Scattered across oilfield sites, tank farms, and isolated process units, standalone pneumatic controllers have been used for decades to locally control level, pressure, flow, and temperature in remote locations. However, these traditional devices have limitations, and many vendors no longer offer or support them.
My article in the April 2024 issue of InTech, titled “When Pneumatic Standalone Controllers Age Out,” discusses alternative process controller designs that address the issues with pneumatic controllers, while providing a host of additional benefits.
Limitations of pneumatic controllers
Since their introduction in the 1940s, pneumatic controllers have been used to keep isolated processes on track. Unfortunately, these controllers degrade in performance over time and eventually fail, and without remote monitoring capabilities, such issues cannot be detected until production is affected.
To make matters worse, with fewer vendors selling these controllers and fewer technicians servicing them, repair or replacement has become increasingly challenging.
A superior alternative
Modern digital process controllers, such as the Fisher FIELDVUE DPC2K, address these shortcomings, offering remote control and monitoring, as well as improved performance. These controllers require minimal electricity, easily supplied by a small solar panel and battery.
Two inputs are available, and each can accept 2- or 4-wire signals. The first input is used for the primary process variable in a PID loop, and the second can be used to accept a remote hardwired PID setpoint, or it be used to pick up a second process variable for internal monitoring. One of these inputs can be replaced with an integrated pressure sensor with ranges from 0-30 to 0-1500 PSI, and this value can be used as the process variable in the PID control loop. The PID control loop output is a 4-20mA signal, or an integrated pneumatic module can provide a 3-15 PSI signal.
These devices meet the latest EPA emissions requirements, whether using air or natural gas as a motive force. They also have an integral valve positioner compatible with the rising stem valves typically provided by most control valve vendors, as well as some rotary valve designs.
Modern technology features
These controller upgrades transcend the limitations of pneumatic controllers. Advanced control options, including anti-reset windup, dynamic reset limiting, and configurable deadband, provide superior setpoint tracking. Additionally, a local interface displays control data and allows for setpoint and configuration modifications at the device.
An Ethernet port enables remote communication via multiple protocols, including Modbus TCP and HART IP. Remote control, monitoring, and device alerts provide plant personnel with easy access and early awareness of developing problems, minimizing downtime and production losses.
Configuration is simple and straightforward, using intuitive menus at the device, or via a free software application with remote access. Maintenance is greatly reduced due to the lack of moving parts, and when needed it requires little technical knowledge since the pressure sensors and pneumatic output sections are modular and can thus be easily replaced.
Common applications
Standalone digital process controllers are well suited for a variety of single-loop applications, including tank farm heaters, heat trace applications, and fuel gas train pressure control. They are especially ideal for backpressure control in remote wellhead oil separator applications, which require tight setpoint tracking.
Additionally, digital controllers may be preferred in tank blanketing applications because blanketing pressure regulators tend to droop at high flows and usually do not offer pressure feedback.
Closing
When the need arises for remote, independent control, consult your valve vendor to learn more about standalone digital process controllers. With improved performance, as well as remote control and monitoring capabilities, they are a worthy successor to the pneumatic controllers of decades past.
