A bonnet is a control valve component that encloses the valve packing and actuator. A bonnet is typically designed to accommodate a variety of temperature ranges. As a result, special considerations must be made when selecting the appropriate bonnet for your control valve.
An extended bonnet is used in extreme temperatures, such as 450°F, and in temperatures below freezing. This type of bonnet keeps the valve packing away from critical temperatures.
An extended bonnet isolates the stem-valve packing from the subzero flow in cryogenic service. This protects the packing from damage.
The internal components of a control valve that come into contact with the process flow are referred to as trims. The bonnet, packing, and bottom gaskets and flanges are not included in the trim.
A trim keeps the connection between valve-plug lift and flow capacity stable. It also ensures that the control valve is closed properly.
The seat is primarily in charge of achieving a tight shut-off. A tight shut-off and proper lift, on the other hand, are dependent on other components of the control valve. These include actuator design, valve-stem packing, and valve body shape.
The preferred level of tight shut-off is determined by the service. The percentage of leaks in the flow when the control valve is closed is used to calculate shut-off. Various industry standards specify the requirements for shut-off for a variety of applications.
ANSI/FCI 70-2-1998 (3), also known as Control Valve Seat Leakage, is a common standard for determining leakage classes. These classes range from Class II (weak shut-off) to nearly zero leakage (Class VI). The operator configures the TSO or tight shut-off requirement in one of the classes based on the application requirements (in most cases, Class IV, V, or VI are chosen).
The valve trim is primarily chosen based on the following factors: fluid operation conditions, basic flow characteristics for a specific trim (as specified by the manufacturer), and the effects of various operating conditions on the basic flow characteristics. These factors indicate the flow characteristics of a trim and are used as a guideline when selecting the appropriate trim.
A lower trim allows for more accurate control of low flows while still allowing for high flows in the near future. The design of this trim reduces the flow through the valve opening. However, because of the shorter distance between the plug and the lift, flow control is more precise.
There is no rule stating that a reduced-capacity trim must be used at a rate lower than a certain turndown rate. However, when accurate control is required at 20% to 25% of valve capacity, a reduced trim may be an excellent solution.
Cages are commonly used in trims because they provide several benefits:
A cage can be designed to ensure that fluid forces on the stem and plug are distributed evenly. This design yields a well-balanced, cage-guided trim.
A cage guides the plug to ensure it is properly positioned and makes proper contact with the valve seat.
A cage can be used to modify the control valve’s original flow characteristics.
The stem and plug of a sliding-stem valve are subjected to pressures that can interfere with actuator control. As a result, high dead-bands and improper stem movement may occur. The fluid surrounding the stem has the ability to lift it up, either sideways or downward, and may even apply torsional pressure.
Several trim designs are available to balance and counteract these pressures. A balanced trim distributes and balances pressure by adjusting the plug or using a cage-guided trim.
Noise is produced by a control valve as a result of aerodynamic effects, mechanical vibrations, or cavitation. Unsteady flow, pressure oscillations, and high velocities produce vibrations that are typically less than 100 decibels (dB) (decibel). A decibel is the maximum level of sound strength from the earphones of a portable radio.
The degree of noise produced by cavitation varies. Increased pressure drop through a control valve causes more noise. In full cavitation, a control valve makes a rattling noise.