Relief Headers

Chapter from the Pressure Relief Design Guide.

RELIEF HEADER DESIGN

Backpressure Limits by Valve Type

Valve Type	Built-Up BP Limit	Superimposed BP Limit	Total BP Limit	Impact of Exceeding	Mitigation
Conventional	10% of set (gauge)	Zero (affects set point)	10% of set	Set point shifts, capacity loss	Replace with balanced bellows
Balanced bellows	Per manufacturer	30-50% of set	50% of set typical	Capacity reduction per API curve	Use pilot-operated or dual valves
Pilot operated	Per manufacturer	60-70% of set	70% of set typical	Minimal impact if within rating	Inherently tolerant of BP
Rupture disk	N/A (no backpressure effect)	Per rating (typ. 2/3 burst)	Per manufacturer	Premature burst if exceeded	Check burst P at expected BP

Header Sizing Methodology

Step 1: Determine simultaneous relief loads
• Which scenarios can occur simultaneously? (Power failure affects multiple PSVs)
• Fire case: all PSVs in fire zone + any non-fire scenarios that occur with fire
• Common contingency: general power failure (most conservative)

Step 2: Size header for maximum simultaneous flow
• Velocity limit: Mach 0.5 maximum in any header segment
• Pressure drop: keep total backpressure within PSV limits
• Segment-by-segment hydraulic analysis from each PSV to final disposal

Step 3: KO drum sizing
• Liquid from two-phase relief must be captured
• Size for maximum liquid rate + hold time (typically 20-30 min)
• Vertical separators: Souders-Brown K = 0.1-0.15 ft/s for demisting
• Liquid level instrumentation with automatic pump-out

Step 4: Disposal system
• Elevated flare: radiation check at grade (API 521)
• Ground flare: noise and radiation enclosure
• Vent stack: dispersion check for toxic/flammable releases

Source: Pressure_Relief_Design_Guide_v1.xlsx · sheet “Relief Headers”

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