Refinery Fuel Gas Header Relief Sizing
Engineering Case Study
Case Study 1: Refinery Fuel Gas Header Relief Sizing
Scenario
A Tier-1 petroleum refinery in Houston, Texas, was upgrading its fuel gas distribution header serving multiple fired heaters. The project required replacement of an aging pilot-operated relief valve (PORV) on the 6-inch header operating at 325 psi(g). Constraints included minimal downtime (≤4 hours), strict API RP 520 Part I compliance, and a requirement to maintain 10% overpressure margin per OSHA PSM standards. The existing valve had undocumented capacity and failed a recent functional test.
Given Data
- Fluid Type:
gas - Required Relieving Capacity:
28,500 lb/h(determined from worst-case blocked outlet + heater tube rupture scenario) - Set Pressure:
325 psi - Overpressure:
32.5 psi(10% of set pressure, per API RP 520 §3.3.2) - Back Pressure:
15 psi(superimposed, from common flare header) - Coefficient of Discharge:
0.92(manufacturer-certified value for API 526 Class 2500 metal-seated PORV)
Calculation
The tool applies the standard gas sizing equation per API RP 520 Eq. 3-1:
$$ A = \frac{W}{C_d \cdot K_{sh} \cdot K_b \cdot K_c \cdot \sqrt{\frac{T Z}{M}} \cdot \frac{1}{P_1} \cdot \frac{1}{Y} $$
While the tool abstracts intermediate factors (e.g., $K_{sh}$, $K_b$, $K_c$, $Y$), it internally computes them using:
- $P_1 = P_{set} + P_{overpressure} + P_{back} = 325 + 32.5 + 15 = 372.5\ \text{psia}$
- $Y$ (expansion factor) derived from $k = 1.3$ (fuel gas), yielding $Y \approx 0.792$
- $T = 110^\circ\text{F} = 570\ \text{R}$, $Z = 0.98$, $M = 22.4$ (typical fuel gas molecular weight)
Using the embedded algorithm with the given inputs, the tool computes:
- Orifice Area = 1.4287 in²
- Valve Size = 2.00 in (NPS — selected from standard API 526 orifice designations: D = 1.39 in², E = 1.89 in² → E-orifice fits; nominal inlet size is 2-inch NPS per API 526 Table 2)
Result and Decision
A new 2-inch × 1.5-inch (inlet × outlet) API 526 Class 2500 PORV with E-orifice (1.89 in²) was selected. It exceeded the minimum required area by 32%, providing margin for future capacity growth and accommodating potential fouling. Installation occurred during a scheduled 4-hour turnaround, and the valve passed hydrostatic and lift tests per API RP 576.
Lesson
Always validate the coefficient of discharge against the actual valve model and trim configuration—not generic tables. In this case, using the manufacturer’s certified $C_d = 0.92$ (vs. default 0.8) reduced the required orifice area by 15%, enabling use of a smaller, more cost-effective valve without compromising safety.