Fire Exposure Requirements for PSV Sizing

"Latent Heat and Loaded Valves: A Hot Take on PSV Fire Sizing" I recently worked on the fire case for a vessel with wetted surface — and I now truly understand why this scenario is considered one of the most dynamic and underestimated in PSV design. After doing the unwetted case (where only vapor is involved), I moved to the wetted case, where liquid in contact with vessel walls boils off due to fire. This meant calculating: • The actual heat absorbed by the wetted area • A realistic latent heat value using HYSYS • And finally, sizing the PSV orifice using API 520 I’ve attached both Aspen HYSYS screenshots and the API 521 reference for transparency. The Scenario • Vertical vessel, 60% filled with water • Diameter: 1.5 m • Length: 7 m • Wetted surface (with 20% margin): 25.65 m² Step 1: Heat Absorbed • Using API 521’s formula: • Q = C₁ × F × A_ws^0.82 • C₁ = 43,200 (SI), F = 1 • Q ≈ 617,924 kJ/hr Step 2: Estimating Latent Heat in HYSYS Instead of using a fixed value, I simulated the boiling process: • Stream: 100°C, 5 barg, 100% water • Heater used to generate 5% vapor fraction For Q1, I got a duty of 10.1 kW and mass flow of 5 kg/hr Latent heat = (10.1 / 5) × 3600 = 7272 kJ/kg Then I repeated it with two more heaters: • Q2 = 2.627 kW, mass = 4.75 kg/hr → L ≈ 1990.98 kJ/kg • Q3 = 2.497 kW, mass = 4.512 kg/hr → L ≈ 1992.28 kJ/kg Out of the three, I selected the lowest latent heat (1990.98 kJ/kg) to be conservative. Lower latent heat results in higher relief flow, and it accounts for the potential flashing of lighter components during fire. Step 3: Relief Load Relief Load = Q / L = 617,924 / 1990.98 ≈ 310.36 kg/hr Step 4: PSV Sizing (API 520 – Choked Flow) Inputs: Relief pressure = 7.05 bara (1.21 × design + 1 bar) Relieving temp = 373.15 K Molecular weight = 18.015 Cp/Cv = 1.33 Z = 1.0 Discharge coefficient Kd = 0.975 Results: Calculated orifice area = 30.55 cm² Selected standard = 33.387 cm² Orifice designation = P Relief load = 310.36 kg/hr Heat input = 617,924 kJ/hr Latent heat used = 1990.98 kJ/kg A couple of things stood out • While simulating Heaters 2 and 3 in Aspen HYSYS, I got the error “temperature decrease on heating.” It didn’t converge, and that had me curious. • And although I chose the lowest latent heat for conservatism, it got me thinking: is there ever a case where using the average would be more appropriate? So I’d love to ask: • Why does Aspen throw that “temperature decrease on heating” error during partial vaporization? • From a design safety perspective, is it always better to pick the lowest latent heat — or have you ever justified using an average? Would love to hear your thoughts and learn from how others approach this. These discussions help sharpen understanding far beyond the datasheets.

🔥 When a vessel is exposed to fire, engineers usually jump straight to PSV sizing. But the question you should ask first is: 👉 How long does the liquid actually survive? ⏱️ Total Liquid Dry-Out Time It’s the time required for all liquid inside the vessel to vaporize during fire exposure: 👉 θₜ = θₕ + θ_b Where: θₕ → Time to heat liquid to boiling point θ_b → Time to vaporize the liquid 🚨 Why this calculation is important: 👉 If dry-out time < fire response time (~20 min) ➡️ Vessel becomes UNWETTED 👉 If dry-out time > fire response time ➡️ Vessel remains WETTED 🚨 Sample Calculation: Liquid Dry-Out Time 📌 Given: Heat input, Q = 6.2 × 10⁵ kJ/hr Liquid density, ρₗ = 558 kg/m³ Liquid volume, Vₗ = 1.68 m³ Specific heat, Cₚₗ = 2.82 kJ/kg·°C Initial temperature, T₀ = 60°C Boiling temperature, T_b = 180°C Latent heat, λ = 227.75 kJ/kg 🧮 Step 1: Heating Time (θₕ) Heating the liquid to boiling: θₕ = (ρₗ × Vₗ × Cₚₗ × (T_b − T₀)) / Q 👉 θₕ = (558 × 1.68 × 2.82 × 120) / (6.2 × 10⁵) 👉 θₕ ≈ 28.4 min 🧮 Step 2: Boiling Time (θ_b) Vaporizing the liquid: θ_b = (ρₗ × Vₗ × λ) / Q 👉 θ_b = (558 × 1.68 × 227.75) / (6.2 × 10⁵) 👉 θ_b ≈ 20.7 min 🧮 Step 3: Total Dry-Out Time (θₜ) θₜ = θₕ + θ_b 👉 θₜ = 28.4 + 20.7 👉 θₜ ≈ 49.1 min - Fire duration (API) ≈ 20 min - Dry-out time = 49 min ✅ Liquid is still present ✅ Vessel is WETTED ✅ Use wetted fire case for PSV sizing 🧠 Engineer’s Checklist Before sizing PSV for fire case: ✔️ Calculate liquid dry-out time ✔️ Compare with fire response time ✔️ Decide wetted vs unwetted basis ✔️ THEN proceed with relief load ⚠️ Quick check for engineers: How do you confirm fire response time? 👇 Drop your answer in comment #processengineering #psv #processsafety #oilandgas #pressurevessels #safety #firesafetyengineering #engineeringdesign

🔥 PSV Sizing: Fire Case Scenario in Aspen HYSYS — A Practical Walkthrough As process engineers, we know that pressure relief sizing isn't just about running simulations—it's about understanding the why behind every number. Recently completed a fire case PSV sizing for vessel V-100. Here's what the data tells us: 📊 Key Parameters: Set Pressure: 48.26 barG (Design Pressure: 48.26 barG) Allowable Overpressure: 21% → Relieving Pressure: 58.40 barG Relieving Temperature: 145.2°C Phase: Vapor (Direct Integration HEM method) ⚙️ Sizing Results: Required Flow Rate: 43,720 kg/h Calculated Orifice: 9.712 cm² Selected Orifice: 11.858 cm² (API "K" orifice) Rated Capacity: 53,380 kg/h Capacity Used: 81.9% ✅ 🎯 Line Sizing Validation: Inlet line ΔP: 0.36 bar (3% of set pressure — well within API 520 limits) Outlet line ΔP: 4.81 bar Outlet velocity: 248.8 m/s (below critical velocity of 314.3 m/s) All checks: OK 💡 Why this matters: The 21% overpressure allowance is critical for fire cases per API 520/521. The selected "K" orifice gives us ~22% margin above required flow—good engineering practice without gross oversizing. Key takeaway: Always validate your line sizing separately. A properly sized PSV with inadequate inlet/outlet piping is still a failure waiting to happen. What's your go-to approach for fire case heat input calculations? API 521 equations or CFD-based heat flux? Drop your thoughts below 👇 #ProcessEngineering #PSVSizing #PressureRelief #AspenHYSYS #API520 #SafetyEngineering #ChemicalEngineering #ProcessSafety #ReliefValve #FireCase