Industrial machines and production systems that are designed to run efficiently and save energy are undoubtedly important considerations in manufacturing today. The reasons not only include rising energy prices and the need to hold down production costs, but also an increasing awareness of the environmental impact on an organization’s sustainability.
All of these issues should not simply be the responsibility of production-floor personnel. Machine and system designers and financial managers of industrial and manufacturing corporations have a stake in this as well.
With pneumatic systems, there are basically three different ways to approach energy savings: leak detection, design, and advanced machine diagnostics. Here’s a closer look at each.
A good leakage-management program is essential. It radically lowers compressed-air consumption and the associated costs as leaks are a waste of compressed air, which translates into lost energy and money. In addition, leaks create pressure losses that can significantly slow production processes and prevent machines from running at peak capacity.
A study by the Fraunhofer Institute in Germany looked at compressed-air systems in the European Union and concluded that 42% of the total potential savings in optimizing pneumatic systems comes from simply implementing a solid leak-detection program. The objective should be to completely check the pneumatic system from the compressor (supply side) to each machine and device (demand side). On the demand side, using a simple ultrasonic leak detector can help manually identify individual leaks on machines — usually for an entire plant. From there, technicians can categorize the leaks by size and importance and record their locations. One of the most common methods is to mark leaks with colored tags containing some basic information:
- Location of the leak on the plant floor (included for the audit report).
- Approximate leak rate in liters/min.
- Classification of leak size and priority for repair.
- Required repair measures and list of parts that need to be replaced.
- Estimated repair time and, if necessary, additional machine downtime.
- Machine accessibility during production.
- Remarks concerning general design-improvement opportunities.
When this information is aggregated in a report and reviewed by stakeholders over a period of time, it highlights problem areas on the production floor. This might include trouble-prone locations within the plant, specific types of machines and equipment, or particular designs that could be causing problems; or necessitate an adjustment of compressor use across production shifts.
Such leak-detection programs yield several benefits. First, leak-detection programs usually need no machine downtime, unless of course detection is to be carried out on parts of a machine that are inaccessible when it’s in operation.
Second, they are a relatively inexpensive. An ultrasonic leak detector is fairly cheap, typically in the $500 to $3,000 range, for a device that is suitable for most locations and can detect most leaks. The headset that comes with a leak detector provides noise cancellation, thus permitting use in noisy production environments where leaks may not be audible.
Finally, once leaks are tagged and documented, in-house repair and maintenance personnel can fix them when normal machine downtime is scheduled.
In addition to leakage management, there needs to be a strong airconsumption management program. Knowing the standard consumption of each machine during different batch processes is a vital prerequisite for properly sizing the compressedair supply and distribution lines and for determining leakage losses.
Experts recommend that such tests be done periodically, both during machine operation and downtime. Characteristics that could provide more insight include consumption per machine cycle, average consumption per minute, average pressure, and minimum and maximum flow rates.
A good air-quality management program rounds out the measures that can be taken in the field. A lot of production and repair personnel still don’t realize that poor-quality air can lead to huge monetary losses. Contaminants such as oil, water condensation, and particulates reduce the service life of components as they accelerate abrasive wear, cause corrosion, and degrade the lubricants built into most pneumatic components.
Keeping contaminants in check helps increase machine availability and process reliability while reducing maintenance costs. Maintenance personnel must regularly inspect filters and airpreparation units, measure pressure dew point and oil content in the air-distribution lines, and document and analyze results.
Even systems with no leaks waste energ y if they’re poorly designed. Engineers can significantly boost efficiency by using the principles of good pneumatic-system design as building blocks to construct a machine. For example, consider the impact of replacing every double-acting cylinder in a machine with a single-acting cylinder; the air savings from eliminating the air-powered retract strokes over the life of the machine would be substantial.
Also consider the fact that single-acting cylinders have one less port per cylinder and use less tubing. Fitting ports that interface with the tubing are notorious for springing leaks. Component costs are also reduced due to one less throttling valve per cylinder. A simple change like this can cut compressed- air consumption by up to 50% and reduce potential leak points by up to 44%.
Today, it is easy to source pneumatic actuators with integrated accessories such as valves and flow controls. With these devices, the hardware for control and actuation of the cylinder rests directly on its own body, therefore eliminating expansive runs of tubing from the controller to the actuator. This, in turn, reduces the probability of leaks and unnecessary pressure drops that degrade machine performance.
Also, when specifying individual valves, engineers must optimize the flow rates of the valve without under or oversizing. Undersized valves can “choke” flow and limit performance. Oversized valves, on the other hand, not only increase air and energy consumption, they unnecessarily add to cost, size, and weight and can slow actuation time. For more details on component sizing and circuit design, see the accompanying sidebar, “Software for sizing, simulation.”
The type of valve can matter, too. For example, specifying a 5/3 solenoid valve has a slight advantage over a 5/2 valve for the same application. Interestingly, the reason is not about air-pressure costs. In a 5/3, when the valve is switched to the centered position during long periods of inoperation, it consumes less electric current compared to a 5/2 valve. Switching to valve manifolds from stand-alone valves ensures space-saving designs and fewer ports and fittings, which means less chance of leaks.
When it comes to monitoring vacuum applications (one of the biggest sources of energy loss of any pneumatic application), always consider using inexpensive vacuum switches that cut off the air supply when the desired vacuum level is reached.
When running machinery in dusty environments, make sure to use lubricated compressed air. A filter-regulator-lubricator (FRL) assembly makes this easy. FRLs prevent dust from entering the cylinder and interfering with the lubrication, which is intended to last the entire working life of the actuator. In addition, keep the system pressure supply in check by using a regulator and digital pressure switch. Simple calculations, such as the total volume of the pneumatic assembly and cycle times, should give a base number for the necessary supply pressure. Make sure the actual pressure supply is close to the base number and not significantly higher. A good FRL easily performs both these functions.
And as with other components, make sure the filters are the right size and that they are replaced periodically, as a clogged filter can create a big drop in supply pressure. Modern filters incorporate digital indicators, which supply an electrical signal when the filter is clogged beyond acceptable limits. These signals can be tied into the process PLC and trigger a message to the operator that a maintenance check is needed. Moreover, these assemblies can also include a moisture drain that removes water from the pneumaticdistribution lines and can operate automatically.
As an added note: Wherever possible, design in guide rods to prevent a cylinder’s piston rods from deflecting, because deflection of the piston rods damages seals and thus induces premature leakage. Guide rods also protect the cylinder from lateral forces and prevent redirection of the cylinder stroke from the workpiece.
The advanced machine-diagnostics concept is a relatively new idea in the pneumatic field. It promotes dedicated and permanent condition monitoring of machines to help detect changes in critical process parameters early enough to prevent downtime and, more critically, avoid catastrophic failures. If downtime still occurs, advanced diagnostics can quickly analyze the cause of failure.
Condition-monitoring systems can provide information as basic as simply indicating something is wrong or as complex as pinpointing the exact component on a subsystem level that is exhibiting a failure pattern. It can also home in on the root cause of failure, the failure type, and detailed steps to fix the problem. Today’s condition-monitoring systems can collect data from multiple pressure and flow sensors embedded over distant machine locations via industrial Ethernet, display data in real time, create baseline thresholds, and churn out diagnostic information, all from one intelligent device and display.
On a subsystem (critical-component) level, machine diagnosis can be implemented by installing an inexpensive flow sensor at the component’s air inlet. Nowadays, flow meters double as air-consumption meters that deliver a pulse every time a preset air consumption level is reached. Pulses are counted in the master controller to check if consumption in critical components is within preset limits.
Similarly, on the “system” level (entire machine), dedicated hardware can monitor parameters such as pressure, flow, air consumption, actuator stroke times, valve cycle times, and vacuum suction and evacuation times. This information can then be used to create baselines for each parameter which can be monitored on a permanent basis. A typical system might display simple colors like green, orange, and red to indicate System OK, Warning, and Alarm, respectively.
The real advantage of such systems is the guidance they give to maintenance personnel. For example, they can tell repair technicians:
- If there is an alarm for a problem, where the problem is.
- The type of problem.
- How to fix it.
- The repair sequence.
Automated systems which do the above are exactly what are needed to keep equipment and plants running in an energy-efficient mode while delivering peak performance. Data logging lets manufacturers archive machine performance and gives them the ability to evaluate the residual life span of equipment.
By implementing leak detection, air management, design engineering, and condition monitoring, companies will not only save thousands of dollars in operation costs over the life of the machine, but also increase productivity by optimizing machine performance.
Software for sizing, simulation
One of the highly recommended ways to select pneumatic components is to use interactive software, which is available from several major manufacturers. Festo’s online sizing tools, for example, include selectors for products such as pneumatic drives, compressed-airpreparation units, grippers, shock absorbers, and rotary indexing tables, as well as calculators for determining air consumption of cylinders and mass moment of inertia.
Such software helps engineers size components and run simulations to test operating parameters for them. Generally, users merely need to enter the general technical conditions, run the simulation, and evaluate the results, such as a survey of pressure and air consumption pro les and stroke times in real time. Then, they can confidently select the appropriate products from a list of suggestions.
Simulation lets designers interactively experiment with components without the cost, complexity, and time required for physical testing. For example, they can nd the products that produce the most throughput or force for the desired stroke, that consume the least amount of air, or the ones that balance these factors for the highest productivity and profit.
Of course, using interactive software to research components can increase the time necessary for engineering and system design. But consider both the fact that the life span of industrial machines can easily exceed 10 years, and the savings the right choice will generate over this time. A cost-benefit analysis or even a Total Cost of Ownership (TCO) analysis should con rm such engineering-design techniques create improvements and save money over rule-of-thumb estimates.