This document is about why every control valve is a flow control valve.

1. Why every control valve is a flow control valve?
Ans- 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:-
Case 1
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.
Case 2
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.