What is the difference between pressure relief valve and pressure control valve?

Control Valves
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A Pressure Relief Valve (PRV) will not relieve the excess pressure till the moment, when the pressure inside the vessel or the container exceeds beyond the designed criteria. So, a PRV will only operate at excess pressures and will remain idle during the rise and build of the pressure in the vessel in the container. A damaged PRV will damage the pressure generating equipment and it may cause injuries to the workers around it.

For example, PRVs are used in Boilers to prevent the generation of excessive pressure in the boiler vessel. Consider a Spring-Loaded PRV, which will expand when the axial load exceeds the designed pressure, thus opening the valve to relieve the excess pressure.

On the other hand, a Pressure Control Valve(PCV) will control the generation of excess pressure in the fluid line. It is mostly used in hydraulic and pneumatic circuits and will not allow the generation of excessive pressure. By the virtue of its internal mechanism, a PCV will increase the flow passage (cross sectional area of the flow passage) to reduce the pressure and decrease the C/S area when the pressure needs to be increased. A damaged PCV will adversely affect the actuation of the hydraulic component and may even damage the fluid lines.

For example, a PCV is used to control the pressure of the fluid going into the cylinder or an hydraulic actuator.

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There are several fundamental differences. Key ones are:

PRV is a static device but PCV is dynamic. In other words if there is no flow, PCV does not do anything but PRV can reduce pressure even if there is no flow out of the reservoir.

PRV typically releases to drain or environment so steam or fluid that goes to PRV is typically lost to the system. PCV acts to change resistance to the flow of the fluid but does not alter its destination. Fluid that passes through

PCV is still going to the destination it was intended for and is not lost.

PRV is usually not used as a pressure control device but as a backup of last resort. If PRV is activated it is usually a sign something went wrong and you hear alarm bells going off . The only exception is your kitchen pressure cooker where PRV is used as PCV. In complex engineering systems, PCV is often the first resort and an active component of the high pressure system. When it is on and in use, it is business as usual.