How to Review P&IDs: A 30-Point Checklist for Process Engineers

A bad P&ID costs you during commissioning. A P&ID review that catches the bad P&ID saves you during commissioning.

I've reviewed hundreds of P&IDs. I've seen vent valves pointing down (they fill with condensate). I've seen check valves drawn backwards (they check nothing). I've seen control valves with no block-and-bypass arrangement (plant shutdown to replace a $500 valve). Every one of these mistakes made it through at least two rounds of internal review before it reached me.

The problem isn't that engineers are careless. The problem is that P&ID review is treated as a visual check instead of a systematic process. Someone glances at the drawing, thinks "looks about right," and stamps it. Six months later, construction RFIs pile up, and someone says "this should have been caught in design review."

This article is a 30-point checklist for reviewing P&IDs — organized by what actually goes wrong, not by what's easy to check.

Before You Start: The Three-Layer Review

A P&ID review has three layers. Most teams only do the first one.

Layer	What You're Checking	Who Does It	Time per P&ID
1. Completeness	Is everything shown that should be?	Process engineer (self-check)	15-30 min
2. Correctness	Is what's shown actually right?	Peer reviewer / senior engineer	30-60 min
3. Operability	Can this actually be operated, maintained, and started up?	Operations representative + process engineer	30-60 min

If you skip Layer 3, you'll find out what you missed during commissioning. Commissioning is the most expensive time to find design errors.

Layer 1: Completeness Checklist (12 Points)

These are the things that should be on the drawing. If they're not, the drawing isn't ready for review.

1. Drawing Borders and Title Block

[ ] Drawing number, revision, and date are correct
[ ] Title matches the system being shown (not "Various Lines" — be specific)
[ ] North arrow (for plot plan references)
[ ] Match lines to adjacent P&IDs are shown and correct
[ ] Legend is complete for all symbols used on this sheet
[ ] Design pressure and temperature are shown for each major system

2. Line Numbering

[ ] Every line has a unique line number
[ ] Line numbers follow the project's numbering convention (not "14\"-Steam-01" on a project that requires "14-ST-001")
[ ] Line size, service, insulation, and heat tracing are identified
[ ] Line breaks/continuations are labeled with the destination P&ID number
[ ] Sloped lines show the direction of slope (arrow with ratio, like "1:100")

3. Equipment Tagging

[ ] Every equipment item has a tag number matching the equipment list
[ ] Equipment nameplate data (design pressure, temperature, capacity, motor kW) appears on or adjacent to the equipment symbol
[ ] Spare equipment is identified as SPARE and shown in dashed lines (per project convention)
[ ] Equipment nozzles are numbered and sized

4. Instrument Tagging

[ ] Every instrument has a tag number
[ ] Instrument tag follows ISA-5.1 convention (or the project's adopted standard)
[ ] Instrument bubbles include the function designation (first letter), measured variable (second letter), and loop number
[ ] Field-mounted vs panel-mounted vs DCS indication is clear (no line = field, single line = panel, double line = DCS)

5. Valve Tagging

[ ] Every valve that appears in the valve list is shown on a P&ID
[ ] Valve tag number, size, and type are indicated
[ ] Critical valves (PSVs, ESDVs, control valves) have complete tag information
[ ] Handwheel/actuator type is indicated for actuated valves

6. Notes and Hold Clouds

[ ] Process notes are clear, specific, and not boilerplate
[ ] Design conditions are noted at system boundaries
[ ] Hold clouds are used for unresolved design items (and a hold list exists separately)
[ ] Insulation and heat tracing requirements are noted

Layer 2: Correctness Checklist (10 Points)

This is where you find mistakes that will cause things not to work.

7. Flow Direction and Line Routing

[ ] Flow direction arrows are consistent with the process description
[ ] Gravity flow lines actually flow downhill (check elevations at both ends — if the destination is higher than the source, the arrow is lying)
[ ] Pump suction lines are as short and straight as shown (check the actual piping arrangement — P&IDs are schematic, not to scale, but impossible routing should be caught)
[ ] Lines don't "dead-end" without a purpose (a line entering a vessel with no outlet and no explanation)

8. Valve Orientation and Accessibility

[ ] Check valves are oriented with the flow arrow pointing in the correct direction. This is the #1 P&ID error. Print the drawing, fold it so you can't see the text, and trace the flow path — does it make physical sense?
[ ] Block valves are positioned where they can be reached (not 4 meters above the platform with no access)
[ ] Control valves have upstream and downstream block valves with a bypass (exception: small-bore lines where the process can be shut down elsewhere)
[ ] Relief valve inlet piping is as short as practical with minimal pressure drop (long inlet lines cause chatter)
[ ] Relief valve outlet piping drains to a safe location (not into a closed system unless designed for backpressure)

9. Instrument Placement

[ ] Flow meters have the required upstream and downstream straight pipe runs (orifice plates: 10D upstream, 5D downstream; Coriolis meters: minimal; vortex meters: 15D upstream, 5D downstream)
[ ] Pressure taps are on the top of horizontal gas lines and the side of horizontal liquid lines (never on the bottom — they clog)
[ ] Temperature sensors are in the pipe, past the point of good mixing, not in a dead zone
[ ] Level instruments on vessels cover the actual operating range, not just the vessel's geometric top and bottom

10. Material and Specification Breaks

[ ] Specification breaks are shown at the correct location (change in pipe class, material, or pressure rating)
[ ] The more expensive material is on the "source" side (e.g., 316L transitions to carbon steel, not the other way around)
[ ] Spectacle blinds, spades, or removable spools are shown at spec breaks where positive isolation is needed

11. Redundancy and Sparing

[ ] Spare pumps are shown with the correct valving for online switchover
[ ] Parallel equipment (filters, strainers, heat exchangers) has isolation valves on both inlet AND outlet — you'd be surprised how often the outlet isolation is forgotten
[ ] Critical instruments (safety interlocks) are not single points of failure without justification

Layer 3: Operability Checklist (8 Points)

This is the layer that separates experienced engineers from CAD operators. You can only review operability if you've operated something. If you haven't, bring someone who has.

12. Startup and Shutdown

[ ] Every pump has a minimum flow bypass or recirculation line. If the discharge block valve is closed and the pump is running, where does the flow go?
[ ] Vessels and columns have a startup line (small-bore bypass around the main feed valve for controlled pressurization and heat-up)
[ ] High-point vents and low-point drains exist at every location where liquid can pool or gas can accumulate. (Horizontal gas lines don't need drains until you discover condensate in one.)
[ ] Steam lines have drip legs with steam traps at all low points and before risers
[ ] The system can be drained completely for maintenance without cutting pipe

13. Normal Operation

[ ] Operators can see the instruments they need without walking 50 meters or climbing three levels (check the plot plan against the P&ID)
[ ] Manual valves that are operated frequently are at grade or platform level, not overhead
[ ] Sample points are accessible, and the sample line is short enough that the sample is representative (a 20-meter sample line with 50 mm diameter holds 40 liters — how long do you have to purge before taking a representative sample?)
[ ] Chemical injection points are downstream of good mixing (turbulence from an elbow, pump discharge, or static mixer), not in a straight pipe where the chemical will stratify

14. Maintenance Access

[ ] Equipment can be removed without demolishing pipe. Check: if this pump needs to be pulled, is there a flange break within 1-2 meters?
[ ] Strainers and filters have isolation valves on both sides AND a drain between them (otherwise you open a strainer full of liquid)
[ ] Relief valves have isolation (block valve below PSV) only if permitted by code, and if so, the block valve is car-sealed or locked open
[ ] Instruments that need calibration (pH probes, pressure transmitters) have isolation valves and can be removed without draining the pipe

15. Emergency Conditions

[ ] Emergency shutdown valves (ESDVs) fail to the safe position on loss of power, instrument air, or signal (fail-closed for feed lines to a reactor, fail-open for cooling water to an exothermic reactor — direction matters)
[ ] Check valve placement prevents backflow from high-pressure systems into low-pressure systems during a pressure loss event
[ ] Fire zones are considered: can a fire in one area be isolated without shutting down the entire plant?
[ ] Relief valve discharge is directed away from operator access areas and ignition sources

How to Run a P&ID Review (The Process, Not Just the Checklist)

Step 1: Print It

Yes, print it. On paper. At least A3 size. A P&ID review on a screen misses things — the human eye scans a printed drawing differently than a zoomed-in monitor view. This is not superstition. I've personally caught flow arrow errors on paper that I missed three times on screen.

Step 2: Review Alone First

Before the group review, the reviewer spends 30-60 minutes alone with the drawing and: - The process description / design basis - The equipment list - The line list - The cause-and-effect matrix (for interlock checks)

Mark up the drawing with red pen. Write questions in the margin. The group review is for resolving questions, not for finding them.

Step 3: Walk the Process

Starting at the main feed inlet, trace every possible flow path: 1. Normal operation path 2. Startup path (bypasses, recirculation lines) 3. Shutdown path (drains, vents, isolation points) 4. Emergency path (relief valves, ESDVs, depressurization)

If you find a line that you can't trace from beginning to end, you've found an error or omission. A line that goes to a piece of equipment and stops without an outlet is not a complete line.

Step 4: Cross-Check Every Item

Go down the equipment list: is every item shown on a P&ID? Go down the valve list: is every valve shown? Go down the instrument list: is every instrument shown?

Missing items mean the lists are wrong or the drawings are wrong. Either way, that discovery is the purpose of the review.

Step 5: The Operations Walk-Through

Bring in someone who will actually operate the plant. Not the design manager. Not the project engineer. An operator or a commissioning engineer. Walk them through a typical shift:

"Show me how you start this pump."
"Show me how you take a sample."
"Show me what you do if this pressure alarm goes off."
"Show me how you isolate this heat exchanger for cleaning."

If they hesitate, point to a valve that doesn't exist, or ask "where's the drain?" — you've found an operability issue.

Common P&ID Mistakes I've Seen (And What They Cost)

Mistake	Consequence	Cost to Fix After Construction
Check valve drawn backward	Flow blocked on startup; pipe section must be cut and re-welded	$5,000-15,000
Missing high-point vent	Gas pocket prevents pump priming; line must be hot-tapped	$3,000-8,000
Control valve with no bypass	Plant shutdown required to replace CV trim; production loss	$50,000-200,000 (downtime)
Relief valve discharge to atmosphere at grade level	Safety incident; relocation of discharge piping	$10,000-30,000
Flow meter with insufficient straight run	Inaccurate measurement forever; meter relocation	$15,000-40,000
Missing drain between isolation valves	Maintenance personnel open strainer under pressure; safety incident	$2,000-5,000 + potential injury
Pipe spec break in wrong direction	Corrosion failure; pipe replacement	$20,000-100,000
Level instrument range doesn't match vessel operating range	Incorrect level reading; potential overflow or pump cavitation	$5,000-20,000

These are not hypothetical. I have seen every single one of these on real projects. The total cost of P&ID errors on a medium-sized project ($50M) is typically $200,000-500,000 in field modifications. A thorough P&ID review costs $5,000-15,000 in engineering time. The ROI math is not complicated.

Tools That Make P&ID Review Faster

Tool	What It Does	Recommendation
Bluebeam Revu	PDF markup with measurement tools, overlay comparison, comment tracking	Industry standard for P&ID review. Worth the license.
AutoCAD P&ID / Plant 3D	Built-in validation: line numbers, tag consistency, connectivity checks	Use the "Check Drawing" function before exporting.
Smart Plant P&ID (Hexagon)	Rule-based validation engine, auto-tagging, consistency across sheets	Enterprise tool. Overkill for small projects.
Red pen on paper	The original, still unbeaten for discovering flow logic errors	Print at A3. You will find things you missed on screen.
P&ID Symbol Library	Pre-drawn ISA-5.1 standard symbols for consistency	Available in our templates pack.

The One Rule That Matters

A P&ID review is not complete until someone has traced every line from its origin to its destination.

Not "skimmed." Not "looked at." Traced. With their finger on the paper. Every branch. Every valve. Every instrument. Every possible flow path.

If you do this for every line on every drawing, you will catch 95% of P&ID errors before construction. If you don't, the construction team will catch them for you — at ten times the cost.

Print the drawing. Get a red pen. Start at the first line on the left. Trace it until it ends. Then do the next one.

That's the whole checklist, really. The 30 points above are just reminders of what to look for while you're tracing.