Transcript Document

Chemical Engineering 3P04
Process Control
Tutorial # 2
Learning goals
1. The feedback cause-effect principle
2. Key element in the loop: The control valve
WHAT DOES A FEEDBACK SYSTEM DO?
Desired value
CONTROLLER
FINAL
ELEMENT
SENSOR
PROCESS
Inputs
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
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
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
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
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
Is this a valid feedback
control loop?
sensor
pump
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?
sensor
F
valve
pump
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?
P
Gas stream
Gas stream
Empty vessel
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
v3
F5
T5
P1
T4
F3
T6
F4
L1
v1
v5
v2
v6
v7
T2
Hot Oil
T8
T9
Hot Oil
v4
F6
T7
L2
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
v3
F5
T5
P1
T4
F3
T6
F4
L1
v1
v5
v2
v6
v7
T2
Hot Oil
T8
T9
Hot Oil
v4
F6
T7
L2
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
Cooling water
into tubes
Hot process fluid
into shell
We want to
control the hot
outlet
temperature.
TC
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
We want to
control the hot
temperature
here.
Hot process fluid
into shell
Cooling water into
tubes
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
We can measure this temperature, but
the heat exchanger does not influence it.
We want to
control the hot
temperature
here.
We cannot control the temperature
using the flows in this figure!
T
(We can look upstream to see if/how it
can be controlled.)
Cooling water into
tubes
Add a sensor and a valve to make this possible.
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
v3
F5
T5
P1
T4
F3
T6
F4
L1
v1
v5
v2
v6
v7
T2
Hot Oil
T8
T9
Hot Oil
v4
F6
T7
L2
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
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.
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.
Valves: What types of fluids do we regulate with
valves?
CO2, N2, H20 + little catalyst
riser
“FCC”
Fluid
Catalytic
Cracker
Burns carbon
to “regenerate”
catalyst
regenerator
Reacts (cracks) petroleum
to more valuable products;
by-product is carbon on
catalyst
Hot oil
Feed oil
air
Catalyst &
steam
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!
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
Valves: How to we “actuate” or open and close valves?
Name
Block
Table 3.1.1. Most common applications of valves in the process industries.
Symbol
Power
Typical process application
Manual
These valves are usually fully opened or closed,
(by person)
although they can be used to regulate flow over
short periods with a person by the valve.
Safety
Relief
On-off
M
Self-actuated
(the difference
between process and
external pressures
results in opening
when appropriate)
Electric motor
(fully open
or closed)
Throttling
control
Usually pneumatic
pressure
These are located where a high (low) pressure in
a closed process vessel or pipe could lead to an
explosion (implosion).
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.
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.
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.
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?
Capacity =
Range =
Pressure drop =
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.
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?
T6
best = safest
Feed
T1
T5
T2
P1
v3
v5
Vapor
product
P  1000 kPa
T  298 K
Methane
Ethane (LK) F1
Propane
Butane
Pentane
T4
T3
v1
F2
L1
v2
F3
v4
A1
Process
fluid
Steam
L. Key
Liquid
product
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.
fo
What are the best
failure positions?
T6
best = safest
Feed
T1
T5
T2
Methane
Ethane (LK) F1
Propane
Butane
fc
Pentane
T4
P1
v5
v3
Vapor
product
Must consider
upstream and
downstream
processes
T3
L1
fc
v1
F2
v2
F3
v4
A1
Process
fluid
Steam
L. Key
Liquid
product
Valve Body: We match the valve body to the fluid type
and process needs?
http://www.tycoflowcontrol-pc.com/products_results.asp?Selection=Double+Flanged+Butterfly+Valve&m=1
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://images.google.ca/imgres?imgurl=http://www.ckit.co.za/Secure/Catalogues/mac%2520steel/fluid%2520control/MS%2520image%2520
24%25202520thumb.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,%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.tycoflowcontrol-pc.com/products_results.asp?Selection=Double+Flanged+Butterfly+Valve&m=1
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://images.google.ca/imgres?imgurl=http://www.ckit.co.za/Secure/Catalogues/mac%2520steel/fluid%2520control/MS%2520image%2520
24%25202520thumb.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,%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.tycoflowcontrol-pc.com/products_results.asp?Selection=Double+Flanged+Butterfly+Valve&m=1
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://images.google.ca/imgres?imgurl=http://www.ckit.co.za/Secure/Catalogues/mac%2520steel/fluid%2520control/MS%2520image%2520
24%25202520thumb.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,%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.tycoflowcontrol-pc.com/products_results.asp?Selection=Double+Flanged+Butterfly+Valve&m=1
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://images.google.ca/imgres?imgurl=http://www.ckit.co.za/Secure/Catalogues/mac%2520steel/fluid%2520control/MS%2520image%2520
24%25202520thumb.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,%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.tycoflowcontrol-pc.com/products_results.asp?Selection=Double+Flanged+Butterfly+Valve&m=1
Typical purchase cost ~ $2000-3000 for
a 4”pipe globe or ball valve with
actuator (installation extra)
http://images.google.ca/imgres?imgurl=http://www.ckit.co.za/Secure/Catalogues/mac%2520steel/fluid%2520control/MS%2520image%2520
24%25202520thumb.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,%2Bpr
ocess%2Bcontrol%26start%3D80%26ndsp%3D20%26svnum%3D10%26hl%3Den%26sa%3DN