Pressure-Reducing 3-Way Valve (IL)
Combined pressure-relief and pressure-reducing valve in an isothermal liquid system
Since R2020a
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Pressure Control Valves
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 A and port T to a threshold, the set pressure. When the pressure between A and T, Pcontrol, exceeds this set pressure, Pset,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, 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 Pcontrol, PA – PT, exceeds the threshold pressure, Pset,reducing, the valve at port P begins to close. When the Pressure transition range is exceeded, or when Pcontrol > Pset,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 Pcontrol is greater than Pset,reducing and below Pmax,reducing. The relief valve response is triggered when Pcontrol is greater than Pset,relief and below Pmax,relief. Pmax,relief is the sum of the Pressure regulation range and Pset,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, Pset,reducing and Pmax,reducing are the first and last parameters of the Pressure differential vector for reducing valve, respectively, and Pset,relief and Pmax,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:
The mass flow rate through the valves is calculated as:
where:
Cd is the Discharge coefficient.
A is the instantaneous valve open area between ports A and P or A and T, as indicated by the subscript.
Aport is the Cross-sectional area at ports A, P & T.
is the average fluid density.
Δp is the valve pressure difference: pA – pB.
The critical pressure difference, Δpcrit, is the pressure differential associated with the Critical Reynolds number, Recrit, the flow regime transition point between laminar and turbulent flow, which corresponds to either the pressure-reducing or pressure relief component of the valve:
where A is either APA or AAT, 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. PRloss is calculated as:
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, clear the Pressure recovery check box. In this case, PRloss 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
When you set Opening parameterization to
Linear
, the valve area for the reducing valve is
and for the relief valve is
The normalized pressure, , is
where the set and maximum pressures are the respective reducing or relief valve settings.
When the valve is in a near-open or near-closed position in the linear parameterization, you can maintain numerical robustness in your simulation by adjusting the Smoothing factor parameter. If the Smoothing factor parameter is nonzero, the block smoothly saturates the control pressure between pset and pmax. For more information, see Numerical Smoothing.
When you set Opening parameterization to Tabulated
data
, Aleak,PA and
Amax,PA are the first and last
elements of the Opening area vector reducing valve parameter,
respectively, and Aleak,AT and
Amax,AT are the first and last
elements of the Opening area vector for relief valve parameter,
respectively. The block calculates the opening area as
For the reducing valve, the block calculates the opening area by using the table lookup method, where:
pcontrol is the control pressure, which is the pressure differential between ports A and T.
pcontrol,TLU,ref = pTLU + poffset.
pTLU is the Pressure differential vector for reducing valve parameter
poffset is an internal pressure offset that causes the valve to start closing when pcontrol,TLU,ref = pset,reducing.
ATLU is the Opening area vector reducing valve parameter.
For the relief valve, the block calculates the opening area by using the table lookup method, where:
pcontrol is the pressure differential between ports A and T.
pcontrol,TLU,ref = pTLU + poffset.
pTLU is the Pressure differential vector for relief valve parameter.
poffset is an internal pressure offset that causes the valve to start opening when pcontrol,TLU,ref = pset,releif.
ATLU is the Opening area vector for relief valve parameter.
Opening Dynamics
If Opening dynamics are modeled, a lag is introduced to the flow response to valve opening. Avalve becomes the dynamic opening or closing area, Adyn; otherwise, Avalve is the steady-state opening area. This area is specific to the reducing and relief components of the valve, APA or AAT, respectively. The instantaneous change in dynamic opening area is calculated based on the Opening time constant, τ:
By default, Opening dynamics are not modeled.
Faults
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
Input
Parameters
Extended Capabilities
Version History
Introduced in R2020a