WOLFRAM SYSTEMMODELER

PartialValve

Base model for valves

Diagram

Wolfram Language

In[1]:=
SystemModel["Modelica.Fluid.Valves.BaseClasses.PartialValve"]
Out[1]:=

Information

This information is part of the Modelica Standard Library maintained by the Modelica Association.

This is the base model for the ValveIncompressible, ValveVaporizing, and 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.

Modelling options

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 Av coefficient (m^2).
  • CvData = Modelica.Fluid.Types.CvTypes.Kv: the flow coefficient is given by the metric Kv coefficient (m^3/h).
  • CvData = Modelica.Fluid.Types.CvTypes.Cv: the flow coefficient is given by the US Cv coefficient (USG/min).
  • CvData = Modelica.Fluid.Types.CvTypes.OpPoint: the flow is computed from the nominal operating point specified by p_nominal, dp_nominal, m_flow_nominal, rho_nominal, opening_nominal.

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.

If 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:

ValveCharacteristics1a.png
Components/ValveCharacteristics1b.png

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):

FilteredValveIcon.png

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.

ValveFilteredOpening.png

Parameters (18)

allowFlowReversal

Value: system.allowFlowReversal

Type: Boolean

Description: = true to allow flow reversal, false restricts to design direction (port_a -> port_b)

dp_start

Value: dp_nominal

Type: AbsolutePressure (Pa)

Description: Guess value of dp = port_a.p - port_b.p

m_flow_start

Value: m_flow_nominal

Type: MassFlowRate (kg/s)

Description: Guess value of m_flow = port_a.m_flow

m_flow_small

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

show_T

Value: true

Type: Boolean

Description: = true, if temperatures at port_a and port_b are computed

show_V_flow

Value: true

Type: Boolean

Description: = true, if volume flow rate at inflowing port is computed

CvData

Value: Modelica.Fluid.Types.CvTypes.OpPoint

Type: CvTypes

Description: Selection of flow coefficient

Av

Value:

Type: Area (m²)

Description: Av (metric) flow coefficient

Kv

Value: 0

Type: Real

Description: Kv (metric) flow coefficient [m3/h]

Cv

Value: 0

Type: Real

Description: Cv (US) flow coefficient [USG/min]

dp_nominal

Value:

Type: Pressure (Pa)

Description: Nominal pressure drop

m_flow_nominal

Value:

Type: MassFlowRate (kg/s)

Description: Nominal mass flowrate

rho_nominal

Value: Medium.density_pTX(Medium.p_default, Medium.T_default, Medium.X_default)

Type: Density (kg/m³)

Description: Nominal inlet density

opening_nominal

Value: 1

Type: Real

Description: Nominal opening

filteredOpening

Value: false

Type: Boolean

Description: = true, if opening is filtered with a 2nd order CriticalDamping filter

riseTime

Value: 1

Type: Time (s)

Description: Rise time of the filter (time to reach 99.6 % of an opening step)

leakageOpening

Value: 1e-3

Type: Real

Description: The opening signal is limited by leakageOpening (to improve the numerics)

checkValve

Value: false

Type: Boolean

Description: Reverse flow stopped

Connectors (4)

port_a

Type: FluidPort_a

Description: Fluid connector a (positive design flow direction is from port_a to port_b)

port_b

Type: FluidPort_b

Description: Fluid connector b (positive design flow direction is from port_a to port_b)

opening

Type: RealInput

Description: Valve position in the range 0..1

opening_filtered

Type: RealOutput

Description: Filtered valve position in the range 0..1

Components (5)

system

Type: System

Description: System wide properties

state_a

Type: ThermodynamicState

Description: state for medium inflowing through port_a

state_b

Type: ThermodynamicState

Description: state for medium inflowing through port_b

filter

Type: Filter

minLimiter

Type: MinLimiter

Extended by (3)

ValveCompressible

Modelica.Fluid.Valves

Valve for compressible fluids, accounts for choked flow conditions

ValveVaporizing

Modelica.Fluid.Valves

Valve for possibly vaporizing (almost) incompressible fluids, accounts for choked flow conditions

ValveIncompressible

Modelica.Fluid.Valves

Valve for (almost) incompressible fluids