In a petrochemical company's heavy oil main pressure alarm interlock system, the pressure of the heavy oil main of the boiler fuel oil dropped and the standby pump could not be started automatically. As a result, the pressure of the heavy oil continues to drop until the boiler interlocking action cuts off the heavy oil and stops, causing a malfunction.
Under normal circumstances, when the pressure of the heavy oil main pipe drops to a certain value, the standby pump should be automatically started to keep the heavy oil at a certain flow and pressure. Now the standby pump has not started, which means that the standby pump has not received the signal of pressure drop, that is to say, the pressure transmitter of this system does not feel the change in the pressure of the main pipe.
The reason is that the isolating fluid in the conduit is discharged, and the heavy oil enters the pressure guiding tube and the positive pressure chamber cavity of the transmitter: Because the isolating liquid is used to measure the pressure of the main pipe, the pressure guiding tube and the pressure gauge are not used for heat preservation and heating, and the freezing point of the heavy oil is relatively low. Therefore, it condenses in the pressure guiding tube and the membrane cavity, and cannot sense and transmit changes in the pressure of the manifold. At the same time, due to the solidification of heavy oil and volume expansion, the force indication of the sensor element is too high, and this value has been maintained.
When the pressure of the main pipe drops, this value does not change, and the standby pump does not start until a shutdown accident occurs.
Purge the pressure guiding tube with steam, and clean the membrane cavity of the instrument with gasoline. Before the instrument is put into use again, the instrument shall be subjected to a static pressure test. After the inspection is qualified, the pressure guiding tube shall be filled with isolating fluid again.