Valve Bonnet: The Secrets of Connection and Sealing

In the industrial field, valves play an indispensable role, and the valve bonnet, as an important part of the valve body assembly, is not to be underestimated. The bonnet not only carries the motion of the valve stem connection rod or rotating valve shaft, but also bears pressure on straight-through or angle-type valve bodies, provides mounting methods for actuators, and accommodates the packing box. This article will delve into the various connection methods of valve bonnets and their sealing principles, helping you better understand this key component.

Valve bonnets come in various types, and their functions differ depending on the type of valve. For example, ball valves and butterfly valves, which rotate 90°, typically do not have a separate bonnet. The packing is contained within the extended part of the valve body itself, or the packing is an independent part bolted between the valve body and bonnet.

For straight-through globe valves and gate valves, the bonnet is made of the same material as the valve body or an equivalent forged material because they are pressure-bearing elements that need to withstand the same temperature and corrosive effects as the valve body.

One of the main functions of the bonnet is to provide a mounting interface for actuators, ensuring the normal operation of the valve. At the same time, the bonnet also accommodates the packing box to prevent medium leakage. The packing is usually compressed by the packing gland, which is pressed into place by the flange on the bracket boss of the bonnet. In addition, the bonnet also provides centering for the cage, further guiding the valve core and ensuring the correct centering fit between the valve stem and packing.

There are various ways to connect the valve body and bonnet, each with unique characteristics and applicable scenarios. Next, we will detail several common connection methods to help you better understand their working principles and advantages and disadvantages.

Bolted flange connection is one of the most commonly used methods for connecting valve bodies and bonnets. This method tightly connects the bonnet and valve body using bolts, forming an integrated whole. Its advantage lies in relatively simple installation and disassembly, which is convenient for maintenance and inspection. When the packing compression needs adjustment, simply loosen or tighten the bolts.

However, this connection method also has limitations. As valve size increases and pressure rises, the load-bearing capacity of the bolts significantly decreases due to creep relaxation, especially when the operating temperature exceeds 350°C (600℉). In addition, the volume and weight of a bolted flange bonnet also increase with valve size.

The threaded bonnet structure is relatively simple and inexpensive. It connects the bonnet and valve body via threads, without the need for additional bolts. However, this method has some issues. The gasket of the bonnet must closely fit the mating surface, and loosening or tightening the bonnet may damage the joint. For larger valves, the torque required to tighten the bonnet becomes very high. Therefore, threaded bonnets are generally only suitable for valves with a nominal size not exceeding DN80.

Welded bonnets are economical and relatively reliable in structure, without limitations on valve size, operating pressure, or temperature. Welding forms an inseparable whole between the bonnet and valve body, greatly improving sealing performance. However, the drawback of welding is difficult maintenance. Once there is a problem inside the valve, the weld must be machined to access the valve interior. Therefore, welded bonnets are typically used for valves that require long-term maintenance-free operation, one-time valves, or valves where sealing the bonnet joint is more difficult than disassembling the valve for inspection.

Pressure-activated self-sealing bonnets are an advanced connection method that uses medium pressure to compress the joint surface, overcoming the disadvantage of large weight in other structures. As medium pressure increases, the bonnet seal becomes tighter. This structure has become the preferred connection method for large high-temperature, high-pressure valves.

Pressure self-sealing uses two types: metal sealing rings and composite sealing rings (flexible graphite + 304 stainless steel wire). Metal sealing rings use mild steel with silver plating on the outer surface. The plasticity of silver allows it to fill microscopic pores on the sealing surface, ensuring sealing performance under high pressure. In high-temperature and strongly corrosive media, the metal sealing ring can use 316 stainless steel with a chrome-plated surface, but this sealing ring may cause wear on the valve body inner wall. To completely solve this problem, Stellite alloy is overlaid at the contact area between the valve body cavity and the sealing ring, effectively preventing leakage caused by valve body wear. Flexible graphite composite sealing rings combine the advantages of metal and flexible graphite, achieving good sealing performance, but require replacement after valve maintenance.

Valve sealing is mainly divided into bonnet sealing and stem sealing. Bonnet sealing refers to the seal between the bonnet and valve body, which can be categorized as forced sealing or self-tightening sealing.

Forced sealing relies on tightening the bonnet bolts to apply compression to the gasket. The preloaded gasket fills the microscopic gaps of the sealing surface, preventing medium leakage. When medium pressure rises and the valve operates, the preloaded contact pressure on the sealing surface decreases, and the gasket rebounds. If the gasket has sufficient resilience, the working sealing contact pressure on the sealing surface always exceeds the medium and operational pressure, maintaining good sealing.

The necessary condition for forced sealing is that the sealing surface maintains a certain residual preloading force under medium pressure. Its feature is that medium pressure always tends to reduce preloaded sealing contact pressure, decreasing sealing performance. Typical structures include flat gasket seals, spiral-wound gasket seals, and serrated gasket seals, usually used for medium-to-low pressure and small-to-medium diameter valves.

Self-tightening sealing, as a high-pressure sealing technology, is commonly used in high-temperature, high-pressure, large-diameter valves. Its principle is that before pressure rises, preloading bolts are first tightened, raising the bonnet and forming sealing conditions between the bonnet and wedge gasket, as well as between the valve body and wedge gasket, the preloaded contact pressure on the sealing surface.

As medium pressure rises, the bonnet moves upward under the medium's pressure, and the sealing contact pressure between the bonnet, wedge gasket, and valve body gradually increases with pressure. In self-tightening sealing, the working contact pressure on the sealing surface consists of two parts: preloaded contact pressure and pressure formed by the medium. A feature of self-tightening sealing is that medium pressure tends to increase the preloaded sealing contact pressure, improving sealing performance. The higher the medium pressure, the greater the working contact pressure and the better the sealing.

To ensure good sealing at high temperatures, austenitic stainless steel or hard alloy can be overlay welded at the contact areas (sealing surfaces) of the valve body, bonnet, and wedge gasket. The overlay thickness is approximately 2 mm. The overlay increases the hardness of the sealing surface and enhances corrosion and erosion resistance. It easily forms a good contact surface and angle with the wedge gasket, improving sealing performance. To prevent excessive sealing force from crushing the sealing surface, a wedge gasket of appropriate material should be selected. The material selection principle is that, while ensuring corrosion and temperature resistance, the surface hardness of the wedge gasket should be lower than that of the sealing layer on the valve body and bonnet. This requires the wedge gasket to be both plastically deformable and sufficiently strong. To meet this, a soft coating (e.g., silver or plated layer) is often applied to the surface of this high-strength material.

Maintenance and care of the bonnet are crucial for normal valve operation. Common measures include:

Regularly check bolt tightness: For bolted flange bonnets, ensure bolts are not loose and are tightened evenly to avoid local over-tightening or loosening, which may compromise sealing.

Inspect packing wear: Packing is a key component of bonnet sealing, and its wear directly affects sealing performance. Replace worn packing promptly and ensure correct installation for uniform force distribution.

Monitor bonnet corrosion: The bonnet may experience corrosion from the medium over long-term use. Minor corrosion can be treated with protective coatings, while severe corrosion may require replacement.

Adjust packing compression reasonably: Packing compression changes over time during valve operation. Adjust compression carefully to maintain good sealing without damaging the packing or causing stem jamming.

Bonnet selection should consider valve type, operating pressure, temperature, and medium characteristics:

Valve type: Ball and butterfly valves typically do not need a separate bonnet, while straight-through globe and gate valves require pressure-bearing bonnets made from the same or equivalent materials.

Operating pressure: Medium-to-low pressure and small-to-medium diameter valves generally use forced sealing bonnets such as flat gasket, spiral-wound, and serrated gasket types. High-temperature, high-pressure, large-diameter valves should prioritize self-tightening wedge gasket designs.

Operating temperature: Above 350°C (600℉), bolt load-bearing capacity decreases due to creep; welded or pressure self-sealing bonnets are preferred. Attention should be paid to sealing ring materials to ensure sealing under high temperatures.

Medium characteristics: For high-temperature, strongly corrosive media, metal sealing rings may use 316 stainless steel with chrome plating, but this may wear the valve body. Stellite alloy overlay on the contact area can prevent leakage from wear. Flexible graphite composite sealing rings combine the benefits of metal and graphite, providing excellent sealing but requiring replacement after maintenance.

As an important component of a valve, the connection method and sealing principle of the bonnet are critical for normal operation. This article has detailed the common types and functions of bonnets, the connection methods between valve body and bonnet, valve sealing principles, maintenance, and care. Through proper selection and correct maintenance, valve sealing performance and service life can be effectively improved. This article aims to provide useful reference for technical personnel, helping them better understand and apply knowledge related to valve bonnets.