The mechanical performance of rotary valves is affected by how they contain pressure or feed materials during processing. Rotary valves require special attention to prevent them from corroding and cross-contaminating product. As a basic aspect of proper rotary valve maintenance, those who work with equipment that features these valves should prioritize protection against corrosion and cross-contamination.
Rotary Valve Maintenance: Preventing Corrosion and Cross-Contamination
Also known as rotary airlocks or rotary feeders, rotary valves can serve for without trouble for many years when properly cared for. Correctly maintaining these valves is of importance in industries where corrosive materials are used, such as the chemical processing and pharmaceutical industries. Rotary valve maintenance is equally important in the food processing and pharmaceutical industries, where cross-contamination can be problematic.
Types of Corrosion & Cross-Contamination
There are numerous ways in which metals, alloys, or other substances can cause corrosion, leading to deterioration of process equipment. Rotary airlock maintenance procedures should deal with the most common types of corrosion. Many of these are interrelated, so if one type of corrosion has been identified, other types may be present as well.
Here are the most common types of corrosion:
- Cavitation corrosion: Caused by imploding gas bubbles on a metal surface, it erodes the metal to cause pitting; It’s a localized problem associated with variations in pressure usually caused by water, an issue that affects pumps without proper water flow and poor surface conditions.
- Concentration cells: A type of galvanic corrosion caused by a metal surface exposed at multiple areas to dissimilar concentrations of conductive solutions, with the metal exhibiting differing electrical properties at these points of distinctive concentrations.
- Crevice corrosion: This occurs at joined surfaces where two metals or a metal and nonmetal meet; this is essentially design-related corrosion that occurs where joints and shapes that form crevices to which oxygen has no easy access. Factors influencing it include:
- Composition or structure of metal alloy.
- Concentrations in the environment of oxygen or chlorides.
- Environmental conditions like pH and temperature.
- Surface roughness or other geometric features that create crevices.
- Erosion corrosion: Caused by corrosive fluids contacting metal surfaces, erosion corrosion deteriorates metals or alloys, causing surface irregularities; preventing this involves improving the design or utilizing more resilient materials.
- Flow-assisted corrosion: Also known as flow-accelerated corrosion, it occurs when the layer protecting a metal’s surface is exposed, due to factors like wind or water.
- Galvanic corrosion: This type of corrosion occurs when two disparate types of metal are submerged in a conductive and corrosive liquid before being exposed to an electrical current; this type of corrosion occurs when a galvanic metal or alloy like 316 stainless steel is used for an impeller in pumps that handle seawater, but the pump’s body is made from austenitic cast iron.
- Intergranular corrosion: A type of selective corrosion, it involves localized corrosion occurring near a metal’s or alloy’s grain boundaries.
- Pitting corrosion: Though the damage appears similar to corrosion caused by cavitation, it’s mostly found on passive alloys and metals where the oxide film is damaged mechanically or chemically so that it cannot re-passivate; pitting can be narrow and deep, perforating metal walls, and it can be prevented by protecting cathodes, controlling chloride concentrations and using appropriate materials for the application.
- Selective corrosion: This involves the internal structure of a metal or alloy, which leads to corrosion; examples include dezincification and intergranular corrosion.
- Stress corrosion: Also referred to as stress corrosion cracking (SCC), it involves a corrosive medium combined with tensile stressors.
- Uniform corrosion: Also known as general corrosion, it affects exposed surfaces made from ferrous metals and their alloys, which aren’t protected by surface coatings or plating techniques.
Cross-contamination occurs primarily due to improper cleaning or disinfection, which can affect the rotary valve. Maintenance to prevent contamination of products during processing generally involves keeping equipment clean, though cross-contamination can also occur due to conditions within the processing environment, which can include chemicals, microbes, and other contaminants. Causes of cross-contamination can include the equipment used for cleaning. Cross-contamination is often classified as either direct or indirect.
In the food processing industry, direct cross-contamination occurs when two types of food contact each other; such is the case when raw meat touches cooked foods. This can cause bacteria or other microbes to transfer from raw to cooked food. In the pharmaceutical industry, cross-contamination occurs when one pharmaceutical ingredient mixes with another undesirable ingredient. Cross-contamination also results when liquids from one substance contaminate another or when the surfaces of two materials mix.
In addition to these types of contaminants that directly taint materials during processing, inadvertent contamination can occur due to improper cleaning or when waste product collects in processing equipment or machinery, such as a rotary valve. Maintenance and cleaning must also be done to prevent dirt, human dandruff, insects, microbes, rodent feces or other contaminants from directly tainting material during processing. For this reason, cleaning is a priority in the food processing and pharmaceutical industries.
Corrosion during production is a problem in many industries. For example, high chloride levels can corrode metal surfaces in the oil and gas industry, causing pitting and rust. Direct cross-contamination from corrosion can also cause color problems in the powder coating sector, causing discoloration of pigments and paints. The best way to deal with corrosion is to prevent it from occurring in the first place.
Rotary Valve Materials
Certain materials aren’t conducive for a rotary valve, depending on the industry. Maintenance issues from caustic chemicals can quickly wear out valves made from cast iron. For this reason, stainless steel is standard in the chemical processing sector or any industry that works with harsh chemicals that can corrode, contaminate or otherwise damage the final product.
Any crack or imperfection can corrode a rotary valve. Maintenance that prevents corrosion by applying a corrosion-resistant coating will prevent destructive materials from gathering in these places and breaking down surfaces. Rotary valve maintenance may include coating the valve or nearby components that are exposed to these harsh substances, preventing such cracks from forming in the first place.
Coatings used to protect rotary valves from corrosion include:
- Standard electroless nickel plating: Economical and broadly effective, this type of coating protects from corrosion, especially that caused by chlorides.
- PTFE coating: A type of plastic coating, polytetrafluoroethylene (PTFE) is known by its brand name, “Teflon” and doesn’t corrode like metal-based coatings.
- Chrome: Ideal for sticky materials like sugar or wet cement. Chrome allows the material to slide off easier, prevents build-up, and reduces flow rate.
Teflon keeps contamination from sticking to surfaces, so is often a good choice for applications like gas and oil refining, mining, pulp processing, tanning, or other industries where hydrogen sulfide can be an issue. However, it’s not good for applications that work with heavy solids like sand or hard water that tend to erode rather than corrode.
Oiling the Valves
Like any piece of mechanical equipment used in production, it’s important to ensure proper lubrication of a rotary valve. Maintenance should include regularly oiling valves to decrease wear and tear, which will keep them running smoothly and extend their lifecycle. Oil should be poured onto the rotors so that it’s evenly distributed, with the excess drained off so only a light film remains. It’s essentially the same principle used when any metal component is exposed to saline or wet environments or even oxygen in the air, which can cause surfaces to corrode.
It’s important to know what corrosive substances are being used in production, which is why providing Material Safety Data Sheets (MSDS) to workers tasked with rotary valve maintenance is imperative. Such documentation allows them to deal directly with materials that can contaminate products with hazardous chemicals. MSDS also helps identify the appropriate manner in which to clean equipment so as not to cause a toxic reaction.
For certain food processing designs of the rotary valve, maintenance includes preventing conflagrations due to combustible material like dust that sometimes is responsible for industrial explosions or fires. In cases like this, rotary valve maintenance also must ensure the valve’s airlock capabilities aren’t affected by cross-contamination during production to help keep equipment clean.
Keep It Clean
Maintenance involves a lot of cleaning when it comes to a rotary valve. Certain quick-clean rotary valve designs make maintaining them easier, enhancing product throughput and inhibiting material from building up. Some rotary valves feature rails to further simplify cleaning and maintenance. The easier it is to clean a rotary valve, the less maintenance needs to be done to prevent cross-contamination.
Certain features can be manufactured into your rotary valves to make cleaning a lot easier for your people on the plant floor. We stand by our U-Shaped rotor pockets as a great way to prevent material build-up and improve flow.
For more information about rotary valve maintenance and how Prater can help, contact us today.