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Control
Valve Packing
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| 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 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.) |
| 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.
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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.
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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.
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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.
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Live-Loaded
Packing Arrangements
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Internal Live-Loading
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External Live-Loading
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Spring-Loaded
Packing
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Jammed Packing
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| The live-loading packing spring is replaced
by a fixed spacer of the same material as the trim material. |
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Dual Packing
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| Pressure inside the valve is alternately greater
than or less than (i.e. vacuum) ambient pressure. |
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Dual Packing
With Leak-Off Connection
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| Valve has a 1/4" (6mm) NPT tapped opening
on its bonnet. Complete with removable steel plug for all body materials.
The opening is located between primary and secondary packing sets when
the valve is equipped with dual packing. |
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Internally Pressurized
Bellows Stem Seal
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| 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 requirements, the bellows seal
will become more popular. |
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Control Valve Concepts
/ Actuator Operating Modes / Positioners
/ Positioner and Controller Operating
Modes / Flow Characteristics / Seat
Leakage / Helpful Hints / Cashco
Terminology / Advanced Topics
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