API 520 - Relief Valves Requiring Capacity Certification
The ASME Code requires that capacity certification be obtained for PRVs designed for liquid service. The procedure for obtaining capacity certification includes testing to determine the rated coefficient of discharge for the liquid PRVs at 10 % overpressure.
The sizing equations for pressure-relief devices in liquid service provided in this section assume that the liquid is incompressible (i.e. the density of the liquid does not change as the pressure decreases from the relieving pressure to the total backpressure).
Where:
π΄ β the required effective discharge area (in2)
π β the flow rate (gal/min)
πΎπΒ β the rated coefficient of discharge
πΎπ€Β β correction factor due to backpressure
β if the backpressure is atmospheric, use a value for Kw of 1.0. Balanced bellows valves in backpressure service will require the correction factor determined from Figure 31. Conventional and pilot-operated valves require no special correction
πΎπ β combination correction factor for installations with a rupture disk upstream of the PRV
β 1.0, when a rupture disk is not installed;
β 0.9, when a rupture disk is installed in combination with a PRV and the combination does not have a certified value;
πΎπ£Β β correction factor due to viscosity
Where:
πΊ1Β β the specific gravity of the liquid at the flowing temperature referred to water at standard conditions
π1Β β upstream relieving pressure (psig)
π2Β β total backpressure (psig)
When a PRV is sized for viscous liquid service, it should first be sized as if it were for a non viscous type application (i.e. Kv = 1.0) so that a preliminary required discharge area, A, can be obtained from the equations above. From API 526 standard orifice sizes, the next orifice size larger than A should be used in determining the Reynoldβs Number, Re, from either of the following relationships.
Where:
π π β Reynolds Number
π β flow rate at the flowing temperature (gal/min)
πΊ1Β β the specific gravity of the liquid at the flowing temperature referred to water at standard conditions
π΄ β the required effective discharge area (in2)
Input Parameters
To create a new case, click the βAdd Caseβ button
Select the API 520 β Relief Valves Requiring Capacity Certification application from the Pipeline Facilities section.
Select either the Flowrate or Effective Discharge Area calculation.
Enter Case Name, Location, Date and any necessary notes.
Fill out all required fields.
Make sure the values you are inputting are in the correct units.
Click the CALCULATE button.
Required Effective Discharge
Relief Valve Set Pressure (psig)
Specific Gravity of Liquid
Liquid Viscosity
Kd- Coefficent of Discharge
Overpressure %
Total Back Pressure (psig)
Outputs/Reports
View the results.
If an input parameter needs to be edited be sure to hit the CALCULATE button after the change.
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To rename an existing file, click the SAVE As button. Provide all case info then click SAVE.
To generate a REPORT, click the REPORT button.
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