ValveLinear Model Modelica Standard Library - Working Principle

Question

I am implementing the ValveLinear model from the Modelica standard fluid library into a model of mine using Dymola. I have some questions regarding its parameters which I can hopefully clear up: The key parameters for this valve are as follows:

    parameter Medium.MassFlowRate m_flow_nominal
    "Nominal mass flowrate at full opening";
  final parameter Types.HydraulicConductance k = m_flow_nominal/dp_nominal
    "Hydraulic conductance at full opening";
  Modelica.Blocks.Interfaces.RealInput opening(min=0,max=1)
    "=1: completely open, =0: completely closed"

The mass flow over the valve is then caclulated as

m_flow = opening*k*dp;

Am I right in assuming that m_flow_nominal is the maximum mass flow rate with a linear drop off in mass_flow down to zero as opening goes from 1 to 0?

Furthermore is dp_nominal the corresponding minimum pressure drop across the valve? (i.e. at fully open). Therefore would we see a linear increase in dp from dp_nominal to some maximum value as opening goes from 1 to 0?

The answer may seem trivial but I have run some examples with valves in Dymola so far and in some cases it seems that dp remains constant across the valve as the opening in varied which doesn't make sense to me.

Answer 1

The nominal mass flow rate and pressure drop are just design values used to calculate the valve coefficient k (fixed relation between pressure drop and mass flow). Since no "nominal opening degree" can be specified in ValveLinear the valve opening in the design point is assumed to be one (fully open valve).

The mass flow rate through the valve is not limited to m_flow_nominal. If you double the pressure drop the mass flow through the valve will double, regardless of the nominal mass flow rate.

An example model is shown below:

1. m_flow_nominal is 5 kg/s and dp_nominal is 10 bar.
2. At time = 0 s the (fixed) pressure drop over the valve is 10 bar and the valve is fully open. Therefore, the mass flow through the valve is 5 kg/s.
3. At time = 1 s the pressure drop over the valve is increased by 50 pct (from 10 to 15 bar). The mass flow increases with 50 pct as well (to 7.5 kg/s).
4. At time = 3 s the valve opening is reduced by 50 % (from fully to half open). The pressure drop remains at 15 bar (of course, since it's a boundary value) while the mass flow rate is reduced to 50 pct (= 3.75 kg/s).

Regarding your second question. The pressure drop is not limited. If the mass flow through the valve is given as a boundary condition (e.g. if source in the model is replaced with a MassFlowSource_T) and the mass flow rate is reduced to half of the nominal value (from 5 to 2.5 kg/s) the pressure drop will also be reduced to half of the nominal value (10 to 5 bar). If the mass flow rate is zero, so will the pressure drop be.

If, on the other hand, you fix the mass flow rate to a value > 0 kg/s and ramp the valve opening towards zero, the pressure drop will approach infinity.

Best regards, Rene Just Nielsen