what is the difference between valves and dampers in piping and instrumentation line?

Answer 1

A damper restricts the flow, and can be adjusted to let more through or less through.

A valve will be opened or closed.

Think of it in the same fashion as in an air conditioning system

Answer 2

The difference is more a matter of application than function. Both valves and dampers can be used to restrict or throttle flow. Valves are used for liquids and compressed gases.

Dampers are used on low pressure gases, air, and in ductwork.

Also a matter of size, small lines valves are more readily available, large rectangular ducts can more readily be fitting with dampers.