Corrosion is a potentially dangerous enemy to the piping of an industrial refrigeration system. Owners spend increasingly large amounts of time and money to control the detrimental effects of corrosion. Its effects are found throughout the refrigeration system. Uninsulated sections of piping and valve trains that continually go through freeze/thaw cycles are prime candidates for corrosion. Valves used in food processing applications where wash-down cycles are a daily operation also are highly susceptible to corrosion. With increasingly stringent standards on cleanliness, refrigerant releases, an increased awareness to safety, and increased industry compliance to preventive maintenance programs, much attention has been given to the control of corrosion.
Combating CorrosionOver the years, valve manufacturers and system owners have tried numerous tactics such as painting techniques and plating to slow the effects of corrosion. In many industries, preventive maintenance programs call for such valves to be repainted on a yearly basis, requiring annual investments of time and money. IIAR Bulletin No. 110,Start-up, Inspection and Maintenance of Ammonia Mechanical Refrigerating Systems,recommends that uninsulated piping be inspected annually.
Another tactic for combating the effects of corrosion is to specify additional piping wall thickness to help prevent a catastrophic failure due to corrosion. Many standards used by engineers today provide for a corrosion allowance based on the expected life and material of the components.
All metals corrode in one shape or form; the key is to contain it to acceptable limits. Because refrigeration valves are made primarily of iron, and iron corrodes relatively easily, this has in part led to thick-walled, heavy castings.
Filtration NeededAnother item associated with high valve maintenance costs is dirt or foreign matter in the system. Whether the concern is slag from welding operations, corrosion from piping, or other contaminants, dirt and foreign matter do not mix well with pilot-operated control valves.
This problem is especially prevalent in startup situations or after a system has been opened up for a modification. Commonly, strainers are installed prior to control and solenoid valves to collect debris in the system and prevent valve malfunction. Often times though, the fits between the moving components of the valve, such as the piston and valve body bore, are smaller than the dirt being taken out by the strainer. Very fine particles pass through the strainer until they encounter a flow path that is smaller or simply a slow enough flow rate for them to precipitate. It is here, in the valve, where they will collect and ultimately cause valves to stick or perform poorly.
With the increasing costs associated with preventive maintenance and startup, neither of these problems is acceptable.
Valve Design Tailored to Tackle BothOne company has addressed both of these issues with a lightweight, corrosion-resistant alloy valve line. By utilizing materials such as thin-walled stainless steel investment castings and aluminum, the line is in compliance with the Pressure Equipment Directive 97/23/EC while reducing overall valve weight by as much as 70 percent and maintaining operating pressures up to 32 bar (465 psig).
The weld-in line is designed overcome the corrosion and filtration problems encountered with other valves and provide long-term, maintenance-free operation. Being weld-in, the valves are not prone to flange leaks and flange-bolts loosening when the system settles after startup or gaskets relax. The regulators have been designed to maintain the same modularity as the company’s existing line, but the weld-in valves offer up to 40 percent less leak points between components. Likewise, modularity is maintained with the use of like components across the entire size range of valves. For instance, the same top portion of a 0.75" valve can be used on a 4", allowing for minimal stocking requirements.
Routine maintenance has not been overlooked. If maintenance should be required, the entire valve can be serviced from one side. Replaceable internal components and specially design wear rings within the valve eliminate the need for removal of the body from the line.
Like the company’s existing line, the valves are pilot operated and designed to function with at a 2 psig pressure drop. However, internally the valve functions differently than a standard pilot-operated valve. Pilot pressure is released from the top of the piston, allowing it to open, rather than applying pressure to the piston to force it open. This accomplishes two objectives:
- It makes the valve much more resistant to dirt.
- It provides better performance in oversize conditions where low flow performance is an issue.
Releasing the pilot flow from the top of the piston also improves valve controllability under low conditions. In a typical pilot-operated valve, where the pressure is applied to the piston valve, stability is limited to 25 percent or more of the rated flow. By releasing the pilot flow off the piston, less volume is required to create the required pressure drop for the valve to function. This causes the valves to be more responsive under these conditions. They have been shown to be stable down to as little as 10 percent of the rated capacity, thus eliminating the need for special reduced-capacity plugs.
The lightweight, corrosion-resistant valves offer system owners performance improvements while attacking two age old industry problems: corrosion and dirt. Corrosion-resistant materials reduce corrosion factors to nearly zero while reducing valve weight appreciably. The pilot flow regime allows dirt to pass through the main port, downstream to the valve, and prevents it from getting into places it does not belong. Compatible with common refrigerants and carbon dioxide, the valve offers an operating range of -76 to 240°F (-60 to 116°C).
For industrial refrigeration applications, the lightweight, corrosion-resistant, patent-pending valve can deliver improved performance and reduced leaks while maintaining simplicity and flexibility.