Stop tuning PIDs to hide mechanical sins

Had a centrifugal pump trip three times last week. HMI lit up like a Christmas tree: low suction pressure, high vibration, seal pot low, VFD current spike. Operations asked for a “smart alarm” and a little more PID love. Well, there’s your problem: suction basket 60% plugged, soft foot on the motor, and January water at 4°C took a bite out of NPSHa. You can’t tune physics.

My rule: if a condition can destroy hardware in seconds, it’s not an advisory, it’s an interlock. Low suction? Trip and latch. Seal pot low? Trip and latch. Want fewer rebuilds? Add a cheap dP across the strainer, put a permissive on starts (Psuction > 45 kPa, seal pot > 0.8 bar, bearing temp < 80°C), and give the VFD a sane ramp. Also, rationalize the alarms: one clear cause, not five chattering echoes.

Where do you draw the line between hard interlocks and operator judgment? What deadband/time filter values have worked for you to avoid nuisance trips without masking real failures? Any low-cost sensors you’ve added that paid back instantly?