Lines, Stainless Steel and Galvanized Pipes

In all cases the factors or criteria for "Specifying" any material is the characteristics of the commodity in the line.

Characteristics include but are not limited to:

• Corrosiveness (Ph factor)
• Pressure
• Temperature
• Velocity
• Specific gravity
• Design life of the plant
• Other
  

Yeah for selecting lined pipes and stainless steel main reason is those both are corrosion resistance. Bu twhen both are corrosion resistant then how do we choose one out these two.. no doubt stainless steel is very costly as compare to line pipes.

And what are different problems we face in galvanised pipes regarding welding and all?

You made two statements:

1, "no doubt stainless steel is very costly as compare to line pipes."

This is not necessarily true. In most areas of the world Stainless Steel pipe is not much more expensive than Carbon Steel. So Lined Carbon Steel pipe would be more expensive than Stainless Steel.

2, "And what are different problems we face in galvanized pipes regarding welding and all?"

First you should avoid welding of Galvanized pipe when ever possible. The reason is the arc or flame used in the welding process reacts with the Zinc in the galvanized coating and form a poison gas that can be inhaled by the welder or others close by. Not good! Galvanized piping is normally only used for low pressure and low temperature piping and is assembled with threaded joints.

Materials compatibility between the pipe inner surface material and the fluid being handled is the primary consideration.

when selecting a material you should begin with an assessment of the corrosvity of the fluid, this to give you an idea of the type of material that you can use (stainless or galvanized for example). the selection of a lined vs a solid material depends on other factors like operating pressure, fabrication cost, type of item (piping, vessel...)

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corrosion control