Today in this article we will discuss about the root cause analysis of an incident in which a tank filled with water started overflowing.
The incident had occurred in a tank in which water was filled. If the tank had some other hazardous liquid, then this could have created fatality.
Problem Statement
A tank was installed in one of the plants in a refinery for storing the water in it. The feed inlet of the tank was controlled using an On-Off valve (butterfly body & rack and pinion actuator). The fail action of the On-Off valve was fail-close.
One day, the field operator informed us that the valve is having some leakage from the actuator side. The instrumentation team checked the valve and found that the actuator of the valve is passing. So, actuator overhauling was needed.
On the next day, the actuator was dropped for overhauling purposes. But the operation requirement was to have the valve in an open condition as the tank was having a low level. Tank filling was in-progress.
So, in valve open condition, the actuator was dropped by giving external air to the actuator to keep the actuator open. After overhauling the actuator, the actuator was reinstalled. The actuator was reinstalled in another shift by a different person.
After a few hours, during the field round, the field operator observed that the tank was overflowing. Immediately the tank inlet was isolated.
When the actuator was reinstalled after the maintenance job, the tank level was 82%. Above 80%, the tank feed inlet valve is in close condition. When the level goes below 25%, the feed inlet valve opens again.
Root Cause Analysis for Valve Problem
So, what can be the probable reason for tank overflow?
Probable Problem 1: The level transmitter showed a low value while the actual level was high.
The level transmitter was working ok. When the tank started overflowing, the level transmitter was showing 101%. The same level was confirmed with the level gauge.
Probable Problem 2: On-Off valve did not close when the level was above 80%.
The feedback from the On-Off valve was close feedback.
Probable Problem 3: The proximity switch malfunctioned and started showing close feedback when the valve was open.
The proximity switch’s functionality was checked. Found ok.
Probable Problem 4: Passing from the On-Off valve body
This might be the probable reason.
Probable Problem 5: Passing problem in bypass valve given for feed inlet valve.
When the isolation valve for the On-Off valve was closed, the tank’s overflow stopped. Hence there was no issue with the bypass line valve.
Probable Problem 6: The actuator direction was incorrect during installation
This can also be one of the reasons.
Observations after taking the On-Off valve for maintenance:
- The actuator was dropped for further checking purposes. The valve was in close condition while dropping the actuator.
- While dropping the body for checking the passing issue, the body was found in open condition.
This means that the actuator’s direction of installation was incorrect.
What went wrong?
During the job, the actuator was dropped in open condition. While reinstalling the actuator, the technician installed the actuator in close condition, while the body was in open condition.
The technician was unaware of this because the body had a coupler of square shape. The technician did not check the condition of the body by removing the coupler.
The technician installed the actuator in close condition. Then he set the feedback and handover the valve for operation.
The direction of every instrument before removing should be marked as well as the proper handover of jobs should be given.
