A poppet valve, sometimes referred to as a mushroom valve, is a tapered plug valve typically found in engines and compressors to control the flow of intake gas and exhaust. This type of valve can also be used to control liquid flow such as in chemical batching or dosing applications. In the closed position, the valve creates a seal on the valve seat and blocks the flow of fluid. In the open position, the valve moves away from the valve seat and allows fluid to escape around the plug. Depending on the application, the valve can be used in the “normally closed” position to control the release of fluid or the “normally open” position to control the stoppage of fluid flow.
The round shape of a poppet allows for an even distribution of heat across the valve, which means the valve evenly expands as temperatures rise. The valve stem acts as a shaft to move the poppet onto and away from the valve seat. A springs surrounding the valve stem imparts energy onto the valve assembly and determines the position of the poppet.
Poppet valves are manufactured in a wide range of sizes to fit a variety of equipment. They are made of different materials to handle different types of environments. Choosing the right material for the application is crucial for ensuring effective equipment. The major poppet valve materials discussed in this article are: PEEK polymer, nylon and metal alloys.
PEEK Polymer
PEEK stands for polyether ether ketone and is a class of polymer that works well in high temperature applications. It is resistant to solvents, refrigerants, steam and other common chemicals. It does not readily absorb moisture nor does it require lubrication. PEEK is typically used in applications where precision and efficiency are necessary and corrosion presents a threat to parts. Its basic structure is resistant to corrosion, and when it is compounded with certain fillers, it can stand up to aggressive conditions without fatigue. When chlorine is present in the application, carbon filled PEEK is recommended.
The mass of PEEK is significantly less than that of titanium and steel. This lightens the total equipment weight and creates less impact when the valve closes. Less impact upon closing means less noise within the equipment. PEEK also has low thermal conductivity, which means it transfers heat at a lower rate compared to materials with high thermal conductivity such as metals. Low heat transfer means that more heat is retained within the equipment after shutdown, which helps make the next startup more efficient.
Due to its favorable characteristics and ability to handle harsh environments, PEEK is also commonly used in bearings, piston components, rotors, vanes, labyrinth seals, start gears and cams.Its strong tensile strength and natural flexibility make it resistance to wear, which saves costs and improves equipment performance.
Nylon
Nylon is another synthetic polymer classified as a thermoplastic. Like PEEK, nylon is lightweight, resistant to corrosion, has low thermal conductivity and can handle high temperature environments. Though PEEK is favored over nylon for high-stress applications, nylon offers more custom options due to its flexible nature.
Nylon 66 is a commonly used form that is composed of two monomers, molecules that can be bonded together to form polymers.Each monomer contains six carbon atoms, which gave the material its name. The molecular structure gives it strength and rigidity even when exposed to high temperatures and chemicals such as hydrocarbons, acids, refrigerant and hydrogen sulfide. Nylon 66 that is reinforced with glass fiber is even stronger and more resistant to wear at high temperatures. It is also resistant to moisture absorption and fatigue, even when exposed to extreme heat for long periods of time.
There are other forms of glass fiber reinforced nylon used to manufacture poppet valves such as cast nylon 6 and nylon 46. Nylon 46 has a higher melting point than either nylon 6 or 66, making it a preferred option for extremely high temperatures.
Metal Alloys
Poppet valves can also be made of a variety of metal alloys. Some examples include:
- Titanium
- Steel
- Nickel
- Chromium
These types of metals are known for withstanding high temperatures and corrosion, making them popular options for exhaust valves in internal combustion engines. Titanium and steel alloys are more cost-effective, but the super alloys such as nickel- and chromium-based alloys are known for their superior strength and greater resistance to corrosion.
Metal poppets are usually paired with metal valve seats, which can create noise if the valves do not seat properly. They usually do not require lubrication, however, since they do not slide across the valve seat.
Selecting the Right Poppet Valve Material
There are several factors to consider when deciding on the right poppet valve material. Such factors include:
- Temperature
- Chemical or substance exposure
- Corrosion
- Fatigue
- Strength
- Cost
- Maintenance
Though most poppet valve materials are manufactured for high temperature applications, it is important to measure or gauge the temperature range of the application to ensure that the maximum temperature does not exceed the melting point of the material. It is also important to understand the characteristics of the fluid passing through the valve to ensure that the material can resist wear, degradation and absorption during exposure.
PEEK polymer and nylon tend to be the materials of choice for compressors and chemical transfer applications. These plastics are favored because of their resistance to corrosion when exposed to harsh chemicals, less mass and low thermal conductivity. They can handle a wide range of environments with less fatigue and wear, which allows them to last longer. The longer a poppet valve lasts, the less costs accrue over time due to less maintenance and repairs.
Why Poppet Valve Material Is Important
Though poppet valves are relatively small compared to other equipment components, they play an essential role in effective operation. When poppet valves fail, fluid can be blocked or allowed to flow freely throughout a system, which can cause serious problems within the equipment and damage related upstream and downstream equipment.Such failure can also render an entire system inoperable and require significant maintenance and costs to repair.
When choosing the right material for an application, thoroughly understand the nature of the fluid and conditions within the equipment. Weigh the application characteristics against the material type. Resist the urge to decide based solely on cost and choose the material most appropriate for the application.