Nitric Acid Reboiling: Small Leaks Can Lead to Big Problems

Alloy Engineering services the Nitric Acid production industry with high temperature fabrications that center on the main reactor, commonly referred to as the converter. The converter, as its name implies, converts a mixture of ammonia and air into NOx at temperatures of approximately 1,650°F across a precious metal catalyst.  Due to the high temperature, converters are commonly double-walled, water-cooled vessels.  

Over the years, Alloy Engineering has fabricated many converter assemblies in several different configurations. One item that is consistent with all the converter designs is a temperature probe installed just below the main reactor. The temperature probe is inserted into a thermowell, which is installed into a nozzle, that makes the penetration through both walls of the converter to measure the temperature of the process gas. The nozzle is a critical piece in that it allows the access from the outside of the converter vessel to the process.

The Call

Recently, Alloy Engineering received a call that one of our most recent converter assemblies to be installed was leaking process gas into the atmosphere after only 4 short months of service (see figure A).  At first, we thought this was impossible because the previous nozzle lasted 50 years without incident.

Fig A.  Process gas is shown leaking from the packing gland from the thermowell nozzle connection

Alloy needed to act fast because our customer’s plant was going down due to this situation. We needed to provide the parts for the repair and we needed some answers as to why this was happening. After reviewing the drawings to confirm the design, our next step was to pull our QC documents to review the material test reports and confirm the material of construction of the nozzle. The material of the nozzle was confirmed with the MTRs to be machined from the specified material.  The final piece of the investigation was to borescope the nozzle to verify the internal condition. The customer performed the borescopic examination to discover the nozzle was under a severe corrosion attack (see figure B).

Fig B.  Borescope of the nozzle indicates a severe corrosion attack.

What did Alloy Engineering need to do next? We will provide the details of the repair and our insights on the issue in a few weeks. Please check back for the conclusion of this post.