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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

 

 

 
     
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