Chemical Engineering 3P04 Process Control Tutorial # 2 Learning goals

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

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

3 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? FC cooling

4 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? FC cooling

5 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? sensor valve L pump pump valve

6 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? valve L sensor Is this a valid feedback control loop? pump pump valve

7 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? sensor F valve pump pump valve

8 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? P Gas stream Gas stream Empty vessel

9 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? v8 F2 F1 T1 T3 T4 v3 T5 F3 T6 F4 F5 P1 v1 v2 L1 v5 v7 v6 T7 L2 T2 T8 Hot Oil T9 Hot Oil F6 v4

10 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? v8 F2 F1 T1 T3 T4 v3 T5 F3 T6 F4 F5 P1 v1 v2 L1 v5 v7 v6 T7 L2 T2 T8 Hot Oil T9 Hot Oil F6 v4

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

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

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

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

15 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 v8 F2 F1 T1 T3 T4 v3 T5 F3 T6 F4 F5 P1 v1 v2 L1 v5 v7 v6 T7 L2 T2 T8 Hot Oil T9 Hot Oil F6 v4

16 2. The key elements and principles of a feedback loop Cause and Effect Exercise: The key elements and principles of a feedback loop 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

17 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. Centrifugal pump (not positive displacement) FC cooling The most common adjustable variable for the process industries is a valve. The valve provides an adjustable resistance to flow through the pipe.

18 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. F Other possibilities for affecting flow rate. cooling Variable speed centrifugal pump (lower energy that pump-valve) Variable speed positive displacement pump.

19 Valves: What types of fluids do we regulate with valves? FCC CO 2, N 2, H little catalyst riser Reacts (cracks) petroleum to more valuable products; by-product is carbon on catalyst Fluid Catalytic Cracker Burns carbon to regenerate catalyst regenerator Hot oil air Catalyst & steam Feed oil 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!

20 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

21 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

22 Valves: How to we actuate or open and close valves? Table Most common applications of valves in the process industries. Name Symbol Power Typical process application Block Manual (by person) These valves are usually fully opened or closed, although they can be used to regulate flow over short periods with a person by the valve. Safety Relief Self-actuated (the difference between process and external pressures results in opening when appropriate) These are located where a high (low) pressure in a closed process vessel or pipe could lead to an explosion (implosion). On-off M Electric motor These valves are normally used for isolating process equipment by ensuring that flows are (fully open not possible. They can be operated by a person or closed) in a centralized control room, who can respond quickly regardless of the distance to the valve. Throttling control Usually pneumatic pressure These valves are typically used for process control, where the desired flow rate is attained by changing the opening of the valve. 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: What are the two main features? Sampson Valves The actuator provides the ability to change the flow resistance, i.e., the size of the opening for flow.

23 Valves: What are the two main features? Sampson Valves 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.

24 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.

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

26 Valves: What are important features for process control? 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. 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.

27 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. What are the best failure positions? best = safest T6 P1 v5 Vapor product Feed Methane Ethane (LK) Propane Butane Pentane T1 F1 F2 Process fluid v1 T2 T4 F3 Steam T5 T3 v2 v3 A1 L1 L. Key v4 P 1000 kpa T 298 K Liquid product

28 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. What are the best failure positions? best = safest T6 P1 fo v5 Vapor product Feed Methane Ethane (LK) Propane Butane Pentane T1 F1 fc F2 Process fluid v1 T2 T4 fc F3 Steam T5 T3 v2 v3 A1 L1 L. Key v4 Must consider upstream and downstream processes Liquid product

29 Valve Body: We match the valve body to the fluid type and process needs? 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

30 Valve Body: We match the valve body to the fluid type and process needs? 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

31 Valve Body: We match the valve body to the fluid type and process needs? 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

32 Valve Body: We match the valve body to the fluid type and process needs? 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? http://www.

selection=double+flanged+butterfly+valve&m=1 Typical purchase cost ~ $2000-3000 for a 4

33 Valve Body: We match the valve body to the fluid type and process needs? Typical purchase cost ~ $ for a 4 pipe globe or ball valve with actuator (installation extra) 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

VB VALVES & AUTOMATION

VB VALVES & AUTOMATION Introduction to Valve What are Valves? Valves are mechanical device that controls the flow and pressure within a system or process. They are essential components of a piping system that conveys liquids,