Chemical Engineering 3P04 Process Control Tutorial # 2 Learning goals 1.The feedback cause-effect principle 2. Key element in the loop: The control valve

PROCESS InputsOutputs FINAL ELEMENT CONTROLLER SENSOR Desired value WHAT DOES A FEEDBACK SYSTEM DO? FEEDBACK CONTROL makes use of an output of a system to influence an input to the same system. CONTROL (verb): To maintain desired conditions in a physical system by adjusting selected variables in the system. input = cause output = effect

FC 2. The key elements and principles of a feedback loop – Cause and Effect Exercise: The key elements and principles of a feedback loop What is being measured? Is this a valid feedback control loop? cooling

FC cooling 2. The key elements and principles of a feedback loop – Cause and Effect Exercise: The key elements and principles of a feedback loop What is being measured? Is this a valid feedback control loop?

pump L valve sensor pump valve 2. The key elements and principles of a feedback loop – Cause and Effect Exercise: The key elements and principles of a feedback loop What is being measured? Is this a valid feedback control loop?

pump L valve sensor pump valve 2. The key elements and principles of a feedback loop – Cause and Effect Exercise: The key elements and principles of a feedback loop What is being measured? Is this a valid feedback control loop?

pump F valve sensor pump valve 2. The key elements and principles of a feedback loop – Cause and Effect Exercise: You want to control the level, but you can only measure the flow in. What is your strategy? Are you using feedback?

Gas stream Empty vessel P 2. The key elements and principles of a feedback loop – Cause and Effect Exercise: The key elements and principles of a feedback loop What is being measured? Is this a valid feedback control loop?

2. The key elements and principles of a feedback loop – Cause and Effect Exercise: The key elements and principles of a feedback loop What is being measured? Is this a valid feedback control loop?

2. The key elements and principles of a feedback loop – Cause and Effect Exercise: The key elements and principles of a feedback loop What is being measured? Is this a valid feedback control loop?

2. The key elements and principles of a feedback loop – Cause and Effect Exercise: The key elements and principles of a feedback loop Hot process fluid into shell Cooling water into tubes We want to control the hot outlet temperature. Add a sensor and a valve to make this possible.

2. The key elements and principles of a feedback loop – Cause and Effect Exercise: The key elements and principles of a feedback loop Hot process fluid into shell Cooling water into tubes Add a sensor and a valve to make this possible. TC We want to control the hot outlet temperature.

2. The key elements and principles of a feedback loop – Cause and Effect Exercise: The key elements and principles of a feedback loop Hot process fluid into shell Cooling water into tubes We want to control the hot temperature here. Add a sensor and a valve to make this possible.

2. The key elements and principles of a feedback loop – Cause and Effect Exercise: The key elements and principles of a feedback loop Hot process fluid into shell Cooling water into tubes We want to control the hot temperature here. Add a sensor and a valve to make this possible. T We can measure this temperature, but the heat exchanger does not influence it. We cannot control the temperature using the flows in this figure! (We can look upstream to see if/how it can be controlled.)

2. The key elements and principles of a feedback loop – Cause and Effect Exercise: Typical disturbances Disturbances affect the important (controlled) variable, but we cannot influence disturbances Identify disturbances that will affect the reactor temperature, T5

Now you can be original. 1.sketch a process system that you have studied in your engineering courses, 2.place a sensor and valve, and 3.determine whether the sensor measurement can be controlled in feedback by adjusting the valve. 4.Identify several disturbances that affect the controlled variable 2. The key elements and principles of a feedback loop – Cause and Effect Exercise: The key elements and principles of a feedback loop

FC cooling Valves: How do we affect fluid flow? This control system requires a flow measurement, and we understand sensors well already. We also need equipment that we can adjust to achieve a desired flow rate. The most common adjustable variable for the process industries is a valve. The valve provides an adjustable resistance to flow through the pipe. Centrifugal pump (not positive displacement)

F cooling Valves: How do we affect fluid flow? This control system requires a flow measurement, and we understand sensors well already. We need equipment that we can adjust to achieve a desired flow rate. Other possibilities for affecting flow rate. Variable speed centrifugal pump (lower energy that pump-valve) Variable speed positive displacement pump.

Other fluids: water, nitrogen, tree pulp and water, blood, sewage, food products (yogurt), highly pure pharma products, hazards (isocyanates) and just about anything else that flows! Valves: What types of fluids do we regulate with valves? “FCC” Fluid Catalytic Cracker

Adjusting valves: Do you believe in automation? Do we run around the plant to adjust the valves when required? Process pictures courtesy of Petro-Canada Products

Adjusting valves: Do you believe in automation? Central control room Overview of entire process Make immediate adjustment anywhere Safe location History of past operation Process pictures courtesy of Petro-Canada Products

We will concentrate on control valves used to “modulate” the flow, i.e., achieve value of flow between maximum (fully opened) and minimum (fully closed) Valves: How to we “actuate” or open and close valves? (fully open or closed)

Valves: What are the two main features? The actuator provides the ability to change the flow resistance, i.e., the size of the opening for flow. The most common actuator is a pneumatic diaphragm. The body of the valve defines the flow path and is selected to achieve the desired fluid flow behavior. Sampson Valves

Valves: What are important features for process control? Capacity Range Failure position Gain Pressure drop Precision Linearity Consistency with process environment Dynamics Cost These are explained in the “pc-education” site. Most engineers select valves, do not design them.

Valves: What are important features for process control? Pressure drop = Capacity = Range =

Valves: What are important features for process control? Pressure drop = The purpose of the valve is to create a variable pressure drop in the flow system. However, a large (non-recoverable) pressure drop wastes energy. Capacity = The maximum flow rate through the flow system (pipes, valves, and process equipment) must meet operating requirements. Range = The range indicates the extent of flow values that the valve can reliably regulate; very small and large flows cannot be maintained at desired values. Range is reported as ratio of largest to smallest.

Valve Actuator: Why is the failure position important? The failure position is the position of the valve plug when the air pressure is zero (atmospheric). This is typically either (fully) open or (fully) closed. Feed Methane Ethane (LK) Propane Butane Pentane Vapor product Liquid product Process fluid Steam F1 F2F3 T1 T2 T3 T5 T4 T6P1 L1 A1 L. Key What are the best failure positions? P  1000 kPa T  298 K best = safest v1v2 v3 v4 v5

Valve Actuator: Why is the failure position important? The failure position is the position of the valve plug when the air pressure is zero (atmospheric). This is typically either (fully) open or (fully) closed. Feed Methane Ethane (LK) Propane Butane Pentane Vapor product Liquid product Process fluid Steam F1 F2F3 T1 T2 T3 T5 T4 T6P1 L1 A1 L. Key What are the best failure positions? best = safest fo fc Must consider upstream and downstream processes v1v2 v3 v4 v5

24% thumb.jpg&imgrefurl= &h=150&w=113&sz=5&hl=en&start=83&tbnid=3ZKYixrpLJ5wTM:&tbnh=96&tbnw=72&prev=/images%3Fq%3Dball%2Bvalves,%2Bpr ocess%2Bcontrol%26start%3D80%26ndsp%3D20%26svnum%3D10%26hl%3Den%26sa%3DN Valve Body: We match the valve body to the fluid type and process needs? Butterfly Globe Gate Ball Fluids: water, nitrogen, tree pulp and water, blood, sewage, food products (yogurt), highly pure pharma products, hazards (isocyanates), polymer melts, and just about anything else that flows!

24% thumb.jpg&imgrefurl= &h=150&w=113&sz=5&hl=en&start=83&tbnid=3ZKYixrpLJ5wTM:&tbnh=96&tbnw=72&prev=/images%3Fq%3Dball%2Bvalves,%2Bpr ocess%2Bcontrol%26start%3D80%26ndsp%3D20%26svnum%3D10%26hl%3Den%26sa%3DN Valve Body: We match the valve body to the fluid type and process needs? Butterfly Globe Gate Ball Question: Would a globe valve be a good choice for affecting yogurt flow? Answer: No! The globe valve has many small “dead ends” where food could collect and not be removed by cleaning fluid.

24% thumb.jpg&imgrefurl= &h=150&w=113&sz=5&hl=en&start=83&tbnid=3ZKYixrpLJ5wTM:&tbnh=96&tbnw=72&prev=/images%3Fq%3Dball%2Bvalves,%2Bpr ocess%2Bcontrol%26start%3D80%26ndsp%3D20%26svnum%3D10%26hl%3Den%26sa%3DN Valve Body: We match the valve body to the fluid type and process needs? Butterfly Globe Gate Ball Question: Would a butterfly valve be a good choice when tight closing is required? Answer: No! The manufacturing would almost never provide a perfect fit.

24% thumb.jpg&imgrefurl= &h=150&w=113&sz=5&hl=en&start=83&tbnid=3ZKYixrpLJ5wTM:&tbnh=96&tbnw=72&prev=/images%3Fq%3Dball%2Bvalves,%2Bpr ocess%2Bcontrol%26start%3D80%26ndsp%3D20%26svnum%3D10%26hl%3Den%26sa%3DN Valve Body: We match the valve body to the fluid type and process needs? Butterfly Globe Gate Ball Question: Would a ball valve be a good choice for low non-recoverable pressure drop? Answer: No! The flow follows a tortuous path and experiences extreme turbulence.

24% thumb.jpg&imgrefurl= &h=150&w=113&sz=5&hl=en&start=83&tbnid=3ZKYixrpLJ5wTM:&tbnh=96&tbnw=72&prev=/images%3Fq%3Dball%2Bvalves,%2Bpr ocess%2Bcontrol%26start%3D80%26ndsp%3D20%26svnum%3D10%26hl%3Den%26sa%3DN Valve Body: We match the valve body to the fluid type and process needs? Butterfly Globe Gate Ball Typical purchase cost ~ $ for a 4”pipe globe or ball valve with actuator (installation extra)

For details on many valves, including principles and advantages and disadvantages, we can access the pc-education WEB site!