Pressure Relief Valve (IL)
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Fluids /
Isothermal Liquid /
Valves & Orifices /
Pressure Control Valves
Description
The Pressure Relief Valve (IL) block represents a pressure relief valve in an isothermal liquid network. The valve remains closed when the pressure is less than a specified value. When this pressure is met or surpassed, the valve opens. This set pressure is either a threshold pressure differential over the valve, between ports A and B, or between port A and atmospheric pressure. For pressure control based on another element in the fluid system, see the Pressure Compensator Valve (IL) block.
Pressure Control
You can use a constant value or a physical signal to control the set pressure:
When you set Set pressure control to
Controlled
, connect a pressure signal to port Ps and define the constant Pressure regulation range. The valve response will be triggered when Pcontrol, the pressure differential between ports A and B, is greater than Pset and below Pmax. Pmax is the sum of Pset and the pressure regulation range.When you set Set pressure control to
Constant
, the valve opening is continuously regulated between Pset and Pmax. There are two options for pressure regulation available in the Opening pressure specification parameter: Pcontrol can be the pressure differential between ports A and B or the pressure differential between port A and atmospheric pressure. The opening area is then modeled by either linear or tabular parameterization. When theTabulated data
option is selected, Pset and Pmax are the first and last parameters of the Pressure differential vector, respectively.
Conservation of Mass
The block conserves mass through the valve such that
The block calculates the mass flow rate as
where:
Cd is the Discharge coefficient.
Avalve is the instantaneous valve open area.
Aport is the Cross-sectional area at ports A and B.
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:
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 Avalve is determined by
the opening parameterization (for Constant
valves only)
and the valve opening dynamics.
Opening Parameterization
When you set Opening parameterization to Linear
- Area vs. pressure
, the block calculates the opening area as
where the normalized pressure, , is
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 - Area vs. pressure
,
Aleak and
Amax are the first and last
parameters of the Opening area vector, respectively. The
smoothed, normalized pressure is also used when the smoothing factor is nonzero with
linear interpolation and nearest extrapolation.
When you set Opening parameterization to
Tabulated data - Volumetric flow rate vs.
pressure
,
the valve opens according to the user-provided tabulated data of volumetric flow
rate and pressure differential between ports A and
B.
Within the limits of the tabulated data, the block calculates the mass flow rate as:
where:
is the volumetric flow rate.
is the average fluid density.
When the simulation pressure falls below the first element of the Pressure drop vector parameter, ΔpTLU(1), the block calculates the mass flow rate as:
where VTLU(1) is the first element of the Volumetric flow rate vector parameter.
When the simulation pressure rises above the last element of the Pressure drop vector parameter, ΔpTLU(end), the block calculates the mass flow rate as
where VTLU(end) is the last element of the Volumetric flow rate vector parameter.
Opening Dynamics
When you select Opening dynamics, the block introduces lag in the flow response to the valve opening. Avalve becomes the dynamic opening area, Adyn; otherwise, Avalve is the steady-state opening area. The instantaneous change in dynamic opening area is calculated based on the Opening time constant parameter, τ:
By default, the block clears the Opening dynamics check box.
Steady-state dynamics are set by the same parameterization as valve opening, and are based on the control pressure, pcontrol. A nonzero Smoothing factor can provide additional numerical stability when the orifice is in near-closed or near-open position.
Faults
When faults are enabled, the valve open area becomes 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 Opening area when faulted parameter:
Closed
— The valve freezes at its smallest value, depending on the Opening parameterization parameter:When you set Opening parameterization to
Linear - Area vs. pressure
, the valve area freezes at the Leakage area parameter.When you set Opening parameterization to
Tabulated data - Area vs. pressure
, the valve area freezes at the first element of the Opening area vector parameter.
Open
— The valve freezes at its largest value, depending on the Opening parameterization parameter:When you set Opening parameterization to
Linear - Area vs. pressure
, the valve area freezes at the Maximum opening area parameter.When you set Orifice parameterization to
Tabulated data - Area vs. pressure
, the valve area freezes at the last element of the Opening area vector parameter.
Maintain last value
— The valve area freezes at the valve 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 parameter, and the maximum is smaller than the Maximum orifice area parameter, in proportion to the Smoothing factor parameter value.
Once triggered, the valve remains at the faulted area for the rest of the simulation.
When you set Opening parameterization to
Tabulated data - Volumetric flow rate vs. pressure
,
the fault options are defined by the volumetric flow rate through the valve:
Closed
— The valve stops at the mass flow rate associated with the first elements of the Volumetric flow rate vector parameter and the Pressure drop vector parameter:Open
— The valve stops at the mass flow rate associated with the last elements of the Volumetric flow rate vector parameter and the Pressure drop vector parameter:Maintain at last value
— The valve stops at the mass flow rate and pressure differential when the trigger occurs:
where
Predefined Parameterization
You can populate the block with pre-parameterized manufacturing data, which allows you to model a specific supplier component.
To load a predefined parameterization:
In the block dialog box, next to Selected part, click the "<click to select>" hyperlink next to Selected part in the block dialogue box settings.
The Block Parameterization Manager window opens. Select a part from the menu and click Apply all. You can narrow the choices using the Manufacturer drop down menu.
You can close the Block Parameterization Manager menu. The block now has the parameterization that you specified.
You can compare current parameter settings with a specific supplier component in the Block Parameterization Manager window by selecting a part and viewing the data in the Compare selected part with block section.
Note
Predefined block parameterizations use available data sources to supply parameter values. The block substitutes engineering judgement and simplifying assumptions for missing data. As a result, expect some deviation between simulated and actual physical behavior. To ensure accuracy, validate the simulated behavior against experimental data and refine your component models as necessary.
To learn more, see List of Pre-Parameterized Components.