The loops with the most severe oscillations listed in order from biggest amplitude to smallest amplitude are pH loops, level loops, flow loops, pressure loops, batch temperature loops, heat exchanger temperature loops, and column temperature loops.
The following is a list of the sources of product quality oscillations in the approximate descending order of frequency of occurrence based on my experience. I have even offered my best guess in parentheses as to the percentage of applications that can be tracked to these root causes for chemical and biochemical products. You may wonder why pH loops didn’t make the top of the list since it has the most severe oscillations. The main reason pH loops are down the list is that most pH loops are in waste treatment (WT). Also, the pH loops in reactors and bioreactors tend to have much lower process gains than WT pH loops and some process regulation from reagent consumption. Interacting temperature loops on furnaces, reformers, and reactors are severe problems but are near the bottom of the list for applications for specialty chemicals and biochemical products because multi-zone or profile temperature control are more prevalent in the petroleum, petrochemical, and bulk chemical industries. The following list is for normal operation of loops with good valves and does not consider oscillations that originate from the startup and shutdown and failure of equipment. Next week we will see the implications of “not so good” valves.
(1) Too much reset action in level loops on surge and feed tanks (40%)
(2) Discontinuities at split range point for pH, pressure, and temperature loops (20%)
(3) Interacting pressure and flow loops on headers (10%)
(4) Too much reset action in overhead pressure loops on columns and vessels (10%)
(5) Set point response of batch temperature loops (5%)
(6) Interacting temperature loops for 2 point composition control of columns (5%)
(7) Interacting temperature loops on furnaces and reactors (5%)
(8) Set point response of batch pH loops (5%)