QUESTION:
We are experiencing tube skin thermocouple failures in fired heaters. The skin thermocouples used are K-type (wire gauge 13AWG), mineral-insulated with sheath material of SS 446 (3/8-in. OD & 1/16-in. sheath thickness). Out of the 16 thermocouples installed in one of the heaters, seven of the skin thermocouples became faulty within three months of installation. One thermocouple became bad, and the second one is erratic immediately after turnaround. Four thermocouples are giving erratic readings after an upset in the heater. Skin temperatures dropped from the normal operating of about 925 °F to 596 °F; then rose to 1003 °F in six minutes; then dropped again to 500 °F in another eight minutes.
After a second heater trip, the seventh thermocouple failed.
- What is the experience with N-type thermocouples? Are they field-proven for five years of trouble-free operation?
- What type of sheath material is best suited with N-type thermocouples?
- Is K-type with Nicrobell alloy-sheathed tube skin thermocouple suitable for five years of trouble-free operation in heaters with skin temperature of around 1000 °F to 1050 °F?
- How is the performance of K-type thermocouples with Inconel sheaths?
- Are there any other better-proven skin type thermocouples for the above service?
Ahmed S.Mohideen, Instrument Design Engineer, Shuaiba Refinery, KNPC, Kuwait
ANSWER:
As shown in Figure 1 below, when the surface temperature of tubes is being measured, the thermocouple must always be shielded from furnace radiation. The thermocouple can be attached to the heater tube surface by being furnished with stainless-steel welding pads or by the use of thermocouple attachment blocks. In your application, I would use the attachment blocks, which are provided with multiple holes, allowing the quick replacement for spare thermocouple elements.
FIGURE 1: THERMOCOUPLE PROTECTION