GENERAL
Valve functions can be defined as ON / OFF service, throttling service (flow control), prevention of reverse flow (or back flow), pressure control, regulation and pressure relief. Valves can be classified as either linear (gate valve) or rotary (ball valve) based on the action of the closure member. They are also classified by the shape of their closure member such as gate, globe, butterfly, ball, plug, diaphragm, pinch, and check.
Their primary function, however, is to control the flow of liquids and gases, including plain water, corrosive fluids, steam, toxic gases, or any number of fluids with widely varying characteristics. Valves must also be able to withstand the pres- sure and temperature variations of the systems in which they are used. Some valves on combined water service mains, and those handling flammable material, may be required to be fire safe or approved for fire protection use.
Pressure regulating valves for water service are discussed in Chap. 9, Plumbing Systems, in the section entitled Water. Pressure regulating valves are discussed in their respective chapters. Pressure relief valves are outside the scope of this hand- book.
CODES AND STANDARDS
The following standards apply to valve construction:
1. AWWA C 500: gate valves for water and sewage systems
2. AWWA C 504: rubber seated ball valves
3. MSS SP 67: butterfly valves
4. MSS SP 80: bronze gate, globe, angle, and check valves
VALVE COMPONENTS
The following are the primary components of a valve.
1. A valve body is the housing for all the internal working components of a valve and it contains the method of joining the valve to the piping system.
2. The closure element, known as the disk or plug, is a valve component that, when moved, opens or closes to allow the passage of fluid through the valve. The mating surface of the disk bears against the seat.
3. The actuator is a movable component that, when operated, causes the closure element to open or close.
4. The stem is a movable component that connects the actuator to the closure element.
5. The bonnet is a valve component that provides a leakproof closure for the body through which the stem passes and is sealed.
6. The seat is a component that provides a surface capable of sealing against the flow of fluids in a valve when contacted by a mating surface on the disk. The seat is attached to the valve body.
7. The stuffing box is the interior area of the valve between the stem and the bonnet that contains the packing.
8. Packing is the material that seals the stem from leaking to the outside of the valve. The packing is contained by the packing nut on the bonnet.
9. The backseat is a seat in the bonnet used in the fully open position to seal the valve stem against leakage into the packing. A bushing on the stem provides the mating surface. Back seating is useful if the packing begins to leak and it provides a means to prevent the stem from being ejected from the valve. Back- seating is not provided on all valves.
10. The stroke of a closure member is the distance the member must travel from the fully opened to the fully closed position.
Their primary function, however, is to control the flow of liquids and gases, including plain water, corrosive fluids, steam, toxic gases, or any number of fluids with widely varying characteristics. Valves must also be able to withstand the pres- sure and temperature variations of the systems in which they are used. Some valves on combined water service mains, and those handling flammable material, may be required to be fire safe or approved for fire protection use.
Pressure regulating valves for water service are discussed in Chap. 9, Plumbing Systems, in the section entitled Water. Pressure regulating valves are discussed in their respective chapters. Pressure relief valves are outside the scope of this hand- book.
CODES AND STANDARDS
The following standards apply to valve construction:
1. AWWA C 500: gate valves for water and sewage systems
2. AWWA C 504: rubber seated ball valves
3. MSS SP 67: butterfly valves
4. MSS SP 80: bronze gate, globe, angle, and check valves
VALVE COMPONENTS
The following are the primary components of a valve.
1. A valve body is the housing for all the internal working components of a valve and it contains the method of joining the valve to the piping system.
2. The closure element, known as the disk or plug, is a valve component that, when moved, opens or closes to allow the passage of fluid through the valve. The mating surface of the disk bears against the seat.
3. The actuator is a movable component that, when operated, causes the closure element to open or close.
4. The stem is a movable component that connects the actuator to the closure element.
5. The bonnet is a valve component that provides a leakproof closure for the body through which the stem passes and is sealed.
6. The seat is a component that provides a surface capable of sealing against the flow of fluids in a valve when contacted by a mating surface on the disk. The seat is attached to the valve body.
7. The stuffing box is the interior area of the valve between the stem and the bonnet that contains the packing.
8. Packing is the material that seals the stem from leaking to the outside of the valve. The packing is contained by the packing nut on the bonnet.
9. The backseat is a seat in the bonnet used in the fully open position to seal the valve stem against leakage into the packing. A bushing on the stem provides the mating surface. Back seating is useful if the packing begins to leak and it provides a means to prevent the stem from being ejected from the valve. Back- seating is not provided on all valves.
10. The stroke of a closure member is the distance the member must travel from the fully opened to the fully closed position.
VALVE BODY MATERIALS
Valves are manufactured in both metallic and nonmetallic materials. Nonmetallic materials consist primarily of thermoplastics.
Metallic
Valve bodies are manufactured of the following materials:
1. Bronze valves are usually limited to the smaller sizes and are used in water service up to 450 F. Different alloys are available for higher temperature and pressure applications.
2. Cast iron is a commonly used material for water and steam up to 450 F and is generally limited to smaller size castings. High tensile strength iron may be used in large sizes.
3. Malleable iron is characterized by pressure tightness and resistance to stress and shock.
4. Ductile iron casting has high tensile strength, good ductility, and good corrosion resistance.
5. Steel is available in a wide variety of alloys that are recommended for high temperature and pressure applications and conditions that may be too severe for iron and bronze bodies.
6. Stainless steel is available in a wide variety of alloys and is often used for pure water and other services requiring noncorrosive materials.
Thermoplastics
Thermoplastics are rapidly gaining popularity in many utility systems. They have proven very successful for carrying corrosive fluids where conventional valves are
not suitable or special alloy metals are very costly. It is estimated that they are suitable for use in 85 to 90 percent of all utility services. There are many different types of thermoplastic materials capable of carrying most chemicals. All standard valve types are available. The valve materials often have additives different from those used for manufacturing piping. Generally avail- able valve types and their sizes are given in Table 2.6. In general, plastic valves are limited to a maximum temperature of 250 F and pressure of 150 psig (1035 kPa).
Valves are manufactured in both metallic and nonmetallic materials. Nonmetallic materials consist primarily of thermoplastics.
Metallic
Valve bodies are manufactured of the following materials:
1. Bronze valves are usually limited to the smaller sizes and are used in water service up to 450 F. Different alloys are available for higher temperature and pressure applications.
2. Cast iron is a commonly used material for water and steam up to 450 F and is generally limited to smaller size castings. High tensile strength iron may be used in large sizes.
3. Malleable iron is characterized by pressure tightness and resistance to stress and shock.
4. Ductile iron casting has high tensile strength, good ductility, and good corrosion resistance.
5. Steel is available in a wide variety of alloys that are recommended for high temperature and pressure applications and conditions that may be too severe for iron and bronze bodies.
6. Stainless steel is available in a wide variety of alloys and is often used for pure water and other services requiring noncorrosive materials.
Thermoplastics
Thermoplastics are rapidly gaining popularity in many utility systems. They have proven very successful for carrying corrosive fluids where conventional valves are
not suitable or special alloy metals are very costly. It is estimated that they are suitable for use in 85 to 90 percent of all utility services. There are many different types of thermoplastic materials capable of carrying most chemicals. All standard valve types are available. The valve materials often have additives different from those used for manufacturing piping. Generally avail- able valve types and their sizes are given in Table 2.6. In general, plastic valves are limited to a maximum temperature of 250 F and pressure of 150 psig (1035 kPa).