Intro4u2u

On the surface, this whole subject can appear to be pretty simple.
However, ensuring that control valves will shut off properly when
installed and throughout their service life requires an understanding
of the physical principles behind shutoff. First, how much leakage
can be permitted for a given control valve application?
ANSI specifies six different leakage classes. A Class I valve
really is not required to shut off at all. From Class II to Class VI, the
allowable leakage decreases, as illustrated in Figure 1. Class II is a
very basic valve that doesn’t ever really have to shut off. Class IV
typically can be achieved with a standard construction control
valve. Class V requires some fairly sophisticated changes to the
seating profiles if the valve is metal-seated, and Class VI normally
requires some type of soft seat (elastomeric seal). While this table
covers a large range of possible leakage levels and is a laudable
attempt at standardization, it has some idiosyncrasies worth noting.
The test medium changes from air to water, and there really is
no good way to correlate results from one type of medium to the
other. Note also that the Class V test is done at actual service
pressure, while the other tests are generally run at 50 psid or less.
Part 1 of this article (VALVE
Magazine, Summer 2003) made the
case that the control valve, as
part of the overall process
control infrastructure, is often
overlooked when end users
consider ways to improve
financial performance in their
plants. One of the prime reasons
for this problem is that control
valves are generally selected
and maintained as if they were
static devices and not part of
the highly dynamic process
control system.
The Control Valve’s
Hidden Impact
on the Bottom Line (Part 2)
The Control Valve’s
Hidden Impact
on the Bottom Line (Part 2)
By Bill Fitzgerald and Charles Linden
FA L L 2 0 0 3 3
This is important, because the test pressure used can have a relatively
serious impact on the valve’s ability to shut off since the
pressure drop across the valve plug and seat results in an unbalance
that will change the seating force between these two parts,
and the seating force is one of the primary determinants for valve
leakage. If you examine a typical valve construction (Figure 2), it
is clear why this happens. The pressure drop either lifts the plug,
tending to open it, or pushes down on the plug, increasing the
seating force and assisting with shutoff.
So, while these ANSI guidelines do help to determine relative
ability of a given valve design to shut off under somewhat consistent
test conditions, they really don’t tell the end user how well a
valve will shut off in actual service conditions, with the exception
of the Class V test where the service medium is a fluid like water.
The Class V test is unique in this respect since it is the only test
done under actual service pressure drops. What this means is that
a valve can pass the test requirement for a given leak class (other
than Class V) but may not meet the real requirements for the
valve in the field. This is an important point since many end users
are under the mistaken assumption that simply specifying an
appropriate leak class will give them what they need. In reality, to
be sure a valve shuts off in service, test it under actual service conditions,
or perform an analysis to show that changes in seating
load in service will not adversely affect shutoff.
This is not as complicated as it sounds since most reputable
manufacturers have developed fairly sophisticated actuator sizing
tools that can easily determine the impact the pressure drop can
have so that it can be compensated for in service.
To summarize, make sure you really understand your valve
shutoff needs given the application, and select a valve that
will not only give you that during the production test, but also
installed in the line. Also, before wasting time servicing a
“leaking” valve, make sure the design is one that will provide
the level of shutoff required. Many of these valves are actually
performing just as designed, and no amount of maintenance is
going to improve performance. You may simply need to change
to a different type of valve trim that is better suited to the leakage
class you need.
Assuming you have properly selected the valve to begin with,
how do you ensure that the leakage level will not increase as the
valve sees service? Here are some basic rules to observe that will
maximize the chances of proper shutoff over the service life of a
typical control valve:

Make sure there is good contact between the seating surfaces
on the plug and the seat ring, a necessary condition for good
shut-off performance. If the actuator and valve are not properly
connected, aligned, and adjusted, these two surfaces may never
come in proper contact, ensuring poor performance. The issue of
axial alignment between the valve and actuator stems deserves
special mention. Take care to ensure the two stems are aligned
properly when the actuator is assembled on the valve body assembly.
More than about 1/16 inch of misalignment on a small valve
or 1/8 inch on a larger model could result in difficulties getting the
valve to shut off (and will also hurt packing performance). If
excessive misalignment is present, corrective action must be taken
to bring the two stems closer together.

Assuming good contact between the mating surfaces, you
need to ensure there is sufficient seat load to bring them together
and hold them tightly under service conditions. The required
seating load is normally a function of the shutoff class and the size
of the port, and is usually given as lbs/lineal inch of seat for a particular
valve. The higher the load per lineal inch, the better the
shutoff. The actuator load also has to compensate for whatever
unbalance is present due to the pressure drop across the valve.
This will ensure the parts stay together and maintain the specified
load.

The mating surfaces must also be machined and prepared
to ensure good solid contact all the way around. Normally, line
Contro l Va l v e ’ s H i d d e n I m p a c t
Figure 1. Leakage specifications Figure 2. Sectional of typical valve construction
4 VALV E MAGAZINE
contact is the best as this provides the highest contact stress on
the parts, which ensures they conform to one another. This
typically is achieved by mismatched angles on the two faces or
having one face cut at an angle and the other on a radius. Note
that lapping of seats is a common practice in the industry, but
some plants rely too much on lapping to bring seats into contact.
Seats should be re-cut first to ensure good contact all the
way around, and then lapping should be done to improve the
surface finish of the mating surfaces. Lapping will not correct
major problems with quality of the seating surfaces. It is a finishing
step.

And finally, there should be no debris caught between the
seat or damage to the seat that could result in poor localized contact
between the two pieces.
If these conditions are met, the valve shutoff performance
should be satisfactory. And to keep it functioning over time, do
everything possible to ensure that these conditions don’t
change. That means a regular check of seat and actuator loading
and, if possible, a seating signature evaluation. Fortunately,
the advent of air-operated valve diagnostics has made checking
seat condition and loading very easy. Simply run a diagnostic
scan, either with a portable testing device or a valve-mounted
smart positioner, to determine whether there is good seat condition
and the proper load.
The output for one of these tests is shown in Figures 3 and 4,
where we have plotted actuator pressure vs. travel for the valve to
show how much actuator load is available to load the seat at the
closed end. Figure 4 shows the expanded view of the seating profile,
which provides clues about the seating condition of the mating
surfaces. As long as these profiles look good, the valve should
function as designed.
One other precaution deserves mention here. This situation
can significantly impact valve shut-off, and it is of particular
concern in power plants, where high-temperature graphite
packing is employed. Many valves as shipped work fine. Over
the years, the valve packing may end up getting adjusted to the
point where, with high-friction graphite, it begins to add significantly
to the overall friction in the valve assembly. If the
friction gets too high, it can begin to detract from the force
available to seat the valve and provide proper shut-off. This
effect is sometimes lost on the end user that tries to correct a
packing problem by over-torquing the packing and creates a
seat load and leakage problem in the process.
Packing is another source of significant leakage in process
plants and is particularly bothersome as it results in a release into
the atmosphere as opposed to the controlled leakage going past
the valve seat. The sealing surface between the stem and the packing
needs to be carefully maintained. The better the finish, the
better the initial seal and the longer the life. Take care not to
damage this surface using things like emery cloth to “polish” it. A
detailed discussion of packing performance is outside the scope of
this article but, in summary, the end user should view packing as
a precision subcomponent in a control valve assembly, which
needs to be tightly controlled to avoid leakage.
Seat Leakage
The issue of seat leakage is important as excessive leakage in a
valve can result in these two negative issues cropping up, both of
which impact the bottom line:

One factor is the loss of fluid downstream and the negative
impact it can have on operational performance. For example,
many critical steam valves in a power plant end up leaking either
to the atmosphere or to the condenser so the generated steam
never gets a chance to get to the turbine. Studies have shown that
a typical power plant can lose 1-2% of its capacity due to control
valve leakage if the characteristics just described are not well
understood.

Another issue is that leakage generally results in trim damage
to both the seating surfaces on the plug and the seat ring. This
translates into additional costs to tear apart the valve for repairs
and can increase the amount of money spent on replacement trim.
Long