How can it be determined if the Element Well has failed?
If flush gas pressure is properly set and the flowmeter is reading the proper flow rate then the Element Well, a pressure-containing component, is intact. A sudden decline of the thermocouple output signal indicates that it is likely that the inner Element Well has failed. Breakage of the Element Well immediately permits the highly corrosive vessel gases to come in direct contact with the thermocouple wires and hot junction. Corrosion immediately begins to attack the thermocouple wires and corrosion failure begins. It is likely that the HRW Refractory Well has failed permitting physical damage to the inner Element Well. The Refractory Well failure is usually the result of severe thermal shock such as from a steam or liquid injection or physical damage to significant shifting/breakage of the refractory. Slow degradation of the thermocouple output indicates that over time, the reactor gases have corroded and poisoned the thermocouple element. This slow degradation is a result of the reaction gases entering the Element Well either through a small crack in the well or in its seal. For this to have occurred, the purge pressure and/or the purge flow probably were not operating properly for many months. And, it is probable that the Element Well is intact. The purge itself is an indicator of the condition of the Element Well as follows: If the purge flow has ceased and cannot be restored, it is likely that the outlet purge path has become clogged with solid sulfur. This sulfur has likely precipitated from the sulfur gases that have leaked into the purge pathway through a fault in either the Element Well or the Element Well seal. This may happen if the purge pressure is not maintained. Sulfur and other gases slowly migrate in through the wall of the Element Well over a period of time by diffusion. This is a normal phenomenon and normally causes no harm since the purge flow constantly carries these corrosive elements away from the T/C before any corrosion occurs. Failure due to this migration of corrosives is likely to have occurred if the purge flow or the purge pressure has not been maintained. If the purge pressure can be maintained and there is no flow (i.e., valve closed), the Element Well is probably intact. If the purge pressure reads approximately the same as the vessel pressure and cannot be effectively elevated, the Element Well has failed and purge gas is flowing into the reactor. In this case, there will be no purge flow to the flowmeter since the outlet vent port will be clogged by sulfur (see below). In this case, repair of the HTP/HTX will have to be withheld until the reaction is shutdown, and the HTP/HTX assembly removed and repaired.