Miscellaneous P&ID Elements

A variety of components, details, and descriptions are shown on typical P&IDs, including the following:

• P&ID revisions. Changes should be clearly identified by sequentially numbered symbols, such as “diamonds,” to pinpoint the location of each revision on the drawing. These changes are listed in the revision block or on a separate sheet issued and filed with the P&ID. Avoid the use of vague terms such as “general revision”

• Line classification changes. Line classification changes may occur where different streams join, for instance:
– Where utility piping ties into process lines with a higher corrosion rate, temperature or pressure
– Lines entering and leaving a vessel where processing changes occur that require additional valves of a higher class than dictated by conditions within the line

P&IDs should be carefully reviewed to ensure that all line classification change symbols are shown. This is particularly important when the plant is modeled, because there are no piping layout drawings (plans and elevations). The model, piping isometrics (spool drawings) and P&IDs are then the only records of line classifications.

• Level, alarm, and shutdown setpoints and operating ranges. Although often shown on other drawings, such as vessel drawings (see Section 134) and the level instrument piping drawings, these should be shown on the process P&IDs when they are critical to the safe or proper operation of the process

• Flanges. Most often, all 2-inch and larger equipment connections and valves are flanged, but there are some which are not, such as welded stub nozzles (a welded line-to-equipment connection often used on high, hard-to-reach vessel connections and between stacked exchanger pairs) and weld-in valves used in higher pressure services

To distinguish welded from flanged connections, either show all the flanges or stipulate that all 2-inch and larger connections are flanged except where a symbol (sometimes “WE” for weld-end) is placed adjacent to the connection. The P&IDs may then be used as a blinding control drawing during shutdowns, and to indicate flanged connections to the design draftsman. When different from the piping classification, flange sizes and rating are also shown on the P&IDs.

• Entering and leaving line designations. Careful labeling of lines as they enter and leave the P&ID allows good continuity from one P&ID to another. Labeling includes the reference P&ID drawing number, the line identification (noted along the line or enclosed in a rectangular “tag” or “balloon”), the to/from equipment number, and a service description

• Equipment internals. P&IDs should include a graphic representation of vessel internals whose function (or lack of it) may impact the operation of the facility. Examples include column and vessel internals, gas and liquid distribution and segregation mechanisms, internal level floats, heat exchanger overflow weirs and tubes, furnace tubes and dampers, and many more. For complicated equipment such as reactors, a separate major equipment P&ID is often prepared to document critical bed temperature points, process gas flow path, quench feed points, etc.

• Detail reduction. Level, pressure, and flow instrument piping details on process P&IDs (e.g., testing and maintenance isolation valving and connections) can be greatly reduced by using auxiliary instrumentation drawings (see Figure 200-10). Repetitive (and sometimes complicated) vent, drain, and sample system details may also be included on a separate schedule

• Reference Drawings. A reference drawing block is often incorporated along the lower edge of the P&ID. It lists major associated drawings—plot plans, piping layout, electrical, etc., with type of drawing, item or area covered, and drawing number. This can help locate associated drawings that may be scattered among hundreds of project drawings. When modifying a facility it is equally important to add new reference drawings to the drawing reference

• Drawing Titles. Many styles are used throughout the Company for drawing titles. Titles may contain helpful information on the type of drawing (P&ID, instrument, piping, etc.) the item or area covered, the project title or plant name, and the name of the facility or division. Depending on the organization, title blocks may be sequenced differently or omit some items