What is specific gravity? what do you mean by 1pascal? Concept of pressure difference?

Sol)

The "specific gravity" of a meterial is the ratio of its density to the density of water at 40 C, 1000 kg/m3, it is a pure number with out uinits.

The "pascal" can be expressed using SI derived units, or alternatively solely SI base units,and a unit  of pressure equal to one newton per squre meter.

The difference between two levels of pressure. During operation, valves often experience different pressure levels downstream and upstream.