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Faster closed-loop control for compressors has helped Alberta EnviroFuels, a division of Keyera Corp. of Calgary, Alberta, Canada, better produce iso-octane, a blending component of automotive gasoline, from a feedstock of field butane. Several of the internal process steps require high-purity hydrogen. This hydrogen stream is acquired by compressing a low-pressure gas stream that contains about 50% hydrogen by weight, and then feeding the compressed stream into a hydrogen recovery unit. Figure 1 shows one of Alberta EnviroFuel’s four hydrogen compressors.
Because the feed stream into the hydrogen recovery unit is very light, process designers selected positive displacement compressors, specifically wet screw compressors, to accomplish the feed compression. Since the demand for high-purity hydrogen can vary from moment to moment, the compression process also needs to provide a method for matching the volume of feed gas delivered by the screw compressors to the demand for purified hydrogen. Instead of supplying a traditional external spillback control valve (which would have been manipulated based on holding a constant discharge pressure), the compressor vendor suggested use of screw compressors with internal slide valves. Extending or retracting these internal slide valves varies the displaced volume (the effective compressor capacity).
“The challenge that we faced,” said Phil Prins, senior process control engineer of Alberta EnviroFuels, “is that while we are always striving for precise control of our process variables—the capacity of the compressors in this case—we often put an even higher premium on smooth control.” And positioning of the internal slide valves with the original control scheme was neither precise nor smooth.
Technically, Alberta EnviroFuels’ screw compressors are two-stage models. The first stage takes the gas to 250 psi and the second stage goes from 250 psi to 500 psi. Each stage has its own slide valve. The motors driving the compressors produce about 1250 hp. The compressor is 8 ft long by 2.5 ft across. Hydraulic actuators control the slide valve since moderately high-pressure oil was already available to seal the rotor gap inside the compressor and lubricate the radial and thrust bearings.
The company initially controlled the slide valves via a distributed control system (DCS) that communicated a position target for each slide valve, using a 4-20 mA analog link to a Modicon programmable logic controller (PLC). In turn, the PLC energized solenoids to activate two-position hydraulic shuttle valves to energize cylinders to move each compressor slide valve in one direction or the other, based on whether the pressure was above or below its setpoint.
“We had a very hard time getting the slide valves to move smoothly and precisely to the target position, and so we couldn’t hold pressures at equilibrium,” said Prins. “In the field we had all sorts of restrictors and check valves attempting to try to convert essentially on/off controls into proportional controls. In the PLC, we had all sorts of timers and deadbands trying to balance process needs with hardware limitations.” The application needed controls to actuate the slide valves with “very precise and smooth movements,” continued Prins. “Dealing with the hydraulics is an area where we don’t have a lot of experience.”
Proportional servo valves
After studying the problem and consulting with engineers from motion control distributor PQ Systems of Burnaby, British Columbia, Canada, it was decided to replace much of the field hardware, and to use direct-drive proportional servo valves so that tighter control of the hydraulics could be accomplished (Figure 2). That raised the question of what would replace the position control software, previously in the PLC. Several PLC- and DCS-based options were considered, but the PQ Systems engineer advised that an easier and more effective way would be to interface the DCS setpoint signal for each slide valve to an electro-hydraulic motion controller, designed for smooth and precise closed-loop control of hydraulic actuators in all environmental conditions.
Oil temperature was one of the variables that caused the original valve control system to behave poorly. During normal compressor operation the oil temperature is fairly consistent. However, when a compressor is in standby mode (not running, but available for immediate start-up), the oil temperature can vary significantly. This can have an effect on the time that the valve takes to move. The old system with the PLC controlling the bidirectional shuttle valves didn’t handle the temperature differences smoothly.
PQ systems recommended a motion controller (Figure 3) that can simultaneously control two motion axes, so one controller can control both slide valves in the application. For feedback on the position of the slide valves, the controller was connected to a potentiometer mounted on each slide, producing an analog voltage relating to slide extension that can be read directly by the controller. Figure 4 shows how the system components are connected.
Another analog input module on the controller connects to the output of the DCS-based process pressure controllers. Since the same DCS control scheme is used in the upgraded system, functioning in the same manner, the compressor’s operator interface needed only very minor changes as a result of the upgrade. This was a big plus for plant operators. The controller improved the way the controls work: The old hardware led to control loops that tended to be undertuned, which meant that the controls didn’t respond well to upset conditions, and operators had to get involved if quick action was needed.
Now with the closed-loop controller running at 1000 loops per second, tuning can be optimized and the controls respond more quickly and automatically to changing conditions. As a result, the hydrogen purification process maintains the target pressure more precisely and responds to environmental changes more quickly. Also, the control system no longer induces its own fluctuations that plagued the previous controls.
PQ Systems applications specialist Rob Nash did the initial controller programming, with the assistance of Delta Computer engineer Don Denman. Those involved said the controller is easy to program, and since the design was initially implemented, Alberta EnviroFuels engineer Prins has made changes to tweak the system’s operation. The controller uses an Ethernet interface, so Prins created an interface with the controller and made software upgrades from his office desk over the corporate network. “That’s a lot more convenient than making trips to the plant floor and back,” said Prins.
The tuning wizard and the plotting functions supported by the software provided “were very helpful features used during commissioning,” said Nash. “The entire commissioning process for both slide valves took only a couple of hours,” he continued. “Without plotting it would have taken much longer, and logistically, since the controller was located across the plant outside of the hazardous area of the compressor and therefore out of sight visually, tuning of a slide valve without plotting capability would have been nearly impossible.” Figure 5 shows a typical plot produced by software.
This application provides an example of how a programmable electro-hydraulic motion controller can be used to bring a process under control that was previously impacting the productivity of a manufacturing operation.
“Comparing the standard deviations of the discharge pressure distribution before and after the controls upgrade, we saw an improvement of about 70%,” said Prins. As Alberta EnviroFuels found out, where precise motion and tight tolerances are involved, it pays to select a control system that is designed for the task. In this case, a motion controller had a clear advantage over a PLC.
- Don Denman is senior applications engineer, Delta Computer Systems Inc. Edited by Mark T. Hoske, content manager, CFE Media, Control Engineering and Plant Engineering, firstname.lastname@example.org.
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