3P04 Tutorial 2 Causeeffectvalve 2008

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





pump

L

valve

sensor

pump

valve





pump

L

valve

sensor

pump

valve





pump

F

valve

sensor

pump

valve


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





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





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







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.



Hot process fluid into shellCooling water

into tubes


TC

We want to control the hot

outlet temperature.




Cooling water into tubes

We want to control the hot temperature

here.





Cooling water into tubes

We want to control the hot temperature

here.


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.)


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



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.


Pressure drop =

Capacity =

Range =


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!




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.




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.




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.




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!

3P04 Tutorial 2 Causeeffectvalve 2008

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