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...... ...... rr[n ............... suction Conditions
4.4.2.1 Static head (Hs)
Increasing the static head available to the pump is a simple (?) matter of
lowering the pump, or raising the suction tank, or the level in the tank.
The physical movement of the tank or pump would usually be a cosily
proposition, yet the raising of the tank levels may be relatively cheap
and simple, and can frequently cure the problem.
Of course, if the suction source happens to be an adjacent river or lake,
there will be no control over the surface elevation and thus, the static
head. The opposite problem of too much fluctuation is possible if the
pump is being fed from a tidal source.
Figure 4.5. Below grade suction tank and pump
Where the pump is located above the level of the suction source, the
Static Head will be a negative value, but all other considerations
discussed above will remain the same.
4.4.2.2 Surface pressure (Ha)
Similarly the surface pressure can be a little tricky to change if the
suction source is some body of water that resists control by mere
mortals. It might be possible however to enclose a man-made tank and
pressurize it, or even introduce a nitrogen blanket. Both of these
possibilities are subject to the limitations of the particular service. For
example, increasing the pressure inside a deaerator would defeat the
whole function of that vessel and must therefore be considered
impractical. However, as this pressure is one of only four factors in
NPSHA formula, it is worthy of some consideration in certain
installations.
4.4.2.3 Vapor pressure (Hvp)
The only way to reduce the vapor pressure of a liquid is to reduce its
temperature. Under many operational conditions this will be
59 m
