Leakages are a perennial concern in any fluid-handling system, and valve bonnets are no exception. Unchecked leaks can result in operational inefficiencies and environmental and safety hazards. We will uncover the common causes of valve bonnet leakages and discuss proactive measures to mitigate these issues. From improper installation to wear and tear, understanding the root causes is crucial for developing effective preventive strategies.
The valve bonnet serves as the protective cover for the valve body, housing critical components such as the stem, packing, and disc. Understanding the nuances of valve bonnets is essential for maintaining optimal performance and preventing costly disruptions. In general, valve bonnets are classified according to how they attach to the valve body. This article will teach you more about valve bonnets, the many types of valve bonnets, the reasons for leaks, and how to fix or prevent them.
What is a Valve Bonnet?
A valve bonnet is a cover that fits over the aperture on top of a valve body. The bonnet and its connection to the body of a valve are exposed to the operating fluid as a pressure-retaining component. As a result, they must endure the fluid’s operating pressure and corrosive effects.
Moreover, the bonnet is cast or forged from material grades similar to the body to avoid placing dissimilar metals in contact. The bonnet is a critical part of the valve as internal components such as the stem and disc are inserted into the valve body before attaching the bonnet to hold all parts together. The connection mechanism can be threaded, bolted , or welded.
Types of Valve Bonnet
A pressure boundary exists between the bonnet and the body. As a result, the type and accompanying gasket must offer adequate sealing at the operating pressure as well as fluid compatibility.
Threaded Valve Bonnet
Threaded bonnets have a straightforward structure and a low-cost design. This system, however, is fragile. The valve bonnet gasket is tightly attached to the rotating surface, which frequently causes friction during tightening and loosening. As a result, the joint surface is likely to be damaged, potentially resulting in leaks. Another disadvantage is that as the valve size grows, the torque required to tighten it increases dramatically. As a result, this bonnet type is typically restricted to nominal sizes of DN80 and less.
Some versions of this design use a separate threaded collar to secure the bonnet to the valve body. As a result, even if the bonnet is repeatedly tightened, there is no relative motion at the joint surface between the bonnet and the valve body. Although this reduces the likelihood of leaking, the constraint on valve sizing remains. As a result, threaded valve bonnets are often used for non-critical services like plumbing and heating shut-off.
Welded Valve Bonnet
Because it eliminates leak pathways, a welding connection for bonnets provides a more reliable bond than threads. Furthermore, a welded connection is neither constrained by operating temperature or pressure nor does it limit the size of the valve. However, valves that use this connection are difficult to service. As a result, welding connections are appropriate for valves that can run for an extended period of time without maintenance, disposable valves, and difficult-to-assemble valves. Welding and threading are combined in some valves to give double leak prevention.
Bolted Valve Bonnet
Bolt connections for bonnets are used in a wide range of diameters and operating pressures. However, as the working pressure increases, so does the size and weight of the valve. Another issue with bolt connections is that the load-bearing capacity of the bolt drops dramatically when the operating temperature surpasses 600 °F (350 °C). In such severe situations, the flange must be hermetically welded to strengthen the bolts. Depending on the working pressure, temperature, and process media, this type of bonnet employs a variety of gaskets.
Pressure Seal Bonnet
This ideal design is for high-pressure applications in pressure classes of 600# to 4500#. It is a unique bonnet design. As the internal pressure increases, the joint surface compresses due to the configuration of the thrust/sealing rings within the bonnet and valve body. The higher pressure increases the sealing capability of a pressure seal bonnet design.
Graphite pressure seal rings are typically reinforced with stainless steel wire. By combining the flexibility of graphite with the strength and anti-corrosive properties of stainless steel, this material produces a better seal. The design of the pressure seal bonnet reduces the need for a larger, heavier structure to contain increasing pressure levels.
Furthermore, pressure seals allow for simple assembly and disassembly by inserting and removing the sealing rings through a valve body orifice. As a result, this design is appropriate for applications that require frequent maintenance. It should be noted that the pressure seal ring must be replaced following repairs. Bolted and pressure seal bonnets are generally the best solution in high-pressure applications. In extremely high operating temperatures, pressure seal bonnets become the only solution.
Bonnetless Valves
Although not a bonnet type, it is important to note that some valves do not have bonnets. As a result, the valve body is the only pressure-retaining component. This reduces leak sites from the bonnet-body joint or gasket. It also speeds up maintenance by allowing for one-step removal of all functional parts and in-line repair. This is normally accomplished through a box aperture wide enough to accommodate all valve pieces. Bonnetless valves are more compact and lightweight, making them ideal for high-pressure applications.
Material Choices for Valve Bonnets
Valve bonnets are made from a variety of materials, the selection of which is heavily influenced by the application and climatic circumstances. Stainless steel, cast iron, and alloys such as titanium or nickel are common materials. The choice is influenced by elements such as corrosion resistance, bearing capacity, durability, and the fluid under control.
Signs of a Faulty Valve Bonnet
Recognizing the symptoms of a defective valve bonnet is critical for efficient and safe operations. A leak is one of the most typical indications. A valve leak can be caused by a variety of difficulties, such as a worn-out bonnet gasket or decaying packing. These are frequently detectable via routine inspections or bonnet tests.
When a gate valve fails to close, it may signal a problem with the bonnet or other interior components. Look for a jammed valve or one that is difficult to operate as well. It could indicate a problem with the bonnet, especially if it’s accompanied by a leak.
Leakages and Repair
A bonnet/body gasket area is prone to leakage as a pressure-retaining component of a valve and requires repair or replacement on occasion. Leaks could occur from either the bonnet or its attachment to the valve body. Leaks can be caused by a variety of factors, some of which are as follows:
- Bonnets are manufactured by either forging or casting. Leakage may occur as a result of poor casting quality, such as slag inclusions, blisters, and loose structures.
- Shock loads such as hammering can cause leakage. This is why it is advisable to avoid impact strikes, as cast iron, in particular, is prone to crack under impact loads.
- In cryogenic valve applications, the bonnet temperature can drop too low, causing the packing to freeze and leak. As a result, engineers should design enlarged bonnets with adequate insulation space. When the air temperature in the working area goes below negative 20°F, cracking might develop.
- Leaks at the bonnet-body joint are possible as a result of stress cracks and slag inclusion from poor-quality welds. Leakage in threaded, bolted, or pressure seals can be caused by uneven contact surfaces caused by corrosion, factory faults, poor gasket installation, or insufficient contact pressure.
The following steps can be taken to prevent leakage:
- Ensure suitable casting quality by conducting tests according to industry regulations before installation.
- Never strike a bonnet or apply excessive loads to a valve. Large-diameter valves with hefty actuators must be installed with a load-bearing bracket or mounting kit.
- In cold weather, valves require insulation and maybe extra heat. Furthermore, if these valves are out of service, technicians should drain any liquid from them.
- When attaching the bonnet to the valve body, the proper procedures must be performed. Engineers must specify appropriate gaskets for the process media, and technicians must perform proper weld testing.
How do you choose the right bonnet for your application?
Choosing the best valve bonnet for your application necessitates taking into account a number of aspects. To begin, determine the working conditions: the type of fluid, the operating pressure and temperature, and the type of valve utilized. Each form of bonnet has a distinct purpose. A long bonnet ball valve, for example, may be the best option for high-temperature applications.
Consider the ease of upkeep as well. Bolted bonnets, for example, are easier to maintain and repair than welded bonnets. Another critical consideration is the material used for the bonnet. It is critical to select a material that can endure the working conditions in order to ensure durability and efficiency.
Remember that the ultimate goal is to produce a secure, tight seal to avoid leaks, and the bonnet is critical to this. As a result, a well-selected bonnet can provide efficient and safe valve functioning.
