The relief valve cannot release pressure
Thread Content
There is a 5-kilogram steam release control valve; when the valve is opened, steam cannot be released. The shut-off valve was closed before that, and after adjusting the valve, it worked properly. Once the shut-off valve was reopened, the control valve functioned normally again. Does anyone know the reason?
It’s probably because of the large pressure difference before and after the valve
If it is found that air defense is not possible, first check whether the valve has actually moved, that is, check the position of the valve stem, using the process of elimination. First rule out issues with the valve body, then check the wiring and signals to eliminate signal problems. It’s still impossible to determine with this approach; let’s wait for the next failure
It is assumed that the valve stem is not moving, and this needs to be confirmed. The control valve failing to operate can occur in many situations. Steam generation is more likely to occur due to prolonged high temperatures, deterioration of the packing at high temperatures, scaling, and other such issues.
Shut off the isolation valve before closing the main valve; when testing the valve, check whether the valve stem moves and whether it matches the feedback signal
Floor 2 is supported; it might be a pressure difference issue. But 5 kilograms of pressure is very low; such a problem should not exist. Previously, for the pump outlet valve, we used this method: we opened the valve by 1% before starting the pump, and then adjusted the valve in case there was an excess flow. I’m not sure if this steam venting method will work: set the control valve to 1% and then open the shut-off valve; this will result in an excess of steam, after which you can adjust the valve to see if it responds.
Can’t release steam after the valve is opened? Was this confirmed on-site, or was it just a signal sent from the control room without anyone going to the site to check the valve? It is likely that the situation wasn’t confirmed; the pressure difference before and after the valve was too large. Additionally, as the valve hadn’t been operated for a long time, its mechanism became stuck. By closing the valve ahead to reduce the pressure difference, the valve was able to function properly again.
It is not possible to determine the condition of the valve with certainty for now; first, it is necessary to confirm whether the valve actually moves. If the valve can be opened and closed properly after closing the shut-off valve upstream of it, then there is likely no major issue with the valve itself. It is also important to check whether the air pressure is sufficient – sometimes, when there is a medium flowing through the valve, insufficient air pressure can prevent the valve from moving. Another possibility is that the connection between the valve stem and the valve core is loose, which may cause the valve core to drop or prevent the flap from moving in response to the steam pressure as the valve stem moves.
When opening a valve in a DCS system, there are many reasons why the valve does not move: 1. The control signal does not reach the actuator； 2. Positioner failure ； 3. The pressure is too low after passing through the filter regulator ； 4. Insufficient thrust or torque of the actuator ； 5. Internal valve failures such as the detachment of internal components and the valve stem. Based on the current analysis, it is estimated to be due to the locator calibration. The author did not specify: the failure location of the valve (FO/FC), the valve type (single-seat valve, butterfly valve), the results of on-site inspection, whether there were any alarms from the actuator, and how it was adjusted to return to normal operation. It is not conducive to problem analysis.