Surge prevention control system for dynamic compressors

A surge prevention control system for use with a dynamic compressor provides a multiple module controller for operating an anti-surge valve to bypass flow around the dynamic compressor. The multiple module controller includes a PID control module and a rate control module. The PID control module controls the anti-surge valve to control the operating point of the dynamic compressor about the surge control line. The rate control module uses the rate of approach of the operating point to the surge control line as its process variable. In the event of high rate of approach, the rate control module takes control of the anti-surge valve. The setpoint of the rate control module is adjusted to open the anti-surge valve to control the rate of approach to the surge control line to minimize the overshoot of the PID control.

FIELD OF THE INVENTION 
The present invention generally relates to control systems for controlling 
the operation of dynamic compressors, and more particularly to control 
systems and methods for preventing surge in dynamic compressors. 
BACKGROUND OF THE INVENTION 
Dynamic compressors are widely used in industrial processes for providing a 
source of compressed gas. In order to avoid interrupting the operation of 
a downstream process which receives the compressed gas, the operation of a 
dynamic compressor has to be well controlled to provide stable output 
pressure or flow rate as required by the downstream process. It is well 
known, however, that if the flow rate of a dynamic compressor drops below 
a certain threshold level for reasons such as changed conditions of the 
downstream process, surge and complete flow collapse can occur in the 
compressor. Besides causing the inevitable consequence of interrupting the 
downstream process, surge can also be a catastrophic experience for the 
dynamic compressor, causing audible noise and strong vibrations in the 
compressor which in serious cases can severely damage the dynamic 
compressor. 
The threshold flow rate below which the dynamic compressor will experience 
surge is a function of the differential pressure across the dynamic 
compressor. The surge condition is often described using a compressor map 
that represents the operation of the compressor in terms of actual flow 
versus polytropic head. It has been found that surge will occur if the 
operating point of a compressor wheel in the compressor map falls within a 
surge zone bordered by a surge line which is well approximated by a 
parabolic curve defined as: 
EQU (actual flow).sup.2 /(polytropic head)=K, 
where K is a constant. 
The commonly employed way to prevent a dynamic compressor from surging, or 
to bring the compressor out of surge, is to open an anti-surge valve 
connected to the compressor output to return a portion of the output flow 
of the dynamic compressor to the compressor inlet. In this way, the flow 
rate of compressor is increased so that the operating point of the 
compressor is moved away from the surge region. In order to control the 
operating point to prevent it from moving into the surge region, systems 
have been developed using proportional-integral-derivative (PID) 
controllers to control the opening and closing of the anti-surge valve. 
Those controllers normally operate after the operating point of the 
compressor passes a predefined surge control line which is disposed in the 
compressor map within a chosen safety margin from the surge line. 
For purposes of effectively preventing surge from occurring, it is highly 
advantageous to be able to anticipate the need for opening the anti-surge 
valve by assessing whether a flow disturbance is likely to cause the 
operating point of the compressor to move across the surge line. A good 
indicator of the likelihood of surge is the rate at which the operating 
point approaches the surge line, i.e., the time derivative of the distance 
between the operating point and the surge limit line. If a surge control 
system can properly respond to a high rate of approach by opening the 
anti-surge valve before the operating point reaches the surge control 
line, the risk of surge can be significantly reduced. 
Due to considerations of stability, PID controllers commonly used for surge 
control are not suitable for the task of responding to the rate of 
approach despite of their "derivative" actions. In most anti-surge control 
applications, the process variable to a PID controller is a calculated 
value based on the measured gas flow. The gas flow rate as measured is 
inherently noisy and has low signal-to-noise ratio. If the derivative 
function of a PID is used to respond to the rate of approach, it would 
open and close the anti-surge valve in response to noise causing 
undesirable interference with the control of the process and needless 
waste of energy. For this reason, the derivative action of the PID cannot 
be relied upon to respond to the rate of approach. 
A method for responding to the rate of approach of the operating point to 
the surge line is disclosed in U.S. Pat. No. 4,949,276 to Staroselsky et 
al. That method involves moving the setpoint of a PID controller for 
controlling an anti-surge valve away from the surge line so that the 
process value of the PID controller would cross the setpoint sooner. As a 
result, the PID controller will act sooner to open the anti-surge valve 
than it would act otherwise. According to this reference, the amount by 
which the setpoint is moved is a function of the rate of approach of the 
operating point to the surge limit. Such a method has several 
disadvantages. For example, having a continuously changing setpoint makes 
it difficult to monitor the operation of the PID controller. Moreover, 
because that method creates an ever changing artificial error for the PID 
controller to respond to, the operation of the PID controller is less 
predictable. Furthermore, because of the requirements for smooth and 
stable operation, a normal PID controller for controlling the opening of 
the anti-surge valve is not optimized to respond to sharp flow 
disturbances, and therefore does not provide optimal response for reacting 
to the rate of approach of the operating point to the surge limit line and 
hence is difficult to set up for effective surge control. 
SUMMARY OF THE INVENTION 
In view of the foregoing, it is a general object of the present invention 
to provide an improved control system for use with a dynamic compressor 
that effectively prevents the compressor from surging. 
To that end, it is an object of the present invention to provide a surge 
prevention control system for a dynamic compressor that controls the 
position of the operating point of the compressor about a surge control 
line and at the same time is optimized to respond to high rate of approach 
of the operating point to the surge line. 
It is a related object of the present invention to provide a surge 
prevention control system for a dynamic compressor that provides optimized 
PID control over the position of the operating point of the dynamic 
compressor about a surge control line, and also provides optimized 
response to high rate of approach of the operating point to the surge line 
that does not adversely impact operation of the PID control. 
In accordance with those and other objects of the invention, there is 
provided a control system for preventing surge in a dynamic compressor and 
employing a multiple module control optimized for both position and rate 
control of the compressor operating point. The compressor has a variable 
operating point definable in a compressor map which includes a stable 
region, a surge region, a surge line separating the two regions and a 
surge control line near but displaced from the surge line. The control 
system operates an anti-surge valve which has an electrical input for 
adjusting the valve opening to controllably increase the flow through the 
dynamic compressor. The control system utilizes a multiple module 
controller which has an input for receiving a control variable indicative 
of the operating point of the dynamic compressor, and has an output signal 
for controlling the valve opening of the anti-surge valve. A PID control 
module in the multiple module controller receives the control variable as 
a process input and has a setpoint corresponding to the surge control line 
to produce a first output signal exerting control action on the operating 
point of the dynamic compressor in the region of the surge control line. A 
derivative module in the multiple module controller receives the control 
variable and produces a rate signal having a magnitude indicative of the 
rate of approach of the operating point to the surge control line. A rate 
control module in the multiple module controller receives the rate signal 
as a process input and produces a second output signal when the rate of 
approach exceeds a setpoint to begin a corrective opening of the 
anti-surge valve. An output signal selector receives a plurality of input 
signals including the first and second output signals and selects one of 
the input signals as the output signal of the multiple module controller. 
It is a feature of the present invention to use a closed loop PID control 
module to control the position of the operating point of the compressor in 
the region of a surge control line, and to use a rate control module to 
directly control the rate of approach of the operating point. The rate 
control module takes over control of the anti-surge valve and starts to 
open the anti-surge valve when the rate of approach becomes excessively 
high, even before the PID control module becomes active. 
It is a feature of the present invention that the rate control module is 
provided with an integrator function which allows the rate controller to 
reduce the error to zero. 
It is another feature of the present invention that the setpoint of the 
rate control module is adjusted according to the proximity of the 
operating point to the surge control line so as to avoid unnecessary 
opening of the anti-surge valve. 
It is yet another feature of the present invention that the gain of the 
rate control module is adjusted according to the operating conditions of 
the dynamic compressor to linearize the action of the rate control module.

DETAILED DESCRIPTION OF THE INVENTION 
Turning now to the drawings, FIG. 1 is a schematic diagram showing a 
dynamic compressor 11 with a surge prevention control system. The surge 
prevention control involves adjusting an anti-surge valve 12 which is 
connected to the output of the dynamic compressor 11. In the system shown 
in FIG. 1, the anti-surge valve 12 is also connected to the inlet 10 of 
the dynamic compressor 11. The anti-surge valve 12 has an adjustable 
opening which is controlled by an electrical signal sent to a control 
input 112 of the anti-surge valve 12. When the anti-surge valve 12 is 
opened, a portion of output flow of the dynamic compressor 11 is bypassed 
around the compressor 11 and returned to its inlet 10 so the flow bypassed 
through the anti-surge valve 12 is recycled. Bypassing flow around the 
dynamic compressor 11 increases the total flow through the dynamic 
compressor 11, which has the effect of moving the operating point away 
from the surge region. It will be appreciated that instead of recycling 
the gas as illustrated in FIG. 1, the flow of the dynamic compressor 11 
can also be increased by simply dumping a portion of the output flow of 
the dynamic compressor 11 via the anti-surge valve 12. When the term 
"bypass" is used herein, unless the context indicates otherwise, it is 
intended to encompass both the preferred form of recycling, as well as the 
less preferred form of dumping. 
To effectively prevent surge in the compressor 11 while at the same time 
minimizing interference with the downstream process 14 which receives the 
compressed gas, the opening and closing of the anti-surge valve 12 should 
be carefully controlled. As shown in FIG. 1, the valve opening is 
controlled by a multiple module controller 20. In accordance with the 
teaching of the present invention, the multiple module controller 20 not 
only operates the anti-surge valve 12 to control the operating point of 
the compressor 11 when the operating point is close to the surge line, but 
also operates the anti-surge valve 12 to directly control the rate of 
approach of the operating point to the surge line when such rate of 
approach is high. In other words, the multiple module controller 20 
anticipates the likelihood of surge by monitoring the rate of approach and 
takes corrective action by opening the anti-surge valve 12 to control the 
rate of approach in the event that the operating point moves rapidly 
toward the surge line. 
In more detail, the dynamic compressor 11 has a gas inlet schematically 
shown at 10, and operates to compress the gas and supply the compressed 
gas via an output 18 to a downstream process 14. A plurality of sensors 
are disposed in the inlet 11 and outlet 18 as is conventional, and are 
adapted to monitor the operating conditions of the compressor. As 
illustrated in FIG. 1, the sensors typically include an inlet temperature 
sensor 91, an inlet pressure sensor 92, a flow sensor 93, a discharge 
pressure sensor 94, a discharge temperature sensor 95. Other types of 
sensors can also be used. The output signals of the sensors are sent to a 
process measurement module 15 which processes the sensor output signals to 
determine the operating conditions of the compressor 11. The output of the 
process measurement module 15 is used by a control variable calculator 16, 
which calculates a control variable according to the measured operating 
conditions. The control variable is then used by the multiple module 
controller 20 to generate an output control signal, which is used by the 
valve positioning controller 17 to control the valve opening of the 
anti-surge valve 12. 
A more detailed view of the multiple module controller 20 is shown in FIG. 
2. The multiple module controller 20 comprises a PID control module 21 and 
a rate control module 40, both of which operate to control the valve 
opening of the anti-surge valve 12 to prevent surge in the compressor 11 
(FIG. 1). The PID control module 21 is optimized for controlling the 
position of the operating point of the dynamic compressor 11 between a 
predefined surge control line and the surge line so as to resist movement 
of the operating point into the surge region. 
Digressing briefly to FIG. 5, there is shown a compressor map in which a 
surge line 70 divides an unstable operating region 73 (the surge region) 
to the left of the surge line from a stable operating region 74 to the 
right of the surge line 73. A surge control line 71 defines an operating 
region between the surge line 70 and surge control line 71 where the PID 
control module 21 is primarily tuned to control the anti-surge valve. 
In practicing the invention, in addition to the PID control module 21, a 
rate control module 40 is operative, regardless of the position of the 
operating point in the compressor map, to respond to a rapid movement of 
the operating point toward surge by assuming control of the anti-surge 
valve 12 to open the anti-surge valve 12 and reduce the rate at which the 
operating point is approaching the surge line 73. In other words, the rate 
control module 40 is optimized to anticipate the likelihood of surge and 
to directly control the rate of approach. Thus, the rate control module 40 
will respond to a high rate of approach by opening the anti-surge valve 12 
even if the operating point is still relatively far away from the surge 
line 73. 
In more detail, as shown in FIG. 2, the multiple module controller 20 has a 
control signal input 120 for receiving the control variable generated by 
the control variable calculator 16. The calculated control variable is 
indicative of the operating point of the dynamic compressor 11 in the 
compressor map. The control variable generally used in compressor surge 
control systems is defined as: 
EQU Control variable=(actual flow).sup.2 /polytropic head. 
It will be appreciated that each value of the control variable defined in 
this way corresponds to a parabolic curve in the compressor map. The 
position of the operating point of the compressor 11 in the map 
corresponding to a given value of the control variable can be uniquely 
determined if the actual flow rate is known. 
The control variable is received by the PID control module 21 as its 
process variable. The control variable is further sent to a derivative 
module 22 which operates on the control variable to produce a rate signal 
having a magnitude indicative of the rate of approach of the operating 
point to the surge line 70 (FIG. 5). The rate signal is then sent to the 
rate control module 40 as its process variable. It will be appreciated 
that the rate signal can indicate a motion of the operating point either 
towards or away from the surge region. Unless otherwise specified, the 
term "rate of approach" used herein refer to the speed of movement of the 
operation point towards the surge region. 
The PID control module 21 exerts closed loop control on the control 
variable to produce proportional, integral, and derivative terms and 
ultimately produces a first output signal for controlling the positioning 
of the anti-surge valve 12. The PID control module 21 is provided with a 
setpoint which corresponds to the surge control line 71 (FIG. 5), and the 
PID control module 21 acts to control the control variable when the value 
of the control variable is between the surge control line 71 and the surge 
line 70. In other words, the PID control module 21 acts to exert control 
action when the operating point of the dynamic compressor 11 assumes a 
position between the surge control line 71 and the surge line 70. The 
setpoint of the PID control module 21 is preferably selected to allow a 
chosen safety margin, typically between % 5 and 15%, between the surge 
line and the surge control line 71. 
The rate control module 40 is also a form of closed loop PID controller 
having proportional, integral, and derivative terms. The rate control 
module 40 uses the rate signal as its process variable and generates a 
second output signal for controlling the opening of the anti-surge valve 
12 when the rate of approach exceeds the setpoint of the rate control 
module 40. In response to a high rate of approach, caused, for example, by 
a sudden drop of the flow rate of the compressor 11, the rate control 
module 40 generates an output signal to open the anti-surge valve 12 to 
increase the flow so as to reduce the rate of approach toward the setpoint 
of the rate control module 40. The setpoint of the rate control module 40 
also functions as a threshold point so that the rate control module 40 
will not open the valve if the rate of approach is below the setpoint. In 
this way, unnecessary opening of the anti-surge valve 12 and interference 
to the downstream process 14 (FIG. 1) can be minimized. 
As shown in FIG. 2, both the PID control module 21 and the rate control 
module 40 generate output signals for controlling the opening of the 
anti-surge valve 12. In the preferred embodiment, the control operations 
of the PID control module 21 and the rate control module 40 are 
coordinated using an output signal selector 25. The output signal selector 
25 typically has a plurality of input signals. For purposes of the present 
invention, the only input signals of interest are those from the PID 
control module 21 and the rate control module 40. However, it will be 
appreciated that other controllers can provide output signals for the 
anti-surge valve 12, and those can include open loop controllers, load 
sharing controllers, and the like. Each of the input signals is normalized 
to correspond to a degree of valve opening of the anti-surge valve 12. One 
of the input signals is selected by the output signal selector 25 as the 
output signal of the multiple module controller 20 for controlling the 
anti-surge valve 12. Preferably the output signal selector 25 is a high 
signal selector in the sense that it selects the input signal that 
corresponds to the largest valve opening among the input signals. 
In this way, the rate control module 40 and the PID control module can 
effectively perform their respective functions when the situation calls 
for either of them. For example, when the rate of approach exceeds the 
setpoint of the rate control module 40 but the operating point is still in 
the stable operation region, the rate control module 40 will be selected 
to open the valve 12 to prevent surge. On the other hand, if the operating 
point is close to the surge line but moving slowly, the PID control module 
21 will be selected to control the valve opening to bring the operating 
point back to the surge control line. On another scenario, if the 
operation point is in the stable region and moving away from the surge 
region the rate control module will try to close the valve rapidly. 
Typically in such a case, the PID control module 21 will tend to close the 
valve at a slower pace. In such a case, the PID control module will have 
control of the valve and close the valve smoothly. 
It has been appreciated that in the past that the rate of approach of the 
operating point to the surge control line 71 (FIG. 5) can be indicative of 
an impending surge. While the prior art as made attempts to use rate 
related information in surge control, it is believed that the control 
approaches have all been indirect, that is they have operated on another 
variable, such as moving the set point for the normal PID controller. 
In controlling directly on rate in accordance with the present invention, a 
special purpose controller is used which takes account of the fact that 
the process signal for the controller is rate information, not position 
information. The controller, like a conventional closed loop controller, 
has a process signal and a setpoint which are compared to produce an error 
signal the magnitude of which drives the output to zero the error signal. 
However, when the process signal is a rate signal, and the controller is a 
PID, the ability to zero the error signal is lost unless special steps are 
taken. 
In accordance with an important feature of the present invention, the rate 
control module 40 is provided with an integrator which provides an 
integral operation in addition to the proportional-integral-derivative 
terms commonly used in PID controllers. The rate control module 40 in the 
preferred embodiment is therefore an IPID controller. This integrator is 
provided so that the rate control module 40 reduces the error defined as 
the difference between its setpoint and its process variable, to zero. The 
integrator is necessary for that purpose because operating a conventional 
PID on a process variable which is a derivative of a field measured or 
calculated signal will provide action on the signal but will not have 
sufficient control response to reduce the error to zero in transient 
situations. For example, if the downstream process using the gas is shut 
down, the flow may drop continuously over a period of time of, for 
example, thirty seconds. If a normal PID is used to control the rate, it 
will hold the valve opening at a constant level if the process input is 
equal to the setpoint. However, if the valve opening is held constant, the 
rate will pick up again, and the rate error will increase accordingly. The 
rate control module 40 is an IPID, is different from conventional PID 
controller in that it will continue to open the valve even if the process 
input equals the rate setpoint, i.e., if the error is zero. Such a 
response is due to the function of the additional integrator in the rate 
control module. In this way, the rate control module 40 is capable of 
reducing the rate error to zero. The effect of the integrator can be 
illustrated, using Laplace transformation notation as follows. The control 
variable, in the preferred embodiment, is defined as: 
##EQU1## 
where WS is the control variable, Q is the actual flow, and H is the 
polytropic head. The process variable for the rate control is defined as: 
EQU Process variable=(s) WS 
The setpoint of the rate control is defined as 
EQU Setpoint=(s) WS.sub.MAX. 
The error term of the rate control is then 
EQU Error=Setpoint-Process Variable=s.multidot.(WS.sub.MAX -WS). 
A conventional PID has a transfer function defined as: 
##EQU2## 
The output of a conventional PID, when presented with the error, will have 
the following form: 
##EQU3## 
It can be seen that the "integral" term is missing in such an output. This 
is significant from a control standpoint since it is appreciated that the 
integral term is the term which ultimately brings the error signal to zero 
in a PID controller. Without an integral term, the PID controller cannot 
control the derivative of a process signal to a desired setpoint, i.e., 
the error cannot be reduced to zero. 
According to the present invention, the rate controller is provided with an 
integrator so that its transfer function has the following form: 
##EQU4## 
When the rate control module operates on the error, the output has the 
form: 
##EQU5## 
It will be appreciated that the "integral" term, represented by I/(1+s), 
is now restored in the output. With the integrator, the rate control 
becomes an integral-proportional-integral-derivative (IPID) type 
controller, instead of a conventional PID type controller. 
One embodiment of the rate control module 40 having an integrator is shown 
in FIG. 3. In this embodiment, the integrator 46 is implemented by adding 
a derivative operator 45 in a feedback path 44 to the PID portion 47 of 
the rate control module 40. Incorporating the derivative function of the 
derivative operator 45 in the feedback path 44 emulates an integral 
operation, which is in addition to the "integral" term in the PID portion 
47. 
As shown in FIG. 3, the PID portion 47 of the rate control module 40 
includes a proportional operator 38, an integral operator 39, and a 
derivative operator 41. The output of the integral operator 39 is 
processed by the derivative operator 45, and the output of the derivative 
operator 45 is sent to the input of the integral operator 39 via the 
feedback path 44. This arrangement of feeding the output of the integral 
operator 39 back to its input via the derivative operator 45 emulates an 
integration function and provides the transfer function described above. 
Such an integration function allows the error of the rate control module 
40 to be reduced to zero. 
Referring in greater detail to FIG. 3, it will be seen that the input of 
the derivative operator 45 is taken at the output of the integral operator 
39, and after the proportional operator 38, but before the derivative 
operator 41 in the PID module 47. Thus, the input to the derivative module 
45 contains position related information. This is because the process 
variable, which is presented to the first summer 34 contains rate 
information which is presented to the integrator module 39 which 
integrates the rate information to produce position related information. 
The derivative operator 45 thereupon notes changes in the integrated rate 
information, and feeds that information back to the input of the integral 
operator 39 by way of summer 35. It will thus be appreciated that position 
related information is inserted into the PID function, and it is that 
information which allows the modified PID (IPID) of the rate control 
module 40 to zero the error signal. 
It will be appreciated that the multiple module controller 20 allows 
transfer between modules and the controller, and if not properly 
implemented, can produce substantial discontinuities in the output signal 
applied to the anti-surge valve 12, when switching from module to module. 
In order to prevent that, in accordance with the present invention, the 
multiple module controller 40 is provided with a tracking function whereby 
the controller 40 determines which of the modules is the active module in 
control of the anti-surge valve 12, and which are inactive. All of the 
modules continue to monitor their input signals, and if steps were not 
taken, would produce output signals which differ from module to module. 
However, in accordance with the invention, the multiple module controller 
40 is provided with tracking means for causing the inactive modules to 
track the output of the active module. To that end, the output of the 
output signal selector 25 is coupled to the tracking inputs of all the 
modules, and a tracking control line is provided for causing all of the 
inactive modules to track the output of the output signal selector 25. 
In greater detail, either of the PID control module 21 or the rate control 
module 40 can have control over the anti-surge valve 12 as long as its 
output signal is larger than that of the other. In order to ensure smooth 
operation of the anti-surge valve 12 when the control is switched from the 
PID control module 21 to the rate control module 40 or vise versa, both 
the PID control module 21 and the rate control module 40 are provided with 
a tracking feature so that their output signals track the output signal of 
the output signal selector 25. As shown in FIG. 2, the output signal of 
the output signal selector 25 is fed back to the PID control module 21 and 
the rate control module 40 via output tracking lines 27 and 29. The output 
signal selector 25 also has tracking control lines 26 and 28 connected to 
the PID control module 21 and the rate control module 40, respectively. 
The tracking control lines 26 and 28 are used for transmitting tracking 
control signals indicating to each of the control modules 21 and 40 
whether its output signal has been selected, i.e., whether it is currently 
in control of the anti-surge valve 12. The control module that is not in 
control will then adjust its output to track the output of the output 
signal selector 25. 
FIG. 3 shows an example of the implementation of the tracking function of 
the rate control module 40 and the PID control module 21 described above. 
In this embodiment, a switch 42 is provided to select between the output 
signal of the output signal selector 25 on the tracking output line 43 and 
the output of the derivative operator 45. When the tracking control signal 
on the tracking control line 26 indicates that the rate control module 40 
is not in control of the anti-surge valve 12 (FIG. 2), the switch 42 is 
controlled to connect the output signal of the output signal selector 25 
on the tracking output line 43 to the summer 35 as the input to the 
integral operator 39. Providing the output of the output signal selector 
25 to the summer 35 serves to preload the integral module 39, causing the 
output signal of the rate control module 40 to be driven to the same level 
as the output signal of the active PID control module 21. Thus, when it is 
desired to switch roles between active and inactive control modules 21 and 
40, their outputs will be at the same level, providing a bumpless 
transfer. However, the control after the switch 40 will be in response to 
the process input and the PID functions associated with the now-active 
control module. 
Adding the integrator function to the rate control module PID, and 
operating the rate control module 40 in parallel with the conventional PID 
module 21 provides a substantially enhanced control system. An output 
signal selector 25, coupled to the outputs of the respective control 
modules, selects the signal from the control module demanding the largest 
valve opening. Thus, the PID control module 21 is operative when the 
operating point is near the surge control line 71 (FIG. 5), and in that 
region, its output signal will normally control. However, with the 
operating point in any part of the compressor map, if the rate of approach 
of the operating point to the surge control line 71 exceeds the setpoint 
of the rate control module 40, the rate control module 40 will produce a 
high output signal and take control of the anti-surge valve 12. 
In a simplified system, the set point for the rate control module 40 can be 
set at a fixed level corresponding to the maximum rate of approach 
acceptable in the compressor system. If the rate of approach exceeds that 
preset value, the rate control module 40 will produce an output signal. If 
its output is larger than the output signal produced by the PID control 
module 21, it will take charge of the anti-surge valve 12 and control its 
degree of opening. 
As a further improvement on that system, however, means are provided for 
adjusting the setpoint of the rate control module 40 so as to render that 
module equally effective both near and away from the surge control line. 
To that end, and in accordance with a feature of the present invention, 
the setpoint of the rate control module 40 is adjusted according to the 
distance between the operating point of the compressor 11 and the surge 
line 70 (FIG. 5). Generally, the setpoint is set high when the operating 
point is far from the surge line 70, and is reduced when the operating 
point is closer to the surge line 70. In this way, higher flow rate 
fluctuation can occur without triggering the rate control module 40 into 
action when the flow is high, thereby avoiding unnecessary opening of the 
anti-surge valve 12. When the operating point is close to the surge line 
70, however, even a low rate of approach may bring the operating point 
into the surge region. Accordingly, the setpoint of the rate control 
module 40 is reduced to provide higher sensitivity when the operating 
point is near the surge line 70. It is important to note that the setpoint 
of the rate control module 40 cannot be allowed to be reduced to zero; if 
it were, the rate control module 40 would keep the anti-surge valve 12 
fully open regardless of the steady state condition of its process 
variable. This condition is avoided by setting a minimum non-zero lower 
limit for the output of the setpoint adjuster 24. 
In the preferred embodiment, the proximity of the control variable to the 
setpoint of the PID control module 21, i.e., the difference between the 
control variable and the surge control line setpoint of the PID control 
module 21, is used as an indicator of the proximity of the operating point 
of the dynamic compressor 11 to the surge line 70 for the purpose of 
adjusting the setpoint of the rate control module 40. An example of the 
functional dependence of the setpoint of the rate control module 40 on the 
proximity of the control variable to the setpoint of the PID control 
module 21 is illustrated in FIG. 4. As shown in FIG. 4, the setpoint of 
the rate control module 40 is variable between a maximum value and a 
minimum value. When the difference between the control variable and the 
setpoint of the PID control module 21 is above a value indicated by the 
point 61, the setpoint is fixed at a maximum value indicated by the point 
63. When difference between the control variable and the setpoint of the 
PID control module 21 is below a value indicated by the point 62, the 
setpoint is fixed at a non-zero minimum value indicated by the point 64. 
When the difference between the control variable and the setpoint of the 
PID control module 21 is in the range between the two points 61 and 62, 
the setpoint of the rate control module 40 is linearly dependent on the 
difference. It will be appreciated that the method of setting the setpoint 
of the rate control module 40 as illustrated in FIG. 4 is only provided as 
an example, and other ways of selecting the value of the setpoint point 
can be used without departing from the scope and spirit of the present 
invention. In the preferred embodiment as shown in FIG. 2, the setpoint of 
the rate control module 40 is adjusted by a setpoint adjuster 24 which 
receives a signal from the control variable calculator 16 and determines 
the difference between the control variable and the setpoint of the PID 
control module 21. 
In accordance with another feature of the present invention, the gain of 
the rate control module 40 is adjusted to regulate the dynamics of the 
rate control module 40 as the process conditions of the dynamic compressor 
11 change. When the gas properties and the pressure and temperature at the 
inlet 10 (FIG. 1) and outlet 18 (FIG. 1) of the compressor 11 change, the 
flow through the anti-surge valve 12 can change significantly. As a 
result, the same amount of valve opening adjustment can have significantly 
different degrees of effect on the flow, depending on the process 
conditions of the dynamic compressor 11. In order to linearize the action 
of the rate control module 40 and to compensate for the process 
variations, the gain of the rate control module 40 is controlled according 
to the following gain function: 
EQU Gain=K.multidot.F(P.sub.s, P.sub.d, T.sub.d, SG), 
where P.sub.s, P.sub.d, and T.sub.d are the suction pressure, discharge 
pressure, and discharge temperature of the dynamic compressor 11, 
respectively; SG is the specific gravity of the gas being processed by the 
compressor 11; K is a constant which is the typical gain for system 
response; F is a function that represents the normalized change in the 
relationship between the valve action and flow as the process conditions 
change, which depends on the flow characteristics of the anti-surge valve 
12. By using a gain that is adjusted according to the response of the 
anti-surge valve 12 to the changes in process conditions, the dynamics of 
the rate control module 40 change appropriately as process conditions 
change. In this manner the rate control module 12 need be tuned at only 
one set of process conditions of the dynamic compressor 11 to generate a 
set of PID dynamics that is operational over the entire operating range of 
the dynamic compressor 11. In the embodiment of the multiple module 
controller 20 shown in FIG. 2, the gain of the rate control module 40 is 
set by a gain adjuster 18. 
FIG. 6 is a block diagram showing the interrelationship of the elements for 
establishing the values of the gain, setpoint and process variable of the 
rate control module 40 in the preferred embodiment of the invention. As 
shown in FIG. 6, a module 81 performs a process measurement to determine 
the operating conditions of the compressor 11. The result of the process 
measurement is used in a module 82 to calculate the control variable 
typically (flow).sup.2 /head. The control variable is then used in the 
setpoint adjuster 24 to establish the setpoint of the rate control module 
40 as a function of the difference between the control variable and the 
setpoint of the PID control module 21. The process variable of the rate 
control module 40 is the derivative of the primary (flow).sup.2 /head 
process variable, and is determined in the derivative module 22. To 
determine the gain of the rate control module 40, the flow characteristics 
of the anti-surge valve 12 are determined in module 85. The valve flow 
characteristics and the result of the process measurement are then used to 
normalize the response of the anti-surge valve 12 as a function of the 
process conditions in module 86. A gain constant for system response is 
established using the output of the module 86. The gain constant and the 
normalized process response are multiplied in module 88 to control the 
value of the gain of the rate control module 40. 
The interaction between the PID control module 21 and the rate control 
module 40 will now be illustrated by way of example. In this example, it 
is assumed that the output signal selector 25 (FIG. 2) receives input 
signals from only the PID control module 21 (FIG. 2) and the rate control 
module 40 (FIG. 2), so that the output signal of either of the two control 
modules 21 and 40 will be selected as the output signal of the output 
signal selector 25 for controlling the valve opening of the anti-surge 
valve 12 (FIG. 2). It is assumed that the setpoint for the PID control 
module 21 is normalized to one hundred (100.0). If the control variable 
has a value smaller or equal to 100.0, the PID control module 21 acts to 
open the anti-surge valve 12 (FIG. 2). If the control variable is larger 
than 100.0, the PID control module 21 acts to close the anti-surge valve 
21. 
The rate control module 40, on the other hand, acts on the rate at which 
the control variable approaches the setpoint of the PID control module 21. 
In this example, the multiple module controller 20 (FIG. 2) is assumed to 
have a loop response time of ten (10.0) seconds. The setpoint of the rate 
control module 40 is set using the following equation: 
##EQU6## 
for a control variable value within the range of 105.0 to 200.0, where 
S.sub.R is the setpoint of the rate control module 40, S.sub.PID is the 
setpoint of the PID control module 21, and CV is the control variable. The 
setpoint is set to a maximum of 10.0 if the control variable exceeds 
200.0, and is set to a minimum of 0.5 if the control variable falls below 
105.0. 
For purposes of illustration, the compressor map of the dynamic compressor 
11 (FIG. 1) controlled by the multiple module controller 20 is shown in 
FIG. 5. The vertical axis of compressor map is the polytropic head, and 
the horizontal axis is the actual flow of the dynamic compressor 11. The 
surge line 70 divides the surge region 73 and the stable operation region 
74. A surge control line 71, which corresponds to the selected setpoint of 
the PID control module 21, is disposed in the stable operation region 74 
at a selected safety margin from the surge line 70. In this example, the 
surge line 70 is assumed to correspond to a control variable value of 
90.0. 
In the compressor map shown in FIG. 5, it is assumed that the points A, B, 
C, D, and E correspond to the control variable values of 150.0, 140.0, 
130.0, 95.0, and 100.0, respectively. Assume that initially the operating 
point of the dynamic compressor 11 is at point A. At point A, the setpoint 
of the rate control module 40 is 5.0. Now assume that the operating point 
moves from point A to point C, at a rate of 4.0 per second. Initially the 
rate control module 40 would not respond to such a movement because the 
rate is lower than its setpoint. However, once the operating point passes 
point B, the control variable drops below 140.0, and the setpoint of the 
rate control module 40 drops below 4.0. The rate control module 40 then 
becomes active and generates an output signal to open the anti-surge valve 
12 because its setpoint has become lower than the reduction rate of the 
control variable. At the same time, the PID control module 21 produces an 
output signal to close the anti-surge valve 12 because the control 
variable is still higher than the setpoint of the PID control module 21. 
In this situation, the output signal of the rate control module 40 is 
selected by the output signal selector 25 and the rate control module 40 
begins to open the anti-surge valve 12 to control the reduction rate of 
the control variable. It will be appreciated that due to the mathematical 
correspondence between the control variable and the position of the 
operating point on the compressor map, controlling the rate of change of 
the control variable is conceptually equivalent to controlling the rate of 
approach of the operating point to either the surge control line 71 or the 
surge line 70. 
When the operating point reaches point C, the setpoint of the rate control 
module 40 is reduced to 3.0. If the operating point stops at point C so 
that the control variable is stable at 130.0, then the rate control module 
40 will start to close the anti-surge valve 12. If, however, the operating 
point continues to move from point C to point D and that the control 
variable drops at a rate higher than the setpoint of the rate control 
module 40, the rate control module 40 will continue to open the anti-surge 
valve 12 in order to control the reduction rate of the control variable. 
Once the operating point passes the point E on the surge control line 71, 
the control variable is reduced below 100.0, the setpoint of the PID 
control module 21. The PID control module 21 then becomes active and 
generates an output signal to open the anti-surge valve 12 in order to 
control the control variable. The output signal of the PID control module 
21 is compared to the output signal of the rate control module 40, and the 
larger of the two output signals will be selected to control the 
anti-surge valve 12. If the operating point stops at point E so that it is 
no longer approaching the surge line 70, the rate of change of the control 
variable is below the setpoint of the rate control module 40, which is 0.5 
at point E. The rate control module 40 then generates an output signal to 
close the anti-surge valve 12. At the same time, the PID control module 21 
generates an output signal to open the anti-surge valve 12 in order to 
move the control variable toward its setpoint of 100.0. In this situation, 
the PID control module 21 will have control of the anti-surge valve 12. 
The switching of control of the anti-surge valve 12 between the PID control 
module 21 and the rate control module 40 depends on how fast the control 
variable changes, the value of the control variable, and the tuning of the 
rate control module 40 and the PID control module 21, as well as the 
system dynamics. If the operating point is moving slowly but is left of 
the surge control line 71, then the PID control module 21 is likely to 
gain control of the anti-surge valve 12. On the other hand, if the 
operating point is very close to the surge control line 71 but is moving 
toward the surge line 70 at a sufficiently high rate, then the rate 
control module 40 is likely to gain control of the anti-surge valve 12. 
The method of the present invention for preventing surge in the dynamic 
compressor 11 (FIG. 1) will now be described in conjunction with FIG. 7. 
In order to control the flow of the dynamic compressor 11 to prevent 
surge, an anti-surge valve 12 (FIG. 1) is connected to the dynamic 
compressor 11. The valve opening of the anti-surge valve 12 is 
controllably adjusted to bypass the output flow around the dynamic 
compressor 11 to increase the flow through the dynamic compressor 11. In 
order to properly control the opening of the anti-surge valve 12, the 
process conditions, such as flow, pressure, and temperature, etc., of the 
compressor 11 are measured (step 102). The result of the process 
measurement is used to produce a control variable indicative of the 
operating point of the compressor 11 in its compressor map (step 103). 
A step 105 exercises closed loop PID control on the control variable 103. 
The step has a setpoint, schematically indicated at 104, which determines 
the point to which the process variable is to be controlled. In the 
preferred practice of the invention, the setpoint 104 is the surge control 
line 71 (FIG. 5), and the step 105 exerts control of the anti-surge valve 
primarily in the region between the surge control line 71 and the surge 
line 70 (FIG. 5). In doing so, the PID control function is used to 
generate a first output signal (step 106) which controls the output of the 
anti-surge valve 12. 
In order to anticipate the risk of surge, the rate of approach of the 
operating point towards surge is monitored (step 107). Preferably, the 
same signal which serves as the process input for the control operation in 
step 105, is used as an input to step 107, and step 107 simply produces 
the time derivative of the process signal. That time derivative serves as 
an indicator of the rate of approach of the compressor operating point to 
the surge line 70 or the surge control line 71. 
In accordance with the teaching of the present invention, the rate of 
approach is directly controlled by performing a second closed loop PID 
operation on the rate of approach (step 108). The PID of step 108 also has 
a setpoint, identified schematically at 108', which is used with the rate 
signal as a process variable to produce an error signal, and to generate 
an output signal from the PID of step 108 capable of controlling the 
anti-surge valve 21. In the simplest implementation, the setpoint 108' is 
simply a fixed level indicating the maximum allowable rate of approach of 
the operating point to the surge control line. Alternatively, the setpoint 
adjusting module 24 of FIG. 2 is preferably utilized. 
The step 108 utilizes a PID control in combination with an integrator as 
described in detail above to produce an output signal capable of reducing 
the error between the setpoint and the process variable to zero. The 
output signal is a measure of the degree of opening of the anti-surge 
valve demanded by the rate control PID, and the generation of that output 
signal is illustrated by the block 109. A selection between first and 
second output signals is then made (step 110), and the output signal 
corresponding to larger opening of the anti-surge valve 12 is selected to 
control the valve opening of the anti-surge valve 12 (step 111). Thus, if 
the first output signal is selected, the first PID operation of step 105 
will have control of the anti-surge valve 12 to control the control 
variable. On the other hand, in the event of high rate of approach of the 
operating point to the surge control line 71, the second PID operation of 
step 108 will produce a larger output signal than that of the first PID 
control. In such a case, the second PID operation takes control of the 
anti-surge valve 12 and opens it to control the rate of approach. 
The foregoing description of various preferred embodiments of the invention 
has been presented for purposes of illustration and description. It is not 
intended to be exhaustive or to limit the invention to the precise forms 
disclosed. Obvious modifications or variations are possible in light of 
the above teachings. The embodiments discussed were chosen and described 
to provide the best illustration of the principles of the invention and 
its practical application to thereby enable one of ordinary skill in the 
art to utilize the invention in various embodiments and with various 
modifications as are suited to the particular use contemplated. All such 
modifications and variations are within the scope of the invention as 
determined by the appended claims when interpreted in accordance with the 
breadth to which they are fairly, legally, and equitably entitled.