A Brief Introduction to Valves

The bonnet is the second most important boundary of a pressure valve. Like valve bodies, bonnets are available in a wide variety of designs.

A bonnet acts as a cover on the valve body, and is cast or forged of the same material as the body. Most common connections are threaded, bolted, or welded. When the valve is manufactured, the internal components, like the stem, disk, etc., are put into the body, and then the bonnet is attached to hold the whole thing together.
There is always a pressure boundary at the attachment point of the bonnet to the body. In other words, the bolts or weld joints that secure the bonnet to the body act as pressure-retaining parts. Although valve bonnets are an integral part of most valves, they are also a source of concern. As well as complicating valve manufacture, bonnets can increase valve size, contribute to the cost of valves, and can cause leaks.

An valve's trim is a term used to describe the parts that can be replaced. The trim of a valve includes a disk, a seat, a stem, and sleeves used to guide the stem.
A valve's performance is determined by the disk and seat interface and the relationship between the disk position and the seat. It is possible to control flow and perform basic motions through the trim. Rotational motion trim designs change the flow opening by sliding the disk closely past the seat. In linear motion trim designs, the disk lifts perpendicularly away from the seat, forming an annular orifice.

The valve disk and seat(s) are critical components within a valve assembly that play a crucial role in controlling the flow of fluids or gases through the valve. Here's a brief overview of these components:

Valve Disk (or Valve Closure Element):

Function: The valve disk, often referred to as the closure element, is the part of the valve that opens or closes to regulate or block the flow of fluids or gases. It acts as a barrier that can be moved into or out of the flow path to control the flow rate.
Design: Valve disks come in various designs, including flat disks, balls, plugs, and more, depending on the type of valve. The shape and design of the disk are selected based on the specific requirements of the application.
Materials: Valve disks are typically made from materials that can withstand the conditions within the valve, such as pressure, temperature, and the nature of the fluid or gas being controlled. Common materials include metals like stainless steel, brass, or plastics.
Sealing: When the valve is in the closed position, the disk forms a tight seal against the valve seat to prevent fluid or gas from passing through. The quality of this seal is critical to prevent leakage and maintain system integrity.

Valve Seat(s):

Function: The valve seat is a stationary component within the valve body against which the valve disk seats when the valve is in the closed position. It provides a sealing surface for the disk, ensuring a tight closure to prevent fluid or gas leakage.
Design: Valve seats are designed to be highly precise and often feature a sealing material or coating that enhances the sealing capability. The design of the seat may be flat, conical, or spherical, depending on the valve type and application.
Materials: Valve seats are typically made from materials that are compatible with the valve disk material and are resistant to wear and corrosion. Common materials include various types of metal alloys, elastomers, and polymers.
Maintenance: Valve seats can wear over time due to friction and the abrasive nature of some fluids. Regular inspection and maintenance may be required to ensure the continued effectiveness of the sealing surface.

Packings are used to seal the stem and the bonnet, and they are equipped with components such as:
The gland follower is a sleeve that compresses the packing by a gland into the stuffing box.
The gland is a kind of bushing that compresses the packing into the stuffing box.
The stuffing box is a chamber that compresses the packing.
There are several types of packing, such as Teflon®, elastomeric material, fibrous material, etc.
When the valve is fully open, a backseat provides a seal between the stem and bonnet, preventing system pressure from building up against the valve packing. It is commonly used in gate and globe valves.
Most valves, like standard Ball, Globe, Gate, Plug, and Butterfly valves, have their sealing assemblies based on shear force, friction, and tearing.
As a result, valve packaging must be properly done to keep the stem from being damaged and fluid or gas loss from occurring. If the packing is too loose, the valve will leak. If the packing is too tight, the valve will be difficult to move, and the stem may be damaged.

ATHENA GROUP is a prominent manufacturer of fluid system components, with a global presence and a well-deserved reputation for producing high-quality products and providing exceptional customer support. The company is headquartered in Milan, Italy, and has a strong foothold in the industrial market.
Over the years, ATHENA GROUP has evolved from being a regional valve and level gauge manufacturer to becoming a robust and diversified global group with a significant presence in the industrial sector.