Operation of the vent valve during planned shutdown of the gasifier
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Recently, I encountered a problem: when planning to shut down the gasification furnace, it is usual to first raise the temperature of the slag and then direct the raw gas to the flare for venting. However, when the emergency shutdown button for the gasification furnace is pressed, issues arise. In some plants, the first step in the shutdown procedure involves closing the manual control valve HV located before the venting pipeline (the solenoid valve of this HV valve loses power for ten seconds, during which the OP value is set to zero; after ten seconds, the solenoid valve resumes power operation, but the valve remains closed). This design helps prevent too rapid a drop in pressure after shutdown. In other plants, there is no such step – one person presses the emergency shutdown button while another person closes the venting isolation valve. But the isolation valve cannot be closed in advance, as that would cause pressure buildup, while delaying its closure again leads to too rapid a pressure drop, which can result in cavitation. Still other plants use interlocking between the pressure venting valve and the isolation valve: when the pressure control valve operates in automatic mode, the isolation valve closes when the OP value output by the pressure venting valve is less than 10, and it opens automatically when the OP value is greater than 10. The above are the various treatment methods currently in use. I hope everyone can discuss their advantages and disadvantages to see if there are any better methods! Dear everyone! Share our opinions, discuss and exchange ideas, and learn together!
We close the valves manually; before shutting down the plant, we disconnect the raw gas first, and then use the vent valve to control the pressure. After pressing the shutdown button, the operator closes the vent valve. It doesn’t seem to be very difficult to do this, and even if the valve is closed a little later, the pressure won’t drop as quickly as one might think – there’s still plenty of time. I haven’t tried using automatic mode yet
“Delay can also lead to too rapid a pressure drop, which in turn makes cavitation more likely. First of all, I would like to say that what you mentioned – namely that too rapid pressure loss leads to cavitation – is impossible. In fact, it’s impossible to happen in theory. After stopping the vehicle, simply close the drain valve manually; there is plenty of time for that. If you have time, you can take a look at the table of saturated vapor pressures of water.
You all say it’s impossible, but I’ve seen it happen in reality. When preparing to shut down the furnace and divert the gas to the flare, the pressure was maintained at 5.7 MPa. After discussing it, the two of us decided to carry out the shutdown procedure together: one person pressed the shutdown button, while the other immediately closed the vent isolation valve once the shutdown process started. There was a time gap of 2–3 seconds between their actions. But once the vent isolation valve was completely closed (after 3–4 seconds), the system pressure dropped sharply to 4.4 MPa. Don’t say it’s impossible – I experienced it firsthand! As soon as the shut-off valves for the oxygen pipeline and the coal slurry pipeline are closed, and the vent valve is not closed in time, the system pressure drops much faster than you would expect!
First of all, don’t you have a pressure control valve after shutting off the valve? After pressing the stop button, manually close the pressure control valve. The operating time of the shut-off valve is relatively long, but with such a large pressure drop, it is recommended that you have an instrument specialist check whether there is something wrong with the valve, whether there are issues with the air supply, and whether the operating time has changed
As already explained in the previous post, a shutdown time of 2–3 seconds for the shut-off valve is considered normal. The process of shutting down and closing the vent PV control valve is even slower, and moreover, that control valve does not provide a proper seal; as a result, its leakage rate is much higher than that of the shut-off valve. It’s definitely not a problem with the valves! You’ll see for yourselves if you try it a few times – once the shut-off valves on the oxygen and slurry pipelines are closed, if the vent shut-off valve isn’t closed promptly, even a short period of time (2-3 seconds) is enough to cause a rapid drop in pressure!
It should be common sense that a shut-off valve closes faster than a control valve!
Cavitation will not occur even at 44 kilograms.
Regarding what you said (that the shut-off valve closes faster than the control valve), this is not necessarily true, nor is it common knowledge. Our control valves can close completely in just 1 second.
The pressure drops so quickly. Also, although the OP values of your two PV valves have been set to zero, what is the time it takes for the valves to close? These two valves must be closed very slowly.