Checklists are a good way to avoid making mistakes in any activity. Lets apply them to Piping and Instrumentation Diagrams (P&IDs) in today’s post. This checklist will help a chemical engineer that has to develop brand new P&IDs, or check other's work. (Click here for a Process Flow Diagram checklist).
P&IDs incorporate a lot of the knowledge in any design. They require a lot of data from many different sources and disciplines. And let's face it, they are one of the few key documents that designers AND business people AND operators will refer to time and time again. You need them as accurate and well thought-out as possible.
As I have worked with them over the years, I built a checklist. Every mistake or near miss, I added to my list of things to look for.
You can take this list, alter it, and make your own checklist. Then, use the checklist each time a drawing passes over your table. Not every check I list below will apply to every drawing, obviously.
Does this drawing include, where appropriate:
- My company’s document/drawing #
- Client document #
- Licensor/Vendor document #
- Title block: including the By/Approved/Checked blocks filled and signed, Date, Rev. all company logos, client signoff block?
- All equipment tagged, title above equip, spelling of names correct?
- Not crowded: Normally have about 4 pieces of equipment per drawing
- Where feasible, streams flags follow the plant conventions. Normally this is coming in from the left side and leaving out the right side of the drawing.
- Stream flags include the fluid, drawing #, note the equipment name and tag the stream is from/going to
- Battery limits are marked.
- Line arrows are on each turn of a line, in the correct direction?
- Vessels - Size, design temperature/design pressure, full vacuum, internals/levels, skirt height to grade
- Heat exchangers - Area, design temp/pressure, duty, motor horsepower (air coolers), conderser/reboilers have elevation, motors have controls
- Heater - size, duty, design T/P, coil arrangement, utilities on a separate page
- Boiler - size, design T/P, superheater outlet T, utilities own page
- Tank – size, capacity, type, heaters/agitators shown
- Pump rated flow, horsepower, differential head. Showing temporary strainers, drains, motor w/ controls
- Turbines - in/out temp/pressure, horsepower, strainers
- Compressor – Actual Cubic Feet per Minute inlet, discharge pressure, driver type, horsepower, strainer, lube/seal oil
- Instruments are right type, local/transmitted, control type, block/bypass valves shown
- Instrumentation signal lines are correct line type, local/DCS code correct
- Control valve type, failure mode shown, block/bypass valves, reducers, the sizes used are the latest from controls department
- Pressure Relief Valves: set pressure, PSV size is latest, inlet/discharge size and pipespec match latest calculations, outlet lines are Car Sealed Open or have no valves (if to atmosphere), the relief outlet goes to drain/flare/atmosphere
- Lines connecting to a main flare line connect to that line from above, even making a detour if necessary. (This is an old drafter’s trick to remind pipers that flare lines need to slope downward to drain properly)
- Block valves: show if they are normally open or closed, car sealed open or closed?
- Drain valves present, use smaller gate valve symbol, they are between checks/gates/etc
- Minor valves are placed logically (check valve near pump discharge, swing blind near a gate valve)
- Spectacle blinds are correctly shown normally open/closed and they are on the side of the valve facing the "danger" (i.e. pressure source, contamination source)
- Steamouts/steam connections where required? Full vacuum rating on equipment when required
- Pipe specs are all shown and correct, spec breaks over valves/PSVs, steam and electrical tracing is shown
- Double-block valves (with 3/4" drain between) where procedures say there are needed (normally 600 psig+ or dangerous fluids like acid)
- Insulation is shown on equipment and pipes?
- Tie Points are provided at all old/new line interfaces
- Lines connect to each other according to project's branch table (e.g. do I need to show an expander before tee?)
- Show slopes, elevations,
- Note whenever any items must be within sight of each other (e.g. ensure level gauge can be seen by someone operating this manual globe valve)
- Utility P&IDs tie in correctly to main drawings, and show connect to utility stations & analyzers
- Header line numbers include all header block valves (so that when the header is built the block valves are installed with the header, not any branch lines)
- Any items rotated for easy access, such as handwheels on valves, are rotated to an angle that is possible considering the number of bolt holes on the lines. (Ex: per ASME B16.5 I see that for 150# carbon steel, 0.5" to 3" lines can rotated 90 degrees , 4" to 8" lines can rotate 45 degrees , 10" to 14" can rotate 30 degrees)
- Neat DWG, no dog-legs/bad-connections
- When lines cross, vertical lines break for horizontal lines except that instrumentation lines break for process lines. (This rule varies by company)
- Matches Process Flow Diagram (PFD) connectivity,
- Equipment names match other documents like the PFD, equipment list, datasheet, etc.
- All Notes/Holds are ok? Is it clear what they refer to? Spelling? Are the general project notes and holds, that must be added to each drawing, in place?
- All the mark-ups were applied by the drafters correctly, spelling is correct
- Check the drafting process did not introduce any random new errors or mistakes. (Sometimes during a revision, a drafter’s template will be set wrong, or a line type will change for no apparent reason, causing a little mistake. And no one will be looking for it, because everyone just focuses on the areas that were marked up!)
- Are revision triangles, clouds, and other markings in the revision process added or deleted as required?
- Do your own mini-HAZOP of the process: can any deviation in normal temperature, pressure, flowrare, operator mistakes, utility failures, etc. cause a safety problem? Are any remaining safety issues going to be properly documented for the operators
- Similarly, can you think of any operability problems? Any common maintenance tasks that cannot be done here? Or any practical problems building or installing what you've planned?
- Taking a look at the set of P&IDs as a whole, the stream connections between the drawings are correct (all inlet streams match to outlet streams somewhere, or at least has an adequate explanation)
- I cannot get rid of any HOLD notes on these drawings
- I feel satisfied that I’ve done enough work to sign this document. I have no ethical qualms or bad gut feelings
As you work more with P&IDs, you will naturally begin to pick up on mistakes as your instincts build up their own checklist.
You can share any omissions from my list with other readers by leaving a comment.
BONUS: Another piece of old lore I read on eng-tips.com long ago:
When you finish a drawing, correct it, and make it perfect. Then put it in your desk and go home and sleep.
In the morning, pull it out, and correct all the mistakes that you missed.
This is very true. If you can sleep on a drawing, you’ll catch more mistakes. Not that modern project deadlines ever let you apply this rule…
Lastly, keep in mind that no matter how hard you try and how experienced you are, there is still a rate of human error. This is a natural part of human nature and it can happen no matter how careful you are. So keep up with having experienced checkers and multiple drawing revisions and design reviews.
Also, check out these links:
2009-11-23: Added spectacle blind
2010-12-11: Added header block valve tip, rotation tip, links to pipingdesigners.com
2011-09-13: Added reference to checking revision triangles and clouds
2015-09-22: Added link to Tie Point Article