Surface finish plays an essential role in the effectiveness of PTFE seals. The different finishes provide different degrees of contact between the two components, which affects the seal’s strength and reliability. In this paper, we will discuss the effect of surface finish on PTFE seal performance and suggest ways to improve seal quality.
The Influences of Surface Finish on Friction
The surface finish is critical for polymer-based seals. Despite being considered a soft plastic, PTFE is significantly harder than traditional o-ring materials. Because of this, if the mating surface is designed for other materials, it can cause the PTFE seals to leak.
As you can see in the graph below, the improvement in the surface finish has a favorable effect on the frictional force required.
The following specifications were used in the force test that resulted in the above data.
Stroke Speed: 4 in/min (102 mm/min)
Shaft Diameter: 0.1875in. (4.762mm)
Ambient Temperature: 73°F (23°C)
Mating Surface Material: 17 -4 PH S.S.
Mating Surface Hardness: -40 Rc
Surface Finish Influences on Wear
The finish of the mating surface is one of the main factors in the PTFE seal wear. Wear on the seal is generally proportional to frictional force. In other words, less friction reduces the wear on the seal.
The following graph shows how a smoother surface finish reduces the PTFE seal wear rate.
To calculate the wear rate in microinches RMS, we used the formula IN3 -MIN/LB-FT-HR -10-9. For microinches Ra, we used the formula CM3 -MIN/KG-M-HR -10-9.
We used the following test parameters during our testing to ensure accuracy for each test.
Surface Speed: 55 fpm (17 m/min)
Loading Stress: 55 lb./in2 (4 bar)
Wear Rate in Air: @ PV 3025 lb./in2 x ft/min (7.5 N/mm x m/min)
Mating Surface Hardness: Rc 42
Matin Surface Maerial 17 -4 PH Stainless steal
Mating Surface Finish: 1.6 to 111.1 microinches RMS (1.4 to 100 microinches RA)
Ambient Temperature: 70°F (21°C)
Ambient Relative Humidity 75% RH
Duration 5 Hours
Seal material FP
Influences on Sealing Ability
Finally, we looked at how the surface finish affected PTFE’s sealing ability. In general, the sealing ability of PTFE is proportional to the fluid’s viscosity. If a media, like gas, has a reduced viscosity, it is more difficult to seal. In these situations, having a smoother finish on the mating surface can help ensure fewer leaks.
We’ve found that different mediums require not only different finish ranges between them but also different finish ranges when considering if the surface is dynamic or static. You can see our suggested finish in the table below.
Media
Dynamic Surface
Static Surface
Gases and Liquids at Cryogenic Temperatures
2 to 4 microinches RMS (1.8 to 3.6 Microinches Ra)
4 to 8 Microinches RMS (3.6 to 7.2 Microinches Ra)
Gas at Non-Cryogenic Temperatures
6 to 12 Microinches RMS (5.4 to 10.8 Microinches Ra)
12 to 32 Microinches RMS (10.8 to 28.8 Microinches Ra)
Liquids
8 to 16 Microinches RMS (7.2 to 14.4 Microinches Ra)
16 to 32 Microinches RMS (14.4 to 28.8 Microinches Ra)
As you can see, the mating surface finishes can profoundly affect PTFE seals. Because of this, if the mating surface is designed for other materials, it can cause the PTFE seals to leak. Finally, we examined how this affected PTFE’s sealing ability. In general, the sealing ability of PTFE is proportional to the fluid’s viscosity.
Radial shaft seals are often found in aggressive environments with high temperatures and pressures. Unfortunately, these environments are also frequently dry-running and unsuitable for traditional radial shaft seals. So, what are the options if traditional radial shaft seals won’t work? Using PTFE lip seals as rotary shaft seals can offer better protection and longer life than conventional radial shaft seals. Understanding these lip seals are essential to helping you make the right choice for your sealing needs.
What Are PTFE Lip Seals
PTFE (polytetrafluoroethylene) lip seals have a lip on the interior diameter (ID) that allows it to fasten to an outer diameter (OD) of a metal casing or shaft. The seal is completed with a gasket sandwiched between the sealing lips.
Not all lip seals are spring-energized. Therefore, the force from radial contact is lower. The lower pressure allows the seal to function at higher surface speeds of up to 10,000 sfpm. This type of seal is for high rates, making it the go-to choice for sealing rotating shafts.
PTFE Lip Seal Fillers
PTFE Lip seals can be enhanced by using fillers instead of virgin PTFE. Fillers can help the seal withstand sintering temperatures. Some of the filled compounds include:
Carbon-Graphite
Carbon Fiber
Aromatic Polyester
Molybdenum Disulfide and Fiberglass
Fiberglass
Graphite
Mineral
Stainless Steel
Each filler offers higher temperature resistance and lower creep rates than virgin PTFE. In addition, some offer lower abrasion or better corrosive protection depending on the filler type. Which type of filler you want will rely on the application where you use the lip seals.
Applications That Use PTFE Lip Seals
PTFE lip seals are found in most applications where speed is concerned. However, they also are ideal for conditions where highly corrosive compounds or in dry, abrasive environments.
Aerospace
The aerospace industry needs seals that can cope with extreme temperatures, surface velocities, and high pressures. External gearbox seals are just one place where PTFE lip seals are used in the aerospace industry.
Lip seals can handle the extremes and offer advanced fuel consumption efficiency. Better efficiency of burning about half-gallon of fuel per mile can save airlines an operating cost of $1.7 million per year.
Other instances in which the aerospace industry uses lip seals include military applications such as:
Lift fans for vertical lift
Wing flap and slats actuator seals
Rotorhead
Equipment in arresting system to catch aircraft as it lands on aircraft carrier decks
Automotive
In the automotive industry, PTFE lip seals are employed in the drive shafts of the top-fuel blowers in NASCAR engines. Traditional crankshaft seals were burning out in these engines due to the high speeds and rapid pressure increases within five minutes at racing speeds.
They are also in e-drive and cooling systems in electric vehicles. Lip seals in electric cars can handle the shaft speeds seen in these automobiles. Electric vehicles can run at over 18,000 RPM, a higher rate than gas-powered cars.
Conclusion
PTFE lip seals are ideal for rotary shafts operating at high speeds and temperatures, such as in the automotive or aerospace industries. Choosing suitable fillers can also help extend the seal’s life. Contact us today, and we can help you determine which PTFE lip seal options you want.
Valve seat failure can lead to costly and time-consuming issues. Under certain circumstances, a ball valve seat failure can cause explosions and lead to life-threatening situations. In this article, we will cover the top five causes of failure.
Material Choice
The material choice of the valve seat can contribute to valve seat problems if you pick the wrong option. Different materials work in different operating conditions, so you want to ensure that you research the material carefully before choosing.
The most common material options are
PEEK
PTFE
TFM
PCTFE
Acetal
Vespel
The wrong material can cause unexpected issues that may damage the hardware of mating components or even physical injury.
Cold Flow
While PTFE, or Teflon, is a common material in valve seats, there could be some cold flow resistance issues. Cold flow is the process when solid material slowly deforms under the influence of long-term mechanical stress.
The cold flow of material during us and cycling causes a slow deterioration in valve performance. Despite cold flow issues, PTFE is still the best choice in many industries. Choose a filled PTFE instead of virgin PTFE to mitigate complications from cold flow. Filled is less susceptible to stress and has better resistance to cold flow.
Excessive Friction
Excessive friction can also cause valve seat issues. Excessive circumferential seal force accelerates wear on the valve, which leads to an increase in torque requirements. The friction between the ball and the valve seat affects how much torque is necessary to turn the ball valve.
When the temperature in the valve increases, the pressure between the valve seat and the ball increases. Increased temperature creates greater friction between the valve ball and the seat.
Eventually, the valve can become locked either open or closed. As the required torque increases, the valve seat is torn apart, and mechanical failure occurs.
Valve Seat Failure: Seat and Seat Carrier Design
The valve seat is one of the most critical components. However, poor seat design can lead to a shortened lifespan, leakage, or catastrophic failure. The catastrophic failure could lead to explosions or life-threatening damage in particular environments.
Soft seat valves typically use metallic seat carriers with the valve seats pressed into them. As with the seat design, the seat carrier design can have similar problems. If the seat carrier design is slightly off, it could make it difficult to determine where the problem lies.
Permanent Deformation
In high-pressure applications, the valve setting of soft seats is necessary. To correctly set soft seals, the valve is repeatedly actuated during part of the build process. This repeated actuation can cause permanent deformation during normal use.
A failure to understand the initial deformation will cause the valve to fail. It won’t fail on initial use, but it will eventually stop working, and the valve will either need to be rebuilt or replaced as a result.
Valve Seat Failure Conclusion
Whether your valve requires PTFE, PEEK, or any other material, you want to ensure you get the appropriate material for your valve seat. Incorrect materials and excessive friction, seat design malfunctions, and permanent deformations can cause failures.
Contact us today to learn more about the valve seats we offer and assist you in finding the appropriate material for your applications.
Teflon (PTFE) billets are compression molded tubes and rods made of Polytetrafluorethylene used in numerous industrial applications covering almost every industry, including chemical processing, automotive, food, aerospace, medical device, semiconductor, and fluid handling. These functional parts are popular due to Teflon’s chemical resistance, extreme temperatures, low friction properties, and ability to mold them into the precise shape and size needed.
Types of Molded PTFE Billets
The molded processing techniques we employ are compression, isostatic, and automatic. The determination of which method to use will depend on the type and size of billets being produced, the industry they are being used in, and the manufacturer’s preference.
Compression
Compression molding for PTFE billets is one of the three main processes, with isostatic and automatic as the other options (we will discuss these in a later article). To make a billet using this process, one must design and create the source mold for the rod or tube. The compression mold is a two-part mold that includes a preform component.
The performance has the same shape as the tube or rod being processed. The material is loaded into the mold, pressed under specific pressure and time depending on the grade of PTFE being processed, and then ejected for the mold.
The pressure causes the material inside the preform to conform to the mold’s shape. As a result, any excess material leaks out of the mold. Once the molding process is complete, the billet is ejected from the mold.
Once the billet is extracted from the mold, it is now in what is called the “Green State,” compact into the desired shape, but no molecular change has occurred; this only happens after sintering in our ovens for a designated time between 650 to 715 degrees F.
The billet is left to cool at room temperature. Once cooled, excess material can be machined off to make a smooth, perfectly formed Teflon billet.
Advantages of using compression molding include:
Strong parts
Lower tooling costs
Broad design options
Lower waste generation
Large part manufacturing
Sintering Molded Tubes, Rods, or Semi-Finished Parts.
Sintering results in a change in the PTFE powder that reorganizes the molecules into a compound. To sinter Teflon billets:
Billets must be supported to ensure that they do not sag
Adding an annealing cycle during the sintering processes will help to stop cracks from forming while the compound bakes. Once the billets are out of the oven and cooled to room temperature, they can be machined.
Molding Process
While all molds have physical property variations, the overall process is similar. Therefore, the variations between compression, isostatic, and automatic molded billets are inconsequential to the prevalent use.
The molding process is two-step. It begins with packing the mold and pressing the powder, called the “green state.” Next, the mold is placed under specified pressures depending on whether the material is unfilled (Virgin) or filled with various fillers, glass, carbon, graphite, etc. pressure of up to i.
After pressing, the item is removed from the mold. Workers must be careful during the ejecting of the tube or rod. Mishandling of the green material could result in cracks. Once the mold is removed, the billet is sintered.
conclusion
Molded PTFE billets are ideal for many industries. The properties of PTFE make them chemical resistant, have a low coefficient of friction, and can be manufactured compliant with FDA, Class VI Medical, NACE, Aerospace, and Semiconductor Standards approved for use in medical and food industries. In addition, clean Room Molding for Ultra Pure applications is available.
Rotary seals are essential to maintaining the life of the equipment. While choosing the appropriate rotary seal materials, the rotary shaft mating surfaces are equally important.
Below, we will discuss rotary seals, materials used for seals, and rotary shaft properties.
Rotary Seals
Rotary seals work to help keep the system lubricated while excluding contaminates. A properly fitting seal can positively impact the life of the lubricant. Oil’s life span at 86°F (30°C) is 30 years. However, as the oil heats up, the life span diminishes rapidly to no more than a 30-day life span.
The addition of contaminants and water also limits the life of the oil and the ball bearings. For example, adding .002% water into the oil lubricant will reduce the ball bearing’s life by 50%. The cause of the ball-bearing integrity loss is called hydrogen embrittlement.
Common Materials Used for Rotary Seals
Rotary seals come in several different materials. The optimal choice is dependent on the environment in which it is used. The most common materials include nitrile rubber, polyacrylate rubber, fluoroelastomers (FKM), and PTFE.
While these materials are a good choice for rotary seals, conventional rubber seals are common in static applications where temperature and chemical compatibility are not a concern. PTFE is the solution in high-speed dynamic service requiring low friction seals or where exposure to severe temperature or chemicals exists. There are different types of PTFE used for rotary shaft seals. Users can choose from
Virgin PTFE
Molybdenum Disulfide Filled PTFE (MoS2)
Carbon Filled PTFE
Carbon and Graphite Filled PTFE
Carbon and MoS2 Filled PTFE
Glass Filled PTFE
Glass and MoS2 Filled PTFE
Polymide Filled PTFE
Properties of Rotary Shafts Mating Surfaces That Affect Sealing Performance
Most often, rotary shafts are metal. However, the rotary shaft mating surface could be made from plastics. No matter what material the shaft is made from, some properties will affect the sealing performance of the shaft.
The properties of the rotary shaft that affect sealing performance are the shaft harness and the shaft roughness. The sections below explain in greater detail how the hardness and roughness of the rotary shaft can affect the rotary shaft’s sealing performance.
Rotary Shaft Hardness
The hardness of the rotary shaft is how deep an indenter can penetrate the surface of a shaft. The shaft’s hardness is measured in the Rockwell C scale. The higher the number, the more complex the surface.
As a general rule, the rotary shaft should always be harder than the seal to ensure the seal wears out before the shaft. Additionally, if you choose a harder surface, there are more options for seal materials.
With a rotary shaft with a hardness exceeding 45 Rockwell C, the seal doesn’t have time to polish and “bed in.” That means that any roughness on the surface will cause issues with the seal, wearing it down quicker than average. A shaft with a hardness under 45 Rc requires a softer seal which doesn’t have as long of a life.
The choice of hardness depends on the environmental pressure and shaft speed. For example:
In environments of 1000 psi with rates up to 150 sfpm, a shaft with 70Rc or greater is necessary
Settings using shaft speed 2500 sfpm and 0 psi need a hardness of 60Rc or greater.
Rotary shaft speeds of up to 150 sfpm and 0 psi need a hardness of at least 35Rc, with lubrication, or 44, with no lubrication.
Rotary Shaft Mating Surface Roughness
The rotary shaft’s roughness refers to the shaft surface’s unevenness. To measure the roughness, measurements of high and low points of the shaft and taking the difference to determine the machined tolerance.
Ideally, a smoother surface will increase the seal life and offer outstanding performance. But on the other hand, when the surface is exceptionally smooth, there is no way for the oil to flow between the mating surface and the seal. As a result, the seal wears out quicker without lubricant between the seal and the mating surface.
Of course, a high roughness level can allow leaks through low points on the shaft. Therefore, the rotary shaft roughness needs to be relatively smooth but not so smooth that the seal cannot be lubricated.
Rotary shafts work with the rotary seals to keep lubrication from dirt and water. Understanding the properties of the rotary shaft mating surfaces helps determine the type of seal material chosen. Contact us today for your rotary seal and shaft mating needs.
Seals used in the oil and gas industry must withstand high temperatures, high pressure, and a chemically hostile environment. The conditions limit the material used to make the seals for this industry. The most common types of materials include
PTFE
PEEK
UHMW
PCTFE
Hytrel
Let’s look further into these materials, their benefits, and their limitations.
What is PTFE?
Polytetrafluroro Ethylene (PTFE) is a synthetic fluoropolymer with high-temperature resistance, commonly known as Teflon. It is a hydrophobic, high-molecular-weight polymer consisting of carbon and fluorine.
Benefits of Using PTFE
PTFE is ideal for use in the oil and gas industry as it is resistant to extreme high and low temperatures. In addition, PTFE has a low coefficient and a low dielectric constant. Finally, the hydro resistance nature of the material makes it a top choice for working with steam or heated seawater.
One of the most significant benefits of using PTFE is the resistance to harsh chemicals. It has the broadest chemical resistance of commercial polymers. For example, seals made of this material are resistant to hydrogen sulfide, ferric chloride, ferrous sulfate, hydrochloric acid, and hydrofluoric acid.
Limitations of PTFE
PTFE’s limitations make it unsuitable for some uses. For example, it is sensitive to creep and abrasion, requiring regular maintenance. PTFE also has low radiation resistance and can corrode and produce toxic fumes as it breaks down.
Properties of PTFE
PTFE has a density of 2200 kg/m3 with a melting point of 327°C (620°F). PTFE maintains self-lubrication, strength, and toughness at temperatures down to -268 °C (-450.67°F). Additional properties include:
What Are Some Common Oil and Gas Applications of PTFE?
PTFE is one of the more common materials used in oil and gas seals. For example, O-rings, slipper seals, backup rings, piston rings, and spring-energized seals use PTFE material. In addition, natural gas, cold media seals, bearings, and wear components also use PTFE for manufacturing.
What Is PEEK?
Polyetheretherketone, or PEEK, is a colorless organic thermoplastic semi-crystalline polymer with excellent mechanical and chemical resistance properties. It’s high-resistance to terminal degradation makes it useful in oil and gas environments.
PEEK has high mechanical strength and is ideal for high vacuum applications. Its robust nature makes it suitable for demanding applications such as the oil and gas industry. It works well in compressors, pumps, and pistons.
Limitations of PEEK
Despite PEEK’s many benefits, there are some drawbacks to using this material. It has low UV light resistance. It is also unsuitable for nitric acid, sulphuric acid, sodium, and halogens. In addition, it is expensive to make and requires high temperatures to process.
Properties of PEEK
PEEK has a high tensile strength of 25000 to 30000 psi. It has a V0 flammability rating of 1.45mm and can withstand high loads for extended periods without residual damage. Additional properties include:
What Are Some Common Oil and Gas Applications of PEEK?
Labyrinth, spring-energized piston seals, backup rings, and seal packing in the oil and gas industry are manufactured using PEEK materials. In addition, it is the material most often chosen for the face seals at the wellhead to contain the high-pressure production of gas and fluid.
What is UHMW Polyethylene?
Ultra-High Molecular-Weight, UHMW, Polyethylene seals are thermoplastic, semi-crystalline materials. It is lightweight with a high-pressure tolerance that makes it ideal for spring energized seals used by the oil and gas industry.
Benefits of Using UHMW Polyethylene
Seals made from UHMW have the benefit of being both abrasion and impact resistant. This self-lubricating material has a low friction coefficient. It withstands extreme colds and high temperatures.
It has a high molecular weight, meaning UHMW is not likely to melt and flow as a liquid. This material cannot be molded by traditional methods, thanks to the high molecular weight. Instead, it is compression molded to make it stronger.
Limitations of UHMW Polyethylene
While UHMW Polyethylene has many benefits for the oil and gas industry, there are some limitations, such as having a lower maximum continuous surface temperature than other materials.
UHMW Polyethylene has a compressive strength of 3000 psi. In addition, it has a maximum safe workload of 1000 psi in some industries. Overload can cause UHMW polyethylene to crack or break.
Properties of UHMW Polyethylene
UHMW polyethylene seals are self-lubricating and have low surface energy, which makes them ideal for the oil and gas industry. Other UHMW polyethylene properties include
What Are Some Oil and Gas Applications for UHMW Polyethylene?
UHMW polyethylene material is used for seals in the oil and gas industry. It is used to make spring energized seals. It is also used for cargo dock impact bumpers and liners.
What is PCTFE?
Polychorotrifluoroethylene (PCTFE) is a chemical compound with a high tensile stretch and good thermal properties. Its chemical-resistant properties make it ideal for use in the oil and gas industry for seals and other components.
Benefits of Using PCTFE
Seals made with PCTFE are nonflammable and heat resistant up to 175°C (347°F). They are also resistant to acetone, hydrochloric acid, sodium peroxide, citric acid, and sulfuric acid. It is water-resistant as well.
PCTFE has a board temperature range with a useful temperature range of -204.4°C (-400°F) to 193.3°C (380°F). When comparing PCTFE vs PTFE, PCTFE is a stronger polymer with better mechanical properties.
Limitations of PCTFE
PCTFE has many beneficial properties for the oil and gas industry. However, some limitations, such as a lower melting point when compared to PEEK or PTFE, might make it less desirable. Seals used in extreme temperatures may need to be a different material.
Additionally, PCTFE is a stiffer material. While this does allow it to maintain its dimensions better, it does break easier than PTFE. Along with being stiffer, it is not as non-stick when compared to PTFE.
Properties of PCTFE
PCTFE has V-0 flammability and a hardness of 67 at 100°C and 80 at 25°C. Other properties include
What Are Some Oil and Gas Applications for PCTFE?
Like PEEK or PTFE, PCTFE is a great material for seals in the oil and gas industry. It’s chemical resistance, which means it can be used in the most volatile industries. Fillers within the seals can enhance some of the properties. PCTFE is also used in component designs and valve seats.
What is Hytrel?
Hytrel is a thermoplastic polyester elastomer that is plasticizer-free. It is a stable material with needed flexibility while handling high temperatures. It is a worthwhile option to consider for the oil and gas industry.
Benefits of Using Hytrel
As a seal material for the oil and gas industry, Hytrel has good chemical resistance. It can withstand exposure to fuel, hydrocarbon solvents, and oil. Additionally, as the material is exposed to higher temperatures, it becomes more rigid. At lower temperatures, it is more flexible.
Hytrel is abrasion resistance. It offers impact and creep resistance to the seals. It also is resilient and excels at providing flex fatigue and tear resistance. Hytrel has a natural spring-like property and has low hysteresis.
Limitations of Hytrel
As there are several models of Hytrel, only a few are best for use in seals. Hytrel 4556, 4056,4068,4069, and 6356 are the ones that work best. However, these don’t always work well in the oil field, so you should know which ones to look for.
Properties of Hytrel
Hytrel has V-0 flammability and a hardness of 67 at 100°C and 80 at 25°C. Other properties include
What Are Some Oil and Gas Applications for Hytrel?
Hytrel is useful as a seal material in the oil and gas industry. As it is resistant to many chemicals, including hydrocarbon solvents, it is ideal for hazardous conditions.
Which Seal Type is Best for the Oil and Gas Industry?
Each seal type has its uses and benefits for the oil and gas industry. However, the best options are PEEK, UHMW, PCTFE, and Hytrel. It is because they have the best physical properties to withstand the harsh conditions in the oil and gas field.
At Advanced EMC Technologies, we offer custom-engineered sealing systems that provide reliable sealing solutions. Our seals perform under high temperatures, high pressure, and chemically hostile environments. Contact us for more information.
Seals for Oil and Gas Industry FAQ
IS PEEK environmentally friendly?
There is no evidence that PEEK has a significant environmental impact in its service life, disposal, or manufacturing. Toxicity is low and does not contain anything known to be toxic. There is low smoke, poisonous gas emissions, and fire when involved in a fire.
What are spring-energized seals?
Spring-energized seals can store mechanical energy by compressing the spring. As a result, they withstand more pressure and heat than their conventional counterparts. In the end, the mechanical energy stored in the seal keeps it from leaking.
Auto molding PTFE seals and seats offer a wide variety of benefits, especially for high-volume production runs. In this blog post, we cover some background on both PTFE and auto molding (also known as compression molding) and discuss why this particular manufacturing process is often preferred by engineers for both seals and ball valve seats.
