Here is a  Valve model that is giving me problem when I try if equations in it to allow  reverse flows. I am aware concepts related to flow reversals have been covered extensively in many of Modelica presentations that explain Streams. This is my attempt to understand them through simple examples.

When I try to use this model "omc" complains that the system is under determined .... needing two more equations. But I am unable to see what other eqns. are needed here.

The model works fine if I replaced the conditional equations with the equations that are in the (deltaP>0) case.  (as long as I don't get into reverse flow conditions and that is what I expected).

My aim is to make the valve work in both flow directions. 



[model Valve
  // A simple Valve (that permits bidirectional flow)
 
  replaceable package Medium = PartialSimpleIdealGasMedium ;

FluidPort     inlet (redeclare package Medium = Medium)  ;
  FluidPort     outlet (redeclare package Medium = Medium)  ;

  parameter Real   cv = 1.0 / sqrt(1e5)     ;   
Medium.ThermodynamicState    state ;
 

  equation
    // No change in Enthalpy in the Valve

     inlet.h_outflow = inStream(outlet.h_outflow) ;  // when flow from out to in
     outlet.h_outflow = inStream(inlet.h_outflow) ;  //      inlet to outlet
     inlet.Xi_outflow = inStream(outlet.Xi_outflow) ;  // pass composition un_altered
     outlet.Xi_outflow = inStream(inlet.Xi_outflow) ;  //     

  // Mass balance
     inlet.m_flow + outlet.m_flow = 0  ;

    // Pressure difference drives the flow
    if ( inlet.p > outlet.p ) then
        state = Medium.setState_phX(inlet.p, inStream(inlet.h_outflow), inStream(inlet.Xi_outflow)) ;
        inlet.m_flow =  Medium.density(state)  * cv * sqrt(inlet.p - outlet.p) ;
    else
        state = Medium.setState_phX(outlet.p, inStream(outlet.h_outflow), inStream(outlet.Xi_outflow)) ;
        outlet.m_flow =  Medium.density(state)  * cv * sqrt(outlet.p - inlet.p) ; // Typical valve behaviour
     end if ;
end Valve;