19.1 Process Flow Diagrams, P&IDs, Scale-Up, and Plant Siting

19.1 Process Flow Diagrams, P&IDs, Scale-Up, and Plant Siting

The NCEES specification names Process design (e.g., scale-up, process or product development, process flow diagrams, P&IDs, specifications, layout and siting considerations, economics) as the first subtopic under Design. Economics is covered in Section 18.3. This section covers the rest — the documents that define a plant, and the two physical arguments (similarity and separation distance) that decide whether the plant works and whether it is safe.

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1. The Document Hierarchy

Design information is developed on three successive drawings, each of which answers a different question.

That last row is the definition question the exam likes. A P&ID showing a stream temperature is a defective P&ID. The reason is maintenance discipline: a P&ID is a permanent as-built document that must remain valid across every operating case, and operating data varies with case. Conversely, a PFD showing every block valve is unreadable and defeats its purpose as a process summary.

The P&ID is also the base document for process hazard analysis (Section 17.1), for mechanical integrity inspection planning (Section 20.2), and for management of change. Plants that let P&IDs drift out of date lose all three.

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2. Reading the Drawings

Instrument tags (ISA-5.1). The tag is read in two halves:

• First letter — the measured or initiating variable: F flow, L level, P pressure, T temperature, A analysis, S speed, W weight.
• Succeeding letters — functions performed: I indicate, R record, C control, T transmit, A alarm, S switch, V valve, Y compute or relay.

So FIC-201 is a flow indicating controller, LAH-104 is a level alarm high, TT-315 is a temperature transmitter, and PSV-301 is a pressure safety valve. The bubble symbol adds location: a plain circle is field mounted; a circle with a single solid horizontal line is accessible to the operator at the primary location (control room); a circle inscribed in a square denotes a shared display / shared control function in a DCS; a dashed line denotes a location not normally accessible to the operator.

Line numbers. A typical line number such as 6"-P-1201-A1A-H encodes, in order: nominal size ((6\text{ in})), service (P = process), line sequence number, piping specification class (which fixes material, pressure rating, flange facing, gasket, and corrosion allowance), and insulation code (H = hot service). The specification class is the item that matters for integrity: it is what says this line is (316\text{L}) rated for Class 300 rather than carbon steel rated for Class 150.

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3. Scale-Up

Scale-up rests on three kinds of similarity, applied in order:

1. Geometric similarity — all length ratios preserved.
2. Dynamic similarity — the governing dimensionless groups ((Re), (Pr), (Fr), (Da), (Sc)) preserved.
3. Kinetic/thermal similarity — velocity, temperature, and concentration profiles preserved.

The permanent difficulty is that these conflict. Section 4.4 showed it for agitation: holding (P/V) constant necessarily raises tip speed, and no single speed holds (P/V), tip speed, blend time, and (Re) constant at once. The engineer's job is to identify which group actually controls the outcome and hold that one.

The surface-to-volume collapse. This is the most important quantitative fact in scale-up. Under geometric similarity, with characteristic length (L):

$V \propto L^3, \qquad A \propto L^2, \qquad \frac{A}{V} \propto \frac{1}{L}$

Worked example. A laboratory reactor is scaled up (100\times) in volume with geometric similarity.

$\frac{L_2}{L_1} = 100^{1/3} = 4.642 \qquad \frac{A_2}{A_1} = (4.642)^2 = 21.54$

$\frac{(A/V)_2}{(A/V)_1} = \frac{21.54}{100} = 0.2154$

Heat generation scales with volume, so it rose (100\times). Heat removal through the jacket scales with area, so it rose only (21.5\times). Cooling capacity per unit of heat generated fell to 21.5% of its laboratory value. A mildly exothermic reaction that a flask dissipates to the bench top with no cooling at all can, at plant scale with the same chemistry and the same jacket temperature, cross the point where heat generation outruns removal and go to thermal runaway — the mechanism developed in Section 14.2.

The design responses are to add internal heat transfer area (coils, external pump-around loops), to cut the rate (semi-batch addition so the reaction rate is set by the feed pump rather than by the inventory), or to change to a continuous reactor with an inherently high (A/V). Building the plant reactor "just like the lab one, only bigger" is the failure.

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4. Layout and Siting

Plot plan decisions are dominated by a single question: when something releases, where does it go, and what does it reach?

Prevailing wind. Establish the wind rose first. Continuous ignition sources — fired heaters, flares, boiler houses, and the electrical substation — are placed upwind of units handling flammable inventory, so that a drifting vapor cloud moves away from them rather than into them. Control rooms and occupied buildings are likewise placed upwind, so that a toxic release travels away from the people who must manage it.

Occupied buildings. API 752 (permanent buildings) and API 753 (portable buildings) require a facility siting study evaluating explosion overpressure, fire radiation, and toxic exposure for credible release scenarios. Buildings that cannot be moved far enough away are designed blast resistant, and portable trailers are the classic finding: a trailer parked beside a unit for a turnaround is exactly the structure that collapses on its occupants.

Spacing and access. Working rules that repeatedly show up as exam judgment items:

• Separation distances between process units, storage, utilities, and the property line come from NFPA 30 and insurer guidance, and they exist to limit fire and explosion propagation from one unit to the next.
• Two means of egress from every elevated platform and every operating level, arranged so that a fire at one cannot block both.
• Fire water and emergency access on at least two sides of every major unit; dead-end roads inside a process area are a finding.
• Gravity flow and hydraulic gradient set elevations: reboiler thermosiphons, column reflux drums, and pump NPSH margins (Section 3.4) all require the vessel to sit high enough. Elevation is a process constraint, not an architectural preference.
• Maintenance access: tube-bundle pull space in front of every shell-and-tube exchanger, crane access to compressors, and drop zones for relief valve removal.
• Grading and drainage slope spills away from equipment supports toward a remote impounding basin, so that a burning pool does not sit under the structural steel that holds up the unit.