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Control
Valves Do What They Are Told!
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| Being the Final Control Element in a system
is not an easy job. To start with, you are blamed for any and all problems
that crop up in the process. You are subjected to corrosion, high velocity,
cavitation, flashing liquids, cryogenic temperatures, high temperatures,
abrasion, and thermal shock. You are expected not only to throttle along
through all this, but most likely, you are also being asked to act as a
block valve and shut off tight. |
| As you work with control valves always keep
in mind that a control valve only does what
it is told to do. |
| A Control Valve is a power-operated device
used to modify the fluid flow rate in a process system. Well, what happens
if the power is cut off? When a Control Valve is sized or selected to do
a particular job, one of the first questions you should consider is how
that valve will respond in the event of a loss of signal or power. This
is called its "fail-safe mode" and knowing the
fail-safe mode is the key to troubleshooting it. |
| In most applications (about 80%), it is desirable
for valves to fail closed. In
other applications, you might want a valve to fail
open or fail in place.
Safety concerns and process requirements will mandate the fall mode of
the valve. |
| When a valve is not sitting in its fail position,
is is being told how and when to move by some external signal. |
| By the comments one hears, you would be led
to believe that control valves sit around and think up things to do on
their own. Perhaps this will some day be true when all control valves are
"smart." |
| If a Control Valve is observed in an unstable
condition or appears to not be responding correctly to an input signal,
remember that something is telling the valve to
behave that way. |
| A control valve is only as strong as its weakest
link. |
| When the 1965 Ford Mustang first appeared,
it was powered by a 6-cylinder engine with a 3-speed transmission - but
it had a 140 m.p.h.(225 k.p.h.) speedometer. The fact that it had a 140
m.p.h.(225 k.p.h.) speedometer did not mean it could actually travel that
fast. In the same way, a control valve with a 600# rated valve body cannot
throttle and shut off against 1440 pounds of pressure. |
| There are two basic
types of control valves: rotary
and linear.
Linear-motion control valves commonly have globe,
gate, diaphragm,
or pinch
- type closures. Rotary-motion valves have ball,
butterfly,
or plug
closures. Each type of valve has its special generic features, which may,
in a given application, be either an advantage or a disadvantage. |
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Linear
Valve Features
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TORTUOUS FLOW PATH
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LOW RECOVERY
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CAN THROTTLE SMALL FLOW RATES
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OFFERS VARIETY OF SPECIAL TRIM DESIGNS
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SUITED TO HIGH-PRESSURE APPLICATIONS
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USUALLY FLANGED OR THREADED
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SEPARABLE BONNET
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Rotary
Valve Features
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STREAMLINED FLOW PATH
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HIGH RECOVERY
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MORE CAPACITY
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LESS PACKING WEAR
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CAN HANDLE SLURRY AND ABRASIVES
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FLANGELESS
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INTEGRAL BONNET
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HIGH RANGEABILITY
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| In addition to linear and rotary, control
valves are also classified according to their guiding
systems and the types of services they are used in. |
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Control
Valve Classification
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Face
to Face Dimensions
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Operating Modes / Positioners
/ Positioner
and Controller Operating Modes / Flow
Characteristics / Seat
Leakage / Packing
/ Helpful
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