Temperature, pressure, flow, level: loop specifics
How typical process loops differ: process dynamics, noise, whether D is needed, where to start tuning. Worked examples for flow, temperature and level with one method.
In short. The method is the same: step test → model → tuning (IMC/SIMC). But loop types differ a lot: flow is fast and noisy (PI, no D), temperature is slow with dead time (PI or PID), level is integrating (usually slow averaging control), pressure ranges from "like flow" to "like level" depending on the medium and volume.
Comparison
| Loop | Dynamics | PV noise | Controller | What to watch |
|---|---|---|---|---|
| Liquid flow | seconds; small θ | high | PI, no D | Ts ≤ 0.1–0.2 s; valve stiction; valve non-linearity |
| Liquid pressure | like flow | medium | PI | water hammer; fast valves |
| Gas pressure in a vessel | seconds to minutes; often integrating | low | PI | vessel volume sets the speed; output limits |
| Level | integrating | waves, foam | P or PI with slow tuning | averaging or tight control is a process decision |
| Temperature | minutes; noticeable θ | low | PI or PID with a D filter | heating/cooling non-linearity, split range, sensor lag |
| Composition, pH | long θ (analyser) | varies | PI, slow; often cascade | analyser θ; strong pH non-linearity |
Example: flow
Normalised model K = 1.2 %/%, τ = 3 s, θ = 1 s. SIMC, τc = θ:
Kc = 3 / (1.2 · (1 + 1)) = 1.25 %/% Ti = min(3, 4·2) = 3 s
Sample time: θ/10 = 0.1 s. If the block runs every 0.5 s, that is half the dead time — increase τc or speed up the task (discretisation).
Example: temperature
Section model: K = 1.8 °C/%, τ = 45 s, θ = 12 s. SIMC PI: Kc = 1.042 %/°C, Ti = 45 s (overshoot 5.5 %, settling 74 s). IMC PID (aggressive): Kc = 1.816, Ti = 51 s, Td = 5.29 s — a faster rise, but D needs a filter and D on measurement (D filter).
Heating and cooling often have different K and τ: tune for the "stronger" direction or use split range with separate gains.
Example: level
Integrating process, k′ = 0.001 1/s, θ = 10 s (integrator model):
| Goal | τc | Kc, %/% | Ti |
|---|---|---|---|
| Tight (level matters, e.g. in a reactor) | 10 s = θ | 1/(0.001·20) = 50 | 4·20 = 80 s |
| Averaging (the vessel smooths the flow) | 100 s | 1/(0.001·110) = 9.09 | 4·110 = 440 s |
With averaging control the level may move within allowed limits, while the flow to the next unit changes smoothly.
Typical mistakes
- D in a flow loop — noise is amplified, the valve wears.
- Tight level tuning when a smooth flow is needed: disturbances are passed down the process chain.
- Ts larger than θ in fast loops (flow, liquid pressure).
- One setting for heating and cooling with very different K.
Limits
The table describes typical behaviour; a particular plant may differ (a short pipe with a large sensor θ, gas with a small volume, etc.). The numbers in the examples illustrate the calculation and are not recommended values for any plant.
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Sources
Updated 2026-10-12