Rupture Disc — Engineering Fundamentals
When to use a disc instead of a PSV, disc types, manufacturing range, combination devices.
1. Disc vs PSV
A rupture disc is a single-use, fast-acting relief device — burst at set pressure, opens fully in milliseconds, never reseats. Compared to a PSV:
- Pro: lower cost, no moving parts (no maintenance), full-bore opening (instant capacity), tight zero leakage.
- Con: must be replaced after burst, no overpressure release if disc fails to burst, manufacturing tolerance on burst P.
Common uses: corrosive / scaling / freezing service that would foul a PSV; very fast overpressure scenarios (deflagration); upstream of PSV for media isolation.
2. Disc types
| Type | Operating P / Set P | Notes |
|---|---|---|
| Forward acting (tension) | ≤ 0.70 | Simple, low cost, no vacuum support. |
| Reverse buckling (compression) | ≤ 0.90 | Higher operating ratio; opens cleanly; needs vacuum support if cycling. |
| Hybrid (knife + reverse) | ≤ 0.95 | Highest operating ratio; for clean service. |
3. Burst tolerance + manufacturing range
Per ASME UG-127, the stamped burst P has ±5% tolerance at the rated temperature. Manufacturing range (MR) is a separate concept — the offset between MAWP and stamped burst, typically 5–20%. Example: MAWP 150 psi, MR 10%, stamped burst = 135 psi → burst window 128–142 psi. The disc must burst before MAWP+10% (UG-125 overpressure limit).
4. Combination (RD + PSV)
Rupture disc upstream of PSV isolates the PSV from corrosive / sticky / freezing media. When the disc bursts, the PSV opens conventionally. UG-127 requires a combination factor K_c = 0.9 on the PSV sizing equation to account for the small flow restriction from the burst disc fragments. A pressure tell-tale (gauge or transmitter) in the space between the disc and PSV warns of a slowly-leaking disc.
5. References
- API Std 520 Part I — Sizing, Selection, and Installation of Pressure-Relieving Devices.
- ASME VIII Div 1 UG-127 — Rupture Disk Devices.
- ASME PCC-3 — Inspection Planning Using Risk-Based Methods (RD inspection scope).
- Vendor lit: Continental Disc, Fike, BS&B Safety Systems.