7.3 Control of Common Process Equipment
Key Takeaways
Stabilize equipment inventory — level, pressure, surge margin — before closing slow composition or quality loops
Pump minimum-flow recycle and compressor anti-surge are fast override loops; a VSD does not retire min-flow at low load
Combustion cross-limiting and burner-management flame safety are different layers; a temperature PID must not bypass purge or the fuel safety shutoff
Distillation dual-composition is interacting because material-balance and energy-balance handles couple; do not use a slow analyzer as an inner cascade loop
Match the loop to the failure mode: runaway cooling constraint, steam-heater stall, defluidization, filter differential-pressure cycle
7.3 Control of Common Process Equipment
Specification 2.B's "common processes" items are not a miniature chemical-engineering textbook. The PE Control Systems question is which loop, which paradigm, and which failure mode. Use feedforward, cascade, ratio, override, split-range, and the discrete/sequential tools from 7.1–7.2. If you start deriving McCabe–Thiele or compressor maps from memory, you are writing the wrong exam.
Pumps
Centrifugal pumps need a minimum continuous stable flow. Below that, recirculation, heat, vibration, and seal failure show up. Capacity is usually a VSD or a discharge control valve. Min-flow is a separate override: a low-flow (or low-dP) controller opens a recycle via high-select with whatever else wants that valve closed. A VSD does not retire min-flow at low header demand; affinity-law slowdown can still leave the pump in a bad region, and dead-head heat is still real. Suction-side NPSH protection is the discrete interlock from 7.2, not a slow level PID.
Compression
Surge is flow reversal and oscillation when flow is too low for the head the machine is trying to make. Anti-surge opens a recycle (or a hot-gas bypass on some machines) before the operating point crosses the surge line. Capacity uses speed, inlet guide vanes, or suction throttle. Anti-surge is a fast override (high-select on the recycle, anti-windup on the capacity controller). Do not "stabilize" anti-surge by detuning it to composition-loop speed. Reciprocating machines add unloaders and clearance pockets — discrete steps with analog suction-pressure regulation.
Combustion
Regulatory firing is temperature (or duty) cascade to fuel flow, air/fuel ratio, and cross-limiting selectors: air leads fuel on increase, fuel leads air on decrease, so you do not go fuel-rich. Flame safety (burner management) is discrete/sequential: purge, light-off, flame-failure trip, safety shutoff valves. The temperature PID is not allowed to crack a safety shutoff during purge or to keep fuel on after flame failure. Mixing BMS and regulatory onto one trip-free valve is a 2.B failure mode, not an efficiency trick.
Evaporation, dehydration, heat exchange, crystallization, filtration, refrigeration, fluidization, hydraulics
Evaporators: body level is inventory (expose tubes and you overheat metal). Steam flow is duty or product concentration. Vacuum/pressure is a fast loop. Lost level is the classic trap, not a missing Brix PID.
Dehydration (glycol contactors, molecular sieves): contactor level and circulation, regenerator temperature, and a cycle sequence for sieves. Water-in-product analyzers are slow outer cascades. Do not let the analyzer loop run faster than the bed or the contactor dynamics, and do not starve contactor level to chase a spec.
Heat exchangers: utility flow or process bypass. Steam heaters stall when condensate cannot leave — condensate level or a stall-resistant trap arrangement matters. Fouling is a slow disturbance; a bypass that inverts process gain (mixed outlet) is a pairing trap.
Crystallization: the quality variable is supersaturation, often a cooling profile, not an aggressive temperature PID that showers fines and fouls the jacket. Agitator proof is an interlock. Sequence holds a cook or a cool-down step.
Filtration: analog differential pressure and flow; sequential backwash or CIP. High dP is both a transition and a feed-stop override. Treating a filter as analog-only misses the cycle.
Refrigeration: suction pressure, evaporator level on flooded systems, compressor anti-surge or hot-gas bypass at minimum load. Hot-gas bypass is a low-load override, not the main capacity handle at high load. Oil return is discrete/sequence as much as analog.
Fluidization: air flow and bed dP prove the bed is fluidized. Temperature is usually outer. Cutting air to control temperature defluidizes the bed; too much air elutriates solids. Air is not just another utility valve.
Hydraulics: load-sensing or pressure-compensated pumps, servo position as a fast inner loop, and a relief as a safeguard, not as the normal modulating pressure controller. Using the relief as a 24/7 regulator is the trap.
Reaction
Exothermic reaction control is temperature cascade to coolant flow, with a cooling-constraint override that can cut feed, go to maximum coolant, or initiate a dump — discrete/SIS as well as analog. Runaway is a cooling-limit problem: feedforward from feed rate that increases reactant faster than the jacket can reject heat is the wrong 7.1 application. Inhibit feed on coolant failure is an interlock, not a tuning constant.
Distillation: material balance, energy balance, dual composition
Inventory first. Reflux-drum level usually moves distillate or reflux. Sump level moves bottoms. Pressure moves condenser utility, vent, or sometimes reboiler duty. Those loops are fast relative to composition. Material-balance handles are product flows; energy-balance handles are reflux and reboiler duty. They couple.
Dual composition — both distillate and bottoms qualities closed tightly — is difficult because the two quality loops interact through those balances. Relative-gain pairings (LV, DV, and similar) exist so you do not assign two composition PIDs to two handles that fight. Exam move: keep inventory tight; control one composition well; treat the second as a constraint, a lab cascade, or a slower inferred temperature. A gas chromatograph is an outer loop, never an inner cascade loop.
Worked: distillation inventory versus composition
A binary column is swinging in both drum level and overhead purity. Closing two analyzer loops first makes it worse: the composition PIDs steal reflux and steam, the drum level winds up, and both qualities oscillate. Correct order: drum level on distillate, sump level on bottoms, pressure on condenser water or vent. Then one temperature (or one analyzer) on reflux or steam — not both at full composition duty until inventory is quiet. Dual-composition, if required, is a later, detuned, decoupled pair, still sitting on a stable inventory shell.
Worked: compressor surge as override
A centrifugal compressor capacity controller wants more discharge pressure and therefore wants recycle closed (or speed raised). As flow falls toward the surge line, the anti-surge PID — fast, often with flow lead — opens recycle. A high-select lets anti-surge win on that valve. The capacity controller must track the selected output. Without anti-windup, when surge clears the capacity PID is still demanding recycle fully closed, slams the valve, and the machine surges again. That is 7.1 override applied to a machine, not a new theory of compression.
| Equipment | Typical loops | Classic trap |
|---|---|---|
| Pumps | Capacity (VSD or discharge), min-flow recycle override, suction trip | Treating VSD as a replacement for min-flow at low load |
| Compressors | Capacity (speed/IGV), anti-surge recycle override | Slow anti-surge, or no anti-windup on capacity |
| Fired heaters | T cascade to fuel F, air/fuel ratio, cross-limit, BMS sequence | Using BMS as the temperature controller; fuel-rich on load increase |
| Distillation | Levels, pressure, one composition; steam/reflux pairing | Dual composition before inventory is tight; GC as inner loop |
| Reactors | T cascade to coolant F; feed/cooling constraint; SIS | Feed increase with no cooling override |
| Heat exchangers | Utility F or bypass; condensate level on steam | Bypass that stalls a steam heater or inverts gain |
| Evaporators | Body level, steam duty, vacuum | Low level, dry tubes, then overheat |
| Crystallizers | Cooling/supersaturation profile, agitator proof | Aggressive T PID → fines and fouling |
| Filters | dP/flow plus backwash sequence | Analog-only thinking; skipped cycle |
| Refrigeration | Suction P, min-load HGBP/anti-surge, evaporator L | Hot-gas bypass as the high-load capacity handle |
| Fluidized beds | Air F, bed dP, outer T | Cutting air on temperature control until the bed defluidizes |
| Hydraulics | Fast position/pressure inner loop; relief as safeguard | Relief valve used as the normal regulator |
| Dehydration | Contactor L, regen T, sieve cycle | Analyzer cascade faster than dryer dynamics |
Work mixed 2.B items — cascade, override, sequences, and these equipment pairings — in the free PE Control Systems practice set.
A binary distillation column has swinging drum level and off-spec overhead. Which control order matches PE Control Systems practice on specification 2.B?
Close dual composition loops on reflux and steam first so quality is correct before inventory is touched
Assign material-balance and energy-balance handles independently because the two quality loops do not interact
Use reboiler duty only for composition and never for pressure, so pressure can wait
Stabilize inventory loops (levels and pressure) first; dual composition is difficult because the two quality loops interact through material and energy balance
A centrifugal compressor capacity controller and an anti-surge controller share the recycle valve. Which description is correct?
Surge protection should be a slow composition-style PID on discharge temperature so the machine stays smooth
Anti-surge is a fast override that opens recycle, often through a high-select, as the operating point approaches the surge line, with anti-windup on the unselected capacity controller
Recycle should remain closed during surge because recycle increases head and makes surge worse
Variable speed eliminates the need to consider surge, so the recycle valve can be locked closed
On a process heater, how should regulatory firing and flame safety be related?
Flame-safeguard logic and the temperature PID should share one trip-free fuel valve so the PID can open fuel during purge if the outlet is cold
Cross-limiting is only a distillation pairing tool and does not apply to fuel and air
Regulatory combustion control — temperature cascade, fuel/air ratio, and cross-limiting — is separate from burner-management purge, light-off, and trip logic; BMS is discrete safety, not a substitute PID
Minimum-flow recycle on the fuel pump is unnecessary if the forced-draft fan has a VSD, even at dead-head
Sections you finish are checked off in the contents.