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Control the burn: Part 1

Published by
World Fertilizer,


Scott W. Sexton, Haldor Topsoe Inc., discusses how digital furnace firebox monitoring technology can extend a machine's lifecycle and improve personnel safety.

Primary reformers are critical to success in the ammonia process. Process upsets from heater problems also disrupt ammonia economics. Process upsets from fires, process tube failures, poor burner operation, and mechanical equipment failures also affect business results. These problems have traditionally required plant personnel to work directly with the reformer firebox to evaluate equipment and process status. However, this troubleshooting method is both risky and subjective.

Digital furnace firebox monitoring technology reduces dependence on human interaction with fireboxes. A system of image collectors annually provides millions of images and data points. Alarms activate when temperatures go out of range or when burners malfunction. Images and data are stored in a database for easy retrieval. Continuous firebox monitoring technology minimises furnace firebox personnel safety risks and personnel are free to perform other important work, such as troubleshooting, diagnoses, data analysis, collaboration with colleagues, and planning.

The Topsoe Furnace Manager (TFM) is the technology designed to capture images imbedded with data. Using a class 1, division 2 system, images are collected using multiple image collectors, which are permanently mounted on the firebox. Digital images are analysed immediately and stored images and data are retained for additional processing. Alarms warn plant operators when limits are exceeded in regions-of-interest areas that have been established with the operator’s insight. Experts outside of the plant can also see problems as they occur and collaborate during troubleshooting.

Asset safeguarding

Preserving high cost assets is a main priority in ammonia plant operations. Reformer catalyst tube metal creep occurs over time, when tubes overheat. The continuous monitoring of a firebox with images shows tube overheating as it occurs, and allows operators to take corrective action based on alarms. Images of poor burner performance provide information to help plan burner maintenance activities. TFM’s data collection capability helps avoid costly shutdowns from tube failure, protecting assets as shutdown is avoided.

Example calculation: For a TFM cost of US$500 000, a payback is realised in 7.5 days by avoiding a plant outage with associated costs and lost margin during downtime shown below.

  • Startup/shutdown costs = US$25 000.
  • Repair costs = US$100 000.
  • Margin loss = US$50 000/day.

There is a potential payback of 7.5 days.

Reformer tube condition monitoring

Time and temperature are fundamental contributors to reformer tube life. The Larson-Miller Parameter (L.M Parameter) is the relationship of time to rupture for metals, with respect to temperature of operation. This can be expressed in the following equation:

P(L.M.) = T[log tr + C]

 

  • T = temperature, K or °R.
  • tr = stress-rupture time, h.
  • C = constant, usually of order 20.

According to the L.M. parameter, at a given stress level the log time to stress rupture, plus a constant (~20) multiplied by the temperature remains constant for a given material. Tube life based on metal creep rate, due to differences in temperature for a given stress, are related. Typically, a 5% increase in absolute temperature will increase the rate of creep by a factor of ten.

Accurately measuring temperature in operation for metal surfaces in a furnace firebox is difficult. The continuous firebox monitoring of reformer tube temperatures, captured with digital images, provides a history of tube temperatures. Tube failure incidents can be avoided using this knowledge. Incident investigations can use the temperature history to rectify reoccuring problems.

The TFM also has asset protection capabilities for startups and shutdowns of the firebox. When typical flame sensing systems are not active after the autoignition temperature is reached, the TFM continues to monitor the furnace through both startup and normal operation.

TFM system capabilities

TFM’s system capabilities provide exponentially more data points than, human monitoring. Over 500 000 images per year per collector and more than 5 billion data points per year per image collector are gathered. Imbedded data includes firebox tube metal temperatures (with alarms), burner characteristics, and general mechanical integrity information, such as tube bowing or refractory damage. This information is remotely accessible and is stored in an easy-to-use database for future reference, 24/7.

Read the article online at: https://www.worldfertilizer.com/special-reports/04012019/control-the-burn-part-1/

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