When comparing the 4 major process variables: temperature,
pressure, flow, and level in a large-scale continuous
process like a distillation column chain, the importance
of the level loop is often over-looked. Typically, the
level loop is controlled by manipulating the inflow
or outflow of the operating unit, and is considered
an easy loop to control.
Why Level Control Can Be Difficult
It is difficult to properly tune a PID controller for
good control performance under all conditions due to
the potential inflow and outflow variations of the operating
unit. Overly tight level control will result in too
much movement of the flow loop, which can cause excessive
disturbances to the downstream operating unit. Thus,
a PID level controller is usually detuned to allow the
level to fluctuate; so the variations of the outflow
are minimized. The detuned PID, however, cannot provide
prompt control against large disturbances, which may
result in safety problems during a plant upset. In addition,
oscillations in level can cause the process to swing,
which also results in a lower yield.
MFA Control Solution
Robust MFA controllers have been used to control the
levels to allow smooth material and energy transfers
between the operating units, and also protect the levels
from overflowing or becoming too dry during abnormal
conditions. The Robust MFA controller can be configured
with the parameters as shown in the following bitmap.
1. Upper and Lower Bound - The bounds for the process
variable (PV) being controlled. These are "intelligent"
upper and lower boundaries that are typically the marginal
values the PV should not exceed. PV is unlike the controller
output (OP) where a hard limit or constraint can be
set. PV is a process variable that can only be varied
by manipulating the OP. Thus, the Upper and Lower Bounds
for PV are very different from the OP constraints.
2. Gain Ratio - The coefficient to increase or decrease
the MFA control action based on the PV. For instance,
a Gain Ratio of 3 is the default setting. As PV gets
closer to reaching its bounds, MFA will react like it
has a gain that is 3 times higher than its setting.
Notice that this is not a gain scheduling approach,
although it appears to be. Gain scheduling will not
be able to resolve the complex problems described.
Boiler Steam Drum Level Control
The steam drum level needs to be kept near the midline
to prevent either heat stress on the boiler water tubes
(level too low) or corrosion (level too high). Improper
control of the level can cause system shutdown, waste
of energy, and a shorter equipment life cycle.
Key variables affecting the drum level are feedwater
inflow, steam outflow and fuel/mix inflow. Each variable
has its own distinctive type of disturbance. Cold feedwater
creates a delay time in the process. A sudden increase
of steam outflow causes a distinctive “shrink
and swell” response. This will confuse the controller
because the process “acting type” will be
CyboSoft’s three-element MFA control system can
effectively control the steam drum level. The MFA level
controller is cascaded with the feed-water controller
to regulate the drum level and compensate for disturbances
from feedwater and steam outflow. The Anti-delay MFA
controller handles the large varying delay time. Feedforward
MFA controllers keep feedwater supply in balance. Table
1 shows what this MFA control solution can accomplish
and the benefits achieved by customers.
Table 1. MFA Control Benefits
for Steam Drum Level Control
of MFA Control
|Anti-delay and Feedforward MFA
effectively handle the “shrink and swell”
||Steam boiler drum level is controlled
within its high and low limits during large steam
pressure load changes and other plant upsets.
|Prevents level running too high
||Safety and productivity of the
steam system is improved.
|Prevents steam systems’
||Production efficiency and throughput
|Helps control steam temperature.
||Energy savings are achieved.
|Improves efficiency & productivity.
||Full investment is returned
in months if not sooner.
Evaporator Level Control
Evaporators are popular operating units for
dewatering, concentration, and crystallization in various
industries. If the density or the consistency of the
product is an important quality variable, the level
control will be much more difficult.
For instance, tomato processing plants use evaporators
to produce tomato paste. The density of the tomato paste
is the most critical quality variable. It is desirable
to control the density within a small range. The manipulated
variables are the inflow and outflow, which affect both
the density and level at the same time. The evaporator,
by its nature, is a multi-input, multi-output (MIMO)
process. When using single-loop controllers to control
the level and density separately, both loops can fight
each other causing major problems.
A multivariable MFA controller can quickly and tightly
control density and level by manipulating inflow and
outflow simultaneously. An MFA constraint controller
protects the evaporator level from running too high
or too low. Table 2 shows what a MIMO MFA controller
can accomplish and the benefits achieved by evaporator
Table 2. MFA Control Benefits
for Tomato Evaporator Control
|Use of MFA Control
|Manipulates inflow and outflow
|Density control is improved by reducing variability
by at least 50%.
|Reduces over-drying the paste.
||Steam consumption is reduced.
|Enables evaporator to be started and maintained
in automatic control.
||Flexible production is enabled and load disturbances
on the steam generation system are reduced.
|Reduces product density variation.
||Product quality and production efficiency is improved
for better profitability.
|Improves efficiency & productivity.
||Full investment is returned in months if not weeks.
Based on the core MFA control method, various MFA controllers
have been developed to solve specific control problems.
This applies to level control applications as well.
1. The Robust MFA controller is well suited to control
the conventional level loops and provides a good protection
from running too high or too low.
2. Anti-delay MFA with Feedforward MFA controllers can
effectively control the steam drum level.
3. MIMO MFA can control the density and level for operating
units like evaporators.
To read more about implementations of CyboSoft’s
MFA level control solutions, click on the following
Adaptive Control of Evaporators
Adaptive Control of Steam Drum Level
Adaptive Control of Air Separation Units
Control and Optimization of Distillation Columns