I am using ANSYS Twinbuilder to model a system containing two reservoirs (open tank) connected to their respectives pumps (2) connected to two Mills (opentank). Everything is modelled in modellica. I want to control the flow depending on the plant operation mode, to every possible combination of tanks using discrete valves. The discrete valves contain variables for ex: flowdir_R1M1, meaning it controls the flow from Reservoir 1 to Mill 1. The code produces the following error:

[error] Index reduction failed 
[error] The system is structurally singular. The following variable(s) could not be matched to any equation:      flowdir_R1M2      flowdir_R2M1      flowdir_R2M2      flowdir_M1R2      flowdir_M2R1      flowdir_M2R2      flowdir_M1M2      flowdir_R1R2      flowdir_R2R1 

code (relevant bit):

model plant
        //Declarations
        ......
        parameter Integer op_mode = 1;
    Boolean flowdir_R1M1;
    Boolean flowdir_R1M2;
    Boolean flowdir_R2M1;
    Boolean flowdir_R2M2;
    Boolean flowdir_M1R1;
    Boolean flowdir_M1R2;
    Boolean flowdir_M2R1;
    Boolean flowdir_M2R2;
    Boolean flowdir_M1M2;
    Boolean flowdir_R1R2;
    Boolean flowdir_R2R1;
    Real rpm_nominal = 500;
    Real pump1_rpm;
    Real pump2_rpm;

//connections
....

initial algorithm
    flowdir_R1M1 := false;
    flowdir_R1M2 := false;
    flowdir_R2M1 := false;
    flowdir_R2M2 := false;
    flowdir_M1R1 := false;
    flowdir_M1R2 := false;
    flowdir_M2R1 := false;
    flowdir_M2R2 := false;
    flowdir_M1M2 := false;
    flowdir_R1R2 := false;
    flowdir_R2R1 := false;
    pump1_rpm := 0;
    pump2_rpm := 0;

algorithm
// Control logic based on mode
  if op_mode == 1 then
    flowdir_R1M1 := true;
    flowdir_M1R1 := true;
    pump1_rpm := rpm_nominal;
    pump2_rpm := 0;
 end if;

equation
....

end plant;

I new to modelica and am not able to pinpoint what’s causing the error. I’d really appretiate your help!