There are two types of seat in a ball valves, soft seated and metal seated. Soft seated ball valves use a thermoplastic material such as PTFE, NBR etc. Metal seated ball valves use metal as seat material such as 316 SS, monel, etc. Each ball valve seat type has its own characteristic and thus having its own advantages and disadvantages. So how to select the best ball valves seat type?
In general application the soft seated ball valves is suitable for a clean fluid and not recommended for a dirty fluid. This is reasonable since some particulate may damage the soft seat material and causing a leakage. In dirty fluid application, the metal seated ball valve is the best choice.
The metal seated ball valve is also suitable for application that has a high erosion or corrosion rate. The metal ball valve will stay much longer than soft seated ball valve in such application. In term of the shut-off capability, the soft seated ball valve is the best. It can achieve the highest level of the seat leakage class while a metal seated ball valve can achieve only up to ANSI/FCI Class V or ISO 5208 Rate D. Thus if the tight shut-off or bubble tight shut-off is required, then there is no choice except using the soft seated ball valves.
In term of the required torque to operate the ball valves, the soft seated ball valve will have a lower torque rather than metal seated ball valves. The high torque of the metal seated ball valve may lead the selected actuator model is bigger than the soft seated ball valve thus it increase space consumption and add additional weight. In general practice, a metal seated ball valve is only used in the severe service condition such as high temperature, high erosion or abbrasion, slurry, etc. In general application of oil and gas, the soft seated ball valve is used. This type of seat will provide an excellent sealing capability with a fair life time in clean service application. While the metal seated ball valve is used when the soft seated ball valve can't handle anymore the process condition.
Some valve manufacturers use a pure soft material as their soft seat material and there are also manufactures that use a metal oring (metal seat ring) and soft material as its seat insert for size 2 inch and above. This type of seat even though it use a metal seat ring but it is categorized as soft seated ball valve due to it use a soft material as final part that in contact with the ball.
Advantages of Metal Seated Ball Valves
The key advantage of metal seated valves when compared to soft seated valves is that they can withstand high temperatures and severe service conditions.
Metal seats can stand up to extreme flashing, hydraulic shock, abrasive process fluid, and high temperatures up to and exceeding 1,000° F. They are also ideal for high erosion or corrosion applications.
Another important factor is that metal seats can be hardened by specialized coatings.
Soft Seated Ball Valves
Soft seats are typically composed of thermoplastic components like PTFE. These valves are appropriate for applications in which chemical compatibility is crucial, and in situations where having the tightest seal is important.
Soft seats, however, aren't suitable for processing abrasive or dirty fluids. These valves are known to break down under conditions like these, resulting in a leaky valve. Complications introduced by soft seated valves are related to the fact that they don’t stand up to applications that challenge their service limits.
Metal seated ball values on the other hand can hold up under high temperatures and extreme service conditions. Well established in the field, metal seated ball valves deliver uninterrupted service with the maximum shut off standards.
The valves also work for longer time periods than soft seated valves. These durable valves can basically handle the majority of abrasive applications.
Metal Seated Valves Coating Options
With metal to metal seating, and depending on the service conditions, applying various coatings make it possible for ball and seat rings to be hard faced on sealing locations.
Examples of these specialized coatings include satellite hard facing, chromium carbide, tungsten carbide and electrolysis nickel plating. Sealing is achieved by the metal-to-metal contact in between two hard coated surface areas.
Metal-Seated Ball Valve Costs
Even though metal seated valves are more expensive, the cost of downtime resulting from failure, coupled with the replacement of soft seated valve break downs, should be factored in. The efficiency and longevity of metal seated ball valves will pay off and counteract the higher price. Bottom line, metal seated ball valves are the best, long lasting, economical options for critical applications.
Recognizing your process condition is critical when it comes to choosing the appropriate seat for your specific application.
If you determine that the best solution is metal seated ball valves you’ll find them to be long lasting, cost effective solutions for critical applications.
