What is the best material for valve stem seals?
Comprehensive Guide to Valve Seal Materials: Viton, PTFE, NBR, and EPDM
In this article, we will explore:
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- The significance of high-quality valve seals
- Different types of seals utilized in valves, their functions, and the valves they are associated with
- Popular materials for valve seals, including their advantages and disadvantages, chemical resistance, essential characteristics, and practical applications
Importance of Valve Seals
Valve seals are crucial for the operational efficiency and longevity of valves. Using faulty or unsuitable seals can lead to serious complications in fluid or gas systems. Here are some key reasons why having a high-quality seal is vital:
- Leak Prevention: Preventing leaks is critical to avoiding waste, minimizing hazards, and saving money.
- Efficiency: Even minor leaks can disrupt pressure or flow rates, reducing overall efficiency.
- Safety: In environments dealing with hazardous chemicals or extreme temperatures, seal failure can pose severe safety risks.
- Extended Valve Life: Selecting the correct seal material enhances the durability of your valve, leading to less downtime and lower replacement costs.
Understanding Seal Types
- Seal: A broad term referring to any element preventing fluid transfer between the valve's joined components.
- Gasket: A compressible sealing device fitted between two surfaces, like the halves of a valve body.
- Plug Gasket: Used in check valves to seal the movable plug.
- End Seal: A gasket located at the junction of the valve and the piping system.
- O-ring: A ring-shaped seal, typically made from elastomers, employed in moving parts or connecting points.
- Ball Seat: A component that encircles the ball in a ball valve, forming a seal when the valve is shut.
- Stem Packing: A compressed seal around the valve stem that helps avoid leaks between the stem and body of the valve.
- Diaphragm: A flexible membrane, often made from elastomers, that plays a crucial role in the activation mechanisms of certain solenoid valves.
Exploring Valve Seal Materials
Viton (FKM):
Viton is a synthetic rubber known for its superior chemical resistance against oils, fuels, and chlorinated solvents.
- Advantages: Offers excellent resistance to chemicals and high temperatures.
- Disadvantages: Tends to be expensive and is less flexible at lower temperatures.
- Common Applications: Widely employed in chemical processing, oil and gas industries, and valves that deal with aggressive fluids.
Examples of Valves:
3-way brass electric ball valves featuring Viton seals, as well as brass solenoid valves with Viton diaphragms.
PTFE (Teflon):
Polytetrafluoroethylene, created by DuPont, is famous for its use in non-stick cookware and is a high-performance thermoplastic fluoropolymer.
- Advantages: Exceptional chemical resistance and very low friction.
- Disadvantages: Reduced mechanical strength, susceptible to deformation under stress.
- Common Applications: Mostly found in food and beverage applications, sanitary uses, and highly corrosive environments.
Examples of Valves:
Swing check valves equipped with PTFE plug gaskets, vertical check valves with PTFE end seals, and manual ball valves with PTFE ball seats, thrust washers, and stem packings.
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NBR (Nitrile Rubber):
Nitrile butadiene rubber (NBR), known as Buna-N, is a synthetic rubber renowned for its resistance to oil and fuel.
- Advantages: Offers good oil and petroleum resistance, making it versatile.
- Disadvantages: Limited chemical resistance, capable of degrading under high temperatures.
- Common Applications: Utilized in hydraulics, pneumatics, and general-purpose valves.
Examples of Valves:
Pneumatic solenoid valves featuring NBR O-rings, low-pressure gas solenoid valves with NBR diaphragms, and 3-way brass solenoid valves containing NBR seals.
EPDM:
Ethylene propylene diene rubber is commonly utilized for valve diaphragms, gaskets, and O-rings.
- Advantages: Excellent performance with water, steam, and mild chemicals.
- Disadvantages: Limited oil and hydrocarbon resistance.
- Common Applications: Frequently found in water treatment, irrigation systems, and steam valves.
Examples of Valves:
Plastic zero differential solenoid valves with EPDM diaphragms, and plastic 2-way solenoid valves that use EPDM seals.
Other Important Factors
Valve Design: The design of a valve significantly affects seal selection. For instance, ball valves employ ball seats and stem seals, while diaphragm valves utilize flexible diaphragm materials. Understanding the structure of the seal and its intended use will guide the appropriate material choice.
Regulatory Standards: In industries with stringent material requirements, such as food production (FDA compliance), pharmaceuticals (USP compliance), or potable water (NSF compliance), it is critical to ensure material conformity. Always verify regulations against the valve's components before installation.
Material Compatibility: Different materials utilized in valve seals exhibit varying degrees of chemical compatibility. Below is a quick reference chart that details which materials are appropriate for specific fluids and gases. For a comprehensive overview, consult our chemical compatibility database.
| Material | Water | Oil | Gas | Acids |
|---|---|---|---|---|
| Viton | Good | Excellent | Fair | Good |
| PTFE | Excellent | Good | Excellent | Excellent |
| NBR | Fair | Excellent | Fair | Poor |
| EPDM | Excellent | Poor | Good | Fair |
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Are There Superior Brands for Valve Stem Seals?
No need to seek the most expensive options when it comes to valve stem seals. Anything of OEM quality should suffice.
As previously discussed, if the valve guide and stem clearance remain at the lower end of the specifications, the "deflector" orientation of the umbrella seals functions effectively, a point emphasized by my late machine shop colleague. For example, even the Chevy O-ring seal is notably economical, sealing only when the valve reaches maximum lift and when port flow is at its peak.
Years ago, I inquired about the PC-style seal, reputed to be "the best" for racing builds in the later s. My colleague indicated that since this style scrapes oil off the stem, minimal oil would reach the guide, which could lead to "less lubrication" and "increased guide wear," not ideal for a non-race motor that we wish to sustain for 80K+ miles between valve servicing.
For longstanding recommendations regarding B or RB gaskets/seals, it's worth noting that Mopar Performance previously offered an "Engine Tear Down" gasket set. This well-advertised kit could be used for those "class winner teardowns" at high-stakes drag racing events, where winners must demonstrate the legality of their engines. It included everything from head gaskets to oil pump pickup tube gaskets. At the time, it was priced around $100.00 USD, providing extensive rebuild components, even with steel head gaskets back then.
Interestingly, I recall the 454 Chevy valve stems being the same diameter as B/RB valve stems, and some were available in red/orange silicone for certain uses. My machinist would expertly trim their length as necessary for Chrysler B/RB heads. To me, that seems like a worthwhile upgrade, while also being OEM, possibly offering some cost savings.
If the guide wear allows oil seepage, the silicone may prove more forgiving as the stem moves within its casting.
When changing seals, you'll require a hand-tightened valve spring compressor to compress the valve spring and remove two locks from the retainer. Once that's accomplished, other methods can assist in keeping the valve stem elevated (compressed air in the cylinder or mechanical assistance) to lightly lubricate the seal/stem prior to installation. Subsequently, re-install the spring, retainer, and locks, ensuring the locks are seated correctly before releasing the valve spring tension. This process might take some practice before achieving a smooth operation.
Always remember to safeguard the upper fenders and anodized aluminum surfaces! Utilizing a bore scope can be beneficial to inspect cylinders for any oil-washed areas on piston tops, which is prudent.
Most startup oil, particularly after an overnight period, likely originates from valve stem/guide interactions. In the mid-s, some OEM engines omitted seals on exhaust valves, leading to brief smoking upon startup after accumulating mileage. This might offer some clarification or justification.
These are merely my thoughts (and those of my late machinist/mentor) based on observations.
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