Pressure Relief Design Guide
HX Overpressure
Chapter from the Pressure Relief Design Guide.
HEAT EXCHANGER OVERPRESSURE PROTECTION
Overpressure Scenarios for Heat Exchangers
| Scenario | Mechanism | When to Evaluate | Sizing Basis | Typical Protection | API Reference | Notes |
|---|---|---|---|---|---|---|
| Blocked outlet | Liquid thermal expansion | All exchangers | Thermal expansion rate | Thermal relief valve (small PSV) | API 521 5.3.1 | Most common HX relief scenario |
| Tube rupture | HP fluid enters LP shell | MAWP ratio >1.5:1 | Full tube bore flow at ΔP | PSV on LP side sized for tube rupture | API 521 Annex D | Major relief load scenario |
| External fire | Fire impingement on shell | All exchangers in fire zone | Fire case heat input | PSV per fire case methodology | API 521 5.15 | Often covered by unit PSV |
| Control valve failure | Full flow to exchanger | If CV failure overpressures HX | Max flow through failed CV | PSV or system design | API 521 5.3 | Check upstream CV failure modes |
| Utility failure | Loss of cooling medium | Coolers in closed circuits | Continued heating + no cooling | PSV or instrument trip | API 521 5.3.2 | Similar to CW failure case |
Tube Rupture Analysis — API 521 Annex D
The 2/3 Rule (API 521 Annex D):
If the LP side MAWP ≥ 2/3 of the HP side MAWP, the LP side is considered adequately protected against tube rupture WITHOUT a dedicated relief device, provided:
• LP side is designed per ASME VIII
• HP side relief is adequate for its own scenarios
• No additional overpressure mechanisms
When 2/3 Rule Does NOT Apply (LP MAWP < 2/3 HP MAWP):
Step 1: Determine tube rupture flow rate (single tube, double-ended guillotine)
Step 2: Calculate two-phase relief rate into LP side
Step 3: Size PSV on LP side for this relief load
Step 4: Check LP side piping and equipment rating for transient pressure
Tube Rupture Flow Calculation:
• Single tube bore: A = π/4 × d_i² (one tube ID area)
• Double-ended: 2 × A (conservative)
• Flow rate: use orifice equation with ΔP = HP design P - LP relief P
• Two-phase flash: HP liquid may flash as it enters LP side
Source: Pressure_Relief_Design_Guide_v1.xlsx · sheet “HX Overpressure”
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