API 520 – Relief Valves Requiring Capacity Certification

Introduction

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).

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

K_v = \left(0.9935 + \frac{2.878}{Re^{0.5}} + \frac{342.75}{Re^{1.5}}\right)^{-1.0}

K_v = \left(0.9935 + \frac{2.878}{Re^{0.5}} + \frac{342.75}{Re^{1.5}}\right)^{-1.0}

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)

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 Reynold’s Number, Re, from either of the following relationships.

Re = \frac{Q(2800 \cdot G_1)}{\mu \sqrt{A}}

Re = \frac{Q(2800 \cdot G_1)}{\mu \sqrt{A}}

Where:

𝑅𝑒 − Reynolds Number

𝑄 − flow rate at the flowing temperature (gal/min)

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

𝐴 − the required effective discharge area (in²)

Case Guide

Part 1: Create Case

Input Parameters

• Relief Valve Set Pressure (psig)
• Required Effective Discharge (GPM)
• Required Effective Discharge Area (in)
• Specific Gravity of Liquid
• Absolute Liquid Viscosity (cP)
• Kd- Coefficent of Discharge
• Overpressure (%)
• Total Back Pressure (psig)
• t – Duration for which Valve is Open (min)

Part 2: Outputs/Reports

Non Viscous Results

• 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