Emergency Depressuring

Blowdown Systems

Depressure inventory fast enough to survive fire exposure while managing low-temperature limits and flare capacity.

Launch Calculator Check low temp

Fire target

100 psig or 50% MAWP

Depressure to the lower value within the allowed time.

Time to depressure

~15 minutes

API 521 benchmark; risk assessments may refine.

Flow regime

Choked then subcritical

Expect choked flow early, subcritical as pressure drops.

Contents

Use this guide when you need to:

  • Set depressuring time/pressure targets for fire case.
  • Size restriction orifices and headers to flare.
  • Check minimum metal temperature during blowdown.

1. Blowdown Principles

Blowdown (depressuring) systems rapidly reduce pressure in process equipment during emergencies—typically fire exposure. The goal: reduce wall stress before fire weakens the metal to failure.

Driver

Fire case

Relieve stress before steel loses strength from heat.

Target

100 psig / 50% MAWP

Depressure to the lower value within the specified time.

Flow path

BDV → RO → KO → Flare

Valve, restriction, knockout, then to flare/header.

Control

ESD triggered

Automatic fail action on emergency shutdown signal.

Why Blowdown?

  • Fire case: Reduce stress before vessel wall loses strength
  • Hydrocarbon release: Minimize inventory available for fire/explosion
  • Maintenance: Depressure for safe entry or repair
  • Process upset: Prevent overpressure during runaway

API 521 Criteria

For fire exposure, depressure to:

  • Target pressure: 100 psig or 50% of design pressure (whichever is lower)
  • Target time: 15 minutes (typical), may vary by risk assessment
Why 15 minutes? Steel loses ~50% of its strength at 1100°F (typical fire exposure temperature). At 72% SMYS design, vessel can fail in ~15–20 minutes without depressuring.

2. Orifice Sizing

Blowdown orifice must allow adequate flow to meet depressuring time target. Size using critical flow assumption for most of blowdown duration.

Simplified Sizing Equation

For ideal gas, isothermal expansion: A = (V / t) × ln(P1/P2) / (C × √(M/TZ)) Simplified (empirical, API 521 Annex): A = 0.0144 × V × √(MW/T) / t × F_A Where: A = Orifice area (in²) V = System volume (ft³) P1 = Initial pressure (psia) P2 = Final pressure (psia) t = Depressuring time (minutes) MW = Molecular weight T = Temperature (°R) F_A = Area factor (1.0 for gas-filled, higher for two-phase)

Example Calculation

Given: V = 1000 ft³, P1 = 500 psia, P2 = 115 psia, t = 15 min, MW = 20, T = 100°F

T = 100 + 460 = 560°R ln(500/115) = 1.47 Using simplified: A = 0.0144 × 1000 × √(20/560) / 15 A = 0.0144 × 1000 × 0.189 / 15 A ≈ 0.18 in² → ½" orifice (0.196 in²)

⚠ Verify with dynamic simulation: Simplified equations assume constant T and single phase. For accurate results (especially with liquid inventory or J-T cooling), use dynamic process simulation.

Compressibility (Z-factor): For systems above 500 psia, include the gas compressibility factor Z in calculations. Real gas deviation from ideal behavior can affect blowdown time by 20–50% at high pressures. Use Standing-Katz chart or Papay correlation.

Pipeline Blowdown (Walworth Equation)

For pipeline segments, the Modified Walworth equation is commonly used:

t = 0.0588 × √SG × L × (√P1 - √P2) × (D²/d²) × Fc × √Z Where: t = blowdown time (minutes) SG = specific gravity (air = 1.0) L = pipe length (miles) D = pipe ID (inches) d = orifice/stack ID (inches) Fc = choke factor (valve type) Z = average compressibility factor

The vessel equations above are for concentrated volumes; the Walworth equation accounts for distributed inventory in pipelines.

01

Define targets. Set P1, P2 (100 psig or 50% MAWP), and depressuring time (e.g., 15 min).

02

Size RO. Calculate orifice area using gas properties and volume; check choked vs subcritical.

03

Verify dynamically. Simulate blowdown to capture cooling, two-phase flow, and flare backpressure.

3. Depressurization Dynamics

Pressure-Time Profile

Blowdown through a fixed orifice follows an exponential decay pattern:

P(t) = P1 × exp(-t/τ) Where: τ = Time constant = V / (C × A × √(T/M))
Blowdown pressure profile showing exponential decay to target pressure over time.
Blowdown pressure profile with exponential decay to target pressure and 15-minute benchmark.

Flow regime

Choked → subcritical

Expect critical flow early; tail is subcritical as P drops.

Flare load

Peak at t = 0

Design flare for initial choked mass flow; decay thereafter.

τ sensitivity

Volume / area

Time constant scales with inventory and restriction area.

Critical Pressure Ratio

Flow becomes choked (sonic) when downstream pressure falls below the critical ratio:

(P2/P1)_critical = (2/(k+1))^(k/(k-1)) For natural gas (k ≈ 1.30): Critical ratio ≈ 0.546 For air (k = 1.40): Critical ratio ≈ 0.528

When P_downstream/P_upstream < 0.546, flow is choked and mass rate depends only on upstream conditions.

Flow Rate During Blowdown

Phase Flow Regime Flow Rate Behavior
Initial (0–80%) Critical (choked) Flow ∝ upstream pressure
Final (80–100%) Subcritical Flow ∝ √(P1 - P2)

Initial Peak Flow Rate

W_initial = C × A × P1 × Kd × √(M / (T × Z)) This is the maximum flow to flare header—critical for flare sizing.

4. Low-Temperature Considerations

Rapid depressuring causes significant cooling due to Joule-Thomson effect and gas expansion work. This can cause:

  • Brittle fracture: If temperature drops below MDMT
  • Hydrate formation: In wet gas systems
  • Ice formation: From trace water

Temperature During Blowdown

Isentropic expansion (theoretical worst case): T2/T1 = (P2/P1)^((k-1)/k) Example: k=1.3, P1=500 psia, P2=100 psia T2/T1 = (100/500)^(0.231) = 0.69 If T1 = 100°F (560°R), T2 = 386°R = -74°F
Isentropic vs. Joule-Thomson: The isentropic equation gives theoretical maximum cooling (adiabatic, reversible). In practice, heat transfer from surroundings reduces cooling. The Joule-Thomson approach (ΔT ≈ 7/SG °F per 100 psi drop) provides a more practical estimate. Use isentropic for conservative material selection; J-T for operational planning.
Pressure and temperature versus time during blowdown with MDMT limit highlighted.
Pressure and temperature vs. time during blowdown, highlighting minimum metal temperature relative to MDMT.

Mitigation Strategies

Strategy Application
Low-temp materials Impact-tested steel, stainless, or nickel alloys
Controlled blowdown rate Staged orifices or larger time target
Depressure to atmosphere Reduces J-T effect vs. flare backpressure
Heat tracing Blowdown piping to prevent ice plugging

⚠ MDMT verification: Always calculate minimum metal temperature during blowdown. If below MDMT, upgrade materials or reduce blowdown rate. Brittle fracture is catastrophic.

5. System Design

Blowdown Valve Selection

  • Type: Ball valve (full-bore, low ΔP) or globe valve (throttling)
  • Actuator: Fail-open (spring-return) for fire case, or fail-closed with ESD override
  • Speed: Fast opening (<5 seconds) for emergency
  • SIL rating: Per risk assessment (typically SIL 2 for fire case)

Restriction Orifice

  • Location: Downstream of BDV (protects valve from erosion)
  • Type: Single-hole or multi-hole plate
  • Material: Stainless or stellite for erosion resistance
  • Sizing: Per blowdown calculation

Piping Layout

  • Header sizing: Limit velocity to avoid noise/vibration (<0.5 Mach)
  • Knockout drum: Capture any liquid carryover before flare
  • Slope: Drain toward KO drum
  • Supports: Design for thermal movement and reaction forces
Blowdown system layout showing vessels tied to blowdown header, knockout drum, and flare.
Blowdown system layout: vessel isolation and BDV through RO to header, draining to KO drum and flare.

References

  • API 521 – Pressure-relieving and Depressuring Systems
  • API 520 – Sizing, Selection, and Installation of PRDs
  • API 537 – Flare Details for General Refinery and Petrochemical Service
  • NORSOK S-001 – Technical Safety
  • ISO 23251 – Petroleum and Natural Gas Industries—Pressure-relieving Systems
  • Sutton, R.P. (1985) – Compressibility Factors for High-MW Reservoir Gases, SPE 14265
  • GPSA

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