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Rupture Disk Specifications & Process Operating Conditions
API RP 520 Part I & ASME Section VIII Div 1 Paragraph UG-127 Sizing Architecture
Rupture Disk Sizing & Burst Tolerance Diagnostics
Marked Burst Pressure (P_b)
--
-- psig burst band
Current Operating Ratio
--
-- max allowable
Operating Ratio Status
--
-- fatigue risk
Minimum Vent Area (A_req)
--
-- req bore
Recommended Nominal Size
--
Standard ANSI flange bore
Relieving Pressure (P1)
--
psig at 10% overpressure
Combination Derate (F_v)
--
ASME UG-127 PRV credit
Interactive Rupture Disk Burst Tolerance & Operating Window Profile
Standard Rupture Disk Nominal Sizing Comparison
Nominal Size (NPS)
Actual Bore (in)
Vent Area (in²)
Vapor Relief Capacity
ASME Sizing Margin
Mathematical Formulations & Code Derivations
Rupture disk sizing is governed by API RP 520 Part I and ASME Boiler and Pressure Vessel Code Section VIII Division 1 Paragraph UG-127. Rupture disks are differential pressure devices that activate without mechanical moving parts, opening fully in less than 2 milliseconds.
1. Marked Burst Pressure & Manufacturing Design Range (MDR):
P_burst = P_set
P_burst_max = P_burst * ( 1.0 + Tol_fraction )
P_burst_min = P_burst * ( 1.0 - Tol_fraction )
2. Operating Ratio & Cyclic Fatigue Boundary:
Ratio_actual = ( P_operating / P_burst ) * 100%
Operating Limits:
Reverse Buckling: Ratio <= 90% to 95%
Forward Prebulged: Ratio <= 70% to 80%
Solid Graphite: Ratio <= 80%
3. Relieving Pressure P1 (10% Overpressure):
P1_psig = 1.10 * P_set
P1_psia = P1_psig + 14.7
4. Vapor Critical Choked Flow Sizing (ASME Kd Method):
A_req = W / [ C * Kd * Fv * P1_psia * sqrt( MW / (T_R * Z) ) ]
Where:
W = Required mass flow (lb/hr)
C = Gas expansion coefficient = 520 * sqrt[ k * (2/(k+1))^((k+1)/(k-1)) ]
Kd = Discharge coefficient (0.62 per ASME UG-127)
Fv = Combination capacity factor (0.90 with PRV, 1.00 sole disk)
T_R = Relieving temperature in Rankine (deg F + 459.67)
5. Liquid Sizing Equation:
A_req = Q_gpm / [ 38.0 * Kd * Fv * sqrt( Delta_P / SG ) ]
When installing a rupture disk upstream of a relief valve, an unmonitored cavity between the disk and valve seat is illegal under ASME UG-127. Any fugitive pressure buildup in the cavity directly elevates the burst pressure of the disk.
If corrosive process vapors develop a micro-pinhole in the rupture disk, gas leaks into the piping spool between the disk and the downstream pressure relief valve. If the spool is not equipped with an active pressure gauge, burst sensor, or excess flow valve per ASME UG-127, pressure builds up to 60 psig. The rupture disk now requires its marked burst pressure PLUS 60 psig to rupture, defeating the emergency relief system and causing catastrophic reactor rupture.
2. Exceeding Operating Ratio on Forward Tension Disks
Forward-acting prebulged metal disks rely on tensile hoop stress to burst. When operated above 70% to 80% of marked burst pressure, thermal expansion and pressure cycling cause ongoing metal plastic deformation (creep). The metal foil thins progressively until the disk ruptures unexpectedly during routine operation, causing unbudgeted emergency plant shutdowns and toxic chemical releases.
3. Vacuum Collapse on Forward Disks Without Vacuum Supports
Forward-acting disks installed on vessels subject to steam-out or vacuum service must be equipped with matched internal vacuum supports. Without a support, external atmospheric pressure (14.7 psi) forces the convex dome inward in reverse compression. The foil distorts, wrinkles, and ruptures at random, uncalibrated positive pressures.
Standard non-scored forward-acting metal disks fragment into jagged metal petals upon bursting. If installed directly upstream of a pressure relief valve, metal fragments fly into the valve inlet nozzle, wedging between the valve disc and seat. The relief valve is jammed permanently open or severely choked, preventing reclosure and spewing hazardous contents into the atmosphere.
5. Omitting Temperature Derating on Marked Burst Pressure
Metal tensile strength drops significantly with temperature. A rupture disk ordered with a marked burst of 150 psig at 70°F will burst at approximately 125 psig if operating at 300°F. Ordering rupture disks without specifying the exact coincident relieving temperature results in disks bursting prematurely during hot summer runs or high-temperature process upsets.
Frequently Asked Questions
What is the difference between reverse-buckling and forward-acting rupture disks?+
Forward-acting (tension-loaded) rupture disks have the process pressure applied to the concave side, loading the metal foil in tension until it stretches and bursts. Because metal under tension is prone to cyclic fatigue and stress creep, forward-acting disks are limited to maximum operating pressures of 70% to 80% of marked burst pressure. Reverse-buckling (compression-loaded) disks have process pressure on the convex side, loading the dome in compression. Reverse-buckling disks resist cyclic fatigue, do not require vacuum supports, and can operate safely at 90% to 95% of marked burst pressure.
What is the ASME Section VIII UG-127 default combination factor (Fv = 0.90)?+
When a rupture disk is installed upstream of a pressure relief valve (PRV) to protect the valve from corrosive process fluids or prevent fugitive emissions, ASME Code Paragraph UG-127 mandates that a derating factor Fv = 0.90 must be applied to the certified relief valve capacity, unless the specific disk/PRV combination has been flow-tested and certified by the National Board with an ASME combination capacity factor.
Why is an interspace tell-tale pressure gauge or sensor mandatory between a disk and PRV?+
Per ASME UG-127(a)(3)(b), the cavity between a rupture disk and the downstream pressure relief valve must be equipped with a pressure gauge, try cock, excess flow valve, or electronic pressure transmitter. If a pinhole leak or corrosion develops in the rupture disk, process gas bleeds into the interspace. Any backpressure accumulated in this cavity adds pound-for-pound to the disk burst rating (e.g. 30 psig interspace pressure on a 100 psig disk requires 130 psig process pressure to burst), creating an illegal and catastrophic overpressure condition.
How does temperature affect rupture disk marked burst pressure?+
Rupture disk foil materials (such as stainless steel 316, Inconel 600, Monel, or Hastelloy C-276) lose tensile strength as temperature rises. A 316 stainless steel disk rated for 100 psig at 72 deg F may burst at only 88 psig at 350 deg F. Rupture disks must always be specified and ordered with their burst pressure referenced to the coincident operating relief temperature at the disk location, rather than ambient room temperature.
What is the Kr flow resistance factor in API 520 sizing?+
The Kr (velocity head loss coefficient) represents the hydraulic resistance of the ruptured disk device when fully open. Per API 520 and ISO 4126-2, Kr is used in the resistance-to-flow method where total relief piping head loss is evaluated: K_total = K_inlet + Kr + K_exit. Typical certified Kr values range from 0.25 to 0.45 for cross-scored reverse buckling disks and 1.0 to 1.5 for non-fragmenting forward acting disks.