the unstable working range by means of a 
blow-off device, wherein a control parameter is determined from measured 
values for at least the compressor flow and the compressor final pressure 
as well as from preset or presettable desired values. A controlled opening 
of the blow-off device is performed by a pump surge limiter on the basis 
of the parameter. The measured values for temperature, flow and pressure, 
which are sent to a pump surge limiter, are coupled with a dynamic 
blow-off line, and the two blow-off fittings of the blow-off device are 
operated in sequence. These control signals are first sent to a small 
blow-off fitting and then to a large blow-off fitting after the opening of 
the said small blow-off fitting, for quick opening of the two blow-off 
fittings. The pump surge limiter receives data from a dynamic blow-off 
line for determining the movement of the compressor working point in the 
direction of the pump surge. This blow off line is dynamic as it takes 
into account the dynamics of the operation, the rate of movement of the 
working point, including quick as well as slow shifts in the working point 
(point of operation of the compressor on a working diagram). The dynamic 
blow off line is similar in operation to control systems using a fixed 
blow off line (See U.S. Pat. Nos. 4,831,535; 4,789,298; 4,796,213; 
4,810,163; 4,944,652 and 4,968,215 which are hereby incorporated by 
reference). The two blow-off fittings are operated in sequence in the case 
of slow shifts in the working point, adjusting first the small blow-off 
fitting and then the large blow-off fitting via I/P converters and 
position controllers. Both blow-off fittings are adjusted simultaneously 
in the case of quick shifts in the working point. 
The invention further includes a device with a pump surge limiter for 
adjusting the blow-off device by means of an actuating device via a 
pressurized medium as well as with control lines for actuating the 
blow-off device in the opening or closing direction. The blow-off device 
comprises a small blow-off fitting and a said large blow-off fitting with 
pneumatic actuating devices. A control component is provided with a 
dynamic blow-off line, in addition to the pump surge limiter. 
The process preferably includes adjusting the blow-off fittings via a 
solenoid valve during a quick shift in the working point. For controlling 
disturbance variables, the closing speed of the large blow-off fitting is 
preferably set to an infinitely high value and is maintained in the 
assumed open position until the small blow-off fitting is closed 
completely or nearly completely. For controlling smaller disturbance 
variables, the large blow-off fitting is preferably set to a slower 
closing speed than the small blow-off fitting, and the smaller blow-off 
fitting is the first to reach the range of control due to its higher 
closing speed. For controlling larger disturbance variables, the large 
blow-off fitting is preferably moved only after the end position or the 
vicinity of an end position of the small blow-off fitting has been 
reached. The position of the final control elements of the blow-off 
fittings is preferably reported back (feed back) to a gradient limiter. 
The gradient limiter is expanded by an additional control component. A 
return line is provided between the large blow-off fitting as well as the 
small blow-off fitting, on the one hand, and the control component of the 
gradient limiter. The solenoid valves of the blow-off fittings are coupled 
via a control line to the control component with the dynamic blow off 
line, and the pneumatic actuating devices of the small blow-off fitting 
are coupled with the pump surge limiter through a control line, and those 
of the large blow-off fitting are coupled with the pump surge limiter 
through another control line. The control line and the another control 
line are preferably each coupled with an I/P converter and a pneumatic 
control medium is transmitted from a compressed air supply source to the 
actuating devices. 
The small blow-off fitting and the large blow-off fitting are coupled with 
the gradient limiter and with the control component via a switching signal 
line. The switching signal line preferably passes over or is split into 
the control line or through an OR element. The control component with 
dynamic blow off line is coupled with the control element via a said 
control line. 
The drawback of the prior art is overcome according to the present 
invention in that a compressor protection system, which operates based on 
the control principle described below and is equipped with pneumatically 
actuated fittings, is nearly as effective as a system with prior-art 
control and with hydraulic blow-off valves. 
If the compressor is operated at the normal working point, the flow removed 
by the process is greater than the flow at the blow-off line, and the 
adjusting or blow-off fitting is closed. If the flow removed by the 
process decreases, it may become necessary to blow off part of the 
compressor throughput via the blow-off fitting in order to guarantee the 
minimum necessary compressor flow. The pump surge limiter opens the 
adjusting fitting for this purpose. 
A control signal of a control component with a "dynamic blow-off line" is 
sent according to the present invention simultaneously to the output of a 
pump surge limiter, which adjusts the blow-off fitting continuously, i.e., 
at a slow regulating speed, and to a solenoid valve, which initiates a 
quick opening. This control has the advantage over the process known from 
U.S. Pat. No. DE 38,11,230 for protecting a turbocompressor by blow-off 
via a blow-off valve in that with the present invention the full dynamics 
of all valves is utilized under critical operating conditions (and such 
critical operating conditions always occur whenever the "dynamic blow-off 
line" responds). 
The command for quick opening is present only as long as the gradient of 
the working point shift is greater than the distance between the working 
point and the pump surge, i.e., as long as the critical operating 
situation is present. If the gradient of the working point shift has 
decreased as a consequence of the beginning movement of the fitting and 
has dropped below the limit value, the solenoid valve switches back again, 
and the fitting returns into the normal controlled operation (closed 
control circuit). As a result, the fitting is ideally adjusted at a high 
speed only as long as it is absolutely necessary. The fitting moves into 
the new stationary working point at the high speed without interruption. 
However, dead times, inertias, friction effects, etc., act in practice, 
and they will cause that the fitting will not possibly open wide enough or 
that it will possibly open too wide. This is subsequently compensated in 
both cases by the normal control. 
It is quite possible and often even the normal case that the "dynamic 
blow-off line" will respond several times during a disturbance in the 
process. As was described above, the permissible limit value for the 
gradient depends on the current distance between the working point and the 
pump surge, and this distance changes continuously during a shift in the 
working point caused by a disturbance in the process. 
Therefore, the present invention combines favorable costs with the 
advantages of an inexpensive fitting in a nearly ideal manner. 
Flaps can be manufactured for flow control at a markedly lower cost than 
valves. However, flaps have the drawback of having a highly nonlinear 
characteristic, which can hardly be influenced by design measures. Even 
though this shape of the characteristic can be linearized by electronic 
linearizing circuits, another essential drawback of flaps still persists. 
This drawback is that flaps have an undefined characteristic in the lower 
opening range, i.e., at opening angles between 0.degree. and 10.degree. to 
20.degree.. Linearization is therefore impossible in this range. In 
addition, it was reported by manufacturers involved that flaps are 
generally unsuitable for control in this range. A system--process with a 
device--which avoids this drawback is therefore shown according to the 
present invention. 
Two regulating flaps, a large one and a small one, are used as the blow-off 
fitting according to the present invention. The large regulating flap is 
designed for about 90% of the rated flow, and the small regulating flap is 
designed for about 10%. A ratio of 80:20 or similar ratios are possible as 
well. The two regulating flaps are operated in sequence (split range). The 
small regulating flap is first opened in the case of a response of the 
pump surge limiter, and the large one will open next. It is achieved due 
to this selection that the undefined range of the characteristic of the 
blow-off fitting occurs at very low flows only and thus it is negligible 
for the practical operation. 
Flaps have been known not to have good regulating properties in the range 
below about 10.degree. opening. This problem is solved according to the 
present invention by operating a large fitting and a small one in series 
on the signal side. This means that the small fitting opens first when the 
output signal of the pump surge limiter decreases (all blow-off/blow-by 
valves are closed at maximum regulator output signal, and they are open at 
minimum signal), and the large fitting opens thereafter. By splitting, 
e.g., into a 10% fitting and a 90% fitting, the critical range with the 
poor control characteristic is reduced to 10% of the value that would 
become established with the use of a single 100% fitting. This is 
sufficient, because the measured signal (flow) is subject to a noise of 1% 
to 2% caused by the measurement anyway, and this noise is reflected by the 
correcting variable to the blow-off/blow-by fitting. 
The pump surge limiter first opens the small regulating flap for this 
purpose. If the flow through this fitting is not sufficient to operate the 
compressor outside the unstable working range, the large regulating flap 
is subsequently opened as well. 
Due to the blow-off fittings being operated in the split range, the 
adjusting times will increase compared with actuation of a single fitting 
only or compared with the actuation of both fittings in parallel. The 
adjusting times double at worst. 
This drawback, as well as the above-described drawback of the more slowly 
controlled adjusting time of pneumatic actuators, are compensated by the 
control signal of the "dynamic blow-off line" acting simultaneously on all 
fittings, i.e., on the large and small fittings. 
The large flap begins to open as soon as the small flap is fully opened. 
This large flap is now possibly operated in the range with poor control 
performance. However, this is noncritical if the regulating speed of the 
blow-off/blow-by fitting in the closing direction is limited to very low 
values. 
Should the large blow-off fitting exactly assume the necessary position 
with the necessary throttling behavior due to the intervention of the 
dynamic blow-off line, no additional control interventions are necessary, 
and the system displays ideal behavior. However, the large blow-off 
fitting will usually open too wide or will not open wide enough. If it 
does not open wide enough or if the throttling action is still too strong, 
the pump surge limiter will open this blow-off fitting wider. However, if 
the blow-off fitting has opened too wide or if the throttling action is 
too weak, the pump surge limiter will execute a closing command. 
In another embodiment of the present invention, the closing speed of the 
large blow-off fitting is limited. This can be performed either by a 
corresponding setting on the drive of the fitting or by limiting the 
gradient of the correcting variable in the regulator. The small blow-off 
fitting can also be provided with a corresponding limitation of the 
closing speed (it is often necessary for safety reasons), but the large 
fitting is set to a substantially slower closing speed. 
Should the "dynamic blow-off line" have opened the two blow-off fittings 
too wide, the pump surge limiter executes a closing command. Since the 
large blow-off fitting is set to a very slow closing speed, it will also 
close only correspondingly slowly. The small blow-off fitting is set to a 
higher closing speed and it will again reach the range of control more 
quickly as a result. As a result, the small fitting has priority during 
the elimination of smaller process disturbances. 
It is also possible to block the closing process of the large blow-off 
fitting in a controlled manner, i.e., to fix the large regulating flap in 
the position it has once assumed, until the small blow-off fitting is 
completely or nearly completely closed. The large blow-off fitting is 
"frozen" in its position as a result, and smaller disturbance variables 
are controlled exclusively via the small blow-off fitting. The large 
blow-off fitting is moved only in the case of larger disturbance variables 
or when the small blow-off fitting reaches an end position or the vicinity 
of an end position. 
As an alternative, the pump surge limiter and the "dynamic blow-off line" 
may also control a blow-off device that has only one blow-off fitting, in 
which case both slow and quick closing processes of the only blow-off 
fitting are possible. 
Exemplary embodiments of the present invention will be explained on the 
basis of two control circuits and a block diagram. 
The various features of novelty which characterize the invention are 
pointed out with particularity in the claims annexed to and forming a part 
of this disclosure. For a better understanding of the invention, its 
operating advantages and specific objects attained by its uses, reference 
is made to the accompanying drawings and descriptive matter in which 
preferred embodiments of the invention are illustrated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Corresponding to FIG. 1, the turbocompressor 1 is connected on the suction 
side to a suction line 10. On the discharge side, the turbocompressor 1 is 
connected to a discharge line 11, which sends the medium compressed by the 
turbocompressor 1 to a downstream process via a nonreturn flap 12. A 
blow-off line 20 leading to a blow-off device 2 is branched off from the 
discharge line 11 before the nonreturn flap 12, and two blow-off fittings 
3, 4 with a common sound absorber 5 as well as with pneumatic actuating 
devices 21 and solenoid valves 24, which are connected to two control 
lines 23, on the one hand, and to the control lines 27 and 28, on the 
other hand, are arranged in the said blow-off device 2. 
The flow of the medium to be compressed, which flows to the compressor 1, 
is determined on the suction side by means of a flowmeter 31 arranged in 
the suction opening of the turbocompressor 1. A thermometer 33 is 
additionally arranged in the suction line 10. The compressor final 
pressure can be determined by means of a pressure gauge 32 connected to 
the discharge line 11. The actual value of the flow currently measured by 
the flowmeter 31 and the desired value of the flow sent by the resolver 
transmitter 38, as well as the gas temperature measured in the thermometer 
33 are sent to a pump surge limiter 41, which is coupled with a "dynamic 
blow-off line" component 42. 
In cooperation with the "dynamic blow-off line" 42, the pump surge limiter 
41 provides for continuous control, i.e., for continuous adjustment of the 
blow-off fittings 3, 4 as a function of the location of the working point 
in the characteristic diagram. To achieve this, the output of the pump 
surge limiter 41 acts on the electropnuematic converters 34, 35, which are 
connected to a pressure supply line 39, via a gradient limiter and via two 
transformers 29 and 30 as well as via control lines 27, 28. The task of 
these transformers 29, 30 is to transform the output signal of the pump 
surge limiter 41 such that the small blow-off fitting 3 will open first, 
and the large blow-off fitting 4 thereafter. A pressurized medium for 
opening or closing is admitted via pneumatic control lines 36, 37 to the 
drives 21, 24 of the small blow-off fitting 3 and of the large blow-off 
fitting 4 in the blow-off device 2. 
A pressurized medium bypass line leads from the actuators 21 to a 
piston-and-cylinder or diaphragm unit 22 for generating the force for the 
adjusting movement of the blow-off fittings 3, 4 in the closing and 
opening directions. 
The output of the control lines 27 and 28 acts on the piston-and-cylinder 
units 22 of the pneumatic actuators 21 via the converters 34, 35. The two 
blow-off fittings 3, 4 are adjusted as a result in "split range" until the 
compressor working point is again set back into the safe range of the 
characteristic diagram. 
In the case of greater deviations, the "dynamic blow-off line" 42 energizes 
the solenoid valves 24 via the control line 23 and initiates a quick 
opening of both blow-off fittings 3, 4 as a result. Once the process 
disturbance has extensively subsided, the "dynamic blow-off line" 42 
switches back, and the blow-off fittings 3, 4 are adjusted only by control 
signals of the control lines 27 and 28. 
As was explained above, the closing process of the large blow-off fitting 4 
can be blocked and this fitting 4 can be fixed in the position it has once 
assumed until the small blow-off fitting 3 is completely or nearly 
completely closed. The large blow-off fitting 4 is "frozen" in its 
position as a result, and the control of smaller disturbance variables is 
performed exclusively via the small blow-off fitting 3. The large blow-off 
fitting 4 is moved only in the case of greater disturbance variables or 
when the smaller blow-off fitting 3 reaches an end position or the 
vicinity of an end position. 
A block diagram of such a structure is shown in FIG. 2. 
It shows the components necessary for controlling the small blow-off 
fitting 3 and the large blow-off fitting 4, which comprise the pump surge 
limiter 41, the "dynamic blow-off line" 42, as well as the gradient 
limiter 50 with the corresponding control lines. 
The control signal for the large flap 4 is energized via a series 
connection of various functional blocks of the gradient limiter 50, which 
was complemented by the relay "REL" 55. 
When the "dynamic blow-off line" 42 responds, both solenoid valves 24 are 
energized via the control line 23, and both blow-off fittings 3, 4 open 
simultaneously at maximum regulating speed. In addition, the integrator 
"NFI" 53 is switched to tracking mode, and the memory contents of this 
integrator 53 are tracked to the currently measured flap position of the 
large blow-off fitting 4. This can also happen for the small blow-off 
fitting 3, but this is not shown in FIG. 2. If the quick opening command 
of the "dynamic blow-off line" 42 disappears, the integrator "NFI" 53 is 
switched back to the integration mode, and the integrator 53 follows the 
output signal of the limiter "LIM" 52. The regulating speeds of the 
blow-off fittings 3, 4 are set in this LIM block 52. 
If the small blow-off fitting 3 is outside the end positions, the relay 
"REL" 55 is switched to zero via the OR element "OR" 56, and the 
integrator "NFI" 53 remains at its initial value. The adjusting signal of 
the pump surge limiter 41 to the large blow-off fitting 4 is released only 
when an end position of the small blow-off fitting 3 is reached, and this 
large blow-off fitting 4 can follow at the regulating speed set in the 
block "LIM" 52 or at a lower speed. If the small blow-off fitting 3 again 
reaches the working range outside the end positions as a result, the relay 
"REL" 55 is again switched to zero when the end positions are left, and 
the large blow-off fitting 4 continues to remain in the required position. 
It is meaningful to provide the feedback of the end positions of the 
blow-off fittings 3, 4 by the end position switches 25, 26 with a 
hysteresis, so that frequent switchovers will be avoided. 
The end position feedback can be picked up just as well from the end 
position switches 25, 26 at the actuator as it can be derived from the 
position feedback 60 with the position transducer 59. 
If a continuous feedback 59 of the measured position of the large blow-off 
fitting 4 is not available, the output of the integrator NFI 53 can also 
be fed back to the follow-up input, instead. 
FIG. 3 shows a control circuit of a turbocompressor with only one blow-off 
fitting. The turbocompressor 1 is connected on the suction side to a 
suction line 10. The turbocompressor 1 is connected on the discharge side 
to a discharge line 11, which sends the medium compressed by the 
turbocompressor 1 to a downstream process via a nonreturn flap 12. A 
blow-off line 20 to a blow-off device 2 is branched off from the discharge 
line 11 before the nonreturn flap 12, and a large blow-off fitting 4 with 
a sound absorber 5 as well as with pneumatic actuating devices 21 and 
solenoid valves 24, which are connected to the control line 23, on the one 
hand, and to the control line 28, on the other hand, are arranged in the 
said blow-off device 2. 
The flow of the medium to be compressed, which flows to the compressor 1, 
is determined on the suction side by means of a flowmeter 31 arranged in 
the suction opening of the turbocompressor 1 in this case as well. A 
thermometer 33 is additionally arranged at the suction line 10, and the 
compressor final pressure is determined by a pressure gauge 32 in the 
discharge line 11. The actual value of the flow measured by the flowmeter 
31 and the desired value of the flow sent by the resolver transmitter 38, 
as well as the gas temperature measured in the thermometer 33 are sent to 
a pump surge limiter 41, which is coupled with a "dynamic blow-off line" 
42. 
In cooperation with the "dynamic blow-off line" 42, the pump surge limiter 
41 provides for a continuous control of the blow-off fitting 4 as a 
function of the location of the working point in the characteristic 
diagram. To achieve this, the pump surge limiter 41 acts on the 
electropneumatic converter 35, which is connected to a pressure supply 
line, via a transformer 30 as well as via the control line 28. The 
transformer 30 forms the output signal of the pump surge limiter 41 such 
that the blow-off fitting 4 will open. 
A pressurized medium bypass line leads from the actuators 21 to a 
piston-and-cylinder or diaphragm unit 22 for generating the force for the 
adjusting movement of the blow-off fitting 4 in the closing and opening 
directions. 
In the case of greater deviations, the "dynamic blow-off line" 42 energizes 
the solenoid valves 24 via the control line 23 in this case as well, and 
it initiates a quick opening of the blow-off fitting 4 as a result. Once 
the process disturbance has extensively subsided, the "dynamic blow-off 
line" 42 switches back, and the blow-off fitting 4 is adjusted only by 
control signals of the control line 28. 
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.