Valve
guide
Tamal
Banerjee gives a comprehensive guide to the world of valves and its
applications in various industrial sectors
Valves
are one of the most important of devices that can control the flow of
both the liquid and gas, through a pipe. Valves can be technically defined
as a device used for closing or modifying the passage through a pipe,
outlet, inlet, or the likes, in order to stop, allow, or control the
flow of a fluid media. It works by having a pivoting flap, which can
only open in one direction, therefore ensuring the product only travels
one way down the pipe. A valve consists of four major components as
described below.
Body: The body is made up of castings, forgings and fabricated/machined
components.
Bonnet: The bonnet is also a pressure part that holds the media
Gland packing: This packing (usually of soft material) is used
for sealing the spindle path so that there is no leakage into the atmosphere
Trim: This consists of plug, seat, spindle for globe and gate
valves and a ball/disc and seat/liner for ball and butterfly valves.
Some of the important criteria that should be considered while selecting
a valve are as follows:
1.Pressure class
2.Whether the construction is forged or cast
3.Body material
4.Bonnet material
5.Trim material
6.End connection
7.Whether it is a manual operation or operated by an actuator.
8.Leakage class desired by customer
9.Media
10.Time required for opening/closing.
There are various governing standards laid down
for valves. The following are the standards for materials as well as
construction:
For Materials: ASTM, AISI, DIN
For Construction/design: ANSI B 16.34, API 600,
ANSI B16.5 (Flanges), ANSI B16.25 (face to face distance) and API 598
/ ANSI B16.104 / FCI 70.2 (seat leakage standard).
The various types of valves and their applications can be summarised
as follows:
Gate valves: Here the orifice size is same as line size. The
disc is at right angles to the straight-line flow.
These valves are ideal for On/Off service only and available in solid
wedge and flexible wedge designs. Good leak tightness is essential and
the valve can be used with electric or pneumatic actuators depending
....CONTD
