Modelica.Fluid.Valves.BaseClasses

Base classes used in the Valves package (only of interest to build new component models)

Information

Extends from Modelica.Icons.BasesPackage (Icon for packages containing base classes).

Package Content

NameDescription
Modelica.Fluid.Valves.BaseClasses.PartialValve PartialValve Base model for valves
Modelica.Fluid.Valves.BaseClasses.ValveCharacteristics ValveCharacteristics Functions for valve characteristics

Modelica.Fluid.Valves.BaseClasses.PartialValve Modelica.Fluid.Valves.BaseClasses.PartialValve

Base model for valves

Information


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:

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

Extends from Modelica.Fluid.Interfaces.PartialTwoPortTransport (Partial element transporting fluid between two ports without storage of mass or energy).

Parameters

NameDescription
replaceable package MediumMedium in the component
replaceable function valveCharacteristicInherent flow characteristic
Flow Coefficient
CvDataSelection of flow coefficient
AvAv (metric) flow coefficient [m2]
KvKv (metric) flow coefficient [m3/h]
CvCv (US) flow coefficient [USG/min]
Nominal operating point
dp_nominalNominal pressure drop [Pa]
m_flow_nominalNominal mass flowrate [kg/s]
rho_nominalNominal inlet density [kg/m3]
opening_nominalNominal opening
Filtered opening
filteredOpening= true, if opening is filtered with a 2nd order CriticalDamping filter
riseTimeRise time of the filter (time to reach 99.6 % of an opening step) [s]
leakageOpeningThe opening signal is limited by leakageOpening (to improve the numerics)
Assumptions
allowFlowReversal= true to allow flow reversal, false restricts to design direction (port_a -> port_b)
checkValveReverse flow stopped
Advanced
dp_startGuess value of dp = port_a.p - port_b.p [Pa]
m_flow_startGuess value of m_flow = port_a.m_flow [kg/s]
m_flow_smallSmall mass flow rate for regularization of zero flow [kg/s]
Diagnostics
show_T= true, if temperatures at port_a and port_b are computed
show_V_flow= true, if volume flow rate at inflowing port is computed

Connectors

NameDescription
port_aFluid connector a (positive design flow direction is from port_a to port_b)
port_bFluid connector b (positive design flow direction is from port_a to port_b)
replaceable function valveCharacteristicInherent flow characteristic
openingValve position in the range 0..1
opening_filteredFiltered valve position in the range 0..1

Automatically generated Mon Sep 23 17:20:48 2013.