Never. Throttling will erode the valve seat and then the valve will never shut off completely.

Overclocking is the process of running components past their specified rates. Throttling is the process of reducing the rate at which the component runs. Throttling is often included in the design specs of a processor, so there is usually no danger in it. When you run components below the rate that they are rated for (even for throttling limits), this is generally known as underclocking.

You cannot increase steam pressure through a valve without adding heat to it. Throttling steam always reduces its pressure.

steam

It is not.. A globe , needle , ball valve are used for throttling

For expansion either a shock absorber , diaphragm tank or air chamber are used to allow for expansion