Surge control for variable speed-variable geometry compressors

Surge control systems prevent and control a surge condition for fixed or variable speed compressors and compressors having variable geometry which provide air to pneumatic loads. A signal proportional to the ratio of the measured compressor outlet pressure to the measured inlet pressure is combined with a signal corresponding to a selected reference pressure to provide a vent valve command signal. When the signal representing the ratio of the measured pressure exceeds the signal corresponding to the selected reference pressure, a surge condition is developing and the vent valve command signal causes a valve to vent a portion of the air to the pneumatic load. The venting of the air reduces the measured pressure ratio and the vent valve command signal decreases, to close the valve. A signal is provided to reduce the effect of the reference pressure ratio if the weight flow rate through the compressor is reduced, as by decreasing compressor speed and/or repositioning the inlet guide vanes of the compressor.

BACKGROUND OF THE INVENTION 
This invention relates to control systems for controlling the operation of 
gas compressor systems to avoid a surge condition and, more particularly, 
to a system for regulating the ratio of the outlet pressure to the inlet 
pressure to prevent surge. 
Gas compressor systems which supply air pressure to pneumatic loads are 
subject to the occurrence of an undesirable condition commonly referred to 
as surge. Although the reason for the occurrence of surge is not fully 
understood, its effect is extremely detrimental. For example, when a surge 
condition occurs in the compressor system, the airflow may suddenly 
reverse and airflow provided to the pneumatic load may cease or be 
interrupted. If the surge condition is permitted to continue, the 
compressor can enter a deep surge condition, causing damage to its 
internal components. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a signal representing a pressure 
ratio, P.sub.r, of the outlet pressure of the compressor to the inlet 
pressure of the compressor is combined with a signal representative of a 
selected pressure ratio, P.sub.r ref., to provide a vent valve command 
signal. The reference pressure ratio P.sub.r ref. may be selected to 
correspond to a maximum flow rate W through the compressor. If the signal 
representing the measured pressure ratio P.sub.r is greater than the 
signal representing the reference pressure ratio P.sub.r ref. for the 
selected weight flow rate W, a surge condition may ensue and the valve 
position command signal causes a valve to vent a portion of the air 
provided to the load. 
The venting of the air reduces the output pressure of the compressor, 
thereby lowering the pressure ratio P.sub.r. As the pressure ratio P.sub.r 
returns to value equal to the reference pressure P.sub.r ref., the valve 
position command signal closes the venting valve and system operation 
along the normal operating line of the compressor resumes. A signal 
representing a reduction in reference pressure P.sub.r is provided to 
accommodate a lesser weight flow rate W if the weight flow rate W is 
reduced, as decreasing the speed of the compressor and/or repositioning 
its inlet guide vanes. Addition of the signal has the effect of reducing 
P.sub.r ref. 
It is an object of the invention to provide an electrical control system 
for preventing and controlling a surge condition in a compressor system. 
Another object is to prevent surge by controlling the pressure ratio of the 
outlet pressure to the inlet pressure by venting a portion of the air 
provided to the pneumatic load. 
Another object of the invention is to control surge when the pressure ratio 
is reduced as by operating the compressor at less than maximum speed or at 
a reduced weight flow rate due to the position of the inlet guide vanes, 
or both. 
Other objects and features of the invention will be apparent from the 
following description and from the drawings. While illustrative 
embodiments of the invention are shown in the drawings and will be 
described in detail herein, the invention is susceptible of embodiment in 
many different forms, and it should be understood that the present 
disclosure is to be considered as an exemplification of the principles of 
the invention and is not intended to limit the invention to the 
embodiments illustrated.

DESCRIPTION OF PREFERRED EMBODIMENT 
Referring to FIG. 1, a surge map for a load compressor is shown. The map 
shows a pressure ratio P.sub.r plotted as a function of airflow rate W. 
P.sub.r is the ratio of the outlet pressure P.sub.out to the inlet 
pressure P.sub.in. Airflow rate W is the weight of the air discharged from 
the compressor as the function of time (as, for example, lbs. per second). 
The airflow rate may also be corrected for temperature and pressure 
variations, in which case the value is denoted by W'. 
Both P.sub.r and W are obtained by measuring various compressor parameters. 
P.sub.in may be obtained by measuring the pressure at the inlet of the 
compressor by a pressure tube. P.sub.out may be similarly measured by a 
pressure tube positioned at the outlet of the compressor. The pressures 
are converted to electrical signals which are manipulated to provide 
P.sub.r. W or W' are proportional to a differential pressure measured at 
the input of the compressor. Hence, the differential pressure at the input 
is converted to an electrical signal and multiplied by a constant to 
provide W or W'. 
The surge line on the map is acquired empirically by detecting and plotting 
values of P.sub.r at which the compressor enters a surge condition for 
selected values of W. The speed of the compressor and the position of its 
inlet guide vanes (IGV) affect the location of the operating position on 
the map, and movement on the map is along the common speed (or common IGV) 
line. For example, at a constant compressor speed, P.sub.r increases 
without an increase in airflow rate until the compressor reaches the surge 
condition, as can be seen by following a common speed line upwardly to the 
surge line, as shown in FIG. 1. 
The magnitude of P.sub.r for a given W can be controlled by controlling 
P.sub.out for a particular airflow rate. This may be accomplished by 
venting a portion of the air provided to the load. As air is vented, 
P.sub.out, and hence P.sub.r, drops, following the common speed line 
downwardly from the surge line. The compressor operating line is drawn in 
the normal operating region of the map and is selected to represent a 
displacement, as 5%, to the right of the surge line. It is desirable that 
the system maintain a pressure ratio P.sub.r greater than the P.sub.r 
value at the intersection of the operating line with the common speed line 
(or inlet guide vane position line) but less than the P.sub.r value at the 
intersection of the surge line and the common speed line. 
In the present invention, the pressure ratio P.sub.r is controlled by a 
venting valve which increases or decreases P.sub.out so that P.sub.r 
equals the P.sub.r value at the intersection of the common speed line with 
the operatng line. The valve is opened or closed when the P.sub.r value is 
in the surge correction region, as shown in FIG. 1. The position of the 
valve determines the value of P.sub.r and is controlled by a surge control 
circuit to be explained in greater detail below. 
If the compressor is operating in surge condition (on the surge line), the 
valve is fully opened. If the compressor is operating in the normal 
operating region (on the operating line), the valve is fully closed. 
Proportional control of the valve position occurs when P.sub.r is in the 
surge correction region. As the valve is opened, the pressure ratio 
P.sub.r drops along the common speed line toward the point of intersection 
with the normal operating line. As the pressure ratio approaches the 
normal operating line, the control valve of the present invention 
proportionally closes the valve and completely closes it when the pressure 
ratio P.sub.r lies at the intersection of the operating line. Thereafter, 
if the pressure ratio increases to enter the surge correction region, the 
control system of the present invention opens a valve in an amount 
proportional to the magnitude of the correction required to drop the 
pressure back toward the operating line. 
An explanation of the operation of various control systems for the 
compressors will now be provided with particular reference to an axial 
compressor. Although an axial compressor will be described in combination 
with the control circuits, it should be understood that the control 
circuits of the present invention are capable of controlling surge for any 
type of compressor having a surge map similar to that shown in FIG. 1. 
Referring to FIG. 2, a surge control system for a fixed speed, fixed 
geometry compressor is shown. A compressor 10 has an inlet 12 and an 
outlet 14 which supplies compressed air to pneumatic loads 16 by a 
pneumatic conduit 18 which is coupled between the load 16 and the outlet 
14. A venting conduit 20 is coupled in parallel with load 16 and has a 
dump valve 22 therein, the position of which determines the amount of air 
vented by vent 24. A pressure sensor 26, which may be a conventional or 
strain gauge transducer, measures the pressure at the inlet 12 and 
converts it to a signal P.sub.in representative of the magnitude of the 
pressure. Similarly, a sensor 28 measures the pressure at outlet 14 and 
generates a signal P.sub.out proportional to its magnitude. 
The singals representing P.sub.in and P.sub.out are applied to conditioning 
circuits 30 and 32, respectively. The conditioning circuits remove noise 
and transients from the signals. The signals are then appllied to a 
divider circuit 34 which divides the P.sub.out signal by the P.sub.in 
signal to provide output singal P.sub.r. The output from divider circuit 
34 is applied to a sumer 36 where it is summed with a singal from P.sub.r 
reference circuit 38. The level of the signal from the P.sub.r reference 
circuit 38 is set equal to the desired pressure ratio P.sub.r ref. at the 
intersection of the common speed line representing maximum flow rate W and 
the operating line (FIG. 1). 
The output of summer 36 is referred to as the vent valve position command 
signal and is negative when the compressor is operating in the normal 
operating region since the value from P.sub.r reference circuit is greater 
than the value from divider circuit 34. If the pressure ratio P.sub.r 
(which follows a common speed or IGV line) exceeds the selected P.sub.r 
ref., the valve position command signal from summer 36 is positive. This 
position on the surge map of FIG. 1 is located in the surge correction 
region and is indicative of an ensuing surge condition. When P.sub.r is in 
this section of the surge map, valve 22 must be opened to reduce P.sub.r 
so that a surge condition does not occur. The magnitude and the polarity 
of the valve position command signal controls the position of valve 22, as 
will be explained in greater detail below. 
The valve position command signal is applied to a summer 39 through an 
amplifier 40. The gain of amplifier 40 is selected in accordance with the 
operating characteristics of the system. A voltage applied to a summer 39 
causes an output voltage to be provided to a valve position control 
circuit 42 through an amplifier 44. The position of the valve is related 
to the voltage applied to valve position control circuit 42 in any 
convenient manner. For example, a positive voltage applied to the valve 
position control circuit 42 may be used to open valve 22 in an amount 
proportional to the mangnitude, whereas zero volts, or negative voltage, 
causes valve 22 to be fully closed. 
Valve position demodulator circuit 46 provides negative feedback to summer 
39 to assure that the valve position with respect to the applied voltage 
is maintained. If P.sub.r increases and enters the surge control region on 
the map of FIG. 1, the valve position command signal is positive and 
causes valve 22 to open. When valve 22 opens, P.sub.r decreases, which in 
turn decreases the magnitude of the valve position command signal causing 
valve 22 to close. When the valve position command signal decreases to 
zero or becomes negative the valve 22 is fully closed and the compressor 
is operating in the normal operating region. 
Referring to FIG. 3, a surge control system for a variable speed, fixed 
geometry compressor 48 is provided. The system shown in FIG. 3 is similar 
to the system shown for the fixed speed, fixed geometry system of FIG. 2. 
The difference between the two systems is easily seen in that valve 
position command signal from summer 36 is modified by a signal 
representing various compressor speeds. Specifically, as the speed of the 
compressor is selectively decreased to a percentage of full speed, the 
value of P.sub.r on the operating line also decreases. FIve different 
speeds are shown in FIG. 1, and each represents a selected percentage of 
full speed. 
The speed of compressor 48 is sensed by a sensor 50 to provide a signal 
proportional to the acutal speed. A speed demodulator circuit 52 converts 
the signal representing the actual speed to a proportional voltage. A 
describing function 54 provides an output to summer 36 through an 
amplifier 56. The gain of amplifier 56 is selected in accordance with the 
operating parameters of the system. As the various speeds are selected in 
decreasing magnitude, the output of the describing function 54 increases. 
The relationship between the input and the output is usually linear, 
although a describing function other than linear may be used if desired. 
The describing function may also modify the input signal for altitude, 
temperature or pressure. 
The valve position command signal from summer 36 is similar to that 
discussed above with respect to the fixed speed, fixed geometry compressor 
(FIG. 2), except that its magnitude is proportional to a reduced value of 
P.sub.r indicative of a reduction in the selected speed. Thus, for the 
five common speeds shown in FIG. 1, the describing function 54 and 
amplifier 56 provide five individual voltage levels, each of which reduces 
the effect of P.sub.r ref. so that the signals generated by the circuit 
correspond to the P.sub.r at the intersection of the operating line with 
the selected common speed line. The valve position command signal from 
summer 36 controls the position of valve 22 in a manner similar to that 
discussed above. 
Referring to FIG. 4, a surge control system for a fixed speed, variable 
geometry compressor 58 is shown. For the purpose of this invention, the 
term "variable geometry compressor" means a compressor having positionable 
inlet guide vanes which control the weight flow rate through the 
compressor. The surge control system shown in FIG. 4 is similar to the 
system shown for the fixed speed system in FIG. 2. The difference between 
the two systems is easily seen in that the valve position command signal 
from summer 36 is modified by a signal representing the various inlet 
guide vane positions of the compressor. Specifically, as the inlet guide 
vanes (IGV) are positioned in a angular relationship with respect to the 
position representing maximum weight flow W, the value of P.sub.r along 
the operating line decreases in a manner similar to the effect of a 
reduction in compressor speed. Five different inlet guide vane positions 
are shown in FIG. 1, and each represents a selected angular relationship 
with repsect to the position representing maximum weight flow W. The IGV 
position is sensed by a sensor 60 to provide a signal proportional to the 
actual position of the inlet guide vanes. An IGV position demodulator 62 
converts the signal representing the IGV position to a proportional 
voltage. Describing function 64 provides an output to summer 36 through an 
amplifier 66. The gain of amplifier 66 is selected in accordance with the 
operating parameters of they system. As the inlet guide vanes are 
positioned to decrease the magnitude of the weight flow rate, the output 
from the describing function 64 increases. 
The valve position command signal from summer 36 is similar to that 
discussed above with respect to FIG. 2, except that its magnitude is 
reduced by an amount proportional to the P.sub.r drop indicative of a 
lower weight flow rate W due to the selection of a particular inlet guide 
vane position. Thus, for the five IGV lines shown in FIG. 1, the 
describing function 64 and amplifier 66 provide five individual voltage 
levels, each of which has the effect of reducing P.sub.r ref. so that the 
circuit provides a signal representative of the pressure ratio having a 
value equal to the value of P.sub.r at the intersection of the operating 
line with the IGV line. 
The operation of the variable speed, variable geometry compressor will now 
be considered. However, before the details of the surge control system for 
the variable speed, variable IGV system are provided, it is helpful to 
consider the nature of the surge control map for such a compressor. If 
both the speed and the geometry of the compressor are permitted to be 
controlled, the surge mapping function becomes three-dimensional. 
Specifically, individual maps, each of which represents a common speed and 
each of which is similar to that shown in FIG. 1, are provided for each 
inlet guide vane position. Thus, the particular value of P.sub.r for a 
selected flow rate W is determined not only by the speed of the 
compressor, but also by the inlet guide vane position. Since both the 
inlet guide vane position and the speed of the compressor affect the 
position of P.sub.r on an operating line for a particular flow rate, 
signals representing both these parameters must be added to the signal 
representing P.sub.r ref., having the effect of reducing P.sub.r ref. to 
accommodate the selected comparison speed and selected inlet guide vane 
position. 
Referring to FIG. 5, the system for controlling surge in the variable 
speed, variable geometry compressor 68 is shown. The system is similar to 
that shown in FIG. 2 except that a signal representing compressor speed 
(similar to that provided by the system shown in FIG. 3) and a signal 
representing inlet guide vane position (similar to that shown by the 
circuit of FIG. 4) are applied to summer 36. The valve position command 
signal from summer 36 is provided in a manner similar to that discussed 
above, but is reduced by an amount proportional to the pressure ratio drop 
due to the particular position of the inlet guide vanes and the selected 
speed of the compressor.