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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.

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.

Packing is a sealing system which normally consists of a deformable material such as TFE, graphite, asbestos, Kalrez, etc. Usually the material is in the form of solid or split rings contained in a packing box. Packing material is compressed to provide an effective pressure seal between the fluid in the valve body and the outside atmosphere.

At one time it was believed that the more packing you had in a control valve the better it would seal. Since FUGITIVE EMISSIONS has become a concern, extensive studies have been made which have shown that better sealing can be obtained by minimizing the number of packing rings.

New standards are being developed to which manufacturers will be asked to test their control valves. Test results from using these standards will allow a user to predict with some certainty how well a particular valve and packing combination will hold up.

Definitions

Consolidation: Packing consolidation is the shortening of a packing stack under load due to the elimination of voids in, between, and around the packing rings. This causes a reduction in packing stress (Radial Load) and consequently an increase in leakage. Consolidation can occur when the packing wears, cold flows, is subjected to thermal gradients, or if a non-uniform stress distribution in the control valves. packing exists.

Extrusion: When packing is loaded to its proper stress level it has a tendency to cold flow and will extrude between the stem and the follower. Any increase in temperature will increase the tendency of the packing to cold flow. PTFE is very susceptible to this because the hotter it is the quicker it will cold flow and because PTFE has an expansion rate roughly ten times that of carbon steel. As the packing tries to expand in the fixed volume of the packing gland, extrusion will occur. This material loss due to extrusion will relieve the axial stress, which relaxes the radial stress and results in a loss of seal.

Migration: Packing migration occurs when a portion of the packing is caught by a rough stem and is removed from the packing box as the stem slides in and out of the packing box. (Applies only to Linear Valves.)

Control valves.Packing System Design Principles

1. In order to minimize stem friction and wear on the packing, the stem surface finish should be in the 8 to 16 RMS range.

2. The stem of the valve should be held concentric with the packing bore. This helps to uniformly compress the packing. This is best accomplished by guiding the stem at the top and the bottom of the packing bore.

3. To minimize packing extrusion under load, the inner diameter of packing spacers should be held as close to the stem diameter as possible. Anti-extrusion washers can also be helpful in minimizing extrusion.

4. It is desirable to use a wiping mechanism. The stem-wiping device should be at least a stroke distance away from the packing to prevent damage to the stem and packing by dragging particles and deposits into the packing area.

Although it is the most expensive way in which to seal off the bonnet assembly from escaping fluids, it is also the most effective way of handling lethal, toxic, explosive, and corrosive fluids. As we head toward "ZERO EMISSIONS" control valves requirements, the bellows seal will become more popular.