All substances have a rated pressure at which deformity or failure
occurs. A knife cuts through bread only when it applies a certain
amount of pressure to the bread. Of course, the pressure rating
for bread is rather low - we can cut through it with a knife or even break
it with our bare hands. Metals have much higher pressure ratings.
For example, steel can be rated at 90,000 PSI. This means that 90,000
pounds of force per square inch must be applied to deform or break the
The outside housing of most pipes have pressure ratings listed. Pipes made from different materials will have different pressure ratings. The rating of copper tubing is 700 PSI. Schedule 40 steel has a rating of 2,000 PSI. The higher rating of the steel pipe means that the pipe is stronger. Of course, factors other than pressure ratings have to be taken into account when choosing piping. For example, though steel pipe is strong, it is not widely used because it is labor intensive and highly corrosive. In contrast, copper pipe is toxic to living organisms and has been known to keep the water flowing through it fresh for over two thousand years.
The pressure ratings of pipes, valves, etc. must be matched throughout a hydraulic system in order for the system to function. For example, if a hydraulic system was rated at 2,000 PSI, you should not install a valve rated at 125 PSI. The pressure of the water will quickly cause the valve to fail since its pressure rating is too low.