Chemical Engineering 3P04 Process Control Tutorial # 6 Learning goals 1.Learn basic principles of equipment in a control loop 2.Build understanding of feedback loop

Loop Elements: Sensor  Computer  Valve Why must we transmit these signals? What is wrong with this picture? Central control room controller

Loop Elements: Sensor  Computer  Valve Why must we transmit these signals? Transmitted to/from Central control room Displayed locally Manual valves

Loop Elements: Sensor  Computer  Valve Why must we transmit these signals? Transmitted to/from Central control room Safety related or time critical Used for control Important for quality, reliability, performance Trouble shoot and monitor longer-term behavior Displayed locally Manual valves

Loop Elements: Sensor  Computer  Valve Why must we transmit these signals? Transmitted to/from Central control room Safety related or time critical Used for control Important for quality, reliability, performance Trouble shoot and monitor longer-term behavior Displayed locally Used for local maintenance/ operation Not safety or time critical Manual valves Infrequently adjusted Not safety or time critical

Central control room Loop Transmission: Why learn about it? We need to understand the “closed-loop” We select equipment to achieve required performance We “trouble-shoot” problems These are our “senses” and our “handles” ?

Central control room Class workshop: What are general features that we seek for the transmission of signals from the sensor  computer and from the computer  valve? Hint: We have lists of features for sensors and for valves already Loop Transmission: Why learn about it?

Loop Transmission: What features do we seek? Accuracy and reproducibility Noise sensitivity Reliability Dynamics Distance Interoperability Safety Diagnostics Cost Class Workshop: Explain these features Typically much better than sensors and valves

Dynamics: Transmission delays are “in the feedback loop”. Delays in transmission are as bad as delays in the process. Good news: Electronic transmission is very fast compared with other elements in the loop. Caution: Old transmission systems using air pressure (pneumatic signals) can be slow for distance over 50 meters. Loop Transmission: What features do we seek?

Distance: Process plants can extend over 1000’s of meters. The transmission must be capable of these distances. Good news: Electronic transmission via “hard wire” has a large enough range. Caution: Pneumatic signals have limited range. Note: Telemetry is not now used for process control. It is used for monitoring remote equipment (wells) Loop Transmission: What features do we seek?

Interoperability When you purchase one loop element from a company, do you want to buy all other elements from the same company for the life of the plant? NO! Standards are recognized so that equipment from various manufacturers can be used interchangeably. This was easy for older, analog technology. Standards are available for digital technology. Loop Transmission: What features do we seek?

Loop Transmission: Two typical designs. Life is exciting during a revolution! Analog transmission Continuous electronic signal Digital transmission Digital numeric representation Older technology, but widely employed and will be in use for decades Newer technology, generally used in new facilities and when replacing analog technology

Loop Elements: A Typical Analog Loop Heating medium fc i/p Digital controller Digital number Thermocouple temperature sensor, mV signal transmitter Analog signal transmission (4-20 mA) Digital number Analog signal transmission (4-20 mA) Pneumatic signal transmission (3-15 psig) Valve stem position 0-100%) D/A A/D Analog to digital conversion Digital to analog conversion

Loop Elements: A Typical Analog Loop

Loop Elements: All digital transmission  -Processor at every sensor and valve

Loop Elements: Life is exciting during a revolution! Why have a micro-processor at every sensor and valve? ValveFlow Sensor

Loop Elements: Life is exciting during a revolution! Why have a micro-processor at every sensor and valve? Valve Flow Sensor Improve accuracy Correct for density changes Diagnose performance and warn when degradation begins Calibrate quickly Power supply error

Loop Elements: Life is exciting during a revolution! Why have a micro-processor at every sensor and valve? Valve Diagnose performance and warn when degradation begins Valve sticking Air pressure low Signal not received Flow Sensor Improve accuracy Correct for density changes Diagnose performance and warn when degradation begins Calibrate quickly Power supply error

Loop Elements: Life is exciting during a revolution! Note that both have two-way communication

Loop Transmission: Two typical designs. Life is exciting during a revolution! Analog transmission Continuous electronic signal Digital transmission Digital numeric representation Older technology, but widely employed and will be in use for decades Newer technology, generally used in new facilities and when replacing analog technology

Chemical Engineering 3P04 Process Control Tutorial # 6 Learning goals 1.Learn basic principles of equipment in a control loop 2.Build understanding of feedback loop Let’s look at some examples from Tutorial #7

FC Flow Control: Centrifugal pump with constant speed (rpm) Orifice plate sensor Globe valve (a)

FC Flow Control: Positive displacement pump Orifice plate sensor Butterfly valve (b)

FC Flow Control: Centrifugal pump with variable speed driver Orifice plate sensor (c) c)The pressure increase from a centrifugal pump depends on the rotor speed – the fast the rotation, the higher the pressure. A variable speed motor can be adjusted to achieve the desired flow rate, which is more energy efficient than adjusting a variable pressure drop (valve) in the pipe. Increasing the speed increases the flow rate. Yes, feedback control is possible.

PC Flows into the pipe Flows exiting the pipe Pressure Control: Manipulate one exiting flow Flexible diaphragm Globe valve (h) h)The pressure in a pipe can be controlled by adjusting one of the flows. We can prove this by formulating a dynamic material balance. Naturally, successful control can only be achieved over a range of flows; when the valve is either fully opened or closed, control is no longer possible. Yes, feedback control is possible. A pressure sensor that deflected because of pressure and converted the deflection to an electronic signal is used in such circumstances. A globe valve is acceptable here.

PC Pressure Control: Manipulate exiting flow from vessel Piezoelectric Globe valve (i) i)The pressure in a vessel can be controlled using the exit (or inlet) flow. The principles are identical to the previous design. Yes, feedback control is possible. A piezoelectric sensor generates a small electronic signal when a pressure is applied; it can be used in this application.

LC Composition Control in isothermal CSTR Manipulate the inlet flow Control C B Ball valve Level maintained constant by LC AC CBCB Reaction: A  B  C (k)

CB can be controlled; increase the flow rate to increase CB CB cannot be controlled by adjusting F CB can be controlled; decrease the flow rate to increase CB

k) The conversion (or extent of reaction) depends on the space time in the reactor. Clearly, the flow rate affects the space time. However, this process is more complex, some might say, “Tricky.” For control to be successful, we need to have a controller gain that has a non- zero gain. The gain can be either positive or negative, but it should not change sign! What happens in this example? The figure below shows that the gain changes sign, because of the two reactions. In two regions, control is possible, but would only function within the region. At the maximum C B point, control is not possible by adjusting the feed flow rate. While control is possible, great care would have to be employed when implementing. A different manipulated variable, such as feed concentration should be investigated. A ball valve would be an acceptable choice. LC AC CBCB