June, 2005

200501JunAll DayToo many start-stop switchesProcess Safety Beacon | When you do a safety check, make sure it is on the right equipment !Lessons:Operational IntegrityLanguage:EN Origin: CCPS Document Type:BulletinTopics:Human Factors


Here’s What Happened:

The evening shift was assigned to clean an agitated mixing vessel. The supervisor asked the lead operator to complete the “Lock out.” The lead operator tagged and locked out the motor starter in the Motor Control Center, verified the motor would not start by pressing the Start button and put a lock and “Danger-Do Not operate” tag on the Start-Stop station near the vessel. The supervisor then issued the Confined Space Entry permit and two workers entered the vessel and cleaned it for the rest of the shift. The oncoming day shift needed to reissue the Confined Space Entry permit. When they tried the Start button on the Start-Stop station, the agitator started! The agitator motor was NOT locked out!

How Did This Happen?

Easier than you might imagine. Did the Lock-out undo itself? No, but the wrong motor was locked out. How can that happen when the starter was labeled the same as the agitator? And, why didn’t the agitator start when the Start button was tested the first time?

Here’s how. Several months before, the agitator motor was changed out to a larger size. The size increase required a larger motor starter and wiring. Because the plant might need the “old” system again some day, it was not removed. Instead, a new Start-Stop station was installed near the vessel, in fact, right next to the old Start-Stop station. The “old” Start-Stop station was on the flange part of a column next to the vessel and the “new” Start-Stop station was in the web of that same column. When the technician locked out and tested the system, he was testing the “old” system which was disconnected. The “new” system was still active!

© AIChE | Sponsored by CCPS Supporters


CCPSCenter for Chemical Process Safety