2-Way Directional Valve (MA)

2-way directional valve in a moist air network

Since R2025a

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Libraries:
Simscape / Fluids / Moist Air / Valves & Orifices / Directional Control Valves

Description

The 2-Way Directional Valve (MA) block represents a valve with two ports, A and B, and one flow path, A–B. The path runs through an orifice of variable width. The input signal specified at port S controls the position of the spool. The valve closes when the spool covers the orifice opening. For more details about how the block calculates flow rate through a variable orifice, see Orifice (MA).

Valve Parameterizations

The block behavior depends on the Valve parametrization parameter:

• Cv flow coefficient — The flow coefficient C_v determines the block parameterization. The flow coefficient measures the ease with which the moist air can flow when driven by a certain pressure differential.

• Kv flow coefficient — The flow coefficient K_v, where $$K_v=0.865C_v$$, determines the block parameterization. The flow coefficient measures the ease with which the moist air can flow when driven by a certain pressure differential.

• Sonic conductance — The sonic conductance of the resistive element at steady state determines the block parameterization. The sonic conductance measures the ease with which the moist air can flow when choked, which is a condition in which the flow velocity is at the local speed of sound. Choking occurs when the ratio between downstream and upstream pressures reaches a critical value known as the critical pressure ratio.

• Orifice area — The size of the flow restriction determines the block parametrization.

Opening Characteristics

The flow characteristic relates the opening of the valve to the input that produces it, which is often the spool travel. The block expresses the opening as a sonic conductance, flow coefficient, or restriction area, depending on the setting of the Valve parameterization parameter. The control input is the orifice opening fraction, a function of the spool displacement specified at port S.

The flow characteristic is normally given at steady state, with the inlet at a constant, carefully controlled pressure. The flow characteristic depends only on the valve and can be linear or nonlinear. To capture the flow characteristics, use the Opening characteristic parameter:

• Linear — The measure of flow capacity is a linear function of the orifice opening fraction. As the opening fraction rises, the measure of flow capacity scales from the specified minimum to the specified maximum.

  

• Tabulated — The measure of flow capacity is a general function, which can be linear or nonlinear, of the orifice opening fraction. The function is specified in tabulated form, with the independent variable specified by the Opening fraction vector.

  

Visualize Orifice Openings

To visualize the spool offsets and displacement, right-click the block and select Fluids > Plot Valve Characteristics. The plot shows the sonic conductance, K_v flow coefficient, C_v flow coefficient, or orifice area as a function of spool position.

To update the data after changing the block parameters, click Reload Data in the figure window.

This figure shows the valve configuration with default values.

Assumptions and Limitations

• The Sonic conductance setting of the Valve parameterization parameter is for pneumatic applications. If you use this setting for moist air with high levels of trace gasses or are modeling a fluid other than air, you may need to scale the sonic conductance by the square root of the mixture specific gravity.

• This block does not model supersonic flow.

• There is no heat exchange between the valve and the environment.

Ports

Conserving

A — Moist air inlet or outlet

moist air

Moist air conserving port associated with the valve inlet or outlet.

B — Moist air inlet or outlet

moist air

Moist air conserving port associated with the valve inlet or outlet.

Input

S — Spool displacement, m

physical signal

Physical signal associated with the spool displacement, in m. A positive value causes the valve to open.

Parameters

Valve parameterization — Variable or constant orifice parameterization

Cv flow coefficient (default) | Kv flow coefficient | Sonic conductance | Orifice area

Method the block uses to calculate the mass flow rate from the pressure difference across the orifice or the pressure difference from the mass flow rate.

Opening characteristic — Method to convert from control signal to measure of flow capacity

Linear (default) | Tabulated

Method by which to convert the control signal specified at port S to the chosen measure of flow capacity.

Spool position at maximum orifice area — Spool offset at maximum area

5e-3 m (default) | positive scalar

Position of the spool travel member when the valve is fully open. The default value represents a zero-lapped system. A positive, nonzero value represents an underlapped, or partially closed, system. A negative, nonzero value represents an overlapped system where the valve remains open over a range of displacements.

Spool travel between closed and open orifice — Maximum spool stroke

5e-3 m (default) | positive scalar

Spool offset at the maximum valve opened area.

Dependencies

To enable this parameter, set Opening characteristic to Linear.

Spool travel vector — Vector of spool positions

[0, .003, .006, .009, .012, .015] m (default) | 1-by-n vector

Vector of spool positions for the tabulated orifice parameterizations. The vector elements correspond one-to-one to the values in the Orifice area vector, Cv flow coefficient vector, or Kv flow coefficient vector parameters.

Dependencies

To enable this parameter, set Opening characteristic to Tabulated.

Maximum Cv flow coefficient — C_v coefficient that corresponds to maximally open component

4 (default) | positive scalar

Value of the C_v flow coefficient when the orifice is fully open and the area available for flow is at a maximum. This parameter measures the ease with which the vapor traverses the resistive element when driven by a pressure differential.

Dependencies

To enable this parameter, set Valve parameterization to Cv flow coefficient and Opening characteristic to Linear.

Cv flow coefficient vector — C_v coefficients that correspond to values in member position vector

[1e-06, .8, 1.6, 2.4, 3.2, 4] (default) | 1-by-n vector

Vector of C_v flow coefficients. Each coefficient corresponds to a value in the Spool travel vector parameter. This parameter measures the ease with which the vapor traverses the resistive element when driven by a pressure differential. The size of the vector must be the same as the Spool travel vector parameter.

Dependencies

To enable this parameter, set Valve parameterization to Cv flow coefficient and Opening characteristic to Tabulated.

xT pressure differential ratio factor at choked flow — Ratio of pressure differentials

0.7 (default) | positive scalar

Ratio between the inlet pressure, p_in, and the outlet pressure, p_out, defined as $$(p_{in}-p_{out})/p_{in}$$ where choking first occurs.

Dependencies

To enable this parameter, Valve parameterization to Cv flow coefficient or Kv flow coefficient.

Maximum Kv flow coefficient — K_v coefficient that corresponds to maximally open component

3.6 (default) | positive scalar

Value of the K_v flow coefficient when the orifice is fully open and the area available for flow is at a maximum. This parameter measures the ease with which the vapor traverses the resistive element when driven by a pressure differential.

Dependencies

To enable this parameter, set Valve parameterization to Kv flow coefficient and Opening characteristic to Linear.

Kv flow coefficient vector — K_v coefficients that correspond to values in member position vector

[1e-06, .72, 1.44, 2.16, 2.88, 3.6] (default) | 1-by-n vector

Vector of K_v flow coefficients. Each coefficient corresponds to a value in the Spool travel vector parameter. This parameter measures the ease with which the vapor traverses the resistive element when driven by a pressure differential. The size of the vector must be the same as the Spool travel vector parameter.

Dependencies

To enable this parameter, set Valve parameterization to Kv flow coefficient and Opening characteristic to Tabulated.

Maximum sonic conductance — Sonic conductance corresponding to maximally open component

12 l/(bar*s) (default) | positive scalar

Value of the sonic conductance when the control signal specified at port S is 1 and cross-sectional area available for flow is at a maximum.

Dependencies

To enable this parameter, set Valve parameterization to Sonic conductance and Opening characteristic to Linear.

Critical pressure ratio — Ratio of downstream and upstream pressures at which the flow first chokes

0.3 (default) | positive scalar

Pressure ratio at which flow first begins to choke and the flow velocity reaches its maximum, given by the local speed of sound. The pressure ratio is the outlet pressure divided by inlet pressure.

Dependencies

To enable this parameter, set Valve parameterization to Sonic conductance and Opening characteristic to Linear.

Subsonic index — Exponent used to characterize mass flow in subsonic flow regime

0.5 (default) | positive scalar

Empirical value used to more accurately calculate the mass flow rate in the subsonic flow regime.

Dependencies

To enable this parameter, set Valve parameterization to Sonic conductance.

ISO reference temperature — ISO 8778 reference temperature

293.15 K (default) | scalar

Temperature at standard reference atmosphere, defined as 293.15 K in ISO 8778.

You only need to adjust the ISO reference parameter values if you are using sonic conductance values that are obtained at difference reference values.

Dependencies

To enable this parameter, set Valve parameterization to Sonic conductance.

ISO reference density — ISO 8778 reference density

1.185 kg/m^3 (default) | positive scalar

Density at standard reference atmosphere, defined as 1.185 kg/m3 in ISO 8778.

You only need to adjust the ISO reference parameter values if you are using sonic conductance values that are obtained at difference reference values.

Dependencies

To enable this parameter, set Valve parameterization to Sonic conductance.

Sonic conductance vector — Sonic conductances corresponding to values in control member position vector

[1e-05, 2.4, 4.8, 7.2, 9.6, 12] l/(bar*s) (default) | vector of positive values

Vector of sonic conductances inside the resistive element. Each conductance corresponds to a value in the Spool travel vector parameter. The size of the vector must be the same as the Spool travel vector parameter.

Dependencies

To enable this parameter, set Valve parameterization to Sonic conductance and Opening characteristic to Tabulated.

Critical pressure ratio vector — Critical pressure ratios corresponding to values in control member position vector

0.3 * ones(1,6) (default) | vector of positive numbers

Vector of critical pressure ratios at which the flow first chokes, with each critical pressure ratio corresponding to a value in the Spool travel vector parameter. The critical pressure ratio is the fraction of downstream-to-upstream pressures at which the flow velocity reaches the local speed of sound. The size of the vector must be the same as the Spool travel vector parameter.

Dependencies

To enable this parameter, set Valve parameterization to Sonic conductance and Opening characteristic to Tabulated.

Maximum orifice area — Maximum orifice opening area

1e-4 m^2 (default) | positive scalar

Orifice area when it is fully open.

Dependencies

To enable this parameter, set Valve parameterization to Orifice area and Opening characteristic to Linear.

Orifice area vector — Vector of orifice opening area values

[1e-10, 2e-05, 4e-05, 6e-05, 8e-05, .0001] m^2 (default) | 1-by-n vector

Vector of orifice area values for the tabulated parameterization of the orifice area. The values in this vector correspond one-to-one with the elements in the Spool travel vector parameter.

Dependencies

To enable this parameter, set Valve parameterization to Orifice area and Opening characteristic to Tabulated.

Discharge coefficient — Discharge coefficient

0.64 (default) | positive scalar

Ratio of actual flow rate to ideal flow rate. This parameter accounts for real-world losses that are not captured in the orifice equation.

Dependencies

To enable this parameter, set Valve parameterization to Orifice area.

Leakage flow fraction — Ratio of flow rates

1e-6 (default) | positive scalar

Ratio of the flow rate of the orifice when it is closed to when it is open.

Dependencies

To enable this parameter, set Opening characteristic to Linear.

Smoothing factor — Numerical smoothing factor

0.01 (default) | positive scalar in the range [0,1]

Continuous smoothing factor that introduces a layer of gradual change to the flow response when the orifice is in near-open or near-closed positions. Set this parameter to a nonzero value less than one to increase the stability of your simulation in these regions.

Dependencies

To enable this parameter, set Opening characteristic to Linear.

Laminar flow pressure ratio — Laminar-turbulent transition pressure ratio

0.999 (default) | positive scalar

Ratio of the orifice outlet pressure to orifice inlet pressure at which the fluid transitions between the laminar and turbulent regimes. The pressure loss corresponds to the mass flow rate linearly in laminar flows and quadratically in turbulent flows.

Cross-sectional area at ports A and B — Valve port areas

0.01 m^2 (default) | positive scalar

Area of the orifice ports A and B.

References

[1] ISO 6358-3. "Pneumatic fluid power – Determination of flow-rate characteristics of components using compressible fluids – Part 3: Method for calculating steady-state flow rate characteristics of systems". 2014.

[2] IEC 60534-2-3. "Industrial-process control valves – Part 2-3: Flow capacity – Test procedures". 2015.

[3] ANSI/ISA-75.01.01. "Industrial-Process Control Valves – Part 2-1: Flow capacity – Sizing equations for fluid flow underinstalled conditions". 2012.

[4] P. Beater. Pneumatic Drives. Springer-Verlag Berlin Heidelberg. 2007.

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using Simulink® Coder™.

Version History

Introduced in R2025a