API 520 - Relief Valves Requiring No Capacity Certification
This method assumes an effective coefficient of discharge, Kd = 0.62, and 25 % overpressure. An additional capacity correction factor, Kp, is needed for relieving pressures other than 25 % overpressure (see Figure 38). This sizing method may be used where capacity certification is not required or was never established.
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 correction factor due to overpressure
β at 25 % overpressure, Kp is equal to 1.0. For overpressures other than 25 %, KpΒ is determined from the equation above.
Where:
πΊ1Β β the specific gravity of the liquid at the flowing temperature referred to water at standard conditions
ππΒ β setΒ pressure (psig)
π2Β β total backpressure (psig)
Input Parameters
To create a new case, click the βAdd Caseβ button
Select the Relief Valves Requiring No 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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