Piping and Tubing
Generally fluids are transferred from one place to another by means of pipe or tubing. These are circular in cross section. Different variety of pipe and tubes are available in the market with varying diameter, thickness and material of construction.
There is no as such difference between the terms pipe and tubing. Generally pipes have larger diameter ranging from 20 to 40 ft and their walls are heavy.
However tubes are coiled shape several hundred feet long and their walls are thin. Tubes cannot be threaded however metallic pipes can be threaded. The walls of pipes are usually slightly rough, however tubes have smooth walls. Pipe lengths are joined by screw flanged or welded fitting. Tubes are usually connected by compression fitting, flare fitting or soldered fitting.
Nowadays pipes and tubes are made from many materials like metals, alloys, wood, ceramics and plastics. Poly vinyl chloride or PVC pipes are largely used for water lines. These days in process plant low carbon steel is utilized for making black iron pipe.
Sizes of Piping and Tubing
Pipes and tubes are differentiated in terms of their diameter and wall thickness. The standard nominal diameter of steel pipe in US is range from 1/8 to 30 in. the term nominal diameter means the actual outside diameter.
The thickness of the pipe is indicated by Schedule number. It increases with the thickness. The Schedule number 10, 20, 30,40,60,80,100,120,140 and 160 of pipes are common in use. The pipes having diameter less than 8 in, in diameter schedule number 40, 80,120 and 160 are common.
Selection of Pipe Size
The size of the pipe selected for the installation oná a particular place depend upon the cost of pipe, cost of fitting charges and the cost of energy needed for pumping the fluid within the pipe. Usually the cost of the pipe and the annual capital charges increases with 1.5 power of the diameter of the pipe. However the power cost for the turbulent flow increases with 4.8 power of the diameter. The optimum velocity of turbulent flow of liquid in steel pipes having diameter larger than one is given as
V= optimum velocity in ft/s
M= mass flow rate lb/s
?= fluid density lb/ft3
V (optimum velocity) for water and similar fluids is
in the range of 3 to 6 ft/s (0.9 to 1.8á m/s).
For air or steam at low to moderate pressure V is 20 to 80 ft/s (6 to 24ft/s)
Low velocities in the range of 0.2 to 0.8 ft/s (0.06 to 0.24m/s) are favored when the flow is by gravity from overhead tank or when viscous liquid is pumped
Joints and Fittings in Pipes and Tubings
The method for joining the pieces of tubes and pipes depend on the property of material of construction and upon wall thickness. Thick walled tubular products are joined by screw, by welding or by flanges. Thin walls tubes are usually connected by welding or soldering or by compression or flare fitting. Pipes which are made by glass or carbon or cast iron are joined by flanges or bell and spigot joints.
Welded joints are stronger than the joints made by screwed fittings welded joints are leak proof. The only disadvantage of welding joint is that they are not opened unless it is destroyed
Alllowance for Expansion
Pipes are made in order to bear high temperature. In some transaction of fluid the temperature is usually very high. The rise in temperature causes the pipe to expand or contract. In case when the pipe is fixed at both ends the pipe may get tear loose, bend or even break. Therefore in common practice pipes are not fixed either they are hanged or loosely rest on the rollers.
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