Control Valve is one of the most important equipment of a plant. Control of pressure, flow, temperature, level, and many other quantities is done using this control valve.
Today in this article we will discuss one incident in which a new industry was observing failure in many control valves one by one which led to process upset many times.
Hunting of Control Valves
Instrument Engineers of a well-known new industry were facing a serious problem of abnormal operation of various control valves across different plants and different sections of the whole industry.
The problem was that the control valve started hunting and not operating properly i.e. command vs feedback error. This led to production losses and sometimes tripping of the whole plant due to disturbance of various parameters like pressure, flow, temperature, or level.
Some of the reasons for the hunting of the control valves were:
- The body of the valve gets stuck
- Actuator passing issue
- Low air pressure setting from the air filter regulator
- Instrument Air tube leak
- Instrument Air tube choke
- Instrument Air pressure fluctuations
- Problematic positioner
Let us deep dive into this problem and see how the Instrument engineer solved the stated problem of control valve hunting.
Instrument engineers carried out a fault tree analysis. Let us have a look at the fault tree analysis.
Fault Tree Analysis:
Fault tree analysis was made for control valve hunting. Fault tree analysis is shown below:
As seen from the above fault tree analysis, various causes were identified for the problem statement “Control valve hunting randomly leading to process disturbance”.
Various reasons were taken into consideration as seen from the above fault tree analysis:
- The body of the valve gets stuck
- Actuator passing issue
- Low Instrument Air pressure set from air filter regulator
- Instrument Air tube leak
- Instrument Air tube choke
- Instrument Air pressure fluctuations
- Problematic positioner
The final conclusion from the fault tree analysis was that the positioner of the control valve was having the issue. Let us see what all things were checked and how this conclusion was made.
Root Cause Analysis
Root Cause Analysis for Control valve hunting randomly leading to process disturbance:
When the Instrument Engineers visited the field to check various control valves having the same problem, they did the following activities to identify the root cause:
- The instrument team gave direct air to the control valve’s actuator. This was done to verify whether the control valves are getting stuck in the body or not. But all the control valves were operating smoothly when checked through direct air.
- Also, the instrument team checked the condition of the actuators. The actuators were not passing.
- The pressure setting of all the air filter regulators was checked. The setting on the air filter regulator was as per the air pressure requirement by the control valve as mentioned on the tag plate.
- All the instrument air path was checked thoroughly. No leakage was found.
- Also, the instrument air tube was checked for choking. No tube was found in choke condition.
- Instrument air pressure was monitored for a few hours and also at different time intervals. But the instrument air pressure was found steady.
- The instrument team checked the positioner’s output. The output air from the positioner to the actuator was fluctuating (verified through the gauge given on the positioner). Hence instrument team removed the positioner and opened the positioner in the workshop.
Findings in the workshop after opening positioner
After opening the positioner, everything was found intact. No physical damage was found in the positioner. After disassembling each and every component of the positioner, one strange thing was observed. The I/P’s air filter was having foreign material deposited on it.
A similar observation was found in many other positioners. So, the instrument team simply replaced the I/P’s air filter in one of the positioners and installed it back. The positioner started working fine.
(Image only for reference)
After meetings and discussions, the instrument team came to the conclusion that the instrument’s air quality was not good. Foreign particles in the instrument air were causing the choking in the I/P filter. Because of this choking, the I/P couldn’t perform properly and hence cause the control valve to hunt.
After this incident, all the Instrument Air Headers were flushed and the operation department also took steps to improve the instrument air quality. After all air header flushing jobs, the control valve hunting issue was observed very rarely.
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