API 520 – Relief Valves Requiring No Capacity Certification

Introduction

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.

A = \frac{Q}{38 K_d K_w K_c K_v} \sqrt{\frac{G_1}{P_1 – P_2}}

A = \frac{Q}{38 K_d K_w K_c K_v} \sqrt{\frac{G_1}{P_1 - P_2}}

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;

𝐾_𝑣 − correction factor due to viscosity

• At 25 % overpressure, Kp is equal to 1.0. For overpressures other than 25 %, Kp is determined from the equation above

Where:

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

𝑃₁ − upstream relieving pressure (psig)

𝑃₂ − total backpressure (psig)

Note: The curve above shows that up to and including 25% overpressure, capacity is affected by the change in lift, the change in the orifice discharge coefficient, and the change in overpressure. Above 25%, capacity is affected only by the change in overpressure. Noncertified valves operating at low overpressure tend to chatter; therefore, overpressure of less than 10% should be avoided.

Case Guide

Part 1: Create Case

Input Parameters

• Required Effective Discharge
• Relief Valve Set Pressure (psig)
• Specific Gravity of Liquid
• Liquid Viscosity
• Kd- Coefficent of Discharge
• Overpressure %
• Total Back Pressure (psig)

Part 2: Outputs/Reports

Non Viscous Results

• 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 Results

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

References

• ASME – Boiler and Pressure Vessel Code, Section VIII
• API – RP 520 Part 2
• ASME B31.8 Gas Transmission and Distribution Piping Systems
• ASME B31.3, B31.4 and B31.8 – Full Encirclement Sleeves (See Appendices)

Appendix

Even though Technical Toolboxes does not provide software for a full-encirclement reinforcing saddles, many operators use them to provide reinforcement for branch outlets in accordance with ASME B31.3, B31.4, B31.8 and other applicable design codes. Full-encirclement reinforcing saddles are designed to fully encircle the run pipe however; they are not designed to be pressure retaining devices. To avoid gas entrapment during welding and to prevent pressure containment, should a leak develop underneath the saddles; these saddles should be provided with a vent to allow escaping product.

A typical field applied Full Encirclement Reinforcement Weldment Saddle is shown below:

FAQ