Title and Link to Calc:

Compressible Fluid Flow : Flow in Conventional Relief Valves, Mass Flow Rate Output : CFF-RP520-0025-1

Description:

Determines mass flow rate in a conventional relief valve of a given flow area for gas or vapors per sections 3.6.2 and 3.6.3 of the reference. Calc determines if flow is critical or subcritical and applies the appropriate method per the reference. Equations 3.2 (US units) and 3.5 (US units) are used in the calc. The equations are rearranged to solve for mass flow rate instead of area.

Alternative Descriptors:

Sizing for Subcritical Flow: Gas or Vapor , Conventional and Pilot-Operated Pressure Relief Valves , Sizing for Critical Flow

Calc Number:

CFF-RP520-0025-1

Calc Status:

Beta

Reference:

Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries, API Recommended Practice 520 seventh edition, January 2000, 3.6.2 and 3.6.3, pgs. 42-50

Reference Example(s) Modeled/Generalized:

Examples 3.6.2.2 and 3.6.3.2 of the reference

Calc Origination Date:

03/27/2006

Calc Originator:

RBC

Calc Revision History:

Calc Guidelines:

User should refer to the specified reference above.

Access:

Activating this calc requires entry of the correct access code. Contact eng-calc@eng-calcs.com

Variables:

Variable Description - Description of the variable
Variable Label Description - The description of the variable as it appears in the calc.
Variable Calc Name - The calc internal variable name
Variable Units - The units for the variable
Variable Calc Upper Limit - The highest allowed value in the calc.
Variable Calc Lower Limit - The lowest allowed value in the calc.
Variable Type - Either "Output" or "Input"
Variable Format - Real Number, Text, or Integer
Variable Notes - Other comments about the variable

Variable Description: Mass Flow Rate through Relief Valve
Variable Label Description: Mass Flow Rate
Variable Calc Name: MFR
Variable Units: LBM/HR
Variable Calc Upper Limit: 100000000
Variable Calc Lower Limit: 0.001
Variable Type: Output
Variable Format: REAL
Variable Notes:

Variable Description: Critical Pressure (Sonic Velocity in Relief Valve Throat Condition)
Variable Label Description: Critical Pressure
Variable Calc Name: CP
Variable Units: PSIA
Variable Calc Upper Limit: 50000
Variable Calc Lower Limit: 0
Variable Type: Output
Variable Format: REAL
Variable Notes:

Variable Description: Critical Pressure Ratio
Variable Label Description: Critical Pressure Ratio
Variable Calc Name: NC
Variable Units:
Variable Calc Upper Limit: 1
Variable Calc Lower Limit: 0
Variable Type: Output
Variable Format: REAL
Variable Notes:

Variable Description: Comments
Variable Label Description: Comments
Variable Calc Name: COM
Variable Units:
Variable Calc Upper Limit:
Variable Calc Lower Limit:
Variable Type: Output
Variable Format: TEXT
Variable Notes: Displays "Input(s) out of range." if an entered input variable(s) are out of range. Displays "Output(s) out of range." if an output variable exceeds its range. The "out of range" variable(s) are displayed in yellow. The limited range values are used in the calculation, not the "out of range" value(s). Depending on flow regime will print one of the following -- "Critical Flow" or "Subcritical Flow".

Variable Description: Effective Discharge Area
Variable Label Description: Effective Discharge Area
Variable Calc Name: REDA
Variable Units: SQIN
Variable Calc Upper Limit: 50000
Variable Calc Lower Limit: 0
Variable Type: Input
Variable Format: REAL
Variable Notes:

Variable Description: Pressure at the pressure relief valve inlet
Variable Label Description: Pressure at PRV Inlet
Variable Calc Name: PIP
Variable Units: PSIA
Variable Calc Upper Limit: 100000
Variable Calc Lower Limit: 0.001
Variable Type: Input
Variable Format: REAL
Variable Notes:

Variable Description: Temperature at the pressure relief valve inlet
Variable Label Description: Temperature at PRV Inlet
Variable Calc Name: PIT
Variable Units: R
Variable Calc Upper Limit: 100000
Variable Calc Lower Limit: 0.001
Variable Type: Input
Variable Format: REAL
Variable Notes:

Variable Description: Downstream Back Pressure
Variable Label Description: Downstream Back Pressure
Variable Calc Name: DBP
Variable Units: PSIA
Variable Calc Upper Limit: 100000
Variable Calc Lower Limit: 0.001
Variable Type: Input
Variable Format: REAL
Variable Notes:

Variable Description: Rupture Disk Correction Factor
Variable Label Description: Rupture Disk Correction Factor
Variable Calc Name: RDC
Variable Units:
Variable Calc Upper Limit: 3
Variable Calc Lower Limit: 0.001
Variable Type: Input
Variable Format: REAL
Variable Notes:

Variable Description: Effective Coefficient of Discharge
Variable Label Description: Coefficient of Discharge
Variable Calc Name: KD
Variable Units:
Variable Calc Upper Limit: 3
Variable Calc Lower Limit: 0.001
Variable Type: Input
Variable Format: REAL
Variable Notes:

Variable Description: Ratio of Specific Heats
Variable Label Description: Specific Heat Ratio
Variable Calc Name: ISK
Variable Units:
Variable Calc Upper Limit: 3
Variable Calc Lower Limit: 1
Variable Type: Input
Variable Format: REAL
Variable Notes:

Variable Description: Compressibility Factor for gas or vapor
Variable Label Description: Compressibility Factor
Variable Calc Name: ZCOM
Variable Units:
Variable Calc Upper Limit: 3
Variable Calc Lower Limit: 0.01
Variable Type: Input
Variable Format: REAL
Variable Notes:

Variable Description: Molecular Weight of Gas / Vapor
Variable Label Description: Gas/Vapor Molecular Weight
Variable Calc Name: MW
Variable Units:
Variable Calc Upper Limit: 10000
Variable Calc Lower Limit: 1
Variable Type: Input
Variable Format: REAL
Variable Notes:

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