Closing the loop to maintain precise hydrogen pressure

Inside Machines: A programmable electro-hydraulic motion controller improved standard deviations of the discharge pressure distribution by about 70%, helping improve manufacturing productivity in an Alberta EnvironFuels application. A closed-loop controller runs at 1000 loops per second to optimize tuning and respond more quickly and automatically to changing conditions.


Figure 1 shows a side view of Alberta EnviroFuels hydrogen compressor. Courtesy: Delta Computer SystemsFaster 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).

Smooth precision

“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

Figure 2 is the Moog Direct Drive Explosion Proof proportional valve selected for the Alberta EnviroFuels compressor control upgrade. Courtesy: Delta Computer SystemsAfter 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.

2-axis control

Figure 3 shows that the motion controller that PQ systems recommended, the RMC75 manufactured by Delta Computer Systems Inc. of Battle Ground, Wash. The RMC75E motion controller supports two axes of control, interfaces directly to analog feedback devices,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.

Figure 4, a control diagram, shows connections between the DCS and motion controller and other system components. Courtesy: Delta Computer SystemsNow 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.

Tuning wizard

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.

Figure 5 is a plot produced by Delta Computer Systems’ Plot Manager software, showing the position feedback input versus target value and motion control output over time for both slide valves. Courtesy: Delta Computer Systems

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, mhoske(at)


See other motion control articles at the bottom.

Key concepts

  • Controllers and actuators need to work together smoothly and precisely in many applications.
  • Work-around solutions included restrictors and check valves.
  • Updates included proportional servo valves and an electro-hydraulic motion controller. 

Consider this

Can a new controller offer higher quality through more precise closed-loop control?

No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2013 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
The true cost of lubrication: Three keys to consider when evaluating oils; Plant Engineering Lubrication Guide; 11 ways to protect bearing assets; Is lubrication part of your KPIs?
Contract maintenance: 5 ways to keep things humming while keeping an eye on costs; Pneumatic systems; Energy monitoring; The sixth 'S' is safety
Transport your data: Supply chain information critical to operational excellence; High-voltage faults; Portable cooling; Safety automation isn't automatic
Case Study Database

Case Study Database

Get more exposure for your case study by uploading it to the Plant Engineering case study database, where end-users can identify relevant solutions and explore what the experts are doing to effectively implement a variety of technology and productivity related projects.

These case studies provide examples of how knowledgeable solution providers have used technology, processes and people to create effective and successful implementations in real-world situations. Case studies can be completed by filling out a simple online form where you can outline the project title, abstract, and full story in 1500 words or less; upload photos, videos and a logo.

Click here to visit the Case Study Database and upload your case study.

Maintaining low data center PUE; Using eco mode in UPS systems; Commissioning electrical and power systems; Exploring dc power distribution alternatives
Synchronizing industrial Ethernet networks; Selecting protocol conversion gateways; Integrating HMIs with PLCs and PACs
Why manufacturers need to see energy in a different light: Current approaches to energy management yield quick savings, but leave plant managers searching for ways of improving on those early gains.

Annual Salary Survey

Participate in the 2013 Salary Survey

In a year when manufacturing continued to lead the economic rebound, it makes sense that plant manager bonuses rebounded. Plant Engineering’s annual Salary Survey shows both wages and bonuses rose in 2012 after a retreat the year before.

Average salary across all job titles for plant floor management rose 3.5% to $95,446, and bonus compensation jumped to $15,162, a 4.2% increase from the 2010 level and double the 2011 total, which showed a sharp drop in bonus.

2012 Salary Survey Analysis

2012 Salary Survey Results

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.