WOLFRAM SYSTEM MODELER
This information is part of the Modelica Standard Library maintained by the Modelica Association.
This is the base model for the
ValveCompressible valve models. The model is based on the IEC 534 / ISA S.75 standards for valve sizing.
The model optionally supports reverse flow conditions (assuming symmetrical behaviour) or check valve operation, and has been suitably regularized, compared to the equations in the standard, in order to avoid numerical singularities around zero pressure drop operating conditions.
The model assumes adiabatic operation (no heat losses to the ambient); changes in kinetic energy from inlet to outlet are neglected in the energy balance.
The following options are available to specify the valve flow coefficient in fully open conditions:
CvData = Modelica.Fluid.Types.CvTypes.Av: the flow coefficient is given by the metric
CvData = Modelica.Fluid.Types.CvTypes.Kv: the flow coefficient is given by the metric
CvData = Modelica.Fluid.Types.CvTypes.Cv: the flow coefficient is given by the US
CvData = Modelica.Fluid.Types.CvTypes.OpPoint: the flow is computed from the nominal operating point specified by
The nominal pressure drop
dp_nominal must always be specified; to avoid numerical singularities, the flow characteristic is modified for pressure drops less than
b*dp_nominal (the default value is 1% of the nominal pressure drop). Increase this parameter if numerical problems occur in valves with very low pressure drops.
checkValve is true, then the flow is stopped when the outlet pressure is higher than the inlet pressure; otherwise, reverse flow takes place. Use this option only when needed, as it increases the numerical complexity of the problem.
The valve opening characteristic
valveCharacteristic, linear by default, can be replaced by any user-defined function. Quadratic and equal percentage with customizable rangeability are already provided by the library. The characteristics for constant port_a.p and port_b.p pressures with continuously changing opening are shown in the next two figures:
The treatment of parameters Kv and Cv is explained in detail in the User's Guide.
With the optional parameter "filteredOpening", the opening can be filtered with a second order, criticalDamping filter so that the opening demand is delayed by parameter "riseTime". The filtered opening is then available via the output signal "opening_filtered" and is used to control the valve equations. This approach approximates the driving device of a valve. The "riseTime" parameter is used to compute the cut-off frequency of the filter by the equation: f_cut = 5/(2*pi*riseTime). It defines the time that is needed until opening_filtered reaches 99.6 % of a step input of opening. The icon of a valve changes in the following way (left image: filteredOpening=false, right image: filteredOpening=true):
If "filteredOpening = true", the input signal "opening" is limited by parameter leakageOpening, i.e., if "opening" becomes smaller as "leakageOpening", then "leakageOpening" is used instead of "opening" as input for the filter. The reason is that "opening=0" might structurally change the equations of the fluid network leading to a singularity. If a small leakage flow is introduced (which is often anyway present in reality), the singularity might be avoided.
In the next figure, "opening" and "filtered_opening" are shown in the case that filteredOpening = true, riseTime = 1 s, and leakageOpening = 0.02.
Description: = true to allow flow reversal, false restricts to design direction (port_a -> port_b)
Type: AbsolutePressure (Pa)
Description: Guess value of dp = port_a.p - port_b.p
Type: MassFlowRate (kg/s)
Description: Guess value of m_flow = port_a.m_flow
Value: if system.use_eps_Re then system.eps_m_flow * m_flow_nominal else system.m_flow_small
Type: MassFlowRate (kg/s)
Description: Small mass flow rate for regularization of zero flow
Description: = true, if temperatures at port_a and port_b are computed
Description: = true, if volume flow rate at inflowing port is computed
Description: Selection of flow coefficient
Type: Area (m²)
Description: Av (metric) flow coefficient
Description: Kv (metric) flow coefficient [m3/h]
Description: Cv (US) flow coefficient [USG/min]
Type: Pressure (Pa)
Description: Nominal pressure drop
Type: MassFlowRate (kg/s)
Description: Nominal mass flowrate
Value: Medium.density_pTX(Medium.p_default, Medium.T_default, Medium.X_default)
Type: Density (kg/m³)
Description: Nominal inlet density
Description: Nominal opening
Description: = true, if opening is filtered with a 2nd order CriticalDamping filter
Type: Time (s)
Description: Rise time of the filter (time to reach 99.6 % of an opening step)
Description: The opening signal is limited by leakageOpening (to improve the numerics)
Description: Reverse flow stopped
Description: Fluid connector a (positive design flow direction is from port_a to port_b)
Description: Fluid connector b (positive design flow direction is from port_a to port_b)
Description: Valve position in the range 0..1
Description: Filtered valve position in the range 0..1
Valve for compressible fluids, accounts for choked flow conditions
Valve for possibly vaporizing (almost) incompressible fluids, accounts for choked flow conditions
Valve for (almost) incompressible fluids