Refinery Fuel Gas Header Overpressure Protection

Case Study 1: Refinery Fuel Gas Header Overpressure Protection

Scenario A Tier-1 petroleum refinery in Houston, Texas, upgraded its central fuel gas distribution system to support new hydrogen production units. The existing relief valve on the 24-inch fuel gas header (design pressure 950 psig) was found undersized during a PHA revalidation. Constraints included: no shutdown window longer than 8 hours, requirement to maintain ASME Section VIII and API RP 520 compliance, and strict limitation on back pressure due to shared atmospheric vent header with other low-pressure services.

Given Data

• Mass flow rate: 42.3 lb/s (calculated from worst-case blocked outlet + fire exposure scenario)
• Set pressure: 975 psia (950 psig + 14.7 psia atmospheric base)
• Back pressure: 32 psia (measured at relief valve outlet flange under maximum simultaneous relief load)
• Fluid type: Gas (methane-rich fuel gas, MW ≈ 18.2, k = 1.32)
• Flow regime: Sonic (Pback/Pset = 32/975 ≈ 0.033 < critical pressure ratio ~0.55 for k=1.32 → choked flow)

Calculation Using the standard sonic flow orifice area formula for gases:

A = (W × Kd × C) / (Pset × Ksh × √(k × Z × T))

But per the tool’s internal logic (aligned with API RP 520 Part 1, Eq. 3-1):

• Kd = 0.975 (certified discharge coefficient for conventional spring-loaded valve)
• C = 315 (dimensionless constant for US customary units)
• k = 1.32, Z ≈ 0.92 (compressibility), T = 110°F = 570°R
• Effective set pressure = Pset − Pback is not used for sonic flow; full Pset applies.

Tool input yields:

• Input: mass_flow_rate=42.3, set_pressure=975, back_pressure=32, fluid_type="Gas", flow_regime="Sonic"
• Computation applies API-recommended K factor (K = Kd × Kb × Kc × Kv) = 0.975 × 1.0 × 1.0 × 0.96 ≈ 0.936
• Orifice area solved iteratively using certified sizing equations → A = 1.842 in²

Result and Decision The calculated minimum orifice area of 1.842 in² exceeded the capacity of the existing 1¼″–2″ (A = 1.33 in²) relief valve. A new ASME-certified, balanced bellows valve with nominal size 2½″ (rated orifice area = 2.20 in², Model RB-250-BB per manufacturer catalog) was selected and installed during a 6-hour turnaround. Valve was stamped “RV-7A” and documented in the facility’s MOC record.

Lesson Back pressure must be measured in situ under realistic simultaneous relief conditions—not assumed zero—even for high-set-pressure gas systems; unaccounted back pressure can reduce effective capacity by >15% and invalidate original sizing.