OPERATION AT THE SUCTION SIDE OF THE PUMP

1
Chapter 5

• OPERATION AT THE SUCTION SIDE OF THE PUMP

2
PUMP LOCATION

• Usually located on top of the reservoir,
• which contains the hydraulic fluid.
• Suction line connects the pump inlet
• to the liquid in the reservoir.
• Energy to move the liquid is applied
• by the atmosphere.
• The atmosphere and fluid in the reservoir operate
  together as an actuator.

3
Measuring Atmospheric Pressure

• Ocean air does exert a pressure, that can be
  measured by a column of mercury (Hg).
• Sea level pressure equals 29.92 inches of Hg
  (14.7 psia) under standard conditions of 68oF and
  36 relative humidity. Elevations above sea
  level will be less.
• Hydraulic pressure is measured in psi or bar.
  Atmospheric pressure and vacuum pressure measured
  in in. Hg.
• To convert 1 psia 2.04 in. Hg

4
Absolute vs. Gage Pressure Scales

• Absolute begins at the point where there is a
  complete absence of pressure.
• Gage begins at the point of atmospheric pressure.
• Convert from Gage pressure to absolute just add
  the standard atmospheric pressure to the gage
  reading.
• Gage scale denoted psig. Absolute denoted psia.

5
Operation at suction side of pump

• When not in operation suction side is in
  equilibrium a no-flow condition indicates zero
  pressure differential between pump and
  atmosphere.
• To work pump generates less than atmospheric
  pressure, system becomes unbalanced and flow
  results.

6
Atmospheric Pressure Use

• Atmospheric pressure applied to the liquid in two
  phases
• Supplying liquid to the pump inlet
• Accelerating the liquid and filling the rapidly
  moving rotating group 1200, 1800 rpm are
  standard speeds.
• Largest portion is used in accelerating the
  liquid into the pump, but the action of supplying
  liquid to the pump comes first.
• If too much pressure is used in the first phase
  then the pump becomes unable to accelerate the
  liquid. Causing the pump to starve and cause
  cavitation.

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Cavitation

• This is the formation and collapse of gaseous
  cavities in the liquid.
• The cavities collapse and generate tons of force
  per square inch.
• Cavities form when the liquid is made to boil,
  due to reaching a low absolute pressure.
• Cavitation interferes with lubrication
  and destroys metal surfaces.
• Indicated by a high shrieking sound
• from the pump as well as a decrease in
• flow rate from the pump and erratic system
  pressure.

interferes with lubrication
8
Liquid Vapor Pressure

• Molecules move at different speeds those that
  move quicker are trying to escape the liquid
  exerting a force to enter the atmosphere,
  creating a pressure.
• Molecules escaping from the liquid is
  evaporation. Molecules returning to the liquid is
  condensation. When these equal an equilibrium,
  the pressure exerted at this point is the vapor
  pressure of the liquid.
• Vapor pressure is affected by temperature and
  pressure.
• An increase in temperature increases the vapor
  pressure, or speed at which molecules escape.
• Boiling point can decrease by increasing the
  pressure.

vapor pressure
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Air Dissolved vs. Entrained

• Hydraulic fluid is made of about 10 air.
• This air is dissolved in the liquid, not
  affecting its volume.
• Ability to hold dissolved air decrease as the
  pressure decreases.
• This dissolved air comes up out of solution and
  does harm to the pump, via cavitations.
• Entrained air is that which is present in an
  undissolved state.
• This is in the form of bubbles, and will cause
  pseudo- cavitations, since it is not affected
  by the vapor pressure.
• Doesnt always come out while in the reservoir
  and will sneak its way into the pump.
• Usually caused by a leak in the suction line or
  seal on the pump.

10
Vacuum Pressure Scale

• Any pressure that is less than atmospheric.
• Zero inches mercury vacuum is the absence of
  vacuum.
• 29.92 inches mercury vacuum is the perfect vacuum
  or zero. absolute pressure
• NOTE Inches mercury vacuum is different than
  in. Hg.

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Pump Suction Specification

• Most pump manufacturers supply specification
  limits as to the less then atmospheric pressure
  that must be present at pump inlet to begin
  filling pumping mechanism.
• In other words, stated in maximum vacuum pressure
  at sea level. If to be used above sea level, the
  barometric pressure must be taken into account.
• Different specifications are given for different
  RPM ratings and/or different fluid types.
• Maximum allowable vacuums on pumps are based on
  petroleum base fluid which has a certain specific
  gravity and vapor pressure.
• Different fluids effect these specifications

12
High Specific Gravity

• Specific gravity is the ratio of weight between a
  volume of water and an equal volume of another
  liquid.
• Heavier fluids require more pressure at inlet for
  fluid acceleration, which decreases maximum
  allowable vacuum.
• SPECIFIC GRAVITIES
• Petroleum base - 0.87-0.90
• Phosphate esters 1.15
• Invert emulsions 0.93
• Water glycol 1.08

Water 62.4 lbs
Oil 56.4 lbs
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High Vapor Pressure

• Petroleum base and phosphate esters have low
  vapor pressures at common operating temperatures.
  They run around a fraction of inch of Hg.
• Water base fluids have higher vapor pressures,
  usually a few inches of Hg.
• The higher vapor pressures cause tendency to
  vaporize and cavitate.
• To avoid this ensure sufficient pressure at pump
  inlet without going below the fluids vapor
  pressure. Accomplished by reducing maximum
  allowable vacuum.

14
Cavitation Detection

• Cavitating pumps have steady high-pitched
• sound.
• Sucked air into a pump has more of an
• erratic sound.
• Small amounts of air sound like a rattle
• or bad bearing.
• Large amounts of air will have a very erratic
  banging or popping sound.
• Most certain way to distinguish is to determine
  the absolute pressure at the pump inlet, making
  sure that sufficient pressure is present, if not
  then cavitation may be occurring.
• May be the result of poor line design or wrong
  fluid viscosity, plugged suction line, or dirty
  filter.

15
Pump Priming

• Priming means the pumping mechanism is filled
  with liquid.
• If the pump is not primed it is required to purge
  air from the suction line and pump cavities
  before operating or permanent damage could be
  done due to lack of lubrication.
• Releasing the air from an unprimed pump can be
  done by loosening the fitting at the pump outlet,
  letting the pump jog till oil squirts out the
  fitting, then tighten.

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Associated terms

• Flooded suction the pump inlet is below fluid
  level in reservoir
• Head pressure pressure exerted at the bottom of
  the fluid column additional source of energy to
  supply the pump.
• Lift the height of a column or body of fluid
  below a given point expressed in linear units.
• Suction- act of the pump generating a pressure
  differential between itself and the atmosphere.
• Suction pressure the absolute pressure of the
  fluid at the inlet pump.

Head pressure/ fluid Lift fluid column
specific Pressure height (in) x 0.036 x
gravity (in. Hg) 0.491