2 replies to this topic

[Alaa55]

We are facing an issue in our plant and would appreciate your advice.

Our level control valve is malfunctioning and suspected to be blocked or internally damaged. As a result, it cannot properly handle the liquid in the knock-out drum. To maintain operation, we have been using a manual bypass globe valve to drain the accumulating liquid from the vessel.

However, when we open this bypass valve, we sometimes encounter high pressure in the downstream vessel, leading to PSV lifting. This problem did not occur when the control valve was functioning normally.
 

Key details that may be relevant:​

• We confirmed there is a liquid seal before opening the bypass valve to avoid gas blowby.
• The bypass valve is only opened slightly (small opening).
• The downstream vessel level is typically around 80% due to a pumping issue.
• The pressure increase in the downstream vessel happens almost immediately after opening the bypass valve, leaving no time to close it before PSV lifts.
• The knock-out drum is designed to separate free water from the feed, which is mainly methane.

Since we will need to operate with the bypass valve until the next shutdown to repair the control valve, we are concerned about the risk of repeated high-pressure incidents in the downstream vessel.

 Question: Based on the above, where do you think the problem lies, and how can we avoid this situation?

I have attached a simple schematic of the process for reference.

 

 Untitled Diagram (4).jpg   38.12KB   0 downloads

Edited by Alaa55, Today, 04:43 AM.

[breizh]

Hi,

You may have equipment failure, wrong tuning on PID parameters, wrong operation.

Is there a chance that the gas outlet is partially plugged, let say demister pugged?

Check what was the original configuration and what are the changes observed. Do you carry gas with the liquid which could have eroded your control valve? To me it's abnormal to have the bypass line partially open unless more material flowing through the separator or gas line obstructed.

EDIT: Globe valve is not adequate to adjust the flow rate.

 

Breizh 

Attached Files

•  Tuning Vessels &tanks.pdf   470.88KB   0 downloads
•  tuning.pdf   1.04MB   0 downloads

[PingPong]

Your simple schematic shows no block valves around the control valve, which would be unusual. Is this really so?

 

Normally there are block valves so that the control valve can be replaced by a spare one and the manual operation of the bypass valve is only requires for a short duration.

 

If there are no block valves then using a manual bypass globe valve to maintain liquid level in the high pressure KO drum until the next plant shutdown would require somebody to locally watch the level gauge and regularly adjust the bypass valve 24 hours per day, 7 days per week, 52 weeks per year. Not a good idea.