Pressure Control
Introduction
Temperature, pressure, flow, and level are the four
most common process variables. Similar to temperature,
pressure is another key process variable because pressure
provides a critical condition for boiling, chemical
reaction, distillation, extrusion, vacuuming, and air
conditioning. Poor pressure control can cause major
safety, quality, and productivity problems. Overly high
pressure inside a sealed vessel can cause an explosion.
Therefore, it is highly desirable to keep pressure in
good control and maintained within its safety limits.
Why Pressure Control Can Be Difficult
The many reasons why a pressure loop is difficult to
control are listed and described in the following table:
| Reason |
Example |
Control
Headache |
| Nonlinear |
Natural gas pipeline. Pressure of a fluidize-bed
boiler. Gas mixing plant. |
A PID or model-based controller may work well
in its linear range and fail in its nonlinear range. |
| Multivariable control |
Multiple gas lines may draw gas from a master
line. When the load changes, they will interact
with each other. |
A multivariable process cannot be effectively
controlled by using SISO controllers due to interactions
among the variables. |
| Large load changes |
Steam generators in co-generation plants have
to deal with large steam load changes due to demand
changes. |
Load changes can cause major disturbances to pressure. |
| Large and varying time-delays |
Pressure in municipal gas grids or a product powder
transport system have large and varying time delays. |
PID cannot effectively control a process with
large and varying time delays. |
| High-speed and open-loop oscillating |
The pressure field and Mach speed value of an
ultra-sonic wind-tunnel used in the aerospace industry
is open-loop oscillating. |
Due to the poor frequency domain behavior of this
process, tying to control an open-loop oscillating
loop can be a nightmare. |
| Nonlinear and high-speed |
Vacuum vessels used in thin film or material deposition. |
It is desirable to reach the vacuum state but
the process is nonlinear. |
MFA Control Solution
The following table provides a roadmap to allow
you to select the appropriate MFA controller to solve
a specific pressure control problem.
| Reason |
Selected
MFA Controller |
What
Can This MFA Do? |
| Nonlinear |
Nonlinear MFA controller or
Robust MFA controller.
|
Nonlinear MFA controls extremely nonlinear processes
with no nonlinear characterization required. Robust
MFA forces the pressure to stay inside the desired
boundary. |
| Multivariable control |
MIMO MFA controller or
Feedback/Feedforward MFA controller.
|
MIMO MFA controls multivariable processes. Interactions
among pressure zones can be decoupled. |
| Large load changes |
Feedback/Feedforward MFA controller or Robust
MFA controller. |
Feedforward MFA makes quick control adjustments
to compensate for the load changes. Robust MFA forces
the pressure to stay inside the desired boundary. |
| Large and varying time-delays |
Anti-delay MFA controller or
Time-varying MFA controller.
|
Anti-delay MFA can effectively control processes
with large time delays. Time-varying MFA can control
processes with large and varying time delays. |
| High-speed and open-loop oscillating |
High-speed Flex-phase MFA controller or Nonlinear
Flex-phase MFA controller. |
After choosing a phase-angle during configuration,
the Flex-phase MFA can effectively control processes
with bad behavior in the frequency domain. |
| Nonlinear and high-speed |
High-speed Nonlinear MFA controller. |
After choosing a Nonlinearity factor during configuration,
the Nonlinear MFA can effectively control this process
with changing nonlinear characteristics. |
Speed Considerations
Since the pressure loop requires fast sample and control
update rates, the PC-based MFA control system using
an HMI or OPC interface may not be fast enough to control
pressure loops. Embedded MFA control products or dedicated
I/O cards in the PC will provide sufficient sample rates
for pressure control. CyboCon HS, CyboCon CE, CyboCon
Dragon, and all embedded MFA control products can be
used for pressure control.
Noise Considerations
Pressure loops are typically noisy. That means, the
pressure PV may jump up and down due to the nature of
the pressure loop and the pressure sensor used. Low-pass
filters can be used to screen the high-frequency noise.
Since the MFA controller is not noise-sensitive, a filter
may not be required unless the S/N ratio (signal-to-noise
ratio) is so high that the control performance is obviously
affected. Some MFA control products like CyboCon can
have a user-entry field to specify when the PV is noisy.
Summary
Based on the core MFA control method, various MFA controllers
have been developed to solve specific control problems.
This applies to pressure control applications as well.
The roadmap above is a guide for selecting the appropriate
MFA controller to keep the pressure loop under good
control.
Case Studies
To read more about implementations of CyboSoft’s
MFA pressure control solutions, click on the following
case studies:
Model-Free
Adaptive Control of Fluidized-Bed Boilers
MFA
Control and Optimization of Crude Oil Separators
MFA
Control and Optimization of Gas Mixing Process
|
