# Hydraulics (HYD)

Summary

• Hydraulics deals with the physical characteristics exhibited by fluids at rest and in motion.
• The branch at the end of a hose line converts the water’s pressure energy into velocity or kinetic energy so that it can form an effective jet or spray.
• Each type/size of nozzle has an optimum operating pressure.
• Water discharging from a nozzle results in a jet reaction.
• Pressure is the force acting over a given surface area. It is usually measured in kiloPascals (kPa).
• The principles of pressure are:

- Pressure is perpendicular to any surface on which it acts.

- At any point, the pressure of a fluid at rest is of the same intensity in all directions.

- Pressure applied from outside a fluid contained in a vessel is transmitted equally in all directions.

- The downward pressure of a fluid in an open vessel is proportional to its depth.

- The downward pressure of a fluid in an open vessel is proportional to the density of the fluid.

- The downward pressure of a fluid on the bottom of a vessel is independent of the shape of that vessel.

• The rule for allowing for height loss or gain when pumping is to add 10 kPa for every metre the nozzle is higher than the pump, or subtract 10 kPa for every metre the nozzle is lower than the pump.
• The five principal laws governing loss of pressure due to friction are:

- Friction loss varies directly with the length of the hose or pipe.

- For the same velocity, friction loss decreases directly with the increase in diameter.

- Friction loss increases directly as a square of the velocity.

- Friction loss increases with the roughness of the interior of the hose.

- Friction loss, for all practical purposes, is independent of pressure.

• Friction loss varies depending on the type of hose, diameter of hose, length of hose line and rate of flow involved.
• For the same flow rate, larger diameter hoses have significantly lower friction loss than smaller diameter hoses.
• The friction loss in any particular diameter hose line will effectively limit the size of nozzle used in association with that diameter of hose.
• Twinning the lines is a method of reducing friction loss in supply lines.
• Pressurised water supply may be obtained from a hydrant system, another appliance’s pump or an elevated tank.
• When the residual pressure in a pressurised supply is approaching zero, it means you are almost overrunning the supply.
• Water may be obtained from a static or open source such as a dam, pool or stream.
• The capacity of a static water source can be calculated (in litres) by multiplying its volume (in cubic metres) by 1000.
• The capacity of a flowing water source can be calculated (in litres) by multiplying the volume (in cubic metres) flowing past any point by 1000.