AcuSafe -> July 2000 Newsletter
  

What Belongs on an Ammonia Refrigeration P&ID? 

The P&ID is a set of drawings that describe a “process;” for ammonia refrigeration, the refrigeration system and its ancillary equipment are considered the process. (This contrasts with the common usage in food plants, where the food is “processed” and the refrigeration system is “just a utility.”) 

A good P&ID:

1. Provides a visual reference to equipment configuration, valving, sensors, etc.
2. Provides useful information to assist in analyzing process hazards (through a PHA study).
3. Supports development of operating procedures (and to a lesser extent, maintenance schedules and procedures).
4. Communicates the configuration of equipment clearly and concisely to improve operator understanding of the process and reduce human errors.
5. Records the current (as-built) state of the process so that changes can be planned safely and effectively.

What must be included (as an absolute minimum)?

1. All process chemical-containing equipment, including pressure vessels, compressors, condensers, evaporators, other heat transfer equipment (desuperheaters, heat recovery water heaters, etc.), pumps, air purgers, chlorinators, vaporizers, transfer/unloading stations, etc.
2. Essential valves, such as isolation valves and control stations, as well as all safety relief valves.
3. Controls (regulators, float switches, etc.) and solenoid valves.
4. Permanent instruments and sensors (pressure transducers, meters, etc.).
5. Legend to symbols and abbreviations.

What should be included?

1. Purge/gauge valves: not providing these details increases the time to write and verify equipment service procedures.
2. Equipment/valve numbering: equipment and especially valves should be labeled, both on the P&ID and on attached tags, to reduce the risk of operator error and simplify the writing of procedures. (Always be careful to ensure that valve tags match the P&ID!)
3. Line designations/purposes: some processes, such as refrigeration, have recognized systems for line designation. For others, use a system that explains the line’s function and optionally includes other information such as temperature and pressure levels, etc.
4. Safety relief valve specifications: information on the relief valve “design and design basis” is required before starting the PHA study. For simple processes, codes and standards such as NFPA 58 (LPG) and ASHRAE 15 (refrigeration) specify the design.
5. Control loops: these can become confusing on some P&IDs, but information on which float switch controls which solenoid valve (for example) must still be developed somehow.
6. Flow direction: at a minimum, always show the permitted-flow direction on a check valve.
7. Line sizes/reducers; expansion tie-ins and block valves, etc.
8. Design working pressure and other pressure-vessel/equipment label information.
9. Support equipment and non-“chemical” lines such as condenser water pumps, secondary heat transfer fluid (glycol/brine) loops.
10. Items included in other equipment: often, P&IDs show screw compressor packages and other complex equipment as a single symbol, even though the package includes motor, compressor section, oil separator vessel, oil cooler, and various controls/sensors. Include, at a minimum, all vessels or other “major” sub-equipment, as well as all valves connecting to the atmosphere and those separating portions of the package from one another.

Any item omitted on the list above should be available in another form (such as a list of safety relief valve specifications). Sometimes, not providing the information can lead to safety hazards. AcuTech and most other consultants typically rely upon the client facility to provide safety information; when such services as valve tagging or P&ID preparation/verification are included, they are explicit line items. Missing information or information not provided in accessible form (such as the location of purge/gauge valves, etc.) can greatly increase both consultant billings and facility staff time to review procedures, etc.

Sometimes, we encounter P&IDs which include information not required by any regulation and which we would not recommend under most circumstances:

1. Isometric-style (“3-D”) drawings: these are very confusing and can obscure the key information in a P&ID – what’s connected to what.
2. CAD/.dwg representations of equipment: a P&ID is intended to be a detailed schematic, not a true down-to-the-millimeter geometric representation.
3. Color: while small amounts of color are useful (e.g., color coding liquid/vapor lines), attempts to color each zone of a plant or each type of line (defrost condensate = canary yellow, while equalization lines = maize) become rapidly confusing. Color P&IDs are also difficult to copy, are vulnerable to fading when posted in the plant, and may be misread by colorblind employees.
4. Large amounts of text: it is not necessary to include complete specifications for every single piece of equipment on the P&ID. Complete specifications must, though, be available for reference. We recommend including in the P&ID sufficient information to rapidly locate specification data (which is why we recommend assigning ID numbers to equipment).
5. Tiny or “spaghetti” drawings: there is no requirement that P&IDs be drawn by professional draftsmen (or even that they be ink- or computer-drawn at all!) but as a practical matter, P&IDs for all but the simplest facilities should be professionally prepared so that they are legible. Do not attempt to include the entire process on a single sheet of paper (unless it is very simple), but ensure that lines may be easily traced without error, both from page to page and on the same page.

After P&IDs are prepared, they should be verified against field conditions. We recommend that each and every line, valve, sensor, or other P&ID item be checked visually against the as-built system, even for supposedly “as-built” drawings. It is common for changes to be made during construction (and these changes are supposed to be controlled by the process risk management program!), and draftsmen are known to make errors in the course of preparing drawings. We typically spot-check P&ID accuracy before starting the PHA study, and we rarely encounter an error-free P&ID. Although it can be expensive (particularly in staff time) to check P&IDs, it is far better to find drawing errors before they lead to, for example, writing incorrect operating procedures.

P&ID verification can be assigned to trusted operators or mechanics, assuming they have been trained to read drawings. While we encourage assigning junior operators to trace P&IDs, always have experienced personnel check their work. Combining P&ID verification with a line-and-valve-labeling project can reduce costs drastically.

The P&ID is an essential tool in preparing and implementing a successful process risk management program.

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