When fluid moves from a high pressure zone to a low pressure zone we call this pressure drop.

When a pressure drop occurs WITHOUT useful work heat is generated. For example, the pressure drop across the ports of a properly functioning motor produces torque at the motor's drive shaft and ultimately useful work.

On the other hand, the pressure drop across a relief valve doesn't produce any work, so this energy is converted to heat - which is an undesirable heat load on the system.

Because a pressure drop without useful work creates heat, an infra-red thermometer can often be used as a quick and effective means of locating abnormal heat load.

For example, if oil is passing over a relief valve, the localized heat generation means this component will be hotter than the rest of the system.
 

By Bredan Casey