Mechanical Science - NA-40001 - Valves

Purpose of an overpressure protection valve

The primary function of a ______ valve is to provide component or system overpressure protection. ______ valves are designed to fully open instantly at the preset pressure setpoint. This allows _______ valves to provide a large volume release path at low overpressure conditions. _______ valves do not have any accumulation like relief valves. The valves open fully and stay fully open until the pressure falls to below the preset pressure; then they close sharply.

The 5 basic parts of a valve are

In this type of stem the stem is threaded into a stationary surface. When either the stem or the hand wheel is turned, the stem will move, repositioning the disk. The threaded surface that allows the stem to rise may be located either in the yoke (yoke bushing) or in the actuator (stem nut).

These valves are used to stop or start fluid flow. The body of a _____ valve is machined to receive the tapered or cylindrical plug. The disk is a solid plug with a bored passage for flow. In the open position, the passage in the plug lines up with the inlet and outlet ports of the valve body. The hole bored in the plug is the same size as the piping or a little smaller. This allows unrestricted flow through the valve when the hole in the plug is aligned with the pipe. There is little turbulence and the pressure loss is small. When the plug is turned 90 degrees from the open position, the solid part of the plug blocks the ports and stops fluid flow.

Valves used to start and stop fluid flow. Uses a rotary motion disk that can be used to isolate flow. Are used in high flow, large diameter, and low pressure systems, such as circulating water. Good for handling of slurries.

These valves are used to start or stop flow. They are not intended to regulate or throttle flow. They are linear motion valves used for positive fluid flow isolation. Usually used as isolation valves. They are normally fully closed or fully open, depending upon their specific application. They are poor throttle valves and can be damaged if misused. They are not effective for use as throttling valves. The major advantages of these valves are that they produce very little pressure drop as flow passes through the valve and they have very good flow isolation characteristics. The major disadvantages of these valves are that they are not suitable for throttling and are prone to vibration when partially open.

The _____ valve begins to open when system pressure reaches the preset pressure setpoint of the valve. The valve will continue to open as the pressure increases, until at some higher pressure the valve is fully open. At that point, the ______ valve is relieving at its maximum rated capacity. The overpressure condition overcomes the spring force applied and lifts the disk, which allows system pressure to be relieved. As the pressure begins to decrease the _____ valve begins to close. When system pressure decreases below the initial setpoint pressure, the ______ valve fully closes.

In this type of stem the stem is threaded into a stationary surface. When either the stem or the hand wheel is turned, the stem will move, repositioning the disk. The threaded surface that allows the stem to rise may be located either in the yoke (yoke bushing) or in the actuator (stem nut).

The binding that can occur when liquid becomes trapped in the valve bonnet. If the valve and valve bonnet are heated, the liquid becomes heated and begins to pressurize the valve bonnet.

Serves as a second disc and seat that mate when the valve is fully opened. The primary purpose of the valve _______ is to remove system pressure from the packing in the event that the packing leakage is excessive and adjustment cannot correct the problem.

Which one of the following list some disadvantages of plug valves?

Provides the seating surface for the disk. In some designs, the body is machined to serve as the seating surface, and these are not used.

Three methods to prevent thermal binding and pressure locking of valves

Part of the valve that mates with the body of the valve and supports the stem, disk, and the device used to position the valve (actuator).

precautions used during manual valve operation

Valves used to start, regulate, and stop fluid flow utilizing a linear motion. They have very good throttling characteristics. The name is derived from its flexible disk, which mates with a seat located in the open area at the top of the valve body to form a seal. Are not used for high pressure or high temperature fluids. Valve fluid is completely separated from the moving valve parts.

The main element of the valve assembly. It directs the flow through the valve and past the disk and its mating surface, the seat.

Techniques for verifying the position of manual valves:

Valves used to make fine adjustments to the amount of the fluid allowed to flow through them. Are used where precise control of flow is desired, such as sampling lines. Because of the precise, delicate nature of these valves, they are not used as isolation valves; another valve in the line stops and starts flow.

Potential causes of valve operator difficulties following maintenance or valve operation

Types of check valves

Valves that are designed to permit flow in only one direction. These valves use the pressure of the fluid flowing through the pipeline to operate them. Pressure applied in the direction of allowed flow causes these valves to open. Pressure applied in the opposite direction causes these valves to close.

How to determine or verify valve position locally.

Which of the following list some advantages of a plug valve?

Opening a valve causes ______ to increase and _______ to decrease.

Valves that are linear motion valves with excellent flow isolation and throttling characteristics. (good regulation valves) The fluid going through the valve follows a torturous flow path. This flow path results in a large pressure loss across the valve.

Two important precautions we need to observe when operating diaphragm valves.

This valve part provides the means to control the flow through the valve. As it is closed, it reduces (restricts) flow until the valve is fully shut and flow is stopped.

In this type of stem the stem is threaded into a sleeve connected to the disk. As the actuator turns the stem, the threaded sleeve and disk rise, but the stem does not rise.

Valves used to start, regulate, and stop fluid flow utilizing a linear motion. They have very good throttling characteristics. The name is derived from its flexible disk, which mates with a seat located in the open area at the top of the valve body to form a seal. Are not used for high pressure or high temperature fluids. Valve fluid is completely separated from the moving valve parts.

When a hot valve is closed and allowed to cool, the difference in the thermal contraction can cause the seats to bind with the disk, so that reopening of the valve may be impossible until the valve body is reheated. This is called:

Closing a valve causes backpressure to ______ and flowrate to _____ ?

Valve part that attaches to the disk and extends out of the valve to allow an outside force to position the disk within the valve. T heads, threaded fittings, or flexible joints typically connect this part to the disk.