When Pneumatic Standalone Controllers Age Out

Summary

They are out there. Largely ignored and scattered across remote oilfield sites, tank farms and isolated process units they sit, quietly hissing now and again, and steadfastly doing their job as they have for years. Unfortunately, their days are numbered.

Affectionally known as “wind powered” and incorporating temperature or pressure sensors, standalone pneumatic controllers have been used to locally control level, pressure, flow and temperature in remote locations for decades, requiring only low-pressure air or line-pressure natural gas for operation. Despite breathtaking leaps in control technology improvements, these controllers are still a simple and acceptable solution for many niche applications.

More than 2.3 million of these devices are still in service but many vendors no longer make them, and more are abandoning the market every year. So, finding a direct replacement can be difficult. In addition, pneumatic controllers have a range of inherent limitations—not the least of which is finding personnel skilled with maintaining this technology—that make direct replacement less than ideal. This article discusses alternative process controller designs that address the issues with pneumatic controllers while providing a host of additional benefits.
 

Quietly doing their job, but for how long?

Pneumatic controllers have been around since the 1940s. Through the years, the pneumatic controller has been minimally upgraded (Figure 1), with some devices incorporating remote setpoints and full PID control algorithms, and a few devices adding some means for remote monitoring. Still, the bulk of applications use a simple local controller keeping a process on setpoint in a remote area. Through good weather and bad, these devices operate 24/7, adjusting a pneumatic valve as necessary to keep the process on track.

Unfortunately, there are aspects of pneumatic controllers that limit their utility and can create issues. Pneumatic controllers incorporate many moving parts, and over the years those parts eventually wear and fail. Control to setpoint degrades over time and ultimately stops, creating production losses and downtime. Most of these controllers provide no means of remote monitoring or adjustment, so when control degradation or an outright failure does occur, it is not noticed until deviation from setpoint creates problems affecting production.

Even when the existing pneumatic controller is an acceptable solution, fixing the unit or finding a replacement has become increasingly challenging as few technicians can work on pneumatics, and many pneumatic controller vendors have exited the market.

Remote control alternatives

Control vendors recognized this developing problem and worked to resolve it. Local controller replacements have become available that satisfy the needs of remote, standalone control—yet take advantage of the latest technology to enable significantly improved control performance, while providing remote access, monitoring and control.

These replacement digital process controllers do require electrical power, but at less than one watt, this can easily be furnished with a small solar panel and battery. However, the addition of such a nominal amount of power yields dramatic improvements in control capability, and it enables remote monitoring and control. These devices also incorporate several configurable I/O options that allow a single device to address a host of control needs (Figure 2).

The latest process controllers incorporate two analog inputs that can accept 2- or 4-wire inputs. The primary input is used as 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 0-1,500 PSI, and this value can be used as the process variable in the PID control loop. The PID control loop output is a 4-2 0mA signal, or an integrated pneumatic module can provide a 3-15 PSI signal.

The device is rated Class 1 Division 1, and it uses low-bleed Quad O certified components, so it can run on air or natural gas and still meet the latest EPA emission requirements. This allows the controller to be used in a broad range of remote-control applications, including oil and gas industry remote sites and other hazardous locations.

These replacement controllers also include an integral linkless positioner feedback to handle control valve positioning duties. The positioner works on rising stem valves from most control valve vendors, as well as some rotary valve designs. The controller can be mounted on the valve (allowing positioner feedback), or it can be remotely mounted if a positioner is not required. When purchased with the integral pressure sensor and pneumatic output module, this single device can replace a pressure transmitter, controller and valve positioner.

Taking advantage of technology

These types of hybrid controller replacements also take advantage of the latest technology to address the inherent limitations of pneumatic controllers. The digital PID loop operates on 50 millisecond cycles, making it suitable for very fast and difficult to control process applications.

The list of advanced control options and parameters includes anti-reset windup, dynamic reset limiting and configurable deadband, enabling this type of controller to provide far superior tracking to setpoint as compared to its pneumatic predecessor. A local digital interface (Figure 3) displays control data, and it allows easy setpoint and configuration modifications at the device.

In many applications, the most important technological advance will be the remote communication capabilities that are now incorporated within the controller. These types of devices can typically communicate via Modbus TCP, HART IP, and/or Modbus RTU RS-485. The Ethernet port can handle multiple protocols simultaneously using Modbus TCP or HART IP, and all digital protocols provide remote control and monitoring capability.

This communication capability allows the local controller to independently perform its control tasks, while providing a means to remotely change setpoints, gather analog signals from up to two devices, detect alarms and equipment alerts, and reconfigure the device if necessary. With this capability, the pneumatic controller is no longer an isolated island of automation, but it can instead provide process data to indicate developing problems, empowering plant personnel to react proactively to minimize downtime and production loss.

These new controllers have also been designed with ease of use and maintenance in mind. 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.

Applications for standalone digital process controllers

Standalone digital process controllers are often the preferred solution for a wide variety of single loop control applications. Common use cases include heaters in tank farms, pressure and temperature controllers for heat trace applications, and fuel gas train pressure controllers. These controllers are also frequently used on equipment skids where local control is required.

Another very common application is a back pressure controller for remote wellhead oil separator applications (Figure 4). While oil/water interface and oil level control can be accomplished by other means, backpressure control on the separator usually demands tighter tracking to setpoint, as can be provided with a PID controller.
These new replacement process controllers are tailor made for this application and are often employed during wellhead electrification projects to limit or eliminate methane emissions. The controllers can also be used in advanced oil recovery projects that require local flow or pressure controls at remote wellhead sites.

Another common application is tank blanketing applications. These advanced pneumatic controllers provide local and independent pressure control, and they also provide a means to remotely monitor tank pressure, and alarm if the blanketing system is not operating correctly. This type of solution is far superior to a blanketing pressure regulator, which tends to droop at high flows and offers no means of pressure feedback.

Conclusion

If your plant has a number of aging pneumatic controllers—or if you encounter an application where remote, independent control is necessary—consult with your valve automation vendor to investigate new standalone digital process controllers. The control capability of these new solutions is far superior to what has been available to date, and their flexibility allows them to be used in a very wide variety of flow, pressure, temperature, and level control applications.

These pneumatic controller replacements also offer the ability to monitor, control and even configure these devices remotely, and they provide a wealth of process data previously unavailable from these remote, single-loop control applications. These venerable pneumatic controllers have done their duty for decades, but as they age out, a very capable and superior replacement is now available.
 
<sub>All figures courtesy of Emerson

This feature originally appeared in the April 2024 isue of InTech digital magazine.</sub>