AN INTRODUCTION TO VALVE TYPES AND APPLICATIONS

AN INTRODUCTION TO VALVE TYPES AND APPLICATIONS

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 pressure 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.

VALVE COMPONENTS
The following are the primary components of a valve.

FIG 1

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. The bonnet is a valve component that provides a leak proof 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. Backseating 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 TYPES

1- GATE VALVES
Gate valves, illustrated in Fig. 2, use a wedge-shaped disk or gate as the closure member operating perpendicular to the flow; it is raised to open and lowered to close the valve. As the disk closes, it fits tightly against the seat surfaces in the valve body. A gate valve is used fully opened or closed only. It should not be used for throttling service (partly open), as the gate will vibrate and quickly become
damaged and subject to wire drawing caused by the velocity of the liquid flowing past the disk.

FIG 2

Disk Design. There are three types of disk constructions: solid wedge, split wedge, and flexible wedge (illustrated in Fig. 3).

FIG 3

Solid Wedge. Solid wedge disks are most prevalent because of their simple and usually less expensive design.
Split Wedge. Split wedge disks, also called double disks, have somewhat better sealing characteristics than solid disks because the two disk halves are forced outward against the body seats by a spreader after the disk has been fully lowered into its seating position. When the valve is opened, pressure on the disk is relieved before it is raised, eliminating the friction and scoring of body seats and disk.
Flexible Wedge. Flexible wedge disks are solid only at the center and are flexible at the outer edge and seating surface. This design enables the disk face to overcome the tendency to stick in high-temperature service where wide swings in temperature occur. This type of disk is generally found only in steel valves.

2- GLOBE VALVES
Globe valves are so named due to the globular shape of the valve body. Globe valves are used where throttling and/or frequent operation is desired. Each uses the same method of closure-a round disk or tapered plug-type disk that seats against a round opening (port). This design deliberately restricts flow, so globes should not be used where full, unobstructed flow is required. 
There are three basic types of globe valve: the standard globe valve (Fig. 4), the angle globe valve (Fig. 5), and the needle valve (Fig.6).

FIG 4

FIG 5

FIG 6

Angle valves are identical to standard globe valves in seat design and operation. The basic difference is that the body of the angle valve acts as a 90 ⁰ elbow, eliminating the need for a fitting at that point in the system. Angle valves also have less resistance to flow than the combination of globe valves and the fittings they replace. Needle valves are generally small in size and are intended to provide
precise flow control. Many turns of the handle are required to adjust flow in order to achieve precise control.

3- PLUG VALVES
A plug valve, shown in Fig. 7, is a quarter-turn valve that uses a tapered cylindrical plug that fits a body seat of corresponding shape. When the port in the plug is aligned with the body opening, flow is permitted in a way similar to a ball valve.A one-quarter (90⁰) turn operates the valve from opened to closed and vice versa.
Plug valves fall into two basic categories, lubricated and nonlubricated

FIG 7

4- BALL VALVES
A ball valve utilizes a ball with a hole drilled through it as the opening/ closing device. It is a quarter-turn valve. The ball seals by fitting tightly against resilient
seat rings on either side. Flow is straight through, and pressure loss depends on the size of the opening in the ball (port).
Ball valves are available in one-, two-, or three-piece body types (Fig. 8).

FIG 8

5- BUTTERFLY VALVES
A butterfly valve has a wafer-shaped body with a thin rotating disk as the closing device. Like the ball valve, the butterfly operates with a one-quarter turn from fully opened to fully closed. The disk is always in the flow path, but since it is relatively thin, it offers little restriction to the flow. When the valve is closed, the disk edge fits tightly against a ring-shaped liner (seat).
These valves generally have one-piece bodies that fit sandwich-style between two pipe flanges.
The two most common body types are wafer body and lug body, illustrated in Fig. 9

FIG 9

6- DIAPHRAGM VALVES
A diaphragm valve uses a rubber, plastic or elastomer diaphragm to seal the stem. The diaphragm not only seals the stem but forms the closure element. There are two styles of diaphragm valves, one having a body with a weir and the other having a straight-through body.Fig 10
Since the diaphragm is not metallic and forms the closure, the valve is severely limited in pressure and temperature. A wide variety of diaphragm materials are available for use with different fluids.

FIG 10

7- PINCH VALVES
A pinch valve (Fig. 11) uses a round elastomeric sleeve connected to the valve body from inlet to outlet that completely isolates the liquid passing through the valve from all internal valve components. Closure is made by a movable closure element outside the sleeve that pinches the sleeve between the element and the valve body. This type of valve is used for slurry and other liquids with highly corrosive properties.

FIG 11

8- CHECK VALVES
Check valves (Fig.12) automatically check or prevent the reversal of flow. Basic types are the swing check, lift check, ball check, and wafer check designs. Another designation used for sanitary waste systems is a backwater valve. The swing check has a hinged disk, sometimes called a flapper, that swings on a hinge pin. When flow reverses, the pressure pushes the disk against a seat. The flapper may have a composition disk, rubber or Teflon, rather than metal when tight closure is required.
Swing checks offer little resistance to flow.

FIG 12

The lift check has a guided disk that is raised from the seat by upward flow pressure. Reversal of flow pushes the disks down against the seat, stopping back flow. Lift checks have considerable resistance to flow, similar to that of a globe valve. They are well suited for high-pressure service.
Another common check is a wafer design which fits between flanges in the same fashion as a butterfly valve. Wafer checks come in two types: a dual flapper that is hinged on a center post and a single flapper that is similar to the standard swing check. They are generally used in larger size piping (4 in and larger) because they are much lighter and less expensive than traditional flanged end swing check valves.
A demand check value is of two-piece construction, with one piece having a spring-loaded closure similar to the air values found on automobile tires. The second piece, when inserted into the first, opens the valve, allowing free passage of air. The demand check valve is used for connecting gauges, allowing removal without permitting air to escape from the pipe.

9- MISCELLANEOUS VALVE TYPES
Various other types of valves are often used in utility systems. They can be either independently installed to operate as self-contained units or controlled electronically from a panel, system signal, or other remote source.
Pump Control Valve
This type of valve is used on pumped systems to control or eliminate surges caused by pump start and stop. It operates by using a spring-loaded closure member that opens or closes slowly to restrict the initial flow of water when a pump starts and stops.
Flow Control Valve
This valve operates by using a calibrated orifice or venturi tube to control the flow of liquid to a predetermined set point regardless of fluctuating line pressure.
Pressure Control Valve
Similar to the flow control valve, this valve limits the pressure of a flowing liquid to a predetermined set point regardless of fluctuating flow rate.
Level Control Valve
This valve accurately controls the level of liquid in a tank or vessel. An altitude valve uses a controlling device to maintain the level, and a float valve uses a movable float on an arm (similar to that in a water closet) to stop the flow at a predetermined level.

FIG 13

10- Conduit Valve
A conduit valve (Fig. 14) is used where an unobstructed opening through the valve is required, such as when pigs are used to clean the pipeline.

FIG 14

11- pressure relief valve

A pressure relief valve is a safety device designed to protect a pressurized vessel or system during an over pressure event. An over pressure event refers to any condition which would cause pressure in a vessel or system to increase beyond the specified design pressure or maximum allowable working pressure

FIG 15

VALVE ACTUATORS
There are three operating methods for valve actuators: multiturn (used for gate, globe, and diaphragm valves), quarter-turn (used for plug, ball, and butterfly valves), and linear (used for gate, diaphragm, and globe valves). The valves can either operate manually or be power actuated. There is no generally recognized code or standard for valve operators outside of those concerned with specific, high-risk industries, for example, nuclear work.
Manual Operation
Manually operated valves are usually used when the valve is easily accessible
Power Actuators
Power actuators are used where valves are remotely located, frequent operation is required, or automatic operation is necessary due to system considerations
Actuators are classified by their source of power: electric, pneumatic, or hydraulic.