{"id":1544,"date":"2018-07-09T16:36:48","date_gmt":"2018-07-09T23:36:48","guid":{"rendered":"http:\/\/rccostello.com\/wordpress\/?p=1544"},"modified":"2018-07-09T16:44:50","modified_gmt":"2018-07-09T23:44:50","slug":"flashbacks-thermal-oxidizers","status":"publish","type":"post","link":"https:\/\/www.rccostello.com\/wordpress\/hazards\/flashbacks-thermal-oxidizers\/","title":{"rendered":"Flashbacks From Thermal Oxidizers and How Redesign Prevents Them"},"content":{"rendered":"

Introduction<\/h1>\n

Flashbacks from Thermal Oxidizers cost the Chemical Process Industries millions of dollars in equipment damage and plant down time.<\/p>\n

Why does this happen? Is it the manufacturers of the thermal oxidizers<\/span> fault?\u00a0 It is normally not the manufacturers fault.\u00a0 The thermal oxidizers operating manual normally says that the client must follow NFPA<\/a> guidelines.<\/p>\n

NFPA 86 says that the operator must insure that the gas stream flowing to the thermal oxidizer never gets above 25% of the Lower Flammability Limit (LFL).\u00a0 If the client has an LFL meter they can operate up to 50% of the LFL.\u00a0 Most plants don’t know where they are operating with respect to % LFL.<\/p>\n

Process upsets is the leading cause of flashbacks where the gas composition goes above LFL.<\/p>\n

System Redesign Improves Safety and Productivity<\/h2>\n

A vertical thermal oxidizer was used in a Gulf Coast chemical plant to incinerate vapors from tank-truck cleaning, tank farm and batch distillation operations.<\/p>\n

Over a period of 2 years, a number of explosions had occurred at the thermal oxidizer.\u00a0The last explosion was the most severe, resulting in the destruction of the spark-proof booster blower that drew air from the tank-truck filling and cleaning operations.<\/p>\n

\"flashbacks\"<\/a>
Thermal Oxidizer<\/figcaption><\/figure>\n

The normal procedure was to blanket the incoming truck with inert gas.\u00a0A special manway cover with connections for inert gas, inert gas exhaust, exhaust air for drying, and cleaning solvent was fitted to the tank truck.<\/p>\n

This purged out all the air containing flammable hydrocarbons.\u00a0Once this inert purge cycle was completed, cleaning solvent was circulated via the manway to a cleaning head.\u00a0The solvent flowed from the discharge of the truck to a tank and was then pumped back to the cleaning head.<\/p>\n

The final cycle consisted of an air drying purge.\u00a0The explosions had all occurred during the air drying purge of the truck.<\/p>\n

In order to solve this problem, the chemical plant made several flashback-prevention modifications.\u00a0 These included the installation of flame arresters, averaging pitot tubes, thermal detectors, a dilution air control valve, draft electronic DP cells, a low-pressure selector, a square-root extractor and a three-way solenoid valve.<\/p>\n

\"flashbacks\"<\/a>
Thermal Oxidizer – After engineering changes<\/figcaption><\/figure>\n

Flashback Mechanism<\/h1>\n

As the drying air pushes out the flammable gas mixture, it moves down the pipe to the booster\u00a0blower and on through more piping.\u00a0If the linear velocity in the pipe is too low, a flashback that results in an explosion can occur.<\/p>\n

Flashbacks occur when the stream velocity near the wall falls below the burning velocity.<\/p>\n

The thermal oxidizer was a vertical unit with a gas-fired burner in the center.\u00a0A donut ring with six nozzles that injected vapors from plant operations surrounded the burner.<\/p>\n

There was one inlet nozzle to the donut ring from the discharge of the booster blower.\u00a0This was later modified to four equally spaced inlets to the donut ring to provide better distribution to the six nozzles inside the thermal oxidizer.<\/p>\n

After the last and most serious explosion, a redesign of the system was undertaken with a primary goal of maintaining high average gas velocity in the pipeline.\u00a0The redesign improved\u00a0the safety of the system through the addition of a number of important features.\u00a0They include:<\/p>\n