User name stevensm
Log entry time 12:32:44 on June26,2004
Entry number 126962
keyword=Septum plc software element failure
I was called at about 0800 regarding the left Septum helium level, which was dropping below 50 per cent. A few questions revealed that the J.T. valve was reading back a negative value. I thought that the problem was the result of 0 offset drift of the module that converts the AC lvdt signal into a DC level that can be read by the plc. I came in and entered the hall to manually reset the valve by going to local control mode and jogging the valve slightly open, causing the readback to develop a positive value, and then returning the valve to remote control. After I did that, I walked away from the controls to accompany the ARM to another location. As we passed the pivot, I heard the helium relief valve chattering on left Q1, and I realized that something had gone wrong. I ran back to the septum console, and saw that the J.T. valve had "run away" to 100 percent open. I immediately put the valve back in local mode and closed it down to a value that would not trip the end station refrigerator. Some additional testing revealed that none of the Septa cryogenic valves were obeying the correctly set "min" and "max" values in auto mode. Apparently the Septum plc program had become corrupted, disabling the valve limit function. All other program functions were operating normally. This is the first time that I have seen a plc program become corrupted, disabling only one program element, and allowing the remaining program to continue operation. Up to now, a program corruption resulted in the program being halted. I ramped the magnets to 0 amps, and replaced the plc cpu with a new, preprogrammed one. All program elements functioned normally, and the J.T. valves obeyed the limits. I exited the hall at about 1130.