Distributed Control / Vibration

Compressor Control Comes Out of the Black Box

Refinery Replaced Proprietary Boxes with Open Systems for Surge and Performance Control

By Paul Studebaker

 

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Does your bucket list include implementing fully redundant integrated turbomachinery control on a two-stage coker wet gas compressor in a refinery? In six weeks? In Mexico?

That's the task described by Paul Fisher, president, Energy Control Technologies (ECT) and Cecilio Gomez, president, Oros Electromechanical (Oros) at the Rockwell Automation Process Solution User Group (PSUG) conference, in Anaheim, California.

ECT deals with axial, centrifugal, screw and reciprocating compressors as well as steam and gas turbines commonly found in floating production, storage and offloading (FPSO), offshore, refining, petrochemical and, especially recently, LNG applications (as a result of increased fracking). ECT specializes in complex applications with networked compressors, load sharing, turbine-driven compressors and plant air systems with mixed compressors. Its products include specialized packages for process, surge and sequence control, HMI, and lubrication and vibration monitoring. Its services range from sales to startups, support and training.

ECT competes with "black-box companies" that provide purpose-built systems for compressor, turbine and surge control. "We keep all the good things and put them in an open environment," said Fisher.

In Mexico, Oros builds and installs the systems and provides communications and training in Spanish. The company has a 23-year history integrating compressor, turbine, fire and gas, and emergency shutdown applications, often involving vibration, lubrication and dry-seal systems.

The coker compressor application described in the presentation required redundant control (including inputs, outputs and power supplies) using open-platform PlantPAx and ControlLogix technology with dedicated compressor control. "That's a little bit of a leftover from the black box," said Fisher. "We did it as dedicated surge control integrated into PlantPAx." The system also had to be SIL-certified, have transmitter failure fallback and be "simple to understand," Fisher said.

"The way the customer operates, they take a long time to develop the specifications and approve a system," Fisher said. "Then they expect you to build, install and commission it in six weeks. It's easy for ECT, but hard for Cecilio."

The high-level control to detect and prevent surges required 40-ms execution and known invariant coordinates. The 10,000-hp, motor-driven two-stage compressor has variable-speed drive, two surge control valves in parallel on the low pressure stage and one valve on the high-pressure stage. "There's a lot of interaction between control and surge," Fisher said.

ECT's SurgePAC controls surge; its PerformancePAC runs the motor drive. The process involves variable molecular weight wet gas and changing out coker drums so, "It's a challenge to keep pressure from fluctuating," said Gomez. The compressor is the limiting factor in production, and the key is to have the compressor operating envelope as large as possible to maximize production by handling the variations without surge.

The HMI shows exactly where the compressor is operating on the invariant diagram. "You can see where you are in relation to the danger point at all times," said Fisher. To establish that envelope, the compressors are surge tested, and the reason for 40-ms response becomes apparent. "We bring the compressor into the danger zone to see where the surge point is and immediately open the valves to bring it back. Then we repeat that at different speeds to find the operating envelope of the compressor."

Fisher and Gomez measure the success of a system by whether or not it is kept in automatic. One important element is excellent surge control; another is reliability. "If the operator doesn't know what's going on, he immediately switches to manual," Fisher said. "Once it goes to manual, it's very difficult to get it back on automatic."

"Standard Rockwell Automation redundancy doesn't meet the needed performance specs, and we have to maintain the 40-ms speed," Fisher said. The solution involves a second I/O chassis, proprietary field termination assemblies and redundant transmitters. These are ECT global-standard solutions, "so any developer and any service technician can handle them," said Fisher.

Redundancy allows the system to stay in automatic through a failure, and the faceplates make it easy to drill down and identify the cause. If card A detects a failure, it shifts control to card B and indicates a fault. "You can easily see if the failure is in a card or in the field," Fisher said.

Now the refinery's two-stage coker wet gas compressor sports state-of-the-art parallel multi-section surge and performance control using an open ControlLogix platform with PlantPAx with speed of response equal or better than a black box. It meets SIL, API and customer specifications. And it stays in automatic.