The Case of the Infectious Sinusoidal Variance

THE MYSTERY

An ACES Control Systems Investigator was called to the scene of an oil-heating steam generator platform, in a processing plant for natural gas liquids (NGLs).

The CSI had been called for routine biennial emissions testing, but he noticed the trend lines didn’t appear to be tracking the way they had been two years earlier. The issue was efficiency: Overall combustion efficiency didn’t seem to be responding properly.

THE CLUES

A plant employee mentioned that they’d had a problem with the control valves, and had to change one of the operators because it had oil in it. What started as simple emissions test turned into a plant excursion complete with fireproof suit.

THE PERP

The CSI examined the operators and found one that was level full of oil. The valve had been installed in such a way that the drain was blocked and allowed the control housing to fill with oil. Upon examination the CSI discovered oil in more of the operators — and water in another crucial operator.

THE SOLUTION

The ACES CSI ragged out the oil and water from all the pockets and holes, installed new parts, and recalibrated and restored the trends back to historical performance and proper operations. In addition, the client installed coalescing vessels on the air lines to bleed out any oil and water that found its way into the system.

THE SECOND MYSTERY

While he was waiting on the emissions check, the CSI observed the rest of the plant’s operations and ascertained that the plant’s control valves had a sinusoidal response that seemed to be tracking the same trend exhibited by the generator.

MORE CLUES

The CSI wasn’t sure which was the culprit in the output fluctuations — the generator or the plant control valves — but he decided to start with the generator.

ANOTHER PERP

The CSI tuned the PID loop for the generator, eliminating the sinusoidal variance in the generator’s fuel control valve — and when the generator flattened out, the entire plant’s output flattened out as well.

THE SOLUTION

After adjusting the PID values on the steam generator, the generator was actually running better than when it was originally installed four years ago. In addition, all the valves in the plant settled down and started behaving in a more stable manner. The client is thrilled with the highly tuned and stable output of their steam generator — and their plant overall.

This case shows how the complex environment of control systems can be simultaneously disrupted by many different suspects — in both hardware and software. The Control System Investigators at ACES are always on the alert for these perps wherever they rear their ugly heads.

CASE CLOSED