Valves
in chemical process industry
VM
Sheth and PR Shroff review valves used in the chemical process industries
and opine which valve is best suited for what application
It
was the need to start and stop fluid flows, that lead to the invention
of valves. Today they are used to stop or control flow in chemical process
industries, power plants, cross-country pipelines, oil and gas installations,
pharmaceutical industries and sewage system.
All
the valves are designed to control flow. However some throttle the flow,
while others perform on-off duties. The valve can be operated manually,
remotely or automatically. They come in a variety of materials ranging
from steels to exotic alloys, non-ferrous metals, plastics, glass and
ceramic. Selecting the right type of valve for duty as well as cost
effective design, is an involved process.
This
article reviews the different types of valves commonly used in industries
and the factors to be considered while selecting the most appropriate
and cost effective valve. We have given specific attention to the materials
used for their construction and various design standards concerning
valves and piping systems.
Types
of valves
Manual
valves may be grouped according to the way the closure member moves
onto the seat. Accordingly valves have been divided into following four
groups.
- Closing
down valves: A stopper like closure member is moved to and from the
seat in the direction of seat axis
- Slide
valves: A gate like closure member is moved across the flow passage
- Rotary
valves: A plug or disc or ball like closure member is rotated valve
within the low passage, around an axis normal to the flow stream
- Flex-body
valves: The closure member flexes the valve body
- Non-return
valves: These valves automatically open with forward flow and closes
with reverse flow
Each
valve group represents a number of distinct types of valves, which use
the same method of flow regulation, but differ in the shape of the closure
member.
Valves
in use
The
majority of valves used in industry
are one of the four, that is gate, globe, quarter turn (plug, ball,
butterfly) and check valves. However, other kinds of valves are used
for specific applications.
All
the valves consist of the same basic components. The body contains the
fluids, which moves along the flow path of the valve. The movement of
the fluid is manipulated by a device, such as disc, plate, and ball,
which is inserted into the flow path. A stem moves the flow-changing
device with either linear or rotary motion. All the joints of the valve
are sealed with suitable seals to prevent any sort of external leakage.
Gate
valve
A
gate valve consists of a body and an internal wedged shaped plate. When
the valve is closed, the plate is wedged between the seats, totally
blocking the flow. In the open position the plate is completely removed
from the path of the fluid flow.
Gate
valves are used exclusively for on-off operation where opening up or
shutting off the flow is relatively infrequent. Gate valves have two
sealing surfaces, one of each side of the gate. In a closed position,
the seals traps liquid in a cavity bounded by the body and two sealing
surfaces. If liquid temperature rises, the fluid will expand and its
pressure will increase. If not vented, the liquid will leak internally
or externally. To avoid such leakage, manufacturers make a positive
vent, a small drilled hole in the upstream seat. Such vents make valve
a one way valve. The one way flow direction of the valve must be marked
on its exterior to prevent installation in the wrong direction.
A
variation of the gate valve, the knife gate, employs a gate with a flat
leading edge rather than a tapered one. With no wedging action to provide
a tight seal, flexible elastomeric or plastic sealing surfaces must
be used. One more variety is parallel slide valve. In parallel slide
gate valve, the shape of the closure is not wedge shaped but parallel.
These valves offer low resistance to flow. These are generally used
in high-pressure application.
Gate
valves start and stop the flow of gases and liquids and are used infrequently.
They are useful in case of fluids with solids in suspension, slurries,
fibres, powders, granules, vacuum, and cryogenic fluids.
Globe
valve
Globe
valve achieves the same level of tightness as a gate valve and can be
used for shut-off conditions. However globe valves are mainly used for
throttling the flow. The flow changing component or plug of a globe
valve has a circular cross-section but its overall shape ranges from
conical to cylindrical or a variety of other forms. It engages a circular
seat, which can be of the same or a different alloy. Each plug shape
produces unique flow characteristics which generates volumetric change
in the flow relative to the amount the valve is open.
....CONTD
