How to Review a P&ID in 30 Minutes

Rev 1 — 2026-04-11 — Alfred / Engineering Library

A structured, time-boxed method for a process engineer or engineering manager to get meaningful value out of a P&ID review in a single 30-minute pass. This is NOT a substitute for a formal IFC review, HAZOP, or PSSR — it is the “is this sheet sane and worth sending to IFR?” first look.

Before you start (2 min)

1. Print the sheet if you can — paper catches things screens don’t. If not, load it on the biggest monitor you have and zoom to read tag numbers without squinting.
2. Pull up next to it:
3. The line list (or line designation table)
4. The equipment list for items on this sheet
5. The PFD for this area
6. The previous rev of the same P&ID, if one exists
7. Grab four colors: red (must fix), orange (should fix), green (question to ask), blue (note/idea).

Pass 1 — Orientation and title block (2 min)

• Title block: right area, right system, right rev, right date?
• Sheet number in the sequence — are there references to “continued on sheet X of Y” that need checking on another tab?
• Legend / symbol sheet version — does it match what’s actually drawn here?
• Revision cloud status — are previous clouds cleared? If clouds from Rev A are still showing on Rev 2, somebody missed a cleanup.
• General notes and reference drawings — read them. The most important engineering decisions are often buried in general notes.

Pass 2 — Main process flow (5 min)

Trace the primary process path from the incoming battery limit to the outgoing battery limit (or tank to tank, column to column, whatever is appropriate):

• Continuity. Every line should start somewhere and end somewhere. No floating pipe stubs. No “to be continued” without a cross-reference.
• Line numbers present and complete. Format matches spec (e.g. 4"-P-12345-1A-H). Line spec, size, insulation, and tracing codes all legible.
• Flow direction arrows on every line, especially into/out of equipment.
• Equipment tagged with full tag, service description, and reference to mechanical datasheet.
• Spec breaks clearly marked at every material or pressure class change. A missing spec break at a class change is one of the most common errors.

Pass 3 — Instrumentation and control (5 min)

Walk every controlled parameter — temperature, pressure, flow, level, composition — and confirm:

• Every control loop has a primary element (indicator), transmitter, controller, and final control element. If it’s a manual control, say so.
• Tag numbering follows the project’s convention (ISA 5.1). Loop numbers make sense (same loop number for the transmitter and valve of the same loop).
• Signal lines (pneumatic, electric, software) drawn correctly per legend. No broken dashes.
• Fail positions shown on every control valve (FC, FO, FL). Check each fail position against what actually keeps the unit safe — a common trap is a fail-closed valve on a feed line to an exothermic reactor that should be fail-open to flare, or vice versa.
• Interlocks have interlock numbers that reference the Cause & Effect matrix / SRS. Follow at least one trip logic from cause to final element.
• Safeguarding instrumentation (SIS) clearly distinguished from BPCS: diamond tags, different line type, 2oo3 voting shown where required.
• Relief devices (PSV, rupture disc) tagged with set pressure (on the drawing or in a callout), orientation, inlet and outlet line spec, and with PSV inlet losses <3% rule respected on the drawing (long inlet runs with reducers are a red flag).

Pass 4 — Utilities and services (3 min)

• Cooling water, steam, condensate, instrument air, nitrogen, flare, drain — each utility should have a clearly labeled origin and battery limit cross-reference.
• Backflow prevention: check valves on every hot/cold utility entering a process line to stop process from getting into the utility header.
• Drains and vents on every trap-prone low point and high point. Equipment drains routed to the correct drain header (oily water, process drain, chemical sewer) per plot.
• Bleeds on every PSV for maintenance isolation, with car-seal-open (CSO) valves.

Pass 5 — Hazards and layers of protection (5 min)

This is where P&ID review earns its keep.

• Over-pressure protection. Every volume that can be blocked in has a relief path or a documented reason it doesn’t need one. Trace the PSV outlet to flare or atmosphere — no dead-ending, no missing tail pipe rating.
• Vacuum / under-pressure protection on vessels that could see a vacuum (steam-out, cooldown, pump suction head loss). Common miss.
• Double block and bleed on flare headers, toxics, and tie-ins for hot work.
• Thermal relief on any line that can be blocked in between two valves and heat-soaked (sun, steam trace, adjacent hot equipment). TSVs or bypass holes required.
• Fire case on vessels. Confirm fire-case PSV sizing is referenced somewhere (usually in the general notes or on a PSV datasheet reference).
• Hot/cold surface protection — trace heated lines, steam lines, and cryogenic lines for insulation/personnel-protection notes.
• Two-phase / flashing service call-outs where applicable.
• Categorization of lines in hazardous service (H, toxic, wet H2S, amine, caustic) — the letter codes on the line number should match the service reality.

Pass 6 — Mark, list, and hand over (8 min)

• Capture every red and orange mark on a numbered comment sheet, one comment per row. Include: sheet number, coordinate (e.g. D-5), tag, description, category (safety/operability/drawing/process), and recommended action.
• Questions to the designer go in a separate list (green marks).
• Take a photo of the marked-up sheet if it’s paper. Email the comment sheet + photo to the lead within the hour.
• Tag any comment that affects another drawing (“check this change on Sh 7 too”) — they propagate fast.

The 12 most common findings (keep this near your desk)

1. PSV inlet pipe has a reducer and a long run — violates API 520 Part II 3% rule
2. Control valve fail position inconsistent with safe shutdown state
3. Missing spec break at a pressure or material class transition
4. Dead leg that can trap water/condensate/solids with no drain
5. Check valve missing on utility tie-in, allowing back-contamination
6. Blocked-in section with no thermal relief or expansion provision
7. Interlock drawn but Cause & Effect reference missing
8. Line number format or insulation code not matching the line list
9. PSV discharge piping not rated for back pressure / no balanced bellows noted
10. Pump minimum-flow recirc line or ARC valve missing
11. Sample point with no isolation, no drain, no block valve
12. Orphaned stub line with no termination

What this review is NOT

• Not a HAZOP. No P&L (Parameter and Guideword) analysis, no deviations.
• Not a model review (3D / P&ID system check).
• Not a piping-to-ISO review. Dimensions and routing live in the iso/model.
• Not a code-of-record compliance review (that takes much longer).

Use this list to triage. A sheet that fails more than two red items should be sent back for rev before a formal review begins — you’re wasting the team’s time running a HAZOP on a sheet that isn’t ready.

References

• ISA 5.1-2009 — Instrumentation Symbols and Identification
• API RP 14C / API 520/521 — Pressure-Relieving and Depressuring Systems
• CCPS, Guidelines for Process Safety Documentation, AIChE/Wiley, 1995
• IEC 61511-1 / ANSI/ISA 84.00.01 — Functional Safety: Safety Instrumented Systems for the Process Industry Sector
• Kletz, T., What Went Wrong? Case Histories of Process Plant Disasters, 5th Ed., Gulf Professional Publishing, 2009 (for the “why does this matter” answers)