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This post was last edited by zhangchao2012 on 2017-3-8 at 13:03. Case of the Week (Accident Handling Training): Fault due to incorrect setting of instrument parameters. 1. Fault description: During the first start-up of a hydrocracking unit in a certain plant, around 14:00 on March 16, 2005, a 200°C thermal cycle was carried out in the distillation unit. The distillation furnace had two main burners, and all the pilot lights were lit. The operator on duty connected the gas pressure control valve PIC1233 with the temperature control valve TIC1278A for the bottom temperature of the second distillation tower T1004 in series; as a result, positive pressure developed in the distillation furnace, causing all the main burners and pilot lights to go out. The on-duty crew immediately shut down the pilot light of the distillation furnace as well as the manual valves in front of the main burners, and then contacted the relevant personnel to conduct an analysis of the explosive gases; once the results were satisfactory, the furnace was restarted and heated up again. By around 16:00, the operation of T1004 returned to normal. 2. Cause of the fault: According to the DCS curves and data, at 13:51:30 on the 16th, the operator on duty switched TIC1278A from manual mode to automatic mode. At that time, PIC1233 was in automatic mode, with a set value of SP at 0.24, a measured value of PV also at 0.24, and a valve position value of OP at 7.66. At 13:51:55, the operator switched the auxiliary gauge PIC1233 from automatic mode to manual control. At that time, the set value for the main gauge TIC1278A was 147°C, the measured value was 147.94°C, and the valve position was 0.24 ； The set value for sub-meter PIC1233 is 0.24, the measured value is 0.24, and the valve position value is 7.66. Under normal conditions, the valve position value of the main table after interlocking should be the set value of the secondary table, while the PIC1233 should maintain its original valve position after interlocking. However, at the moment of serial control, the set value of the PIC1233 is reset to zero, causing the control valve to return to its default position. At this point, the main table TIC1278A causes the valve position to change as the measured value is higher than the set value. The valve should operate in a reverse manner, meaning that when the measured value is higher than the set value, the valve position should decrease; however, in reality the valve operated in the opposite direction, with its position rising rapidly from 0.24 to 0.37. The valve position is sent to the auxiliary controller PIC1233; once the valve operates in the direct-acting mode, its opening degree increases rapidly. At 13:52:41, 46 seconds after the sequential control was initiated, the operator noticed abnormal behavior in PIC1233 and switched to manual control for that device; at that point, the valve’s opening degree had reached 48.52. 13:52:56, the operator manually reduced the valve position to 46.52. At 13:55:52, the valve position dropped to 6.52. 3. Fault analysis: 1) Incorrect setting of internal parameters of the instrument. The main valve TIC1278A should operate in reverse action, but due to incorrect internal parameter settings, this valve actually operates in direct action. The set value of the auxiliary controller PIC1233, after being controlled in series, should immediately track the valve position value of the main controller. Since the PIC1233 and the fuel oil pressure control PIC1236 have a switch for serial control with the main gauge TIC1278A, the instrument set the parameters of this switch incorrectly; as a result, after serial control, the set value of the auxiliary gauge could not immediately track the valve position value of the main gauge, and the set value reverted to zero. Furthermore, since the operator on-site does not have the authority to set the instrument control parameters, with the main gauge value at 3, T1 at 5, and T2 at 0.5, the valve position moves rapidly and experiences significant fluctuations. According to instrument checks, the same issue was present in the temperature control and gas pressure control sequencing of F1002, and it has now been resolved. 2) The team failed to monitor the equipment closely, and handled faults improperly. Firstly, PIC1233 and TIC1278A should not be blindly interconnected for control during the operation of the fractionation system at 200°C. Since few main burners in the furnace are lit at this stage, the amount of gas used is also low; as a result, the opening degree of the gas pressure control valve is very small, making it unsuitable for cascade control. Secondly, after a failure occurred in the serial control between the main and auxiliary tables, the team operators failed to detect it in time and address it promptly. The reverse operation of the main gauge, and the resetting of the set value after serial control of the auxiliary gauge – these are both very obvious gauge errors; they can be detected immediately if one pays close attention to the gauges after enabling serial control. The response time of the team operator is 46 seconds. It may seem like this amount of time is short, but in reality, for internal operations, it’s enough time for many things to happen. For example, the valve position of the gas pressure control valve can open by 48% suddenly within this time period, resulting in an immediate positive pressure in the furnace that extinguishes the pilot light. If sufficiently “lucky,” the pilot light is not completely extinguished during the sudden opening of the valve; the team operator does not reduce the valve position to its normal value until 3 minutes after switching PIC1233 back to manual mode. During this time, the large amount of gas that flows into the furnace from the main burner will inevitably cause an explosion. Furthermore, since the operator does not yet have the authority to adjust the instrument control parameters, they are not aware of the instrument’s operating status. Therefore, extra care and caution must be exercised when the instrument is first put under manual control or even under automatic control; this is also a basic requirement regarding the operator’s sense of responsibility during the commissioning process. Weekly Case of 2017, Device Guessing Summary Post http://bbs.hcbbs.com/thread-1740967-1-1.html