In April 2008, a leak was detected at the flange of a heat exchanger in a coal gasification plant near Terra Haute, Indiana, USA, apparently caused by expansion of the vessel. When two operators attempted to tighten the bolts an explosion occurred, killing both workers, shutting down production and subjecting the company to tens of thousands of dollars in Occupational Safety fines for poor maintenance practices.
This is the kind of ‘nightmare scenario’ that has driven operators of refractory processes all over the world to seek new and better methods for failure prevention. Hot gases will instantly find and push into any gaps in the vessel lining, no matter how small. The combination of extreme pressure and temperature can rapidly and invisibly cause further breakdown and lead to the kind of catastrophic failure that occurred in Indiana.
While avoiding catastrophic breakouts is the ultimate goal, newer technologies offer additional benefits including more efficient maintenance, greater production up-time, increased refractory lining life and better prediction of capital relining or replacement timing.
The core concept of refractory monitoring is measurement of vessel skin temperature. Temperature increases are sure indicators of cracks or other faults in the vessel’s lining, and the amount and rate of increase can be interpreted to determine the scope of damage and its prognosis.
Earlier methods involved placing instruments directly on the skin of the vessel, beginning with tubes filled with Eutectic salt that melted at an alarm temperature, releasing a flow that triggered an alarm. This was later replaced with sensor cables and thermocouples attached to the vessel surface with welded railings, offering improvement in accuracy. However, the ability of this method to detect refractory damage in a timely and effective manner was limited by several factors:
- Readings were taken only once in 250 cm2 – a low density that left most of the vessel surface unmeasured.
- No warning is provided before alarming when threshold temperature is exceeded.
- Cable design temperatures (both operating and peak) are significantly lower than expected vessel surface temperatures.
- Cables can lose contact with vessel shell, making detection impossible.
- Installation requires plant shut down, scaffolding and risks damage to vessels.
- Mechanical damage during installation can cause false hot spot alarms.
- Surface mounted cable and sensors are subject to wear and typically must be replaced after about three years of operation.
The latest technology for critical vessel temperature monitoring – multiple infrared thermal imaging cameras – can effectively solve all of these issues. The most important difference is that the industrial-grade cameras are mounted at a safe distance from the vessel and require no physical contact with the hot surface. No mechanical modifications or surface preparations are required and there is no risk of missed measurements due to broken contact. Installation, expansion and minimally required maintenance can be performed under a hot permit, with no need to access the vessel itself, reducing plant downtime.
With a measurement range of 0 – 500 °C, the cameras can detect all temperatures anticipated on vessel skins, from the typical operating temperature of 280 °C to the design maximum of 460 °C. By positioning cameras around the vessel at all levels, including critical Cone and Dome areas, a thermal imaging system provides complete, continuous surface coverage. Measurements are taken every 16 cm2 – more than 15 times greater density than provided by surface sensor systems. This allows earlier detection of smaller lining faults.
Each camera records over 110 000 individual measurements, ensuring that even small degradations can be detected. Infrared thermal cameras operate consistently and dependably for more than 10 years with almost no maintenance.
All measurements from the cameras are fed by Ethernet or fibre optic link to a control room based graphical software system that provides a wealth of information in addition to instant alarming of hot spots. Data can be available on internal and external networks.
The software provides detailed graphical displays of the system and each individual camera, with up to four configurable areas of interest for each camera. Alarms can be based on maximum or minimum temperature, average temperature or rate of change. Hotspots at pre-alarm stages can be tracked, and alarming is provided for any camera that loses power or communication.
Data from each camera is recorded and trended for up to five years to provide actual performance records for each vessel lining. This information allows plant engineers to schedule maintenance and/or replacement based on statistical analysis of actual conditions rather than generalised standards. Engineers will know if they need to accelerate work on an unexpected crack to prevent catastrophic failure, or if they can safely extend a vessel lining’s life to save money.
For all the reasons discussed above, thermal imaging has become the de facto standard for refractory vessel temperature measurement in coal gasification, petrochemical refining, power generation, chemical and coal processing, waste management, and fertiliser and plastics production.
The known advantages of gasification have accelerated the adoption of this technology throughout the world. Add in the perceived green nature (or renewable tag) to this application, the benefits are clear to see. The burning of waste products in a gasification process and subsequent capture of gas (known as syngas), which can be subsequently combusted at a much higher temperature for greater efficiency, is another plus.
In addition, the high temperatures generated in this process often refine the gas by removing certain corrosive gases such as chlorine, plus corrosive ash containing potassium, producing clean gas from otherwise heavily polluted fuels.
The ever increasing use of the gasification process to generate electricity from waste products has naturally led to the improvement in the technologies used for controlling the process and also to a vast improvement in the refractory monitoring process to cope with the high temperatures involved in this process.
Opposition to the building of gasification plants in urban areas still prevails in certain countries based on the misconception that they are no different to incinerators, emitting large amounts of harmful toxic gases. In fact, a greater risk than any toxic gas release occurs from mismanagement of the technology by not employing a comprehensive refractory thermal monitoring system on the highly pressurised vessels.
Written by Stuart Harris.
Read the article online at: https://www.worldcoal.com/mining/05112012/refractory_critical_vessel_monitoring/