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:

𝐴 − the required effective discharge area (in²)

𝑄 − the flow rate (gal/min)

𝐾_𝑑 − the rated coefficient of discharge

𝐾_𝑤 − correction factor due to backpressure

‒ 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

𝐾𝑐 − combination correction factor for installations with a rupture disk upstream of the PRV

‒ 1.0, when a rupture disk is not installed;

‒ 0.9, when a rupture disk is installed in combination with a PRV and the combination does not have a certified value;

𝐾𝑝 − correction factor due to overpressure

‒ at 25 % overpressure, Kp is equal to 1.0. For overpressures other than 25 %, K_p is determined from the equation above.

 

Where:

𝐺₁ − the specific gravity of the liquid at the flowing temperature referred to water at standard conditions

𝑃_𝑆 − set  pressure (psig)

𝑃₂ − total backpressure (psig)

Input Parameters

1. To create a new case, click the “Add Case” button
2. Select the Relief Valves Requiring No Capacity Certification application from the Pipeline Facilities section.
3. Select either the Flowrate or Effective Discharge Area calculation.
4. Enter Case Name, Location, Date and any necessary notes.
5. Fill out all required fields.
6. Make sure the values you are inputting are in the correct units.
7. 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

1. View the results.
2. If an input parameter needs to be edited be sure to hit the CALCULATE button after the change.
3. To SAVE, fill out all required case details then click the SAVE button.
4. To rename an existing file, click the SAVE As button. Provide all case info then click SAVE.
5. To generate a REPORT, click the REPORT button.
6. The user may export the Case/Report by clicking the Export to Excel/PowerPoint icon.
7. To delete a case, click the DELETE icon near the top of the widget.

Non-Viscous Application

• Kp – Correction Factor due to Overpressure
• Percent of Gauge Back Pressure (%)
• Kw – Back pressure Correction Factor
• Relieving Pressure (psig)
• Required Discharge Flow Rate (gal/min)
• Next larger Standard Orifice Size

Viscous Application

• Calculated Reynolds Number
• Kv – Viscosity Correction Factor
• Required Discharge Flow Rate (gal/min)
• Next Larger Standard Orifice Size

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