Almost every centrifugal pump requires some form of minimum flow protection. Low flow rate can damage the pump through high temperature or flow instabilities that cause excessive vibration. The simplest form of control is a simple orifice plate that dumps a continuous flow rate back to the pump suction. This is acceptable for small pumps,…
The other strategy is adaptive tuning. Here, we assume that the DCS can provide three separate PID tuning constants. The zones, shown in figure below, are selected to span the operating conditions from 20,000 kg/h (gain of 6) to 110,000 kg/h (gain of 0.8), with a uniform ratio from one zone to the next. Now that we have…
In Part 2 we saw that our temperature control process behaved very badly at the low flow rate associated with startup. The root cause of the poor behaviour is that the process behaviour no longer works well with the PID controller. In this section we will look at how to make the process behave better.…
In Part 1, we saw that the hot oil flow rate through the heat exchanger was nearly linear with valve position. Our experience with the exchanger is that the one set of PID tuning parameters is wonderful at one operating point, but fails at another. Why? Good control requires repeatable performance from the PID algorithm…
Heat exchanger temperature control is common to all process industries. However, many temperature control loops have a difficult time maintaining setpoint. The controller performance is excellent at normal plant rates, but is unacceptable at lower rates for plant startup. Sometimes the temperature control loop sees frequent visits from the Control Engineer for re-tuning, or perhaps…
Many of us have experience with processing facilities of some kind. By this, I mean petrochemical plants, refineries, food processing facilities and distilleries (to name a few). How many of us are privy to the day-to-day frustrations from the Operating teams? The seals on P-123 have failed again. I thought Engineering already put in tougher…
Happy New Year! I created a video series that describes how process hydraulic behaviour can cause perfectly good PID controllers to become unreliable. Facilities can increase capacity by addressing these issues, often with no additional capital. Part 1: Control Problems The symptoms of a control problem are described. Part 2: Tuning Strategies Two different tuning…
Thanks for the opportunity to present OTSG water side dynamics to the class. The presentation slides can be downloaded here. Check out the OTSG tab on my web page: the link to the YouTube training videos are at the bottom of the page. The explanation for changing the BFW temperature by starting/stopping the pump is…