Pumps, Cavitation, etc

1
Pumps, Cavitation, etc

• Industrial Hydraulic Technology
• Chapter 5

2
The Mercury Barometer
Vacuum
Atmospheric pressure is balanced by the column of
mercury. The height of the column changes to
accommodate the current atmospheric pressure.
Reading
The column of liquid has weight (force) and
develops pressure.
3
Pump Action Creates a Partial Vaccum
Partial Vacuum
4
Atmospheric Pressure primes the pump.
Because atmospheric pressure is greater than the
pressure of the oil, the oil moves up the pick-up
conductor.
5
Cavitation
Liquid moving
The liquid must accelerate from rest. If it
moves too slow, the inlet pressure will go low
enough to make the oil boil at a low temperature.
Liquid at rest
6
Indications of Cavitation

• Shrieking sound.
• Decrease in pump flow rate
• System pressure is erratic.

7
Result of Cavitation

• Interferes with lubrication
• Destroys metal surfaces

Pump life is reduced, harmful pieces of pump
metal will move to other parts of the system.
8
Boiling and Degasification
Molecules of air and oil.
High concentration of oil molecules in the gas
phase
Bubbles contain oil molecules and air molecules.
Not Boiling
Boiling
Molecules of oil and dissolved air.
9
Conditions for Boiling and Degasification

• Higher Temperature
• Lower Pressure

10
Air in the Oil

• Air and oil can be a solution with air dissolved
  in the oil.. Hydraulic oil normally contains
  about 10 dissolved air.
• Air and oil can be a mixture with air mixed into
  the oil (entrained air). Entrained air comes in
  through leaks between the reservoir and the pump.

11
Pseudo-Cavitation(looks like cavitation)

• From entrained air, NOT from boiling or
  degasification.
• Has same effect as cavitation.

12
Vacuum Pressure

• Zero inches (Zero mm) of mercury is atmospheric
  pressure.
• 29.02 inches (760 mm) of mercury is a perfect
  vacuum.

13
Vacuum Pressure
As good as it gets!
Atmospheric
Partial Vacuum
14
Vacuum Gages

• To convert inches of mercury to absolute
  pressure, subtract from 30.
• To convert mm of mercury to absolute pressure
  subtract from 760.