Piping & Instrument Diagrams Study Notes

Piping & Instrument Diagrams (P&IDs)

Overview of P&IDs

  • P&IDs are essential for both batch and continuous processes, including pharmaceutical and refinery operations.

  • They function as the “blueprints” for a process, providing a detailed schematic that defines the design of the process.

  • Every component in a plant, such as pipes, valves, instruments, pumps, etc., is represented in P&IDs.

  • Allow for analysis, review, and modifications before significant financial investment in equipment, materials, etc.

    • Review Types:

    • Design Reviews

    • Operability Reviews

    • HAZOP / PHA (Hazard and Operability Study / Process Hazard Analysis) Reviews

Importance and Details of P&IDs

  • P&IDs detail:

    • Piping (including valves, sizes, etc.)

    • Instrumentation and controls

    • Equipment information

  • Relationship to other documents:

    • Predecessor: Process Flow Diagrams (PFDs)

    • Successors: Piping Drawings, Equipment Drawings, Cause & Effects (C&Es)

Symbolism in P&IDs

  • P&IDs employ standardized symbols to represent various components of a process system:

    • ISA - Instrument Society of America standards are generally followed.

    • Variations might exist based on specific companies or projects.

  • Examples of representations include:

    • Legend sheets

    • Various equipment symbols like slug catchers, heat exchangers, etc.

Piping Class and Specifications

  • Piping specifications are outlined systematically, including:

    • Line sizes and identifications (e.g., 4”HL-2101-B2-1-2”C)

    • Components of line information include size, fluid handled, pressure rating (Maximum Allowable Working Pressure, MAWP), metallurgy of the pipe, insulation specifications, and identifying numbers.

    • Criteria for line sizing includes considerations for velocity and pressure drop:

    • Velocity criteria: less than 60 fps for gases, less than 10 fps for liquids, and less than 100/rho^0.5 for others.

    • Pressure drop criteria: typically less than 0.5 psi per 100 ft of pipe.

    • Pressure ratings defined under ANSI classes (e.g., 150#, 300#).

Metallurgy Considerations in Piping

  • Metallurgy of piping is determined based on:

    • Required fluid handling capabilities, temperature tolerances, and reactivity with different materials.

    • Examples of piping materials and specifications:

    • Carbon Steel (CS), Stainless Steel (SS), NACE specifications for sour environments, etc.

    • Specific guidelines detail usage of various materials depending on fluid properties and environmental conditions.

Valve Specifications and Types

  • Types of valves shown in P&IDs include:

    • Control valves, shut-off valves, and other variants such as ball, gate, globe, butterfly, and check valves.

  • Details on valve specifications provide:

    • Metallurgy, seal materials, bore size (full or reduced), etc.

  • Critical considerations involve:

    • What scenarios might necessitate full bore vs. reduced bore valves?

Instrumentation in P&IDs

  • Instrumentation encompasses a variety of sensors and systems, indicating:

    • Temperature, pressure, flow, level, and other specialized analyzers (e.g., for fire, gas).

    • Types include:

    • Flow meters (orifice, turbine, etc.),

    • Level measurement methods (differential pressure cells, floats),

    • Control systems featuring temperature, pressure, and flow parameters.

  • Instrumentation specifics may include safety logic, control diagrams, and cascade setups.

Layout Considerations

  • Layout must comply with various operational criteria:

    • Safety to prevent hazards and minimize risk across processes.

    • Capital cost efficiency through strategic plant configuration to minimize runs of piping and electrical interconnections.

    • Operability to enhance maintenance and functional access for repair and operational tasks.

Conclusions and Case Studies

  • Real plant problems emphasize:

    • Importance of understanding existing systems and modifications.

    • Example: A simple adjustment in valve control can yield significant financial benefits (example: $4MM/year increased income just by adjusting one valve).

  • Class exercises and layout strategies reinforce practical applications and decision-making processes for optimal facility design.

3D CAD Visualization

  • Use of 3D modeling assists in navigating complex facilities with multiple systems, demonstrating operational designs to stakeholders and managers effectively.

  • Fly-over models can summarize facility layouts and interconnections to aid comprehension of configuration and logistics.

Piping Specifications Summary

  • Outlines of thickness, insulation types, and standards for various service limits including pressures, temperature ranges, and corrosion allowances.



What this lecture is about

This lecture covers three connected topics: P&IDs, Plant Layout, and how they all come together in a real design. Think of it as the lecture that bridges the gap between the process flowsheet (PFD) and the actual physical plant.

Part 1: P&IDs — What they are and why they matter

The PFD shows you what the process does. The P&ID shows you how it actually works in practice. The P&ID is the true engineering blueprint — it shows every pipe, every valve, every instrument, every control loop. Literally everything that exists in the plant is documented on the P&ID.

The key thing to understand is the hierarchy:

PFD comes first → P&ID comes next → then piping drawings, equipment drawings, and Cause & Effects (C&Es) documents come after

So if someone asks "what document comes before a P&ID?" the answer is the PFD. If they ask "what documents does the P&ID lead to?" the answer is piping drawings, equipment drawings, and C&Es.

P&IDs follow ISA symbology — the Instrument Society of America sets the standard symbols, though individual companies sometimes have their own variations. Every P&ID has a legend sheet that explains what each symbol means.

Piping on a P&ID

Every pipe on a P&ID has a line designation that tells you six things about it: the pipe size, the fluid it carries, the pressure rating (called MAWP — Maximum Allowable Working Pressure), the metallurgy (what material it's made of), the insulation type, and an identifying number. So when you see something like "4"HL-2101-B2-1-2"C" on a P&ID, each part of that code is telling you something specific about that pipe.

Pipe sizing is governed by two criteria — velocity limits and pressure drop limits. For gases you want velocity under 60 fps, for liquids under 10 fps. Pressure drop should be under 0.5 psi per 100 feet of pipe. These aren't arbitrary — exceeding them causes erosion, noise, or excessive energy loss.

Pipe pressure ratings follow ANSI classes — 150#, 300#, 600# and so on. A critical exam trap here: ANSI 150# does NOT mean the pipe is rated for 150 psig. The actual pressure rating depends on temperature and material. This is defined in a pipe spec document.

Metallurgy matters because the pipe material must be compatible with the fluid, temperature, and any corrosive properties. Carbon steel works for standard non-corrosive service. For sour service (H2S environments), you need NACE-rated materials with corrosion allowances. For very corrosive fluids like rich amine, you need stainless steel 316L. For cold service down to -325°F, you need 304 stainless.

Insulation on pipes serves three purposes — hot insulation reduces heat loss, cold insulation prevents heat gain, and personnel protection insulation keeps workers from burning themselves on hot surfaces. Larger pipes need more insulation thickness because they have more surface area losing heat, and the economics of preventing that heat loss justify the additional thickness.

Valves on a P&ID

Every valve is shown on the P&ID with its type and size using standard symbols. The main categories are control valves (automatically operated based on a controller signal), shut-off valves (block valves used to isolate equipment), pressure safety valves or PSVs (which automatically open to relieve overpressure and protect equipment), and manual valves (operated by hand).

Valve types include butterfly, ball, gate, globe, and check valves — each suited to different applications. A valve spec gives you the detailed specification — metallurgy, seal material, whether it's full bore or reduced bore.

A full bore valve has an opening the same diameter as the pipe — you use this when you need to pig the line (run an inspection or cleaning device through it) or when you can't afford any flow restriction. A reduced bore valve is cheaper and smaller but creates some pressure drop — acceptable for most applications where pigging isn't required.

Instrumentation on a P&ID

Instruments measure and control process variables. The main ones are temperature, pressure, flow, and level — but you also have analyzers that measure composition, and specialty detectors for fire, gas, H2S, and chlorine.

The instrument tag numbering system is important to understand. A tag like "FIC-101" tells you: F = flow (the measured variable), I = indicating (it displays the reading), C = controller (it also controls something), and 101 is the loop number. Common first letters: T = temperature, P = pressure, F = flow, L = level, A = analyzer. Common function letters: I = indicator, C = controller, T = transmitter, R = recorder, A = alarm, H = high, L = low.

Safety instruments are especially important. TAH means Temperature Alarm High — it alerts the operator. TAHH means Temperature Alarm High High — this is a more serious alarm that typically triggers an automatic shutdown. The double letter always means a more critical safety level.

Control loops shown on P&IDs include cascade loops where one controller output becomes the setpoint for another controller, and high select logic where the system picks the highest of multiple signals for a specific purpose.

Part 2: Plant Layout

Layout is about where you physically put equipment in the plant. Three things govern every layout decision: safety, cost minimization, and operability.

Safety in layout:

Some equipment is more hazardous than others and needs to be spaced away from ignition sources. The classic example given is high pressure natural gas equipment near a furnace — if the gas leaks and finds the furnace, you have a disaster. Spacing guidelines give minimum and average distances between different equipment types. Fired heaters need the most spacing because they are permanent ignition sources.

Area classification divides the plant into hazardous and non-hazardous zones. Class I means flammable gases or vapors may be present. Division 1 means ignitable concentrations exist under normal operations. Division 2 means it's possible but not normal. The gas groups tell you what kind of hazard — Group A is acetylene, Group B is hydrogen, Group C is H2S, Group D is other hydrocarbons like methane and propane. Electrical equipment must be rated for the area classification it operates in — a motor in a Division 1 area must be explosion-proof.

Hazardous and non-hazardous areas must be segregated as much as possible. Separate drain systems prevent flammable liquids from reaching non-hazardous areas.

Cost in layout:

Equipment should be arranged in logical process order to minimize the length of piping between connected units — shorter piping means less material cost, less heat loss, less pressure drop. Electrical cable routing is also a cost consideration.

You can use elevation to save plot space — stacking equipment vertically. But you have to consider structural costs. A shell and tube heat exchanger is relatively light and can be elevated. A large reciprocating compressor is heavy and vibrates — putting that on an elevated structure would be very expensive and structurally challenging.

Operability in layout:

This is essentially common sense applied to engineering. You wouldn't put an air cooler right next to a furnace exhaust stack — the hot exhaust air would reduce the cooler's effectiveness. Drains need to slope downhill by gravity — you can't have a drain line going uphill. Equipment needs maintenance access — shell and tube heat exchangers need space in front of them to pull the tube bundle out for cleaning. Pumps and compressors need crane access for major maintenance. Filter housings need space to remove and replace filter elements.

Layout components:

Pipe racks carry pipes between pieces of equipment — they have main racks and branch laterals feeding individual equipment. Sleepers are ground-level pipe supports. Skids are pre-fabricated assemblies of equipment and piping built in a shop and transported to site — they save field labor and improve quality. The control room is the central hub where operators monitor and control the process — it has strict construction requirements to keep it safe and habitable during an emergency. The MCC (Motor Control Center) is where all the electrical motor starters and switchgear are housed.

The real plant problems — exam gold

The lecture gave three real case studies that illustrate why P&IDs and layout matter.

Problem 1: They needed to tie a new line into existing piping. They had to find the exact tie-in location by reading the P&ID and then physically locating it in the plant. This teaches you that P&IDs are living documents that must match the physical plant.

Problem 2: A plant had been modified years ago — someone had changed valve positions to increase capacity but sacrificed recovery. Over time, people forgot why the change was made. By understanding the process thermodynamics and reading the P&IDs carefully, engineers identified that simply opening one 4" valve and closing another generated $4 million per year in additional income. This is the most powerful example in the lecture — it shows that understanding process fundamentals plus reading P&IDs can unlock enormous value.

Problem 3: P&IDs can be misunderstood. A wavy line symbol on a P&ID means something specific, but if someone misreads it, the wrong equipment gets installed. This teaches you to always verify the legend sheet and not assume.

The layout class exercise — know how to answer this

The exercise asks about a plant with two feed tanks, pumps, a pre-heater, a reactor, a cooler, product storage, and pipeline pumps.

Pipeline pumps should be located near the pipeline connection, not the storage tank — you want to minimize the length of high-pressure pipeline piping, and the pump is essentially the start of the pipeline, not the end of the storage system.

Feed pumps should be located near the feed tanks — pumps need a positive suction head to work properly, and keeping them close to the tanks minimizes suction line losses and avoids cavitation problems.

The pre-heater should be located near the reactor — it's heating the feed just before it enters the reactor, so you want minimal piping between the pre-heater outlet and the reactor inlet to avoid losing the heat you just added.

The document hierarchy — put it all together

PFD → P&ID → Piping Drawings + Equipment Drawings + C&Es → 3D CAD Model → Final Plant

The lecture walked through a butane splitter project showing all of these documents in sequence — from the simple PFD, through multiple detailed P&IDs for each section, to the layout drawings, piping plans, 3D CAD views, and finally photographs of the completed plant. This was showing you that every drawing serves a specific purpose and they all connect.

Likely exam questions:

"What is the difference between a PFD and a P&ID?" — PFD shows process logic and material balance. P&ID shows every pipe, valve, instrument, and control detail for construction and operation.

"What does a line designation on a P&ID tell you?" — size, fluid, pressure rating, metallurgy, insulation, identifying number.

"What are the three criteria governing plant layout?" — safety, cost minimization, operability.

"What is Class I Division 1 area classification?" — location where ignitable concentrations of flammable gas exist under normal operating conditions.

"What is ANSI 150# and does it mean 150 psig?" — it's a pressure class designation. No, it does not mean 150 psig — actual rating depends on temperature and material.

"Why would you use a full bore valve?" — when you need to pig the line or cannot afford any flow restriction.

"What is TAH vs TAHH?" — Temperature Alarm High vs Temperature Alarm High High — TAHH is a critical safety shutdown level.

"Why do larger pipes need more insulation?" — greater surface area means more heat loss, and the economics of preventing that loss justify thicker insulation.

"What are the key advantages of using flexible piping?" — they allow for easier installation and can accommodate movement and thermal expansion, reducing stress on joints and fittings.

Conclusion

  • P&IDs serve as invaluable tools throughout the lifecycle of process design, maintenance, and operational excellence in industrial settings. Their detailed nature supports effective communication among engineers, operators, and safety professionals, ensuring robust and reliable plant operations.