Fractionator Relief

Chapter from the Pressure Relief Design Guide.

FRACTIONATOR RELIEF LOAD DETERMINATION

Relief Scenarios for Fractionation Columns

Scenario	Description	Typical Controlling?	Relief Rate Basis	Key Assumptions	API 521 Section	Notes
Cooling water failure	Loss of overhead condenser cooling	Often controlling	Vapor rate at zero reflux	All latent heat to relief	5.3.2	Most common controlling case
Power failure	Loss of all electric-driven equipment	Sometimes controlling	Heat input continues, no reflux	Steam reboiler maintains duty	5.3.3	Check what trips on power loss
Reflux failure	Loss of reflux pump(s)	Rarely controlling alone	Column vapor rate at no reflux	Reboiler still firing	5.3.2	Subcooled reflux adds margin
Blocked outlet	Downstream valve closed	Column-specific	Full column throughput	Liquid backs up, then relieves	5.3.1	May pressurize to relief set
Abnormal heat input	Reboiler overfiring / hot feed	Rarely	Max reboiler duty × safety factor	Worst-case heat input	5.3.4	Check fired heater max duty
External fire	Fire impinging on column shell	Supplemental device	Per fire case methodology	Wetted surface calculation	5.15	Often sets second PRV
Tube rupture	Reboiler tube failure	Rarely controlling	Full HP inventory into column	High P utility into low P column	5.3.6	Check 2/3 rule applicability

Cooling Water Failure — Detailed Methodology

This is typically the controlling relief scenario for most fractionation columns.

Step 1: Determine total overhead vapor rate at operating conditions (from simulation or test data)
Step 2: Assume total loss of condensing — all overhead vapor goes to relief
Step 3: Calculate relief conditions:
• Pressure: set pressure + allowable overpressure
• Temperature: bubble point at relieving pressure (this is higher than normal)
• Composition: overhead vapor composition
Step 4: Size relief valve for the vapor rate at relief conditions

Key Considerations:
• If overhead system has air-cooled exchangers, partial credit may be taken for natural draft cooling (typically 20-30% credit)
• Subcooled reflux provides transient relief — steady-state assumption is conservative
• For total condensers, ALL vapor must be relieved; for partial condensers, only the portion normally condensed
• Check if tower internals will flood — liquid backup may reduce vapor relief rate

Source: Pressure_Relief_Design_Guide_v1.xlsx · sheet “Fractionator Relief”

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