Technical material
Introduction Process engineers are concerned about safety, and we spend a lot of our time considering overpressure protection. A simple example is thermal expansion of a blocked in heat exchanger. The cold cold side warms up, the cold fluid expands a tiny bit and the exchanger ruptures. The solution is to install a small PSV…
I was digging through an old hard drive and found something that made me smile. It was a GIF animation that I created 20 years ago (give or take a few weeks). No, not something with the qualities to go viral. This was the continuation of simulation work that I did for my PhD thesis.…
A variable frequency drive is often used to control the flow rate for a centrifugal pump. We can save power by adjusting the pump speed to control flow rate. But this method does not work in all applications. This video explains where a VFD works, and where it doesn’t.
Pipeline supply pumps must meet an interesting set of operating conditions. The flow rate is generally low and the required discharge pressure is high. This requires either a small multistage centrifugal pump or a positive displacement pump, such as a plunger pump. A plunger pump with a VFD is quite suitable for delivering fluid to a pipeline. However,…
Controller tuning can a bit of a trial-and-error exercise. There is often no trust that the PID tuning settings recommended by the Engineer will be successful. This is where some modelling is useful. If the simulation results are acceptable, then the real application stands a good chance of working. If the simulation results are not…
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…