【正文】 ed properly to work well. ?If the valves is installed backward it will tend to wear unevenly and push the flowcontrol element down. ? 46 Chapter 2. Valves 2. Globe valves The typical globe valve consists of: disc body seating area stem bon packing Stuffing box Packing gland Hand wheel ? 47 Chapter 2. Valves 2. Globe valves The disc is attached to the stem in three ways: Slip joint Threading Onepiece manufacturing The disc can be classified as plug, ball, position, or needle shape. ? 48 Chapter 2. Valves 3. Ball valves ?The hollow ball rotates into the open or closed position. ?A ball valve does not lift the flowcontrol device out of the process stream. ?Ball valves provide very little restriction to flow and can be fully opened with a quarter turn on the valve handle. ?In the closed position, the port is turned away from the process flow. In the open position, it lines up perfectly with the inner diameter of the pipe. ? ?Ball valves take their name from the ball shaped, movable element in the center of the valve. 49 Chapter 2. Valves 3. Ball valves ?Ball valves are not designed for excessive temperatures. ?It should not be used for throttling service. ?Ball valves do not generally seal as well as globe valves in highpressure service. ?Some ball valves (multiport valves) are designed with multiple ports, so an operator can switch fluid sources without stopping flow. ? ?The ball valves only require onequarter turn on a handle. 50 Chapter 2. Valves 4. Check valves ?Flow lifts the disc and keeps it lifted until flow stops or reverses. ?A typical check valve design is the swing check, which has a hinged disc that slams shut when flow reverses. ?A check valves is designed to prevent reverse flow and to avoid possible contamination or damage to equipment. 51 Chapter 2. Valves 4. Check valves ?Lift checks are ideal for systems in which flow rates fluctuate. ?Like the swing check, it is designed to close when flow reverses. ?The lift check is more durable than the swing check. ?In the horizontal or vertical lift check design, a piston or ball is lifted up and out of the seat by process flow. ? ?Another design is the lift check, which has a disc rests on the seat when flow is idle and lifts when flow is active. Special guides keep the disc in place. 52 Chapter 2. Valves 5. Butterfly valves ?During a throttling operation, a butterfly valve handle should be carefully secured. They have nonuniform flow characteristics. ?A onequarter turn is all it takes to fully open or close the valve. ?They are designed to be operated at low temperature and low pressure. ? ?Butterfly valves are monly used for throttling and on/off service. The body of this type of valve is relatively small when pared with other valves. It occupies much less space in a pipeline. 53 Chapter 2. Valves 6. Plug valves ?Plug valves provide very little restrictions to flow and can be opened 100% with a onequarter turn on the valve handle. ?The plug valve takes its name from the plugshaped flowcontrol element it uses to regulate flow. ?Plug valves depend on a lubricant inside the body of the valve. The lubricant helps the valve provide a leakfree seal. ? ?Quickopening, onequarter turn plug valves are very popular in the process industry. ?The plug valve is designed for fuel gas piping systems, lowpressure situations, slurry and lubrication, on/off, lowtemperature service. 54 Chapter 2. Valves 7. Diaphragm valves ?The stem is attached to a device called the pressor. ?Diaphragm valves use a flexible membrane and seat to regulate flow. ?In a chemical plant, a variety of slurries, corrisive or sticky substances, are transferred from place to place. ?Standard valves would have a difficult time with this type of product. ?Diaphragm valves are specially designed for the job. ?The pressor pushes on a flexible diaphragm. ?The internal parts of the valve never e into contact with the process material. ?The diaphragm valve seats are made of chemicalresistant plastic, rubber, or neoprene. ?Diaphragm valve can handle corrosive fluids.