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What are Valve Characteristics Inherent Characteristics The three most common valve characterizations are equal percentage, linear, and quick opening Equal percentage - flow capacity increases exponentially with valve trim travel. Equal increments of valve travel produce equal percentage changes in the existing Cv. Professor Douglas Cooper, author Loop Pro-Trainer
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CBE / MET 43322 Feb 12
(Transfer functions and Block Diagrams)
Professor Douglas Cooper, author Loop Pro-Trainer
What are Valve CharacteristicsInherent Characteristics
• The three most common valve characterizations are equal percentage, linear, and quick opening
2Professor Douglas Cooper, author Loop Pro-Trainer
What are Valve CharacteristicsInstalled Characteristics
• In many process applications the pressure drop across a valve varies with the flow. In these instances an equal percentage valve will act to linearize the process.
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Equal percentage valves are the most commonly used control valves. • How do you know what inherent valve characteristic to choose to get a
linear installed characteristic?– The correct selection of valve characteristic to linearize the process
gain will ease the tuning process and make for a robust system.– Most times this selection is through experience, guesswork or the
valve manufacturer’s recommendation.Professor Douglas Cooper, author Loop Pro-Trainer
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Feedback Block DiagramTransmitter
Transducer
Valve
Energy Transfer Controller
Process
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Feedback Block Diagram
11s
11
sK
s+
+
si
sQ
sWVK
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sMor T
6
Feedback Block Diagram (simplified)
11s
1
sK
s+
+
si
sWVK
sMcK
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sE+ sR
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7
Closed Loop Transfer Function (let R(s)=0)
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sK
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si
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sE+ sR
TK sC
sKKKsKs
ss TcVi
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8
Closed Loop Transfer Function (let R(s)=0)
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sWVK
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TcV
i KKKsK
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Open Loop vs Closed Loop
Transfer Function (R(s)=0)
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s
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sWVK
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sss
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Closed Loop:
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time (t)
Y(t)
Open - loop
Closed - loop
t
time
AK *
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Transfer Functions (Chap 3-5)
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1
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sWVK
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s
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Define:
For heated, stirred tank:
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