control valves do what they are told

7/29/2019 Control Valves Do What They Are Told

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Control Valves Do What They Are Told!

Being the Final Control Element in a system is not an easy job. To start with, you are blamed forany 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 thermalshock. 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 co n t r o l va l ve o n l y d o es w h a t i t i s t o l d t o d o .

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 aparticular job, one of the first questions you should consider is how that valve will respond in theevent of a loss of signal or power. This is called its "fail-safe mode" and knowing the fail-safemode is the key to troubleshooting it .

In most applications (about 80%), it is desirable for valves to fa i l c losed. In other applications, youmight want a valve to f a i l open or f a i l i n p lace . Safety concerns and process requirements willmandate 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 bysome external signal.

By the comments one hears, you would be led to believe that control valves sit around and think upthings 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 toan 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-speedtransmission - but it had a 140 m.p.h.(225 k.p.h.) speedometer. The fact that it had a 140m.p.h.(225 k.p.h.) speedometer did not mean it could actually travel that fast. In the same way, acontrol 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 valvescommonly have globe, gate, diaphragm, or pinch - type closures. Rotary-motion valves haveball, butterfly, or plug closures. Each type of valve has its special generic features, which may, ina given application, be either an advantage or a disadvantage.


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Linear Valve Features

TORTUOUS FLOW PATH

LOW RECOVERY

CAN THROTTLE SMALL FLOW RATES

OFFERS VARIETY OF SPECIAL TRIM DESIGNS

SUITED TO HIGH-PRESSURE APPLICATIONS

USUALLY FLANGED OR THREADED

SEPARABLE BONNET

Rotary Valve Features

STREAMLINED FLOW PATH

HIGH RECOVERY

MORE CAPACITY

LESS PACKING WEAR

CAN HANDLE SLURRY AND ABRASIVES

FLANGELESS

INTEGRAL BONNET HIGH RANGEABILITY

In addition to linear and rotary, control valves are also classified according to their guidingsystems and the types of services they are used in.


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Control Valve Classification

source :http://www.maintenanceresources.com/ReferenceLibrary/ControlValves/CashcoControlPg1.htm

control valves do what they are told

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