Question { 10116 }

what is the difference between specific gravity and density
of a substance?

Answer

Specific gravity is the Ratio of Density of a liquid
(Kg/cubic metr) to the density of Standard liquid (water -
1000 kg/cubic metr)

Density of liquid is defined as Mass per Unit volume
(Kg/cubic metr)
We take Standard liquid as WATER ( Hydraulics), and
Standard Gas as Air (Thermodynamics), as Gas or Liquid both
are Fluids.
Reason for taking std reference as Water or Air --
Water is Universal Solvent,chemically inactive,freely
available etc.
Air posseses all thermodynamic properties, also does not
affect chemically to components,freely available etc.

Question { BEL, 6269 }

For avoiding cavity what should be NPSH?

Answer

Cavitation takes place when liquid pressure falls below its
Vapour Pressure forming bubbles of liquid.
To avoid cavitation (NPSH)available > (NPSH)required.
We know that...
NPSH is Net Positive Suction Head.
Therefore,
NPSH = (H)atm - (H)vapour - (H) manometric
the above equation must come Positive value to avoid
cavitation!!!