Pressure Relief Valve (G)

Pressure relief valve in a gas network

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Simscape / Fluids / Gas / Valves & Orifices / Pressure Control Valves

Description

The Pressure Relief Valve (G) block represents an orifice that opens with inlet pressure to keep the pressure from reaching extreme levels. At normal pressures, the valve is closed and only releases leakage flow. Above a specified pressure setting, the valve begins to open, which allows the gas to vent from the region of high pressure. The opening area increases with inlet pressure up to a maximum value, at which point the valve is fully open and pressure rises unabated.

The valve responds to inlet pressure relative to the outlet or inlet pressure relative to the environment. The chosen pressure measurement is the control pressure of the valve. The Control pressure specification parameter determines which of the measurements the block uses during simulation.

The relationship between the opening area and the pressure drop depends on the parameterization of the valve. That relationship can take the form of a linear analytical expression or a tabulated function.

The flow can be laminar or turbulent, and it can reach up to sonic speeds. The maximum velocity happens at the throat of the valve where the flow is narrowest and fastest. The flow chokes and the velocity saturates when a drop in downstream pressure can no longer increase the velocity. Choking occurs when the back-pressure ratio reaches the critical value characteristic of the valve. The block does not capture supersonic flow.

Valve Opening Fraction

The inlet pressure is the valve control signal. The more the inlet pressure rises over the pressure setting of the valve, the greater the opening area becomes. The amount that the control pressure exceeds the pressure setting determines how much the valve opens. When the Opening characteristic parameter is Linear, the block calculates the valve opening fraction to scale the flow characteristic parameters. The valve opening fraction is

$$\widehat{p}=\frac{P_{Ctl}-P_{Set}}{\Delta P},$$

where:

• P_Ctl is the control pressure.

  When Opening pressure specification is Pressure difference of port A relative to port B,

  $$P_{\text{Ctl}}=P_{\text{A}}-P_{\text{B}},$$

  where P_A is the absolute pressure at port A and P_B is the absolute pressure at port B.

  When Opening pressure specification is Gauge pressure at port A,

  $$P_{Ctl}=P_A-P_{Atm},$$

  where P_Atm is the atmospheric pressure specified in the Gas Properties (G) block of the model.

• P_Set is the set pressure.

  When Opening pressure specification is Pressure difference of port A relative to port B, P_Set, is the value of the Set pressure differential parameter

  When Opening pressure specification is Gauge pressure at port A, P_Set is the value of the Set pressure (gauge) parameter

• ΔP is the value of the Pressure regulation range parameter.

Numerical Smoothing

When the Opening characteristic parameter is Linear and the Smoothing factor parameter is nonzero, the block applies numerical smoothing to the normalized control pressure, $$\widehat{p}$$. Enabling smoothing helps maintain numerical robustness in your simulation.

For more information, see Numerical Smoothing.

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 a gas 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 a gas 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 a gas 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. The opening is expressed as a sonic conductance, flow coefficient, or restriction area, determined by the Valve parameterization parameter.

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 valve opening fraction. As the opening fraction rises from 0 to 1, 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 pressure differential vector or the Opening pressure (gauge) vector.

  

Momentum Balance

The block equations depend on the Orifice parametrization parameter. When you set Orifice parametrization to Cv flow coefficient parameterization, the mass flow rate, $$\dot{m}$$, is

$$\dot{m}=C_v{N}_{6}Y\sqrt{(p_{in}-p_{out}){\rho }_{in}},$$

where:

• C_v is the flow coefficient.

• N₆ is a constant equal to 27.3 for mass flow rate in kg/hr, pressure in bar, and density in kg/m³.

• Y is the expansion factor.

• p_in is the inlet pressure.

• p_out is the outlet pressure.

• ρ_in is the inlet density.

The expansion factor is

$$Y=1-\frac{p_{in}-p_{out}}{3p_{in}{F}_{\gamma }{x}_T},$$

where:

• F_γ is the ratio of the isentropic exponent to 1.4.

• x_T is the value of the xT pressure differential ratio factor at choked flow parameter.

The block smoothly transitions to a linearized form of the equation when the pressure ratio, $$p_{out}/p_{in}$$, rises above the value of the Laminar flow pressure ratio parameter, B_lam,

$$\dot{m}=C_v{N}_6{Y}_{lam}\sqrt{\frac{{\rho }_{avg}}{p_{avg}(1-B_{lam})}}(p_{in}-p_{out}),$$

where:

$$Y_{lam}=1-\frac{1-B_{lam}}{3F_{\gamma }{x}_T}.$$

When the pressure ratio, $$p_{out}/p_{in}$$, falls below $$1-F_{\gamma }{x}_T$$, the orifice becomes choked and the block switches to the equation

$$\dot{m}=\frac{2}{3}{C}_v{N}_6\sqrt{F_{\gamma }{x}_T{p}_{in}{\rho }_{in}}.$$

When you set Orifice parametrization to Kv flow coefficient parameterization, the block uses these same equations, but replaces C_v with K_v by using the relation $$K_v=0.865C_v$$. For more information on the mass flow equations when the Orifice parametrization parameter is Kv flow coefficient parameterization or Cv flow coefficient parameterization, see [2][3].

When you set Orifice parametrization to Sonic conductance parameterization, the mass flow rate, $$\dot{m}$$, is

$$\dot{m}=C{\rho }_{ref}{p}_{in}\sqrt{\frac{T_{ref}}{T_{in}}}{\left[1-{\left(\frac{\frac{p_{out}}{p_{in}}-B_{crit}}{1-B_{crit}}\right)}^2\right]}^m,$$

where:

• C is the sonic conductance.

• B_crit is the critical pressure ratio.

• m is the value of the Subsonic index parameter.

• T_ref is the value of the ISO reference temperature parameter.

• ρ_ref is the value of the ISO reference density parameter.

• T_in is the inlet temperature.

The block smoothly transitions to a linearized form of the equation when the pressure ratio, $$p_{out}/p_{in}$$, rises above the value of the Laminar flow pressure ratio parameter B_lam,

$$\dot{m}=C{\rho }_{ref}\sqrt{\frac{T_{ref}}{T_{avg}}}{\left[1-{\left(\frac{B_{lam}-B_{crit}}{1-B_{crit}}\right)}^2\right]}^m\left(\frac{p_{in}-p_{out}}{1-B_{lam}}\right).$$

When the pressure ratio, $$p_{out}/p_{in}$$, falls below the critical pressure ratio, B_crit, the orifice becomes choked and the block switches to the equation

$$\dot{m}=C{\rho }_{ref}{p}_{in}\sqrt{\frac{T_{ref}}{T_{in}}}.$$

For more information on the mass flow equations when the Orifice parametrization parameter is Sonic conductance parameterization, see [1].

When you set Orifice parametrization to Orifice area parameterization, the mass flow rate, $$\dot{m}$$, is

$$\dot{m}=C_d{S}_r\sqrt{\frac{2\gamma }{\gamma -1}{p}_{in}{\rho }_{in}{\left(\frac{p_{out}}{p_{in}}\right)}^{\frac{2}{\gamma }}\left[\frac{1-{\left(\frac{p_{out}}{p_{in}}\right)}^{\frac{\gamma -1}{\gamma }}}{1-{\left(\frac{S_R}{S}\right)}^2{\left(\frac{p_{out}}{p_{in}}\right)}^{\frac{2}{\gamma }}}\right]},$$

where:

• S_r is the orifice or valve area.

• S is the value of the Cross-sectional area at ports A and B parameter.

• C_d is the value of the Discharge coefficient parameter.

• γ is the isentropic exponent.

The block smoothly transitions to a linearized form of the equation when the pressure ratio, $$p_{out}/p_{in}$$, rises above the value of the Laminar flow pressure ratio parameter, B_lam,

$$\dot{m}=C_d{S}_r\sqrt{\frac{2\gamma }{\gamma -1}{p}_{avg}^{\frac{2-\gamma }{\gamma }}{\rho }_{avg}{B}_{lam}^{\frac{2}{\gamma }}\left[\frac{1-B_{lam}^{\frac{\gamma -1}{\gamma }}}{1-{\left(\frac{S_R}{S}\right)}^2{B}_{lam}^{\frac{2}{\gamma }}}\right]}\left(\frac{p_{in}^{\frac{\gamma -1}{\gamma }}-p_{out}^{\frac{\gamma -1}{\gamma }}}{1-B_{lam}^{\frac{\gamma -1}{\gamma }}}\right).$$

When the pressure ratio, $$p_{out}/p_{in}$$, falls below$${\left(\frac{2}{\gamma +1}\right)}^{\frac{\gamma }{\gamma -1}}$$ , the orifice becomes choked and the block switches to the equation

$$\dot{m}=C_d{S}_R\sqrt{\frac{2\gamma }{\gamma +1}{p}_{in}{\rho }_{in}\frac{1}{{\left(\frac{\gamma +1}{2}\right)}^{\frac{2}{\gamma -1}}-{\left(\frac{S_R}{S}\right)}^2}}.$$

For more information on the mass flow equations when the Orifice parametrization parameter is Orifice area parameterization, see [4].

Mass Balance

The block assumes the volume and mass of fluid inside the valve is very small and ignores these values. As a result, no amount of fluid can accumulate in the valve. By the principle of conservation of mass, the mass flow rate into the valve through one port equals that out of the valve through the other port

$${\dot{m}}_A+{\dot{m}}_B=0,$$

where $$\dot{m}$$ is defined as the mass flow rate into the valve through the port indicated by the A or B subscript.

Energy Balance

The resistive element of the block is an adiabatic component. No heat exchange can occur between the fluid and the wall that surrounds it. No work is done on or by the fluid as it traverses from inlet to outlet. Energy can flow only by advection, through ports A and B. By the principle of conservation of energy, the sum of the port energy flows is always equal to zero

$${\varphi }_{\text{A}}+{\varphi }_{\text{B}}=0,$$

where ϕ is the energy flow rate into the valve through ports A or B.

Assumptions and Limitations

• The Sonic conductance setting of the Valve parameterization parameter is for pneumatic applications. If you use this setting for gases other than air, you may need to scale the sonic conductance by the square root of the specific gravity.

• The equation for the Orifice area parameterization is less accurate for gases that are far from ideal.

• This block does not model supersonic flow.

Ports

Conserving

A — Valve entrance

gas

Gas conserving port associated with the opening through which the flow must enter the valve.

B — Valve entrance

gas

Gas conserving port associated with the opening through which the flow must exit the valve.

Parameters

Opening pressure specification — Choice of pressure measurement to use as valve control signal

Pressure difference of port A relative to port B (default) | Gauge pressure at port A

Choice of pressure measurement to use as valve control signal. The block uses this setting to determine when the valve should begin to open. In the default setting, Pressure difference of port A relative to port B, the opening pressure of the valve is expressed as a pressure drop from inlet to outlet. In the alternative setting, Gauge pressure at port A, it is expressed as a gauge inlet pressure.

Valve parameterization — Parameterization used to specify flow characteristics of valve

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

Method to calculate the mass flow rate.

• Cv flow coefficient — The flow coefficient C_v determines the block parameterization.

• Kv flow coefficient — The flow coefficient K_v, where $$K_v=0.865C_v$$, determines the block parameterization.

• Sonic conductance — The sonic conductance of the resistive element at steady state determines the block parameterization.

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

Opening characteristic — Method to calculate the opening characteristics of the valve

Linear (default) | Tabulated

Method the block uses to calculate the opening area of the valve. The linear setting treats the opening area as a linear function of the orifice opening fraction. The tabulated setting allows for a general, nonlinear relationship you can specify in tabulated form.

Set pressure differential — Threshold differential for valve operation

0 MPa (default) | positive scalar

Pressure drop, from inlet to outlet, required to open the valve. This value marks the beginning of the pressure regulation range, over which the valve progressively opens in an effort to contain a pressure rise. The opening area of the valve depends on the difference between the actual pressure drop to the value specified here.

Dependencies

To enable this parameter, set Opening pressure specification to Pressure difference at port A relative to port B and Opening characteristic to Linear.

Pressure regulation range — Width of the pressure range over which the opening area is variable

0.1 MPa (default) | positive scalar

Valve operational range. The valve begins to open at the set pressure value, and is fully open at p_max, the end of the pressure regulation range, where p_max = p_set + p_range.

Dependencies

To enable this parameter, set Opening characteristic to Linear.

Set pressure (gauge) — Inlet gauge pressure required to open the valve

0.1 MPa (default) | positive scalar in units of pressure

Inlet gauge pressure required to open the valve. This parameter marks the beginning of the pressure regulation range, over which the valve progressively opens in an effort to contain a pressure rise The opening area of the valve depends on the difference between the actual inlet pressure to the value specified here.

Dependencies

To enable this parameter, set Opening pressure specification to Gauge pressure at port A and Opening characteristic to Linear.

Opening pressure differential vector — Vector of control pressures at which to specify valve opening data

0.2 : 0.2 : 1.2 MPa (default) | vector

Vector of control pressures at which to specify the chosen measure of valve opening—sonic conductance, flow coefficient, or opening area. This vector must be equal in size to those containing the valve opening data. The vector elements must be positive and increase monotonically from left to right. These pressure values are the pressure drop from inlet to outlet.

The first element of this parameter is the pressure setting of the valve, at which the valve begins to open. The last element is the maximum pressure, at which the valve is fully open. The difference between the two is the pressure regulation range of the valve.

Dependencies

To enable this parameter, set Opening pressure specification to Pressure difference at port A relative to port B and Opening characteristic to Tabulated.

Opening pressure (gauge) vector — Vector of control pressures at which to specify valve opening data

0.2 : 0.2 : 1.2 MPa (default) | vector

Vector of control pressures at which to specify the chosen measure of valve opening—sonic conductance, flow coefficient, or opening area. This vector must be equal in size to those containing the valve opening data. The vector elements must be positive and increase monotonically in value from left to right. These pressures are the gauge pressure at the inlet.

The first element of this parameter is the pressure setting of the valve, at which the valve begins to open. The last element is the maximum pressure, at which the valve is fully open. The difference between the two is the pressure regulation range of the valve.

Dependencies

To enable this parameter, set Opening pressure specification to Gauge pressure at port A and Opening characteristic to Tabulated.

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

4 (default) | positive scalar

Value of the C_v flow coefficient when the restriction area available for flow is at a maximum. This parameter measures the ease with which the gas 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.

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. If you do not have this value, look it up in table 2 in ISA-75.01.01 [3]. Otherwise, the default value of 0.7 is reasonable for many valves.

Dependencies

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

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 regimes.

Dependencies

To enable this parameter, set Opening characteristic to Linear.

Cv flow coefficient vector — Cv coefficients corresponding to values in opening fraction vector

[1e-06, .8, 1.6, 2.4, 3.2, 4] (default) | vector of positive numbers

Vector of C_v flow coefficients. Each coefficient corresponds to a value in the Opening fraction vector parameter. This parameter measures the ease with which the gas traverses the resistive element when driven by a pressure differential. The flow coefficients must increase monotonically from left to right, with greater opening fractions representing greater flow coefficients. The size of the vector must be the same as the Opening fraction vector.

Dependencies

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

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

3.6 (default) | positive scalar

Value of the K_v flow coefficient when the restriction area available for flow is at a maximum. This parameter measures the ease with which the gas 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 — Kv coefficients corresponding to values in opening fraction vector

[1e-06, .72, 1.44, 2.16, 2.88, 3.6] (default) | vector of positive numbers

Vector of K_v flow coefficients. Each coefficient corresponds to a value in the Opening fraction vector parameter. This parameter measures the ease with which the gas traverses the resistive element when driven by a pressure differential. The flow coefficients must increase monotonically from left to right, with greater opening fractions representing greater flow coefficients. The size of the vector must be the same as the Opening fraction 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 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 opening fraction 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 Opening fraction vector parameter. The sonic conductances must increase monotonically from left to right, with greater opening fractions representing greater sonic conductances. The size of the vector must be the same as the Opening fraction 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 opening fraction 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 Opening fraction 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 Opening fraction vector parameter.

Dependencies

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

Maximum orifice area — Flow area corresponding to maximally open component

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

Cross-sectional area of the orifice opening when the cross-sectional area available for flow is at a maximum.

Dependencies

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

Discharge coefficient — Discharge coefficient

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

Correction factor that accounts for discharge losses in theoretical flows.

Dependencies

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

Orifice area vector — Vector of orifice opening area values

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

Vector of cross-sectional areas of the orifice opening. The values in this vector correspond one-to-one with the elements in the Opening fraction vector parameter. The first element of this vector is the orifice leakage area and the last element is the maximum orifice area.

Dependencies

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

Laminar flow pressure ratio — Pressure ratio at which flow transitions between laminar and turbulent regimes

0.999 (default) | positive scalar

Pressure ratio at which flow transitions between laminar and turbulent flow regimes. The pressure ratio is the outlet pressure divided by inlet pressure. Typical values range from 0.995 to 0.999.

Cross-sectional area at ports A and B — Flow area at gas ports

0.01 m^2 (default) | positive scalar

Area normal to the flow path at each port. The ports are equal in size. The value of this parameter should match the inlet area of the components to which the resistive element connects.

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 R2018b

R2022b: Model gas systems using improved flow computations

In previous versions, the blocks in the gas library used a conversion factor to implement different orifice parameterizations in terms of sonic conductance. You can use the Valve parameterization parameter to specify the parameterization.

The new parameterizations also include new parameters:

• xT pressure differential ratio factor at choked flow

• Discharge coefficient

• Leakage flow fraction. This parameter replaces the Cv coefficient (USCS) at leakage flow, Kv coefficient (SI) at leakage flow, Sonic conductance at leakage flow, and Restriction area at leakage flow parameters.

To improve clarity:

• The Opening parameterization parameter has been renamed to Opening characteristic.

• The Reference temperature and Reference density parameters have been renamed to ISO reference temperature and ISO reference density, respectively.

• The Cv coefficient (USCS) at maximum flow and Kv coefficient (USCS) at maximum flow parameters have been renamed to Maximum Cv coefficient and Maximum Kv coefficient , respectively.

• The Sonic conductance at maximum flow parameter has been renamed to Maximum sonic conductance.

• The Pressure control specification parameter has been renamed to Opening pressure specification. The options for this parameter have been renamed to Pressure difference of port A relative to port B and Gauge pressure at port A.

• The Control pressure vector parameter has been renamed to Opening pressure differential vector or Opening pressure (gauge) vector, depending on the choice for Opening pressure specification.