Intro4u2u

Let’s face it. Control valves never make anyone’s
top 10 list of favorite topics. Yet while they may
be considered boring, control valves can impact
your bottom line in ways you may not have considered.
A brief example illustrates why.
Figure 1 documents control loop performance testing
within a power plant and reveals how control valves can
affect a process. The loop in this case consists of a controller,
a flow sensor, and a control valve that changes flow in
the loop.
The bottom graph shows the process variable, that is,
the process characteristic that we are attempting to control.
It also shows a typical process constraint that the process
variable cannot exceed in this case without problems cropping
up. The solid line in the middle is the set point, which
is the desired level for the process variable. Clearly, there
is a lot of error between the set point and the process variable,
indicating that the performance of the process control
system is substandard. There is high steady-state cycling
around the set point and then every 125 seconds or so there
is a major correction that overshoots the set point followed
by a reversing action.
The top curve is the controller output signal to the control
valve. There is a lot of variability on this curve as well,
but it is difficult to determine whether this variation is causing
the cycling in the process variable or simply trying to correct
for it. It is clear that the controller is sensing the error
between the set point and the process variable and slowly
The Control Valve’s
Hidden Impact
on the Bottom Line (Part 1)
The Control Valve’s
Hidden Impact
on the Bottom Line (Part 1)
Many of the profitability issues facing
the process industries today can be
linked directly to control valve performance.
Are there valve-related
issues in your plant that you should be
concerned about? Part 1 of this article
will address dynamic performance
of the control valve and how it impacts
your bottom line. Part 2 will speak to
other valve characteristics that are
typically ignored when selecting control
valves, such as leakage past the
seat, long-term reliability, and maintainability.
It will conclude with some
case studies that illustrate the dramatic
impact that control valves can
have on the bottom line.
By Bill Fitzgerald and Charles Linden
© 2003 Valve Manufacturers Association. Reprinted with permission.
changing its signal to the control valve. Unfortunately, there
is no change in the flow, so the controller keeps changing its
output. Eventually, it appears that the valve breaks loose and
jumps well past where it should. The set point finally does
change but goes past the set point, and the controller has to
reverse its signal, causing the valve to jump back in the other
direction.
Because of the type of performance, the process variable is
not controlled very well, which means that feedstock or
energy may be wasted. It also means that the set point needs
to be moved away from the process constraint to insure that
we don’t go past this level of flow during the fluctuations.
This can also hurt the bottom line for many customers since
efficiencies are normally directly related to maximizing things
like flow, pressures, and temperatures. This derating of individual
loops to account for variability can have a subtle but
very marked impact on profitability by reducing throughputs
and yields. Valve-caused process fluctuations also degrade
overall process reliability by wearing out mechanical equipment
well ahead of its predicted time for maintenance.
This is a very important point. Control loops that fluctuate
result in an unstable plant. All major components in the
plant—like pumps, compressors, vessels, and safety relief
valves—are subjected to these fluctuations. These service
excursions have been shown to age the equipment prematurely,
which means higher maintenance costs and loss of system
performance. Like an automobile, a plant that is brought
up smoothly, and that is able to operate for long periods at
steady state, will have much lower repair bills, so smooth
process control is also critical in optimizing maintenance
costs.
Surprisingly enough, even though this type of process control
performance is costing a lot of money, it is typical of 30-50% of
the loops that we check year in and year out. Why is this happening?
Why are we not getting our return on investment out of
our process control systems? We believe that many of the problems
can be traced back to the lowly control valve and how it is
% of Max.
selected and maintained. Many control valves are purchased
with what we call “static specifications,” like required flow
capacity, pressure ratings, and fluid type. In reality, if we are to
avoid the kind of process-control-related problems just illustrated,
there needs to be a lot more attention paid the dynamic
characteristics of the valve. These include:
• Installed gain
• Friction, hysterisis, and deadband
• Dynamic flow conditions
• Process-side error
• Stroking speed and overshoot
• Positioning resolution
• Force vs. travel profile
• Load sensitivity
We’ll go over these in more detail in the following paragraphs.
Introducing Control Valve Gain
The gain of a control valve is the derivative or slope of the
valve’s flow characteristic, or more simply, the change in flow
for a given change in travel. If gain is relatively constant over
most of the valve travel, we can easily tune the loop for optimum
control without worrying about changes in gain vs.
travel affecting settings. Changes in gain that occur as the
valve strokes can cause the loop to become either nonresponsive
or unstable.
To determine gain, we first have to understand what the
inherent flow characteristic looks like for each valve. Figure 2
shows typical curves by plotting flow coefficient (CV) values vs.
travel. We have to be careful how we use this data, however. CV
values are determined with a constant pressure drop across the
valve. In real life, most valves are coupled to pumps and see a
decreasing pressure drop with increasing travel. This means that
the installed dynamic flow characteristic is different from the
inherent characteristic curves shown in Figure 2.
Figures 3, 4, and 5 illustrate what we mean. An equal
percentage trim becomes more linear if there is a decreasing
pressure drop upon valve opening. (Incidentally, this is why
many throttling valves are sold with equal percentage trim.)
Linear trim looks like quick opening if used with a pump and
quick opening trim gets even worse if installed with a pump.
The steep slope of the quick opening characteristic when the
valve is in its first 25% of travel makes it a very high gain
device. But the gain or slope changes to almost zero as the
valve opens beyond 25%. This type of change in gain for different
points in travel makes the loop very hard to tune. To
simplify tuning, the preferred characteristic is one where the
derivative or slope of the flow curve changes very little over
the normal working range of the valve. In this case, if we
assume that we are seeing a reduction in pressure drop as the
valve opens, then a valve with an inherent equal percentage
characteristic will look most linear when installed. The linear-
installed characteristic gives us a relatively constant slope
over the travel, which is easier to tune.
Understanding Gain vs. Characteristic
For a better understanding of what this discussion of gain
and flow characteristic really means, let’s look at the two different
types of control valves.
Figure 6 illustrates the LoopVue diagnostic program,
which lets us model the installed gain for a valve as it operates
over its normal control range. The top curve shows
installed gains for 8-in. butterfly valves used in a Containment
Air Cooler (CAC) application. The travel span for
operation between 1650 and 1950 gpm is only 5% or 6% for
each CAC valve. The process gain, labeled as KP, varies from
1.3 to 2.7 over the same flow range.
The high process gain, together with the deadband usually
encountered in the drive train of butterfly valves, would typically
cause measurable limit cycling with this type of valve.
Note also that each valve must stroke approximately 80% of
full travel to move from wide open to the throttling position.
Another problem evident here, which is typical of oversized
valves, is the essentially zero process gain at wide open.
If gain scheduling cannot be used in the controller, then the
control loop will respond slowly near the wide-open condition,
even if the control valve responds instantly to the controller
output. High controller gain might be used to
compensate, but it could lead to valve overshoot and large
cycles at the throttling position, as can be seen in the examples
in Figures 7 and 8.
The lower half of Figure 7 shows the LoopVue diagnostic
results for a modern Vee-port valve design with full 90°
travel. The travel range for 1650-1950 gpm varies from
approximately 8% for the #11 CAC valve to 11% for the #14
CAC valve. Process gains vary from 0.8 to 1.3 in the same
flow range, which is a much smaller variation in gain. This
means easier tuning of the loop.
The Vee-port valves would need to stroke only about 40%
from wide open to reach the throttling flow of 1800 gpm.
Their maximum flow is trending to at least 2300 gpm, allowing
more design margin than with the butterfly valve. The
bottom line is that this type of design will give much better
control and will provide a much better return on investment.
Another Factor: Hysteresis/Deadband
Another factor that plays heavily in determining how well
a valve can control is hysteresis/deadband. This is a quantitative
indication of how much a valve’s actual position deviates
from the desired position. It is defined using a standardized
ISA test procedure, and in general it measures the friction
and “looseness” that exists in a control valve’s drive train.
The test does not give an exact indication of control capability
because it is generally conducted without flowing load and
ignores what we call process-side error. However, in general,
the lower the hysteresis/deadband number, the better the
control. A typical test is shown in Figure 9.
Many valves in service can have hysteresis/deadband values
greater than 10% due to either design or maintenance
problems. With values that high, it proves very hard to control
within better than +/-10% accuracy, especially if there is
higher friction present under load.
Process-Side Error
Although not technically a valve characteristic, process-side
error is included here because it is a good general indicator of
process control performance, and it is impacted by valve
performance. To determine the value for a given loop, we begin
with small variations in the input signal to the valve and then
watch how the process reacts. With many control valves, the
input signal to the valve has to change by more than 5% before
we see any change in process variable. In cases like this, the
result is very poor process control with the process variable
wandering all over the place—Figure 10.
Stroking Speed/Overshoot
Figure 11 shows the valve response to a step change in
instrument signal and gives a general idea of the speed of
response for the valve. We see a lot of valves on fast processes
that should be able to respond to a load or set-point change
within a second or two, but actually take 30 seconds or more
to get to their new position. Or, they go well past the actual
required position, which can also cause problems. Once
again, poor response means poor control.
Positioning Resolution
This is another good benchmark for valve response.
Figure 12 tells how well a control valve assembly can follow
small adjustments in input signals. If a valve’s minimum
stroke resolution is 1/16th of an inch, then we can control
no closer than half of this amount in terms of flow. If the
valve installed gain is high, this small movement can mean
a big change in flow, which sets up a limit cycle as the valve
jumps past the set-point and then has to move back in
the opposite direction. Again, there’s an impact on control
and reliability.
Force vs. Travel Profile
For a valve to control smoothly, the valve flow characteristic
should also be free of any negative gradients,
which tend to make the valve unstable and hard to control.
Negative gradients result from flow forces on the controlling
element within the valve body and can result in
large changes in valve position that do not result from a
change in input signal. This results in additional process
error.
Load Sensitivity
The valve actuator needs to be stiff enough to resist movement
when the flowing load changes. In most cases, this is
not a problem as OEMs tend to design their actuators with a
lot of stiffness. However, in unbalanced flow-down applications
this can explain poor control stability and may need to
be checked.
As you can see, there are a lot of characteristics that can
contribute to poor control that are not normally considered
when specifying a control valve. As a result, we believe that
many customers are not sure right now if they are getting
everything they can out of their process control investments.
If they are not, it is impacting their bottom lines.
The potential causes for this lack of performance are
many. The valve may not have been purchased to the right
specification. Maybe the maintenance program needs to be
more proactive and more focused on dynamic process control
performance. Or it could be that the operating conditions for
the loop have changed, since most plants are being asked to
behave in ways that are different from their original charters.
In any case, it may be time for a process control assessment
centered on the control valve. Start with the current
application information and work from the basement up to
make sure you’ve got the right valve and that your purchasing
and maintenance programs identify and focus on what you
really need from your process control system. There is some
homework to be done here, but as you’ll see in the second
part of this article, the payback can be huge.