Every control valve is a flow control valve because the control valve directly controls the flow of fluid by varying passage size. By controlling flow rate, other process quantities such as pressure, temperature and liquid level of the process fluid are also controlled. Other control valves such as pressure control valve, temperature control valve and liquid level control valve control the flow rate of fluid according to process requirements. This can be explained by the following cases:-
Let us consider a liquid-vapor separator where control valves are located at both liquid and vapour outlets. Level control valve is also present at the bottom of the separator. At normal conditions, the valve is 50% open. When the inlet flow rate increases, the liquid level also increases. Due to increase in fluid level, the opening of level valve increases to maintain the level of the liquid. The liquid flow is altered by altering Cv of the valve.
We also have PSV at the top of the separator. MAWP of the PSV is 6 bar. If the pressure of the separator exceeds 6 bar, the PSV opens and releases excess vapour to reduce the pressure of the separator. In this case we are maintaining the pressure by altering the flow of the vapour.
So from above cases, we can conclude that every control valve (LCV, PSV and TCV) are flow control valves.