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 |
A binary distillation column has swinging drum level and off-spec overhead. Which control order matches PE Control Systems practice on specification 2.B?
A centrifugal compressor capacity controller and an anti-surge controller share the recycle valve. Which description is correct?
On a process heater, how should regulatory firing and flame safety be related?