On-site instrument failure analysis (one question per week) – Question 9 (answers released)
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This post was last edited by Yiding on 2009-11-23 at 10:45. Fault description: The pneumatic diaphragm control valve used in the control circuit of the acidic water inlet system of an acid water treatment unit in a certain factory is a normally open air-operated valve. In winter (with an ambient temperature of -10°C), process operators reported that the control valves were not functioning. After inspection by the instrumentation technician, the gas supply pressure gauge showed normal readings. What could be the reason why the control valve is not functioning? Answer: The instrument air line was contaminated with water, and the instrument technician failed to drain it in a timely manner. This allowed water to enter the diaphragm head of the control valve; with an ambient temperature of –10°C, the diaphragm froze, resulting in the control valve not functioning. Solution: 1. Check the cause of water ingress and address it. Inspect and discharge the instrument air lines and filters. 2. Open the diaphragm head of the control valve and clean the water that enters the diaphragm head. 3. Operate the control valve normally.
Possible reasons: 1. Freezing of the instrument air line up to the diaphragm head. 2. Water in the process pipeline medium, leading to freezing of the process medium. 3. Use steam for purging and then open the drain valve.
It is possible that the valve stem packing is leaking or frozen, causing the control valve to fail to operate.
First, acidic water freezes at minus ten degrees, which prevents the valves from functioning. Second, in the fault condition the valve should remain open (FO); it is recommended to install electric heating at the bottom of the valve seat to prevent the fluid from freezing.
Firstly, since this acidic water treatment plant can operate at minus ten degrees, it must have been designed with low-temperature operation in mind; otherwise, other control valves would also encounter problems. Given that the instrument air pressure gauge is functioning properly, the likelihood of a fault in the instrument lines leading from the instruments to the diaphragm heads is also low. In that case, the problem must lie with the valve itself; the cause could be a fault in the actuator or the signal conversion switch, or there might be an issue with the signal wires.
The acidic water inside the valve body freezes, causing the valve to fail to function
The valve core gets stuck due to the freezing of acidic water inside the valve body
I think the diaphragm becomes brittle at low temperatures and gets damaged under high forces
First and foremost, it is certain that there is water in the instrument air supply. Next, checks should be carried out step by step: 1. Check whether there is pressure at the output of the valve actuator; if not, it indicates that the actuator has frozen； Otherwise, 2. Freezing of water inside the diaphragm head of the control valve causes the valve to fail to operate ； 3. Another possibility is that the diaphragm of the regulator has broken, resulting in air leakage ； As for acidic water freezing and causing the valves to fail to function, there is no such possibility, as the water is constantly in motion and cannot freeze. If it really froze over, the control valve would have frozen and broken long ago.
The compressed air has frozen or the valve core is damaged
Personally, I think the instrument air containing water has frozen up, blocking the air path of the valve positioner or electrical converter.