Ball valve seats that show signs of swelling, blistering, or &#;popcorning&#; have been permeated at a molecular level. Needless to say, this can cause some serious issues such as leaks and catastrophic failure. The solution is to find a ball valve seat material that is highly resistant to permeation and an excellent choice would be PCTFE. In this week&#;s blog post, we will talk about PCTFE Ball Valve Seats and how they are used in Low Permeation Applications.
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Introduction
Certain types of media may permeate the ball valve seat, leading to swelling, blistering, and leakage. Applications such as chemical processing and petrochemical transport may require a seat material that is resistant to permeation but still exhibits key properties such as low friction, compressive strength, and resistance to deformation is still needed.
How Permeation Works
Permeation refers to the molecular level penetration of gases, vapors, and liquids through a solid material via diffusion. In diffusion, molecules pass from an area of high concentration to an area of low concentration. This can be extremely problematic when a ball valve is being used because of the potential distortion and leaking of the ball valve seat.
Keep in mind that permeation can take place through a surprising variety of materials, including metals and polymers. In addition, some materials are only semipermeable, which means that only ions or molecules with certain properties can pass through the material.
The rate of permeation is directly related to crystal structure and porosity, which is why factors such as density and molecular structure are important when selecting materials for applications where low permeation is important.
Why Permeation is a Problem for Ball Valve Seats
Gas permeation can not only compromise gas stream purity but also result in dimensional changes of the ball valve seat. One form of these dimensional changes is swelling, which can occur if the permeating media becomes a part of the molecular structure of the material. In reinforced polymers, such as glass-reinforced PTFE, swelling can cause separation between the glass fibers and the PTFE matrix.
Another common manifestation of permeation is referred to as &#;popcorning&#; or &#;popcorn polymerization&#; which occurs due to a polymeric chemical reaction. And among the most notorious source of problems with popcorning and swelling are monomers with extremely small molecular sizes such as Butadiene and Styrene.
Both popcorning and swelling will lead to leakage, and over time popcorning will completely destroy the ball valve seat. This makes the choice of ball valve seat materials extremely important for applications where this is a problem.
PCTFE for Low Permeability Ball Valve Seat Applications
One of the best materials for a ball valve seat application where permeability is a problem would be PCTFE (Polychlorotrifluoroethylene), a thermoplastic chlorofluoropolymer. PCTFE is sometimes referred to as Modified PTFE or PCTFE, as well as by trade names Kel-F, Voltalef, and Neoflon. PCTFE is often thought of as a second-generation PTFE material that maintains the chemical and thermal resistance of PTFE along with its low friction. It is also similar to other fluoropolymers such as PFA or FEP.
One of the defining characteristics of PCTFE is that it has a much more dense molecular structure and a low void and micro-porosity content when compared to similar ball valve seat materials. This gives it a very low permeability coefficient, which means that the likelihood of it swelling or popcorning is far lower than other materials. For example, its permeability for O2, N2, CO2, and H2 are 1.5 x 10-10, 0.18 x 10-10, 2.9 x 10-10, and 56.4 x 10-10 darcy, respectively.
PCTFE also provides improved toughness and strength along with good deformation recovery and excellent creep and cold-flow resistance. In addition, it has a wide operating temperature range of -100°F to 500°F. In fact, it performs extremely well at cryogenic temperatures. Because of its low friction, it also results in a very low ball valve operating torque. PCTFE also exhibits zero moisture absorption and is non-wetting.
PCTFE works well in operating environments where other polymers may fail. For example, it is well adapted to nuclear service that may involve high radiation exposure, is non-flammable (D 635), and is resistant to attack by the vast majority of chemicals and oxidizing agents. The only chemicals that might lead to slight swelling are ethers, esters, aromatic solvents, and halocarbon compounds.
In addition to its use in applications requiring low permeability, PCTFE is also considered an excellent choice for applications that need a low-outgassing material and is commonly used in semiconductor applications. Also note that there are PCTFE grades that are FDA approved, such as Fluorolon PCTFE .
Conclusion
Fuel processing and transport, chemical processing, petrochemical systems, and emissions control are just a few of the applications where low permeation materials may be necessary. For such applications, PCTFE is an excellent option for ball valve seat materials because it combines the basic properties necessary for a seat with an extremely low rate of permeation.
If you need a solution to blistering, swelling, or popcorning of a ball valve seat, contact the experts at Advanced EMC. Our sealing team will work with you to find the right ball valve seat material for your application.
There are fundamental factors that need to be considered when selecting an instrumentation valve. Some of the most important criteria required for valve selection are listed below. Each application will place higher importance for each of the following factors:
- Basic Valve/Packing Designs
- Pressure Fluctuation
- Pressure and Temperature Considerations
Other key factors include flow media, codes and standards, actuator design options, and flow analysis and orifice sizing.
It is critical that all aspects of the system be assessed for proper design prior to conducting a valve specification. Understanding the variety of operating conditions will greatly affect the final design selection.
Primary Instrumentation Valve Designs
Understanding the variety of ways an instrumentation valve&#;s packing has been designed and the types of valve designs that are available play an important role in the specification process. At its basic core, valves consist of a control component such as a ball or a needle, and sometimes incorporate an automatic actuation device for valve operation, as opposed to manual operation. The instrumentation valve seat controls the flow of the material while the packing limits the leakage between the seat and actuation elements. The overall performance can be dramatically affected by factors such as materials of construction, rigidity, and location on the valve stem and proper valve selection. Among all of the valve designs on the market, generally speaking, the following four valve types will be used in instrumentation applications for flow control, analysis, testing and ensuring maximum safety.
Ball Valves
A ball valve consists of a ball-shaped component to stop or start fluid flow. In terms of operation, when the valve handle is turned to the open position, the ball rotates to where the hole through the ball moves in line with the valve body inlet and outlet. When the valve is shut, the ball is rotated so that the hole closes the flow opening of the valve body and the flow is stopped. Most ball valve actuators are of the quick-acting type, which only require a quarter turn of the valve handle to open and shut the valve.
The most popular instrumentation ball valves are rated for pressures up to 3,000 PSI and temperatures to 350°F (176°C) with PFA (perfluoro-alkoxy polymer) seats and are available in single (in-line), two-way angle pattern and three-way connections. High quality ball valves have pneumatically tested seats at 1,000 PSI with nitrogen at zero leakage. These ball valves should have hydrostatically tested bodies at 3,000 PSI and seats tested at 2,000 PSI.
Contact us to discuss your requirements of surface safety valve actuator. Our experienced sales team can help you identify the options that best suit your needs.
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4 Advice to Choose a Water Wave Stainless Steel SheetsPopular ball valve designs are available in 316 stainless steel and brass, and incorporate one-piece packing for clean and accurate samples as well as designs that allow for a drop-in fit for easy replacement of the packing.
Double Block and Bleed Valves
A double block and bleed valve or a DBBV takes the place of 3 separate valves, 2 isolation valves and 1 drain valve. Double block and bleed valves provide isolation from both the upstream and downstream process flow and pressure.
A double isolation and bleed valve stops process fluid from getting to an area that is being worked on. Both in-line valves are closed while the bleeder is open. If fluid were to leak past the first valve the bleeder would drain off before it pressurized the cavity between the upstream and downstream valves. If the bleeder, which may be a needle valve, were to be plugged the downstream valve would keep the process fluid from getting past it.
Needle Valves
A Needle Valve uses a precision engineered tapered pin to gradually open a space for fine control of flow. The flow can be controlled and regulated with the use of the needle. A needle valve has a relatively small orifice with a long, tapered seat, and a needle-shaped plunger on the end of a screw, which precisely fits the seat for valve shut-off.
When the needle valve screw is turned the plunger retracts allowing flow between the seat and the plunger. However, precise regulation of the flow rate can be controlled when the plunger is completely retracted. Needle valves are often used in vacuum systems where precise control of gas flow at low pressure is required. They are often used as component parts of other, more complicated valves systems.
Instrumentation needle valves are available with screwed bonnet or integral bonnet. Most are rated for pressures up to 6,000 PSI and temperatures to 450&#;F (232&#;C), and are available in straight and angle patterns.
Check Valves
A check valve (aka one-way valve) is a valve that normally allows media (liquid or gas) to flow through it in only one direction.
Check valves incorporate two-port valves, meaning they have two openings in the body, one for fluid or gas to enter and the other for the media to exit. There are various types of check valves used in a wide variety of applications. Check valves are available in many sizes, yet are typically very small. Most check valves work automatically and do not have a valve handle or stem.
Check valves close from the weight of the check mechanism when back pressure on a spring occurs, allowing the media to flow in only one direction.
Check valves have calibrated cracking pressures, which is the minimum upstream where the valve will operate. Most check valves are rated for pressures of up to 6,000 PSI and temperatures to 350&#;F (177&#;C). Depending on the application, cracking pressures are usually set at 1/3, 1, 5 10, 15, 25 and 50 PSI.
Manifold Valves
Manifolds are used in my many processing applications. They are often used to mount valves or to consolidate the plumbing configuration to interface between valves and ports that are being plumbed into. A manifold is a single block or adjoining blocks, which interface for the valves to mount to or ports for fluid transfer. The ports are then plumbed to the manifold to complete the circuit.
Because manifold valve systems can be used in modules, other valve types are easily interchangeable and easily mounted to the manifold to customize a system operated by actuators. Valves that are connected in series to manifolds can have a variety of NPT, Metric or ORB ports. This alleviates the need for developing a more complex system of various valve types with different ports, simplifying the system and saving costs.
A manifold can be used without other valves as a fluid cavity with two or more ports combined in series to reduce plumbing. For instance a return line manifold with multiple smaller ports connecting to a larger tank port can eliminate the need for a series of fittings. This lowers cost and reduces potential leak points.
Most manifolds are available in 2, 3 or 5 valve configurations:
- Two valve manifolds are used with instruments such as pressure gauges, pressure transmitters and pressure switches.
- Three valve manifolds are the most common. They can incorporate test ports on the process side and drain ports on the instrument side.
- Five valve manifolds are typically used with differential pressure instruments where drain valves are required on the instrument side. They are also common for flushing of the system and the prevention of loss of expensive fluids.
- Three and five valve combination manifolds are used with differential pressure instruments such as differential pressure transmitters, switches and pressure gauges.
Primary Design Features
Most manifold designs have case hardened swiveling stem plugs. This design is ideal for most low, medium and high pressure applications.
- High pressure gas applications typically incorporate a swiveling plug with a soft seat design. The seat materials are reinforced polytetrafluoroethylene (RPTFE), delrin and PEEK.
- High quality manifolds have a case hardened ball plug in 316 SS. Manifolds made of Hastelloy C, Monel, Inconel and titanium have tungsten carbide ball plugs.
- Manifold valves should be designed with the &#;thread above the seal&#; so that the stem threads are not wetted by the fluid flowing in the system.
- Gland sealing arrangements vary based on temperature ranges. For temperatures up to 180 °C, the material used is RPTFE. For temperatures in the 180 °C to 270 °C range, graphitized seals are used. Beyond 270 °C and up to 540 °C, Grafoil seals are used.
The choice of materials is based on the working fluid and the installation location, where atmospheric conditions may vary and require specific materials to prevent corrosion and withstand vibration.
Common manifold materials include:
- Carbon steel ASTM A105
- Stainless steel ASTM A479, A182, F304, F304L, F316, F316L and F321, and are available with conformity to NACE MR 01 75 for corrosion resistance.
When extreme corrosion resistance is required, materials are usually:
- Monel with conformity to NACE MR 01 75 for corrosion resistance
- Inconel, Hastelloy C and titanium
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