Relief Valve
The Relief Valve junction allows users to model a device that opens at a specified pressure in the system.
The Relief Valve Properties window follows the first of the two basic Properties Window formats, displaying the connected pipes in a fixed format. The Relief Valve junction adopts a flow direction from the connecting pipes.
A Relief Valve junction in AFT xStream will always behave as an Exit Rupture Disk, with the relief valve located at the end of a pipe leg. Below the Set Pressure, the valve is closed and behaves as a Dead End. When the Set Pressure is reached, the valve will instantly open and remain open for the rest of the MOC Transient simulation. If the valve is open during the AFT Arrow Steady Solution, it will remain open for the entire MOC Transient simulation.
Valve Setpoints
The valve setpoints determine when the relief valve will open. These setpoints are defined using pressure.
There are two setpoints the user needs to define:
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Exit Pressure - the ambient pressure downstream of the relief valve when open.
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Set Pressure - the upstream pressure at which the valve opens.
Note: The Relief Valve junction operates exclusively using stagnation pressure. The Set Pressure and Exit Pressure are defined as stagnation pressures and all calculations are based on stagnation pressure.
The Relief Valve loss information is entered on the Loss Model tab. The Basis Area for each Loss Model is by default the upstream pipe.
The following loss model options are available:
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CdA (Constant) - the relief valve instantly changes from closed to a fixed CdA. This CdA is defined as the product of an Orifice Effective Area (A) and a Discharge Coefficient (Cd).
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The Orifice Effective Area can come from an API 526ANSI/API 526 7th Edition - Flanged Steel Pressure-relief Valves, 2017, published by American Petroleum Institute, 1220 L Street, NW, Washington DC, USA letter designation or it can be User Specified.
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The Discharge Coefficient must always be specified by the user. The value is typically specified by the manufacturer.
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K Factor - the relief valve instantly changes from closed to a fixed K Factor.
Note: The CdA for sonic choking may be different from the subsonic CdA loss model option in xStream. The discharge coefficient can vary at different pressure ratios due to the vena contracta moving closer to or farther from the orifice restriction. For the highest accuracy the CdA used for subsonic and sonic losses should be tested and entered separately. See the "Modeling Choked Flow Through an Orifice" white paper on AFT's website for more information.
Discharge Coefficient Loss Model
Note: The following is true for both API 526 and User Specified CdA data sources.
When determining the pressure loss across the junction, AFT xStream calculates a subsonic discharge coefficient area (CdA) for the orifice and applies the following set of isentropic compressible flow equations to solve for the static pressure and Mach number at the vena contracta
Where ṁ̇ represents the mass flow rate, To is the stagnation temperature, Po denotes the stagnation pressure, M is the Mach number, γ is the specific heat ratio, R is the gas constant, Z is the gas compressibility factor, and P is the static pressure. Note that the specific gas constant, R, is defined as the universal gas constant divided by the molecular weight of the gas.
For information on the discharge coefficient loss model see the Discharge Coefficient Loss Model topic. For information on the orifice loss when the flow is choked see the Sonic Choking description.
Special Conditions
The relief valve has two Special Conditions:
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Failed Open - the valve is forced to remain open regardless of pressure conditions.
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Ignore Relief Valve - the valve is forced to remain closed regardless of pressure conditions.
For more information see Special Conditions.
