Mechanical Sciences Valves
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The pressure above the preset pressure at which a relief valve is fully open (expressed in percent of preset pressure)
The pressure difference between the preset pressure setpoint and the actual pressure at which the relief valve reseats (expressed in percent of preset pressure)
The main element of the valve assembly. Houses valve seat and disk.
Mates with the body of the valve and supports the stem, disk, and valve actuator.
Mechanical disk shaped device positioned in flowpath to control fluid flow through the valve.
The pressure at which the relief valve reaches maximum capacity (fully open).
The stem position is independent of valve disk position.
The pressure setpoint at which a relief or safety valve begins to relieve pressure.
The stem rises or lowers with the position of the valve disk.
The seating surface for the disk.
Seat Rings (Valve)
Attaches to the disk and extends out of the valve body to allow positioning of valve disk.
Explain the effect of valve position on backpressure and flow rate in piping systems.
- Closing a valve causes:
- D/P or headloss across the valve to increase
- Backpressure to increase
- Flow and downstream pressure to decrease
List the five basic parts of a valve.
- Seat rings
State the purpose of valve packing.
Packing provides a seal between the bonnet and stem to prevent the fluid from escaping from the valve along the stem.
Explain the significance of rising and non-rising stem valves when determining valve position.
- In rising stem valves, 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.
- In non-rising stem valves, 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.
State the purpose and usage of valve backseats.
the backseat serves as a second disc and seat that mate when the valve is fully opened. The primary purpose of the valve backseat is to remove system pressure from the packing in the event that the packing leakage is excessive and adjustment cannot correct the problem.
Used to start or stop flow. They are not intended to regulate or throttle flow.
Most widely used flow regulating valves. They are linear motion valves with excellent flow isolation and throttling characteristics.
Used to make fine adjustments to the amount of the fluid allowed to flow through them.
Used to start and stop fluid flow. A rotary motion valve that can be used to isolate flow.
Used to stop or start fluid flow. The names are derived from the shape of the disk.
Ball and Plug Valves
Used to start, regulate, and stop fluid flow. A linear motion valve. They have very good throttling characteristics. Used in radiation areas
Designed to permit flow in only one direction. These valves use the pressure of the fluid flowing through the pipeline to operate them.
List the common types of check valves.
Swing, Testable Swing, Lift, Stop
State the purpose of a pressure reducing valve.
Pressure reducing valves are specially designed valves that automatically reduce downstream fluid pressure to a set desired pressure
Describe the function and operation of a relief valve.
The primary purpose of a relief valve is to provide component or system overpressure protection.
Describe the function and operation of a safety valve.
The primary function of a safety valve is to provide component or system overpressure protection. Safety valves are designed to fully open instantly at the preset pressure setpoint. This allows safety valves to provide a large volume release path at low overpressure conditions. Safety 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.
Describe how thermal binding and pressure locking of valves occurs.
- 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.
- If the valve and valve bonnet are heated, the liquid becomes heated and begins to pressurize the valve bonnet. If there are no means of pressure relief via valve packing leakage or disk/seat leakage, the pressure build-up could prevent valve operation.
LIST three methods to prevent thermal binding and pressure locking of valves.
To prevent thermal binding during cooldown, the affected valves can be cracked opened and re-shut. This should be done periodically during the cooldown. Also, using the proper closing torque can prevent the binding of the seat and disk. Using a relief valve or vent valve on the valve bonnet to equalize or reduce the pressure build-up can prevent pressure binding as well.
Describe the proper methods or techniques for verifying the position of manual valves.
- 1.When locally determining valve position physically check the valve position by turning the valve handwheel a short distance in the “shut” direction.
- 2.Use system indications to check valve positions
- 3.Check the position of a rising stem valve by observing whether the stem is up or down.
- 4.Non-rising stem valves sometimes have small mechanical pointers that are operated by the valve actuator simultaneously with valve operation.
- 5.Power actuated valves typically have a mechanical pointer that provides local valve position indication.
- 6.Limit switches are also often attached to the valve stem to provide local and remote position indication.
IDENTIFY potential causes of valve operator difficulties following maintenance or valve operation.
factors that could affect the stroke time of a valve include valve actuator (motor, solenoid, or air) damage, packing tightness, stem binding, initiating signal adjustments, and limit switch adjustment.
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