«

»

Jul
10

Is This the Time – Part 2?

I remember my very first time …. for simulation ….. for processes …. for chemical not biological. I was working as lead instrument and electrical engineer on what was then the largest acrylonitrile and hydrogen cyanide plant in the world and I decided maybe the huge compressor that provided the air for reaction with propylene and ammonia was worth a closer look. If the compressor tripped a whole day’s production was lost plus startup was the most dangerous mode of operation. Then there were the consequences of surge. The amount of energy from flow reversals was enormous. Surge cycles reduced the compressor efficiency and could damage the rotor from excessive vibration. Even though it wasn’t in my job description, I decided as an extracurricular activity to write a dynamic model of the compressor. I learned that surge can occur in seconds after crossing the surge set point and that once the compressor got into surge you could not rely upon feedback control could not get it out. The nearly full scale flow reversals every couple of seconds was just too much for a flow controller. I learned the importance of a feed forward signal from reactor feed valve position and an open loop backup. After a year in field construction for installation and startup, I was invited either to do an encore for the next plant in England or based on my simulation program venture take a job in engineering technology. Even though the overseas assignment was tempting, the chance to work with some of the brightest minds in process modeling and control (Henry Chien, Larry McCune, Bob Otto, Terry Tolliver, and Vernon Trevathan plus the legacy of Joel Hougen and Ted Williams) was too much to pass up. Dynamic simulation became the most important tool for me for more than 30 years.

I have used models of compressors many times in some pretty exciting situations. No other disturbance is faster. The precipitous drop in flow that precedes surge occurs in 0.05 seconds. To simulate these dynamics you need a momentum balance besides a material and energy balance as shown in the ACSL program on page 121 of the E-book: http://www.modelingandcontrol.com/compressorcontrolstudent/page-121.asp. The phenomenon was worse than falling off a cliff with a bungee cord with a perpetual rebound.

For a summary of some my compressor startup experiences check out “Compressor Surge Control – Traveling in the Fast Lane” on page 19 of the E-book: http://www.modelingandcontrol.com/FunnyThing/page-19.asp