The main P&IDs show process flow, mechanical equipment, and instruments and controls. For small plants the main P&ID is all that is required.
Major Equipment P&IDs
Major processing equipment such as compressors, reactors, furnaces, treaters, and refrigeration systems are often placed on separate P&IDs (See Figure 200-6). This accomplishes the following:
• Provides the space to show the interrelationships of complex mechanical elements with their instrumentation and supporting supply systems
• Shows precise location details, particularly for critical temperature points
• Unclutters the main P&IDs
Equipment not directly in the main processing stream is often referred to as auxiliary equipment. Examples are seal, flush, and purge systems; lube oil, hot oil, and oil mist systems; and glycol heating systems (see Figure 200-7). These may be placed on separate P&IDs to reduce crowding on the main P&ID or when they serve equipment on different P&IDs. When small, they may be combined on a single drawing with other auxiliary systems.
When auxiliary equipment is supplied assembled in a “package unit” from a vendor, it should be depicted within a dashed-line box, with attention given to the following
Company/vendor interface areas:
• Equipment supplied at the boundaries. Otherwise, pickled pipe may arrive without mating flanges, the pipe material may be wrong, or both Company and vendor may supply block valves
• Instruments. Otherwise, both parties may supply duplicate sets, or Company supplied instruments may not fit vendor-provided connections
Plot Limit Block Valve Manifold P&IDs
This is a type of geographical layout (see Section 222). In major petroleum and petrochemical processing facilities individual plants or groupings of plants are set up as isolable entities. A single major assemblage of block valves at the end of a central pipeway, the plot limit block valve manifold (plot limit manifold), ties the individual plant headers into an interconnecting pipeway system serving other facilities (see Figure 200-8). With a few exceptions (primarily underground lines) all lines in the plant pass through the plot limit manifold. This facilitates supervisory review of plant isolation prior to a major planned shutdown. For small plants the plot limit block valves may be shown on the process P&IDs themselves. For larger plants a plot limit manifold drawing is prepared.
These specialty drawings show, on one drawing, the relationship between control systems located in different plants.
Utility Distribution System P&IDs
These are usually laid out geographically to preserve the sequencing and relative locations of all elements (see Figure 200-5). In mid-sized plants, several utility systems (steam and condensate, all gases, water, etc.) may either be layered on a single drawing in separate well-defined strips or superimposed.
To reduce clutter, only the tie-in portions of utility systems should be shown on the main P&IDs. These should include all valving and instrumentation associated with the control or isolation of the processing equipment—checks, block valves, flow indicators, etc.; the utility P&IDs themselves show little valving. The tie-ins should be labeled with the utility P&ID line and drawing numbers, and, if desired, their service.
Small, in-plant utility facilities are usually shown on their associated utility P&IDs—instrument air dryers, fuel gas knock-out drums (separators that remove entrained water from the gas), condensate dryers, etc. Larger utility supply and processing systems are usually shown on separate process P&IDs—water treatment plants, boiler plants, etc.
Relief System P&IDs
Figure 200-9 shows a typical geographical layout for a relief system. Relief valves and bypasses are not shown here, but are included on the main process P&IDs—the usual practice for process operations information.
All calculated relief loads should be recorded on this drawing, since they are not always found in the plant design records. Relief system P&IDs are very helpful in determining relief system modifications when adding major equipment in the future.