With issues such as temperature extremes, washdown capabilities and cleanliness considerations, components for this industry must be thoughtfully selected.
By Richard McDonnell, Market Development Manager, Motion Systems, Parker Hannifin Corp.
Recently, a food processing OEM was designing and installing a new line of extremely high-speed filling machines intended to deliver higher performance with a smaller shop floor footprint. Redesigning the pneumatic valve system in the OEM’s machinery using collective wiring and stainless-steel filter-regulators cut the size of the overall footprint nearly in half and minimized both build and installation times. By relying on a supplier that understood their overall processes and helped them choose the right components, the OEM was able to realize a variety of bottom-line advantages:
- They halved the size of machine’s control panel.
- Their costs for components decreased by almost $1,000 per machine.
- Each machine required one hour less wiring time and one hour less plumbing time.
- There was an operating cost savings of $2,200 per machine ($22,000 per year).
The combination of timely advice and intelligently chosen pneumatic components is critical to the ultimate success of systems like these. For any given application, selecting the optimal pneumatic component depends on understanding two main issues:
- The required operating performance and parameters
- The operating environment.
For example, when considering the operating performance requirements for pneumatic cylinders, design engineers first must determine parameters such as available air pressure, desired speed and cylinder power, and maximum theoretical force. This force must be 50-100% greater than the basic force required, to account for effects such as friction, potential lateral loads on the piston rod and braking effects at the end of the stroke. These values are typically included on the manufacturers’ datasheets; if not, the manufacturer can offer guidance on calculating them.
Standard pneumatic cylinders have an end cover, cylinder, piston, piston rod and front gable, along with a piston rod bearing and scraper seals. Their general design adheres to ISO standards. In most applications, the cylinder and end caps are made of aluminum, to provide the level of strength required without adding excessive weight; chrome-plated steel or stainless steel are often used for the piston rod.
Pneumatic cylinders’ performance characteristics are often specified for an operating temperature range of ‑20° to 80° C; however, more challenging applications require cylinders that can exceed these limits. To meet these demands, a growing number of pneumatic cylinder manufacturers have developed products designed for environments as low as -40° or as high as 150° C, which can require the use of higher performance lubricants and special sealing materials.
All too often, the operating environment is overlooked in the process of selecting a pneumatic cylinder or accessory. Environmental factors may include ambient temperature, vibration, operating media/chemicals and other fluids or materials present. However, a variety of other aspects of the operating environment should also be considered. For example, in humid operating environments or those that pose a risk of salt spray, all external cylinder surfaces need to be coated, and the piston rod should be made of stainless steel.
Requirements for cleanliness
Applications in food and beverage production typically require specific certification of the pneumatic cylinder and other associated equipment and special clean design features that minimize entrapment points for bacteria. Multiple national and international standards must be taken into account. For example, 3‑A Sanitary Standards Inc. is dedicated to advancing hygienic equipment design for the food, beverage, and pharmaceutical industries.
An online database simplifies identifying vendors of components that comply with 3-A certification. Other important standards and guidelines are published by NSF (National Sanitation Foundation) International, the International HACCP Alliance, the American National Standards Institute (ANSI), and the Safe Food Alliance.
Food production environments necessitate frequent wash-downs of the work area, which can lead to damage to static and dynamic gaskets and seals. Constant exposure to damp and the caustic sprays of hydrogen peroxide and other cleaning materials used in wash-down cycles can eat away at unprotected materials. These environmental challenges have made stainless steel the most commonly used material for all food processing applications. Although stainless steel is more expensive and more costly to machine than aluminum, it can resist the steam, high pressure water, and caustic cleaners often used in food and beverage production. This makes it easier for designers to meet stringent requirements for hygiene and corrosion resistance, which justifies the higher initial investment for many machine designers.
Fortunately, however, some new, less costly alternatives to stainless steel pneumatic cylinders are available for food and beverage processing applications, Figure 1. They feature cylinder barrels and end covers made of anodized aluminium, with stainless steel piston rods, piston rod nuts, and end cover screws. Pistons, piston bearings, and piston rod bearings made of polyoxymethylene, an FDA-approved thermoplastic with high stiffness, low friction, and excellent dimensional stability. Polyurethane scraper rings, piston seals and cushion seals are used to seal in keep the transparent food-grade grease.
No matter which component is being specified, it’s critical to understand the details of the food processing application and what is required—such as pressure, temperature, flow, and port sizes, configurations, and locations. Too often, filters or valves are chosen based on cost or size alone, forcing maintenance personnel to spend extra time on maintenance as a result of the system designer’s less than optimal choice. Longevity and repeatability are basic requirements for any good pneumatic solution. Choose components that have been thoroughly tested and designed to withstand the toughest conditions for operation, vibration, and impact.
In food processing applications that involve sub-freezing temperatures, specify products designed to withstand these conditions without sacrificing performance or reliability, Figure 2. In a filter-regulator, for example, the air must be dry enough to avoid ice formation. If temperatures reach 32° F or lower, performance will be degraded because liquid particles become gelatinous or frozen.
When specifying pneumatic cylinders, take a careful look at the ingredients to which they may be exposed; for example, ice, sugar crystals or dough can form deposits on piston rods that will interfere with their operation. To alleviate this problem, choose options that allow installing metal scraper rings (rather than rubber or plastic ones) with the piston rod seal to remove the residue before it can damage the cylinder assembly.
Watch out for water
The accessories and options for pneumatic components are frequently neglected, so it’s important to ensure the entire product can withstand the environment where it will operate to avoid forcing maintenance personnel to waste time replacing parts. For example, the adjustment knob or T-handle of a typical regulator is made of a composite material. The caustic chemicals used in wash-down can corrode many types of plastic, so in addition to a stainless steel regulator, the knob should be made of stainless steel or other compatible material.
Filter-regulator options such as tapped manual drains or automatic stainless-steel drains are widely used to get rid of excess liquid and prevent water from draining onto the floor. Look for non-relieving regulators that do not release gases or liquid into the atmosphere. Whenever possible, select pre-lubricated or lubrication-free mechanisms that use food-grade grease and don’t require periodic lubrication.
Although some pneumatic valves meet NEMA protection standards or IEC/IP ratings, most are designed to be mounted in an enclosure to protect them during wash-downs. Check the design of this enclosure for any crevices between the valves and subplate or manifold bases and other non-smooth surfaces that can harbor bacteria. For those who use serial communications with their valves, these electronics also require protection.
Components that require lubricated compressed air or periodic manual lubrication should be avoided when working in food processing to minimize the risk of product contamination. Lubricant in the compressed air can collect near exhaust ports, and manually applied lubricant can spill onto or collect on multiple components.
Using dry air in non-lubricated applications is critical; condensation can corrode system components, increasing maintenance costs and reducing system efficiency. Also, unless distribution air lines are made of stainless steel, aluminum, or high-strength plastic, water can create pipe scale that can work its way into components and cause malfunctions. Water is a poor lubricant; when emulsified with residual compressor oils, it becomes a milky substance that must be drained away. Depending on the amount of liquid produced, this can lead to problems with OSHA. In addition, there should never be any contact with synthetic emulsions in food processing. Dry, filtered, non-lubricated air usually eliminates these issues.
Making smarter choices
Over and above issues related to a component’s required performance and the environment in which it must operate, food processing OEMs are increasingly demanding systems that can connect seamlessly and communicate intelligently with other systems within the facility.
For example, IO-Link smart control of pneumatic valve manifolds makes process data easily accessible and allows monitoring by PLC. This point-to-point communication technology for sensors, actuators and control devices can be built into most PLC configurations or incorporated into most existing industrial Ethernet and fieldbus networks. IO-Link technology can be especially beneficial for machine applications that require frequent changeovers; both reconfiguration time and unplanned downtime can be reduced significantly.
Although the choice of pneumatic components for any industrial application is often a matter of making tradeoffs to work within budget restrictions, system designers can prevent future frustration and ensure optimal equipment performance by resisting the temptation to cut corners on corrosion resistance, cleanliness issues, and long-term maintenance costs while laying the groundwork for additional expansion and enhanced system communication.
Parker Hannifin Corp.
Filed Under: Cylinders & Actuators, Fluid Power World Magazine Articles, Slider