Apparatus for the prevention of critical process variable excursions in one or more turbomachines
View Patent ↗Many variables in processes such as those using turbocompressors and turbines must be limited or constrained. Limit control loops are provided for the purpose of limiting these variables. By using a combination of closed loop and open loop limit control schemes, excursions into unfavorable operation can be more effectively avoided. Transition between open loop and closed loop may be enhanced by testing the direction and magnitude of the rate at which the limit variable is changing. If the rate of change indicates recovery is imminent, control is passed back to the closed loop limit control function.
1. An apparatus for providing limit control, not antisurge control, of a compression process having a limit variable, L, values of said limit variable being divided into a first region wherein closed loop limit control is used and a second region in which open loop limit control is used, the apparatus comprising:
(a) a first calculator for calculating the value of the limit variable, L, based on parameters associated with a turbocompressor;
(b) a closed loop limit controller in effect when the value of the limit variable, L, is in the first region; and
(c) an open loop limit controller in effect when the value of the limit variable, L, is in the second region.
2. The apparatus of claim 1 including means to return control to the closed loop controller when the value of a limit variable, L, returns in the first region.
3. The apparatus of claim 1 wherein the open loop limit controller comprises means for changing a value of a manipulated variable as quickly as possible a predetermined increment.
4. The apparatus of claim 3 including a second calculator for determining a variable predetermined increment during operation.
5. The apparatus of claim 1 including a suction pressure sensor for sensing a suction pressure of the turbocompressor as the limit variable, L.
6. The apparatus of claim 1 including a discharge pressure sensor for sensing a discharge pressure of the turbocompressor as the limit variable, L.
7. The apparatus of claim 1 wherein the turbocompressor comprises a plurality of stages and the apparatus additionally comprises an interstage pressure sensor for sensing an interstage pressure of the turbocompressor as the limit variable, L.
8. A apparatus for providing limit control of a process having a limit variable, L, values of said limit variable being divided into a first region wherein closed loop limit control is used and a second region in which open loop limit control is used, the apparatus comprising:
(a) an open loop limit controller, in effect when the value of a limit variable, L, is in the second region;
(b) a derivative calculator for calculating a value of a first temporal derivative, dL/dt, of the limit variable, L; and
(c) a closed loop limit controller in effect if the value of the first temporal derivative, dL/dt, has a sign indicating the value of L is changing toward the first region.
9. The apparatus of claim 8 wherein the values of the limit variable, L, are divided into three regions: a first region wherein closed loop limit control is used and a second region in which open loop limit control is used, and a third region wherein no limit control is required, the apparatus additionally comprising:
(a) means for setting a closed loop limit control set point in a neighborhood of a boundary between the first and third regions;
(b) means for setting an open loop limit control set point toward the second region relative to the closed loop limit control set point; and
(c) an open loop limit controller in effect when a value of a limit variable, L, is at the open loop limit control set point or on an opposite side of the open loop limit control set point relative to the closed loop limit control set point.
10. The apparatus of claim 8 additionally comprising a comparator for testing a magnitude of dL/dt before providing closed loop limit control.
11. The apparatus of claim 8 additionally comprising a function calculator for determining a set point for the closed loop limit controller as a function of dL/dt.
12. The apparatus of claim 8 additionally comprising a function calculator for determining a set point for the open loop limit controller as a function of dL/dt.
13. The apparatus of claim 10 additionally comprising a logic function for bounding a set point for the closed loop limit controller.
14. The apparatus of claim 11 additionally comprising a logic function for bounding a set point for the open loop limit controller.
15. The apparatus of claim 8 additionally comprising a manipulated variable, M, adjusted to control the value of the limit variable, L.