There are various types of valve designs that can be used in demanding, hazardous and corrosive applications. However, there is a case to be made for the inherent benefits of a top-entry design in severe service applications. While top-entry ball valve designs naturally offer the benefit of inline reparability, this is just one of the advantages gained from this style of valve. Top-entry ball valves offer an engineered seating technology. This is different from a traditional floating ball valve seal common on other types of designs. This is one of the most important design differences of the soft seated ball valve designs. The top-entry design also minimizes potential points of leakage out to atmosphere. This is a primary concern when considering fugitive emissions and gases or vapors.
With the introduction of top-entry ball valves, many of the inconveniences and reliability issues experienced by industrial facilities can be minimized. These valves, easy to maintain, are resilient and are effective for safely operating in severe services like ammonia, hydrofluoric acid, chlorine, ethylene oxide, phosgene, oxygen and numerous other applications.
Different Types of Ball Valve Designs
There are four common ball valve designs which include end-entry valves, split-body valves (two-piece), three-piece valves and top-entry valves. Like all comparisons, each of these options have advantages and disadvantages. The question users must answer is: “What attributes of a given ball valve design are the most important to the application?” For users with severe service applications, reliable shut-off, reducing fugitive emissions and the ease/speed of inline maintenance are typically at the top of the list.
The end-entry valve is popular due to its compact, unibody design that prevents piping stresses from being applied to the critical ball and seat seal. End-entry valves use a design where the ball and seats are inserted and removed via the end of the valve. A drawback to this design is that the user must remove the valve from the pipe to service it. If the threads on the end piece are corroded, removal of the end piece and eventual replacement of the ball and seats can become difficult.
The seats of an end-entry valve are like the two- and three-piece designs, which are packed into the valve body and will have continuous pressure on the seats causing cold flowing problems. As end-entry valves are usually only available in flanged ends, top-entry valves offer more versatility with the different end configurations.
Similar to the end-entry valve is the split-body valve (two-piece). Split-body ball valves use a two-piece body design which is separated to replace the ball and seats. It is typically used for general service applications and common for larger valve diameters. It is easier to handle the valve body halves during routine maintenance when compared to the one-piece body. However, the flanged connection in the center of the valve body adds a potential leak point and is subjected to the stresses of the piping system. Like the other ball valve configurations where the seats are under constant load, cold flow problems will more quickly occur, leading to premature seat leakage. These valves are typically supplied only in flanged end, which limits the end connection configurations and is not serviceable inline.
The most common and economical ball valve design in the industry today is the three-piece ball valve. Three-piece valves use a three-piece body design which allows the center section to swing out by loosening one bolt and removing the other fasteners, thus permitting the seats and ball to be replaced in line. An advantage of this design is gained when considering screwed and welded piping.
Unfortunately, this type of valve usually requires disassembly prior to welding or the replacement of body seals (dummy seals) after welding. On specific top-entry valves, they can be welded in place without requiring removal or replacement of the seats and bonnet gasket. This prevents a user from negating the factory acceptance test performed by the OEM and saves time and money on installation cost. Like the end-entry and split body design, three-piece ball valve seats are under constant load. As such, cold flow problems will more quickly occur and lead to premature seat leakage. This valve design inherently has a higher risk of atmospheric leakage. There are two potential leak points in the body that need to be maintained. Like the split-body design, the stresses in the piping system are directly applied to the body fasteners, which in turn effects the reliability of the body seals. It is critical the body fastener torques are maintained on this design to avoid an unintended leak to atmosphere.
Finally, there is the top-entry ball valve, which has been designed for severe service applications and offers reliable shut-off, fugitive emissions compliance and ease of inline maintenance. Like the end-entry, the top-entry benefits from a compact, unibody design that prevents piping stresses from being applied to the critical ball and seat seal. Unlike the other designs, the top-entry feature creates the ability to maintain the valve in line without removing the valve from the piping system. The top-entry design uses a top bonnet that allows for quick access of the ball and seats and the bonnet itself houses the stem and stem seals. The act of removing the bonnet thereby gives access to all components needing maintenance. Additionally, piping stresses do not impact the bonnet fasteners that maintain the critical seal between the body and bonnet joint. This makes it easier to maintain a reliable joint seal and minimizes the risk of an atmospheric leak between the valve body and bonnet seal.
The concept of interchangeable or modular bonnet designs is also a characteristic of top-entry ball valves. The top-entry design allows the user to easily convert or upgrade from one style of packing system (bonnet) to another without having to purchase a new valve. With the many bonnet design options available on top-entry valves, the bonnet selection can be tailored to the specific operating parameters of the application. This makes upgrading from a standard bonnet design to an extended or severe service bonnet simple. Upgrades to the bonnet are offered to allow for extensions to the stem to be used in applications requiring the use of insulation on the piping system or for severe service applications requiring specialized stem seals, such as v-ring stem seals, which are commonly used to meet fugitive emissions standards in the industry such as the International Organization for Standardization (ISO) 15848 and American Petroleum Institute (API) 622.
Benefits of Engineered Seating Technology in a Top-Entry Valve
End-entry, split-body and three-piece body ball valves use conventional seating technology. With this technology, constant pressure is placed on the ball in both the open and closed position. Time, temperature and pressure will lead to cold flowing the seats more quickly. The result over time is a decrease in the dimensional stability of the seats, which can greatly affect shut-off. Conventional floating ball valves also rely on the line pressure to push the ball into the downstream seat. This reliance on line pressure to seal can become a problem under low pressure or low pressure drop conditions. Particularly, as the seats wear, shut-off is compromised and higher pressure is required to seat the ball to maintain tight closure.
Top-entry ball valves require a clearance for the ball and seat to be installed in the seat cradles of the valve body. There are essentially two methods to accomplish this. The more common seating technology used by top-entry ball valve manufacturers is the wedge seat design. Like a plug valve, the seats are wedged into a tapered body and held in place by a spring. Like the plug valve, this type of seating results in a reliable seal, but with increased loading on the seats and increased operating torque. The benefit is a tight-seat seal on the upstream and downstream seat independent of the line pressure.
There is an alternate ball/seat design to allow top-entry valves to be assembled and increase seat life and minimize operating torque. The geometry between the ball/seat interface can be changed so there is a clearance when the ball is in the open position and a camming action against the seats when the ball is rotated to the closed position. This ball design mechanically compresses the upstream and downstream seats to create a tight, dependable seal independent of line pressure. Two benefits of this method are operating torque requirements are reduced and the positive seal from the camming action allows the valve to seal reliably at low pressures. This design has been proven to meet and exceed industry seat shut-off standards such as API 598 and American National Standards Institute (ANSI)/Fluid Controls Institute (FCI) 70-2. Because this design only applies pressure to the seats when the valve is in the closed position, the seats do not have to be loaded continuously. Therefore, the seat life for this design is the most favorable when compared to conventional ball valve seating technology.
While differing valve designs have advantages and disadvantages, the advantages of top-entry ball valves have been tailored to fit the needs of severe service applications. Their ease of inline maintenance, reliable tight shut-off and fugitive emissions compliance are ideal for critical applications. This allows users to focus less on valve maintenance and concentrate time and effort on improving their operational efficiency.