Pressure-Reducing 3-Way Valve (IL)

Combined pressure-relief and pressure-reducing valve in isothermal system

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Description

The Pressure-Reducing 3-Way Valve (IL) is a combination of a pressure-relief and pressure-reducing valve. It maintains pressure at the valve outlet, port A, by restricting the inflow area at port P and venting the flow at port T.

Valve Functionality

Valve operation is triggered by comparing the pressure difference between port T and port A to a threshold, the set pressure. When the pressure between T and A, P_control, exceeds this set pressure, P_set,reducing, port P begins to close. A transition pressure range defines the pressures the valve experiences when both valves at ports P and T are closed. When the pressure difference between ports A and T exceeds the pressure transition range, port T opens. That is, $$P_{set,relief}=P_{set,reducing}+P_{range}+P_{transition}.$$ The Pressure regulation range is specified for both the pressure-reducing and the pressure-relief valves. The valve parameters, such as Leakage Area and Maximum Opening Area, are the same for all ports.

To simulate pressure relief or pressure reduction with respect to another system element, see Pressure Compensator Valve (IL). To simulate pressure reduction between the valve outlet and atmosphere, see the Pressure-Reducing Valve (IL). To simulate pressure relief with respect to a valve or between the valve outlet and atmospheric pressure, see Pressure Relief Valve (IL).

Pressure Control

When P_control, P_A – P_T, exceeds the threshold pressure, P_set,reducing, the valve at port P begins to close. When the Pressure transition range is exceeded, or when P_control > P_set,relief, the valve at port T begins to open. Both valve closing and opening are parameterized in two ways:

• When Set Pressure control is set to Controlled, connect a pressure signal to port Ps, set the constant Pressure regulation range, and set the constant Pressure transition range. The pressure-reducing valve begins to close when P_control is greater than P_set,reducing and below P_max,reducing. The relief valve response is triggered when P_control is greater than P_set,relief and below P_max,relief. P_max,relief is the sum of the Pressure regulation range and P_set,relief.

• When Set Pressure control is set to Constant, valve closing at port P is continuously regulated by either a linear or tabular parameterization. Similarly, relief valve opening at port T is parameterized linearly or by table lookup. An example of linear parameterization of the reduction valve (solid line) and relief valve (dotted line) is shown below.

  

  When the Tabulated data option is selected, P_set,reducing and P_max,reducing are the first and last parameters of the Pressure differential vector for reducing valve, respectively, and P_set,relief and P_max,relief are the first and last parameters of the Pressure differential vector for relief valve, respectively. An example of tabular parameterization of both the reducing and relieving valves are shown below.

  

Mass Flow Rate Equation

Momentum is conserved through the valve:

$${\dot{m}}_A+{\dot{m}}_{reducing}+{\dot{m}}_{relief}=0.$$

The mass flow rate through the valves is calculated as:

$${\dot{m}}_{reducing}=\frac{C_d{A}_{PA}\sqrt{2\overline{\rho }}}{\sqrt{P{R}_{loss,reducing}\left(1-{\left(\frac{A_{PA}}{A_{port}}\right)}^2\right)}}\frac{\Delta p_{reducing}}{{\left[\Delta p_{reducing}{}^2+\Delta p_{crit,reducing}^2\right]}^{1/4}},$$

$${\dot{m}}_{relief}=\frac{C_d{A}_{AT}\sqrt{2\overline{\rho }}}{\sqrt{P{R}_{loss,relief}\left(1-{\left(\frac{A_{AT}}{A_{port}}\right)}^2\right)}}\frac{\Delta p_{relief}}{{\left[\Delta p_{relief}{}^2+\Delta p_{crit,relief}^2\right]}^{1/4}},$$

where:

• C_d is the Discharge coefficient.

• A is the instantaneous valve open area between ports Aand P or A and T, as indicated by the subscript.

• A_port is the Cross-sectional area at ports A, P & T.

• $$\overline{\rho }$$ is the average fluid density.

• Δp is the valve pressure difference: p_A – p_B.

The critical pressure difference, Δp_crit, is the pressure differential associated with the Critical Reynolds number, Re_crit, the flow regime transition point between laminar and turbulent flow, which corresponds to either the pressure-reducing or pressure relief component of the valve:

$$\Delta p_{crit}=\frac{\pi \overline{\rho }}{8A}{\left(\frac{\nu {\mathrm{Re}}_{crit}}{C_d}\right)}^2,$$

where A is either A_PA or A_AT, corresponding to the reducing or relief component of the valve, respectively.

Pressure loss describes the reduction of pressure in the valve due to a decrease in area. PR_loss is calculated as:

$$P{R}_{loss}=\frac{\sqrt{1-{\left(\frac{A}{A_{port}}\right)}^2\left(1-C_d^2\right)}-C_d\frac{A}{A_{port}}}{\sqrt{1-{\left(\frac{A}{A_{port}}\right)}^2\left(1-C_d^2\right)}+C_d\frac{A}{A_{port}}}.$$

Pressure recovery describes the positive pressure change in the valve due to an increase in area. If you do not wish to capture this increase in pressure, set Pressure recovery to Off. In this case, PR_loss is 1.

The opening area A is determined by the opening parameterization (for Constant valves only) of the reducing valve (P to A) or relief valve (A to T) and the valve opening dynamics.

Opening and Closing Parameterization

Linear parameterization of the valve area for the reducing valve is

$$A_{valve}={\widehat{p}}_{reducing}\left(A_{leak}-A_{\max }\right)+A_{\max },$$

and for the relief valve is

$$A_{valve}={\widehat{p}}_{relief}\left(A_{\max }-A_{leak}\right)+A_{leak}.$$

The normalized pressure, $$\widehat{p}$$, is

$$\widehat{p}=\frac{p_{control}-p_{set}}{p_{\max }-p_{set}},$$

where the set and maximum pressures are the respective reducing or relief valve settings.

At the extremes of the relief and reducing valve pressure ranges, you can maintain numerical robustness in your simulation by adjusting the block Smoothing factor. With a nonzero smoothing factor, a smoothing function is applied to every calculated pressure, but primarily influences the simulation at the extremes of these ranges.

When the Smoothing factor, f, is nonzero, a smoothed, normalized pressure is instead applied to the valve area:

$${\widehat{p}}_{smoothed}=\frac{1}{2}+\frac{1}{2}\sqrt{{\widehat{p}}_{}^2+{\left(\frac{f}{4}\right)}^2}-\frac{1}{2}\sqrt{{\left(\widehat{p}-1\right)}^2+{\left(\frac{f}{4}\right)}^2}.$$

In the Tabulated data parameterization, A_leak,PA and A_max,PA are the first and last parameters of the Opening area vector reducing valve, respectively, and A_leak,AT and A_max,AT are the first and last parameters of the Opening area vector for relief valve, respectively. The smoothed, normalized pressure is also used when the smoothing factor is nonzero with linear interpolation and nearest extrapolation.

Opening Dynamics

If Opening dynamics are modeled, a lag is introduced to the flow response to valve opening. A_valve becomes the dynamic opening or closing area, A_dyn; otherwise, A_valve is the steady-state opening area. This area is specific to the reducing and relief components of the valve, A_PA or A_AT, respectively. The instantaneous change in dynamic opening area is calculated based on the Opening time constant, τ:

$${\dot{p}}_{dyn}=\frac{p_{control}-p_{dyn}}{\tau }.$$

By default, Opening dynamics is set to Off.

Faulty Behavior

When faults are enabled, the reducing and relief valve open areas become stuck at a specified value in response to one of these triggers:

• Simulation time — Faulting occurs at a specified time.

• Simulation behavior — Faulting occurs in response to an external trigger. This exposes port Tr.

Three fault options are available in the Valve areas when faulted parameter:

• Reducing valve closed and relief valve open — The reducing valve freezes at its smallest value and the relief valve freezes at its largest value, depending on the Opening parameterization:

  • When Opening parameterization is set to Linear, the reducing valve area freezes at the Leakage area and the relief valve area freezes at the Maximum opening area.

  • When Opening parameterization is set to Tabulated data, the reducing valve area freezes at the first element of the Opening area vector and the relief valve area freezes at the last element of the Opening area vector.

• Reducing valve open and relief valve closed — The reducing valve freezes at its largest value and the relief valve freezes at its smallest value, depending on the Opening parameterization:

  • When Opening parameterization is set to Linear, the reducing valve area freezes at the Maximum opening area and the relief valve freezes at the Leakage area.

  • When Orifice parameterization is set to Tabulated data, the reducing valve area freezes at the last element of the Opening area vector and the relief valve area freezes at the first element of the Opening area vector.

• Maintain last value — The reducing and relief valve areas freeze at the open area when the trigger occurred.

Due to numerical smoothing at the extremes of the valve area, the minimum area applied is larger than the Leakage area, and the maximum is smaller than the Maximum orifice area, in proportion to the Smoothing factor value.

Once triggered, the valves remain at the faulted area for the rest of the simulation.

Assumptions and Limitations

Friction between the valve and fluid, the hydraulic force of the fluid on the valve components, and the effect of fluid inertia are neglected.

Ports

Conserving

A — Liquid port

isothermal liquid

Liquid exit port of the valve.

P — Liquid port

isothermal liquid

Liquid entry port to the valve.

T — Liquid port

isothermal liquid

Liquid relief port of the valve.

Input

Ps — Controlled pressure differential

physical signal

Varying-signal set pressure for controlled valve operation.

Dependencies

To enable this port, set Set pressure control to Controlled.

Tr — External fault trigger

physical signal

Physical signal port for an external fault trigger. Triggering occurs when the value is greater than 0.5. There is no unit associated with the trigger value.

Dependencies

This port is visible when Enable faults is set to On and Fault trigger is set to External.

Parameters

Parameters

Set pressure control — Valve operation method

Constant (default) | Controlled

Valve operation method. A Constant valve opens or closes linearly over a fixed pressure regulation range and pressure transition or in accordance with tabulated pressure and opening area data that you provide. A Controlled valve opens or closes according to a variable set pressure signal at port Pset over a fixed pressure regulation and pressure transition range. The selected setting applies both to the reducing and relief valve operation.

Opening parameterization — Type of opening parameterization

Linear (default) | Tabulated data

Method of modeling the valve opening or closing. Valve opening is either parametrized linearly, which correlates the opening area to the provided pressure range, or by a table of values you provide that correlate the valve opening area to pressure differential data.

Dependencies

To enable this port, set Set pressure control to Constant.

Set pressure differential for reducing valve — Set pressure for valve closing at port P

0.6 MPa (default) | scalar

Pressure differential between port T and port A. When this set pressure differential is surpassed, the valve at port P begins to close. The closing is parametrized linearly or by lookup table as defined in the Opening parameterization.

Pressure regulation range — Valve operational pressure range

3e-2 MPa (default) | scalar

Operational pressure range of the reducing valve. The pressure regulation range lies between the Set pressure differential and the maximum valve operating pressure. At the end of the Pressure regulation range, the pressure-reducing valve is closed and the Pressure transition range begins.

Dependencies

To enable this parameter, set

• Set pressure control to Controlled, or

• Set pressure control to Constant and Opening parameterization to Linear.

Pressure transition range — Pressure range of valve with ports P and T closed

2e-2 MPa (default) | scalar

Pressure range of the 3-way valve. This parameter defines the pressure range, which begins at the end of the Pressure regulation range, over which both ports P and T are closed. Below this range, the reducing valve at port P is open to flow, and above this range, the relief valve at port T opens.

Dependencies

To enable this parameter, set either:

• Set pressure control to Controlled

• Set pressure control to Constant and Opening parameterization to Linear

Maximum opening area — Maximum opening area of ports

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

Cross-sectional area of the valve (P-A or A-T) in its fully-open position.

Dependencies

To enable this parameter, set either:

• Set pressure control to Controlled

• Set pressure control to Constant and Opening parameterization to Linear

Leakage area — Gap area when in fully closed position

1e-10 m^2 (default) | positive scalar

Sum of all gaps when the valve is in fully closed position. Any area smaller than this value is saturated to the specified leakage area. This contributes to numerical stability by maintaining continuity in the flow.

Dependencies

To enable this parameter, set either:

• Set pressure control to Controlled

• Set pressure control to Constant and Opening parameterization to Linear

Pressure differential vector for reducing valve — Differential pressure values for tabular parameterization of reducing valve

0.6 : 0.02 : 0.7 MPa (default) | 1-by-n vector

Vector of pressure differential values for the tabular parameterization of opening area. The vector elements correspond one-to-one to the values in the Opening area vector reducing valve parameter. Pressure differential vector values are listed in ascending order and must have the same number of elements as the Opening area vector reducing valve parameter. Linear interpolation is employed between table data points.

Dependencies

To enable this parameter, set Set pressure control to Constant and Opening parameterization to Tabulated data.

Opening area vector reducing valve — Vector of valve opening areas for tabular parameterization of reducing valve

[1e-05, 8e-06, 6e-06, 4e-06, 2e-06, 1e-10] m^2 (default) | 1-by-n vector

Vector of opening area values for the tabular parameterization of opening area. The vector elements must correspond one-to-one to the values in the Pressure differential vector for reducing valve parameter. Areas are listed in descending order. Linear interpolation is employed between table data points.

Dependencies

To enable this parameter, set Set pressure control to Constant and Opening parameterization to Tabulated data.

Pressure differential vector for relief valve — Differential pressure values for tabular parameterization of relief valve

0.68 : 0.02 : 0.78 MPa (default) | 1-by-n vector

Vector of pressure differential values for the tabular parameterization of opening area. The vector elements correspond one-to-one to the values in the Opening area vector relief valve parameter. Pressure differential vector values are listed in ascending order and must have the same number of elements as the Opening area vector relief valve parameter. Linear interpolation is employed between table data points.

Dependencies

To enable this parameter, set Set pressure control to Constant and Opening parameterization to Tabulated data.

Opening area vector for relief valve — Valve opening areas for tabular parameterization of relief valve

[1e-10, 2e-06, 4e-06, 6e-06, 8e-06, 1e-05] m^2 (default) | 1-by-n vector

Vector of opening area values for the tabular parameterization of opening area. The vector elements must correspond one-to-one to the values in the Pressure differential vector for relief valve parameter. Areas are listed in ascending order. Linear interpolation is employed between table data points.

Dependencies

To enable this parameter, set Set pressure control to Constant and Opening parameterization to Tabulated data.

Cross-sectional area at ports A, P & T. — Orifice area at conserving ports

inf m^2 (default) | positive scalar

Cross-sectional area at the entry and exit ports A, P, and T. This area is used in the pressure-flow rate equation that determines the mass flow rate through the orifice.

Discharge coefficient — Discharge coefficient

0.64 (default) | positive scalar

Correction factor accounting for discharge losses in theoretical flows. The default discharge coefficient for a valve in Simscape™ Fluids™ is 0.64.

Critical Reynolds number — Upper Reynolds number limit for laminar flow

150 (default) | positive scalar

Upper Reynolds number limit for laminar flow through the valve.

Smoothing factor — Numerical smoothing factor

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

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

Pressure recovery — Whether to account for pressure increase in area expansions

Off (default) | On

Accounts for pressure increase when fluid flows from a region of smaller cross-sectional area to a region of larger cross-sectional area. This increase in pressure is not captured when Pressure recovery is set to Off.

Opening dynamics — Whether to introduce flow lag due to orifice opening

Off (default) | On

Accounts for transient effects to the fluid system due to the valve opening. Setting Opening dynamics to On approximates the opening conditions by introducing a first-order lag in the flow response. The Opening time constant also impacts the modeled opening dynamics.

Reducing valve initial opening area — Open area at port P at the start of the simulation

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

Initial cross-sectional area at port P at the time of dynamic opening. This value is used to calculate the instantaneous opening area for the dynamic opening between P and A at the following time step.

Dependencies

To enable this parameter, set Opening dynamics to On.

Relief valve initial opening area — Open area at port T at the start of the simulation

1e-10 m^2 (default) | positive scalar

Initial cross-sectional area at port T of opening at the time of dynamic opening. This value is used to calculate the instantaneous opening area for the dynamic opening between A and T at the following time step.

Dependencies

To enable this parameter, set Opening dynamics to On.

Opening time constant — Valve opening time constant

0.1 s (default) | positive scalar

Constant that captures the time required for the fluid to reach steady-state when opening or closing the valve from one position to another. This parameter impacts the modeled opening dynamics.

Dependencies

To enable this parameter, set Opening dynamics to On.

Faults

Enable faults — Fault option

Off (default) | On

Enable externally or temporally triggered faults. When faulting occurs, the valve area normally set by the opening parameterization will be set to the value specified in the Opening area when faulted parameter.

Valve areas when faulted — Set faulted characteristics

Reducing valve closed and relief valve open (default) | Reducing valve open and relief valve closed | Maintain last value

Sets the faulted valve type. You can choose for the valve to seize when the valves are opened, closed, or at the area when faulting is triggered.

Dependencies

To enable this parameter, set Enable faults to On.

Fault trigger — External or temporal trigger option

Temporal (default) | External

Whether a fault trigger occurs due to an external event or at a specified time.

When set to External, port Tr is enabled. A physical signal at port Tr that is greater than 0.5 triggers faulting.

When set to Temporal, when the Simulation time for fault event is reached, the valve area will be set to the value specified in the Opening area when faulted parameter.

Dependencies

To enable this parameter, set Enable faults to On.

Simulation time for fault event — Time at which faulting is triggered

5 s (default) | positive scalar

When the Simulation time for fault event is reached, the valve area is set to the value specified in the Opening area when faulted parameter.

Dependencies

To enable this parameter, set Enable faults to On and Fault trigger to Temporal.