Is hydraulic fluid flammable

The most commonly used hydraulic fluids are oils, so yes they are flammable. However, most any fluid COULD be used as a hydraulic fluid depending on the system design requirements. So it really depends on exactly what type of fluid is being used whether it is flammable or not.

Water is a "hydraulic fluid" and is not flammable.

Under NFPA standards, petroleum-based hydraulic fluids are not "flammable" if their flash point is above 100o F (37.8o C); they would be "combustible". NFPA 1 (2009): 3.3.153.2 Definition of Flammable Liquid