Oil & Gas Onshore Process Skid — ESD / F&G / HIPPS
Project Summary
| Field | Detail |
|---|---|
| Application | Onshore O&G process skid with Emergency Shutdown (ESD), Fire & Gas (F&G), and High Integrity Pressure Protection (HIPPS) |
| Industry | Petroleum / Oil and Gas — upstream or midstream onshore |
| Safety standard | IEC 61511 (SIS application) + IEC 61508 (foundation) |
| Hazardous area | Zone 1 / Zone 2 gas atmosphere — IEC 60079 series |
| US installation | NEC Art. 500–505 (hazardous locations) |
Standard Stack
| Standard | Role |
|---|---|
| IEC 61511 | SIS application lifecycle — HAZOP through proof testing |
| IEC 61508 | Logic solver qualification foundation |
| IEC 60079-0 | Ex equipment marking and selection |
| IEC 60079-10-1 | Hazardous area zone classification |
| IEC 60079-11 | Intrinsic safety for field instruments |
| IEC 60079-14 | Ex installation design and verification |
| IEC 60079-17 | Ex inspection and maintenance |
| IEC 60204-1 | Electrical equipment of the skid |
| NEC Art. 500 / 504 / 505 | US hazardous location wiring |
| NEC Art. 250 | Grounding and bonding |
Design Workflow
Phase 1 — Hazard Analysis and SIL Determination
Step 1: HAZOP
- Identify process deviations (high pressure, low flow, leak)
- Assign consequence severity to each deviation
- Record IPLs (PRVs, operator response, BPCS) already in place
Step 2: LOPA (IEC 61511 §9)
- For each intolerable consequence: calculate residual risk after IPLs
- Determine if a SIF is required and what SIL target it must meet
- SIL 1: PFDavg 0.1–0.01 | SIL 2: 0.01–0.001 | SIL 3: 0.001–0.0001
Step 3: Safety Requirements Specification (SRS)
- Document each SIF: process demand, safe state, response time, SIL target
- Specify logic solver inputs/outputs, field device requirements
Phase 2 — SIS and Ex Equipment Design
Step 4: SIF Architecture Selection
- SIL 1: 1oo1 often sufficient; verify PFDavg with reliability data
- SIL 2: 1oo2 or 2oo3 voted — choose based on safe failure fraction (SFF)
- SIL 3: 1oo2D or 2oo3 with HFT ≥ 2; logic solver must be IEC 61508 certified
Step 5: PFDavg Verification
- Calculate PFDavg for each SIF architecture using supplier λSD/λSU data
- Include proof test interval (PTI) — longer PTI increases PFDavg
- Confirm PFDavg falls within required SIL band
Step 6: Zone Classification (IEC 60079-10-1)
- Identify all release sources on the skid
- Assign grade of release (continuous / primary / secondary)
- Assess ventilation, determine zone extents
- Produce classified area drawing
Step 7: Ex Equipment Selection (IEC 60079-0)
- Match EPL to zone: Gb → Zone 1, Gc → Zone 2
- Match gas group: IIA / IIB / IIC per process fluid
- Match T-code: T-code max surface temp < autoignition temp of gas (with margin)
- For IS field devices: select ia level for Zone 0/1
Step 8: IS Loop Design (IEC 60079-11)
- For each IS field device: select zener barrier or galvanic isolator
- Verify entity parameters: Uo ≤ Ui, Io ≤ Ii, Ci + Ccable ≤ Co, Li + Lcable ≤ Lo
- Document IS loop calculation sheet for each loop
Phase 3 — Installation and Commissioning
Step 9: Installation per IEC 60079-14
- IS cables on separate trays / conduits from non-IS wiring
- IS earth point: resistance ≤ 1 Ω (zener barrier circuits)
- Equipotential bonding: all metallic structures bonded
- Cable glands: Ex d certified, correct for cable type and gas group
Step 10: Initial Verification (IEC 60079-14 §6)
- Verify all certificates are current and correct
- Inspect flame paths (Ex d), IS earth resistance, entity compliance
- Issue commissioning certificate
Step 11: SIS FAT / SAT (IEC 61511 §12)
- Factory Acceptance Test: test each SIF end-to-end
- Site Acceptance Test: repeat after installation, confirm response times
- Witnessed proof test for each SIF before process start-up
SIS Architecture Diagram
graph TD
subgraph "Field — Zone 1 (Gb)"
PT1[Pressure Transmitter<br/>Ex ia IIB T4 Ga]
PT2[Pressure Transmitter<br/>Ex ia IIB T4 Ga]
XV1[Shutoff Valve<br/>Ex d IIB T4 Gb]
end
subgraph "Safe Area — Marshalling"
B1[Zener Barrier<br/>IS Earth ≤1Ω]
B2[Zener Barrier<br/>IS Earth ≤1Ω]
end
subgraph "Safe Area — Logic Solver"
LS[Safety PLC<br/>SIL 2 Certified<br/>IEC 61508]
end
PT1 -->|IS loop| B1 --> LS
PT2 -->|IS loop| B2 --> LS
LS -->|SIF output| XV1
Key Engineering Decisions
Zener barrier vs. galvanic isolator for IS instruments: Zener barriers are lower cost but require a dedicated IS earth ≤1 Ω. If the IS earth cannot be guaranteed (old facility, multi-unit grounding), use galvanic isolators. Galvanic isolators also eliminate ground loops that cause 4–20 mA measurement errors in long cable runs.
Zone 1 vs. Division 1 on a US installation — a classification decision, not an approval shortcut:
Either classification system may be used for a Class I location; OSHA’s 29 CFR 1910.307 permits both. Zone (Art. 505) keeps the area classification, protection concepts, and EPLs in IEC vocabulary, so an IEC-marked equipment population needs no translation into Class/Division/Group terms (Gb corresponds to Zone 1). Division (Art. 500) uses the traditional US vocabulary and is more familiar to most US inspectors, and requires conduit seals at every explosionproof enclosure entry.
Choosing Zone does not change who approves the equipment. Under 29 CFR 1910.399 “approved” means acceptable to the authority enforcing OSHA Subpart S, and acceptability normally rests on certification, listing, or labelling by an OSHA-recognised NRTL. OSHA’s NRTL programme guidance states that equipment intended for potentially explosive atmospheres in the United States must bear the mark of an NRTL recognised for that equipment type; an ATEX certificate does not substitute for it, and the CE mark is unrelated to US product-safety requirements. 29 CFR 1910.307(g) also requires Class I Zone equipment to be listed and marked in the US form — class, zone, AEx, protection technique, gas group, temperature class.
On this skid, that means: if the transmitters, barriers, and F&G detectors are being sourced from an EU vendor on the strength of an ATEX or IECEx certificate, budget a US listing path for each item and confirm the NRTL mark and its scope on the actual product before it is specified — not at commissioning. Then confirm the marking form and the classification drawings with the AHJ. The classification-system choice and the equipment-approval question are settled separately. See the IEC 60079 ↔ NEC 500/505 crosswalk for the full five-layer separation and the narrow non-NRTL exceptions.
SIL 2 logic solver selection: Avoid using a standard PLC for SIL 2 — it requires a full IEC 61508 assessment of the PLC hardware and software. Use a safety-certified logic solver (e.g., SIL 2 certified SIS controller) where the supplier provides the IEC 61508 certificate and safety manual.
See Petroleum / O&G industry overlay →
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