Post on 23-Dec-2015
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
Inputs Outputs
FINALELEMENT
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 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?
v1
Hot Oil
v2
v3
L1
v7
v5 v6
Hot Oil
F1 T1 T3
T2
F2
T4T5
F3 T6
T8
F4
L2
v8
T7
P1F5
F6T9
v4
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?
v1
Hot Oil
v2
v3
L1
v7
v5 v6
Hot Oil
F1 T1 T3
T2
F2
T4T5
F3 T6
T8
F4
L2
v8
T7
P1F5
F6T9
v4
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 shellCooling 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
v1
Hot Oil
v2
v3
L1
v7
v5 v6
Hot Oil
F1 T1 T3
T2
F2
T4T5
F3 T6
T8
F4
L2
v8
T7
P1F5
F6T9
v4
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
regenerator
riser
airFeed oil
CO2, N2, H20 + little catalyst
Catalyst & steam
Hot oil
Reacts (cracks) petroleum to more valuable products; by-product is carbon on catalyst
Burns carbon to “regenerate” catalyst
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
Table 3.1.1. 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 not possible. They can be operated by a person 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: 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
MethaneEthane (LK)PropaneButanePentane
Vaporproduct
Liquidproduct
Processfluid
Steam
F1
F2 F3
T1 T2
T3
T5
T4
T6 P1
L1
A1
L. Key
What are the best failure positions?
P 1000 kPa
T 298 K
best = safest
v1 v2
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
MethaneEthane (LK)PropaneButanePentane
Vaporproduct
Liquidproduct
Processfluid
Steam
F1
F2 F3
T1 T2
T3
T5
T4
T6 P1
L1
A1
L. Key
What are the best failure positions?
best = safest
fo
fcfc
Must consider upstream and downstream processes
v1 v2
v3
v4
v5
http://www.tycoflowcontrol-pc.com/products_results.asp?Selection=Double+Flanged+Butterfly+Valve&m=1
http://images.google.ca/imgres?imgurl=http://www.ckit.co.za/Secure/Catalogues/mac%2520steel/fluid%2520control/MS%2520image%252024%2520-2520thumb.jpg&imgrefurl=http://www.ckit.co.za/Secure/Catalogues/Mac%2520steel/Fluid%2520control/MS%2520fluid%2520control.htm&h=150&w=113&sz=5&hl=en&start=83&tbnid=3ZKYixrpLJ5wTM:&tbnh=96&tbnw=72&prev=/images%3Fq%3Dball%2Bvalves,%2Bprocess%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!
http://www.tycoflowcontrol-pc.com/products_results.asp?Selection=Double+Flanged+Butterfly+Valve&m=1
http://images.google.ca/imgres?imgurl=http://www.ckit.co.za/Secure/Catalogues/mac%2520steel/fluid%2520control/MS%2520image%252024%2520-2520thumb.jpg&imgrefurl=http://www.ckit.co.za/Secure/Catalogues/Mac%2520steel/Fluid%2520control/MS%2520fluid%2520control.htm&h=150&w=113&sz=5&hl=en&start=83&tbnid=3ZKYixrpLJ5wTM:&tbnh=96&tbnw=72&prev=/images%3Fq%3Dball%2Bvalves,%2Bprocess%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.
http://www.tycoflowcontrol-pc.com/products_results.asp?Selection=Double+Flanged+Butterfly+Valve&m=1
http://images.google.ca/imgres?imgurl=http://www.ckit.co.za/Secure/Catalogues/mac%2520steel/fluid%2520control/MS%2520image%252024%2520-2520thumb.jpg&imgrefurl=http://www.ckit.co.za/Secure/Catalogues/Mac%2520steel/Fluid%2520control/MS%2520fluid%2520control.htm&h=150&w=113&sz=5&hl=en&start=83&tbnid=3ZKYixrpLJ5wTM:&tbnh=96&tbnw=72&prev=/images%3Fq%3Dball%2Bvalves,%2Bprocess%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.
http://www.tycoflowcontrol-pc.com/products_results.asp?Selection=Double+Flanged+Butterfly+Valve&m=1
http://images.google.ca/imgres?imgurl=http://www.ckit.co.za/Secure/Catalogues/mac%2520steel/fluid%2520control/MS%2520image%252024%2520-2520thumb.jpg&imgrefurl=http://www.ckit.co.za/Secure/Catalogues/Mac%2520steel/Fluid%2520control/MS%2520fluid%2520control.htm&h=150&w=113&sz=5&hl=en&start=83&tbnid=3ZKYixrpLJ5wTM:&tbnh=96&tbnw=72&prev=/images%3Fq%3Dball%2Bvalves,%2Bprocess%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.
http://www.tycoflowcontrol-pc.com/products_results.asp?Selection=Double+Flanged+Butterfly+Valve&m=1
http://images.google.ca/imgres?imgurl=http://www.ckit.co.za/Secure/Catalogues/mac%2520steel/fluid%2520control/MS%2520image%252024%2520-2520thumb.jpg&imgrefurl=http://www.ckit.co.za/Secure/Catalogues/Mac%2520steel/Fluid%2520control/MS%2520fluid%2520control.htm&h=150&w=113&sz=5&hl=en&start=83&tbnid=3ZKYixrpLJ5wTM:&tbnh=96&tbnw=72&prev=/images%3Fq%3Dball%2Bvalves,%2Bprocess%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
BallTypical purchase cost ~ $2000-3000 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!