Continuous Galvanizing Line – Nitrogen Knife Optimization


Mining and Metals

Business Issue

A steel company built a brand new plant that involved a continuous hot dip galvanizing and galvannealing line.  A critical section of the line was the nitrogen knives that control the zinc coating weight on the metal sheet. The pressure control system of the nitrogen knives was not able to respond in a stable manner, which did not ensure smooth transitions during grade changes. This issue was causing a large amount of defective material that had to be re-processed.

Project Summary

The project involved conducting a thorough control performance audit of the pressure control system of the nitrogen knives at the zinc pit of the continuous galvanizing line. Based on the results from the audit, an optimization action plan was performed in order to speed up the response of the pressure loops without instability during grade transitions.

In order to assess the health of the equipment and control loops performance, tests were conducted for loop analysis and control. Any field equipment that was identified as being “unhealthy” was repaired and calibrated. The existing control strategy was also reviewed and modified so that the performance of the pressure loops was optimized. Inappropriate final control elements were changed for more suitable design.

On-site optimum loop tuning was performed to generate a smooth grade transition.

Scope Responsibility

Cornerstone Controls was responsible for the following areas:

  • Performance tests on pressure control valves and control loops to assess the health of equipment and control loops.
  • Review of pressure control equipment and modifications.
  • Specify and provide more suitable final control elements.
  • Review and improve pressure control strategy on the PLC system.
  • Dynamic performance tests at different grade changes.
  • Optimum tuning of nitrogen knife pressure control loops.
  • Sustained performance checks of the loops during scheduled plant outages.


  • Optimization project produced significant savings in economic plant performance.
  • Substantial reduction of defective materials during grade transitions.
  • Response time of the pressure loops was decreased by more than 70%.
  • A smooth transition during grade changes was achieved.
  • Production records were attained after the optimization was performed.