CHAPTER 1 VALVES

Swing check

A swing check valve allows full, unobstructed flow in the allowed direction and automatically closes as flow decreases. Turbulence and pressure drop within the valve are low. Flow from inlet to outlet pushes the disk open. Flow from outlet to inlet pushes the disk closed and provides a positive seating force. Recommended for lines having low velocity flow.

PRESSURE DROP IN A GLOBE VALVE

A valve throttles or reduces flow by increasing the head loss (pressure drop) in the fluid system. The major disadvantage of a globe valve is the high head loss experienced by the fluid as it passes through the valve. This characteristic may cause the globe valve to be excluded from applications where a large pressure drop will inhibit system operation

Butterfly valves

Butterfly valves provide significant advantages over other valve designs in weight, space, and cost for large valve applications.

Check Valves

Check valves are designed to permit flow in only one direction. These valves use the pressure of the fluid flowing through the pipeline to operate them. There are three basic types of check valves: a. Swing check b. Lift check c. Stop check

Diaphragm valves

Diaphragm valves are particularly suited for the handling of corrosive fluids, fibrous slurries, radioactive fluids, or other fluids that must remain free from contamination or which must not be allowed to leak out. An advantage of a diaphragm valve is that the fluid is completely separated from the moving valve parts; therefore, the valve internals and moving parts are not subject to contamination and degeneration Never backseat a diaphragm valve This may tear the operating stem out of the diaphragm Don't close a diaphragm valve too hard. Excessive seating force can damage or rupture the diaphragm

GAGGING DEVICE

For certain types of high pressure testing, safety valves and relief valves may be prevented from lifting by a gagging device. The test may require that the test pressure be higher than the relief valve lifting setting. The valve stem is prevented from lifting (gagged) by screwing the gagging screw (bolt) through the threaded hole in the center of the valve upper bonnet.

Gate valve

Gate valves are used as flow isolation valves. They are not recommended for use as throttling devices. Gate valves are generally used in systems where low flow resistance for a fully open valve is desired and there is no need to throttle the flow.

Globe valves

Globe valves are used as throttling and isolation valves. Globe valves are used in systems where good throttling characteristics and low seat leakage are desired and a relatively high head loss in an open valve is acceptable.

Lift check valve

Lift check valve bodies generally resemble those of the globe valve Because of the turns the fluid must make while flowing through the valve body, lift check valves, like globe valves, cause a relatively large pressure loss They are recommended for use with steam, air, gas, water, and on vapor lines with high flow velocities

Needle valves

Needle valves are used for throttling application only. They provide fine control and have high pressure drops. They are typically not used for isolation. Ball valves and plug valves are quick acting, allowing quarter turn on‑off operation.

BALL VALVE

Plug valves and ball valves are usually oriented such that the handle will be 90° to the pipe when the valve is closed and line up with the pipe when it is open

Pressure reducing valves

Pressure reducing valves are specially designed valves that automatically reduce downstream fluid pressure to a set desired pressure (as long as the supply pressure is at least as high as the desired pressure).

ACCUMULATION IN RELIEF VALVES

Relief valve accumulation is that pressure above the preset pressure, at which the valve is fully open. Accumulation is normally expressed in percent of the preset pressure.

Stop check valves

Stop check valves more closely resemble globe valves than do lift check valves and are capable of being forcibly closed by an actuator The stop check valve is a check valve capable of stopping flow in both directions The stem is not connected to the disk and functions only to close the valve tightly or to limit the travel of the valve disk in the open direction When the stem is withdrawn, the disk is free to operate depending on direction of flow, functioning like a lift check

STATE the purpose of valve packing

The PURPOSE is to keep the fluid that is flowing through the valve from leaking out around the stem.

PLUG VALVE

The body of a plug valve is machined to receive the tapered or cylindrical plug. In the open position, the passage in the plug lines up with the inlet and outlet ports of the valve body. When the plug is turned 90° from the open position, the solid part of the plug blocks the ports and stops fluid flow.

BRIDGEWALL MARKING

The outside of the valve body usually has either an arrow showing the correct flow direction or a "bridge wall" marking which is a schematic picture of the valve's internal construction. This bridge wall marking indicates flow is from left to right.

SAFETY VALVE POP ACTION

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

Relief valve

The primary purpose of a relief valve is to provide component or system overpressure protection If the pressure below the disk overcomes the spring pressure holding the disk down, the valve begins to open The overpressure condition overcomes the spring force applied and lifts the disk, which allows system pressure to be relieved When system pressure decreases below the preset setpoint pressure, the relief valve closes

State the purpose and usage of valve backseats.

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.

PRESSURIZED DISK

Using an equalizing line, relief valve, or vent valve on the valve bonnet to equalize or reduce the pressure build‑up can prevent pressure binding as well. This differential pressure comes from: 1.Much higher system pressure on one side of a valve 2.Liquid trapped in the valve bonnet is heated and pressurizes the bonnet 3.Thermal binding during cooldown occurs and can be prevented by periodically cracking open the valve and reshutting it to equalize the D/P

What are the BASIC parts of a valve?

· Body · Bonnet · Disk · Seat rings · Stem.