Chlorine dioxide (ClO2) gas is unstable and decomposes, sometimes violently, into Cl2 and O2 (with a 50% increase in gas volume). When storing chlorine dioxide (ClO2), it is commonly kept in CHILLED WATER because its solubility in water significantly increases at lower temperatures, making it more stable and allowing for higher concentrations to be safely stored; essentially, cold water helps to preserve the ClO2 in solution. However, ClO2 is very reactive and breaks down quickly in the presence of sunlight or unchilled water. In water, it reacts to form chlorite ions.
In this process, it is stored as 8 grams per liquid (GPL) in chilled water before use in a pulp-bleaching plant. The decomposition (explosion) is worse as the water temperature rises.
A “top hat” (inverted bucket) water seal was weighted to 250lb to protect the storage tanks from overpressure. It was designed to blow clear in an explosion. That summer, we had many hot days, and we experienced a few “puffs,” which lifted the bucket off and vented the tank into the atmosphere. To catch the weight and bucket, we designed and installed the steel cage shown in the picture below. NOTE: This was a safety-critical design change!
When we eventually got our first real decomposition, the bucket couldn’t blow clear, and the expanding gases had to follow an S-shaped path around the water seal annulus. See the marks on the weights.
The tank was an API 650 mild steel tank lined with polyester resin which was sprayed on after the steel tank internals had been sandblasted. We used a climbing platform stage to raise and lower quickly. To support this platform, we needed to stiffen the roof with 6×3 channels, which are also shown in the pictures. This made the roof/wall joint stronger than the wall/floor joint, which meant the tank failed at the base, which is not the design basis of the API 650 code.
Lessons learned…
Many Thanks to Paddy Durham @ Gauge Refrigeration (Sponsor of SAFTENG) for sharing this lesson-learned event.
Contact Paddy for your “down-under” refrigeration needs:
Mobile: +61(0)4 2222 9828
Email: [email protected]
Website: www.ammonia.solutions