Tuning Practice 6 — Flow — VFD pump, no valve

The job

What you are looking at

There is no control valve. The controller output is a speed reference to a variable-frequency drive, and the pump speed sets the flow directly. A flow transmitter downstream is the measurement. This is the noisiest measurement in the set — noisier than the flow loop you just tuned with a control valve.

That fuzz on the trend is real, and it is supposed to be there. A flow measurement is genuinely noisy. The liquid is turbulent, and the transmitter reports what it actually sees, moment to moment. So the flow line will never be a smooth curve the way the level and temperature loops were — it will always look slightly hairy, even when the loop is tuned well and sitting right on setpoint. Nothing is broken, your display is not glitching, and it is not something you can tune away.

Watch what it does to the controller output. Proportional passes that fuzz straight through to the valve or drive. Rate — the derivative term — responds to how fast the measurement is changing, and noise changes fast, so rate multiplies it. Turn rate up on a noisy flow loop and watch the output trace before you decide whether rate belongs here.

The disturbance is downstream restriction — a hand valve somewhere else in the system, or another branch drawing flow. It does not simply add or subtract flow. It changes how much flow each percent of pump speed actually buys you, so a tuning that behaves well wide open may behave quite differently once the line is throttled.

The controller is on factory defaults and the acting direction has not been commissioned. All three tuning terms are unlocked.

This is practice, not a test. Nothing on this page is scored and nothing is submitted anywhere. Judge your tuning by the trend in front of you: how quickly the measurement reaches the setpoint, how far past it goes, whether it settles, and how the loop behaves when the load changes. Change something, watch it, change it again.

How to work through it

Steps

Press Start. The loop begins balanced at setpoint, so nothing will move until you disturb it — move the setpoint, or change the load, and watch which way the measurement goes.
Set the acting direction for this loop.
Tune it until the flow holds setpoint steadily without the output chattering.
Now change the Resistance slider and see whether your tuning still behaves the same way. Try it at more than one setting, and settle on tuning you are willing to leave in the controller across the whole range.
Before you call it tuned, change the load and confirm the loop brings the measurement back and holds it. A loop that only looks good on a setpoint move is not proven.

All three tuning terms are available on this exercise, including the derivative (rate) term.

Adjustment range available for each tuning term on this exercise
Tuning term	Lowest	Highest
Proportional gain (Kp)	0	2
Integral gain (Ki, 1/s)	0	1.35
Derivative gain (Kd, s)	0	0.6

These ranges are set for this loop only. Another exercise will have different ones, because a different process needs different numbers.

Simulator

For your own record

Commissioning notes

Nothing here is graded or sent anywhere. Write down what you settled on and why, so you have it to compare against the next exercise.

Which way did this controller have to act, and how did you work that out? What did you leave the tuning set to? Record the numbers themselves, in whichever form the controller was showing them. What did the loop do when you changed the load, and were you satisfied with the recovery?

Your notes stay in this browser, on this computer, and they belong to this exercise alone — each exercise keeps its own set. Open this exercise again on this computer and they will be here waiting for you. They are not sent anywhere and they are not graded, and they will not follow you to another machine, so copy anything you want to keep elsewhere.