The main components of the valve include two types of functions, one is the pressure retainer, which includes the valve body, bounty, cover bolts, discs and other non-pressure retaining parts of a valve, such as valve seat (s), stem, yoke, packing, screw and Nuts, bushings, handwheels, actuators and so on.
In all of these removable and replaceable internal parts, some of them will be in direct contact with the fluid that passes through them. These internal parts of the valve are collectively called the valve trim. These operating units control the flow of liquid and gas through a valve. They are the parts that are most exposed to the elements of the process and are therefore very vulnerable to wear over time. The stem, plug and seat are the three basic elements in a high pressure control valve trim. So by changing the valves, the trim components may also be different except for the disc, the valve seat and the stem. This will be common to all lions. As trim parts, disc movements and flow control are possible.
And some other components of the milk trim as special milk included,
- Sit back
- Holder pins
- Internal springs
Milk trims based on milk type
See Table 1, which shows the milk trim elements for different types of milk.
Table 1: Trim the valve against different types of objects
The importance of valve trim design
Trim plays an important role in milk characteristics. Determines the flow rate and separation of valves. The shape of the valve trim determines the characteristics of the valve flow. The valve trim characteristic is related to the percentage of flow and movement of the valve stem between 0 and 100%.
The trim material is sometimes the same as the body material or the bonnet and sometimes different. Due to the different properties, the components need to withstand specific forces and conditions. So they are made of different materials. When suitable machining materials are selected, the properties of the flow medium such as the chemical composition of the fluid, pressure, temperature, flow rate, velocity and viscosity of the fluids are taken into account.
Valve trim components and valve features
The plug is the part of the valve that, depending on its position, passes, opens or stops the flow, is part of the valve holding pressure. The types of plugs on the valve specify the name of the valve. A ball plug is called a ball valve. When the valve is closed, a plug is placed in front of the fixed seat; in the closed position, the plug performs the function of maintaining pressure, while when the valve is open, the plug does not perform the function of holding the pressure. . By moving the valve stem, it can be removed from the valve seat, except in check and stiffness valves, the plug is removed from its seats by the flow and pressure of the fluid. Milk plugs are usually cast, forged or made.
The valve seat is one of the non-pressure parts that hold the valve. The valves will probably have one or more seats, as in the case of a globe or a rotating return valve, there is a seat that seals with a disc to stop the flow. While for the gate valve, there are two living levels that will be in contact with the seats. Likewise, plug-in valves and ball valves will have multiple sites, depending on the number of ports. Valve seats can be integrated, renewable or replaceable rings. Small valves are provided with screw, broom or welded valves, and large valves may have any of the seats used in small valves and have integrated casting or forging seats along the body of the valve. .
The leakage rate in the valves is directly proportional to the sealing effectiveness. Between the disk and its seat. Acceptable leakage rates are specified by MSS SP61, API 598 and ASME B16.34 milk standards.
The rear seat is another non-pressure valve. The rear seat consists of a stem shoulder. When the stem is in the fully open position, it forms a seal. Therefore, leakage from the milk shell to the packaging chamber and the environment is prevented.
The actuator stem connects the valve and disc handlebar, the plug or the ball. Valve is required to open and close the movement. Thus, the opening and closing of the milk stalk took place. In the gate valve or globe, linear movement of the disc is required to open or close the valve. While in a plug, ball and butterfly valve, the valve disk is rotated to open or close the valve. Stop valves and check valves have no stems.
Bolts include bolts, nuts and washers. They hold the hood and body and create a tight seal between them. Bolts should be selected according to the program code and standards in accordance with the applied materials.
The flange connects the gland and the hood and tightens the screws.
The yoke is also called the yoke arms. The actual mechanism of the valve is attached to the body or cap of the valve with a yoke. On many valves, the yoke and bounty are designed as one piece. The upper part of the yoke consists of a yoke nut and a yoke bush. Yoke is made hard enough to withstand the forces, moments and torque generated by the actuators.
This is the part where the internal threaded nut is held above the yoke where the milk shaft passes. In some valves such as gates and diaphragms. The nut rotates and the stem moves up and down, depending on the direction of rotation. On some lions, the yoke is held in place permanently and the stem rotates through it. To have the least effort to activate, yoke bushes are made of softer material than the stem. Antifreeze sleeve bearings are installed on the valves, which require a lot of effort to open or close. This friction between the hardened stem and the bush minimizes the yoke.
Used to support the tuber bush to hold the tuber pack under tension.
Depending on the application of packing shoot applications such as
- Prevent leakage from environment to environment.
- Prevent air from entering the valve during vacuum applications.
Available in stuffing box. The packing loops are tightened by tightening the packing nuts or packing bolts. Compression here should be enough to create a good seal. Stem packing is made of graphite or PTFE as required.
Types of control valves
The shape of the valve trim determines the flow of valve properties in the control valve. There are three main types of control valve trim.
- Snap trim (also known as quick cut)
It opens quickly and is used for isolated services. Which includes basic operations such as draining fluids, reducing pressure and measuring.
- Nominal trim (also called linear trim)
It is used for gasification of liquids, control of liquid level and in applications where water hammering is difficult.
3. Trim with equal percentage
They are used to control the pressure or flow of gases and vapors in gas valve applications.
Select trim against corrosion and erosion
Valve trims are selected based on service conditions such as high temperature, pressure, chemical composition, flow rate, etc. The stability of materials for corrosion resistance can be determined by corrosion resistance tables and selection based on them.
Erosion is caused by high-velocity liquids with abrasive particles that are harder than lathes.
Metal valve cutting material
AISI 316 stainless steel is the most common decorative material. Metal trim is suitable for all-round selection in public services for about -195 degrees Celsius to + 400 degrees Celsius and liquids with moderate corrosion. These include steel alloys, titanium, cobalt-chromium alloys, nickel-boron alloys.
Non-metallic cutting materials
Operating temperature is the most important factor in choosing non-metallic decorative materials. Polytetrafluoroethylene (PTFE), Teflon and so on.
Valve trim chart
To standardize lathes, the API assigns a unique number to each set of valve trim materials. The lathe number or combination number consisting of materials such as disc, stem, rear seat and sleeves is grouped and assigned to a number.
- API 600 & 602 provides a list of trim materials that can be used in milk.
- ASTM A410 (13Cr), ASTM A316, 20 alloy (19Cr-29Ni), and Monel (Cu-Ni alloy) are commonly used trim grades.
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