CHAPTER 6
VELOCIS SOLUTIONS @2018
4
Figure 7.4: Suction system of a pump
The net positive head available is given by the following equation:
𝑁𝑃𝑆𝐻
𝑎𝑣𝑎𝑖𝑙
= 𝑃
𝑠
𝜌
⁄ + 𝐻 − 𝑃
𝑓
𝜌 −
⁄
𝑃
𝑣
𝜌 − ℎ
𝑎𝑐
⁄
Where:
𝑁𝑃𝑆𝐻
𝑎𝑣𝑎𝑖𝑙
− - net positive suction head available at the pump suction, [m]
𝑃
𝑠
− pressure above the liquid in the feed vessel, [N/m
2
]
𝐻 − height of liquid above the pump suction, [m]
𝑃
𝑓
− pressure loss in the suction piping, [N/m
2
]
𝑃
𝑣
− vapor pressure of the liquid at the pump suction, [N/m
2
]
𝜌 −
density of the liquid at the pump suction temperature, [kg/m
3
]
ℎ
𝑎𝑐
− acceleration head (reciprocating pumps only), [m]
For a closed tank system in which liquid is in an equilibrium with gas, P
s
is equal to P
v
. In this case,
line specification is directly affects NPSH
avail
. Increase in line size decreases pressure drop and
higher elevation gives rise to increase net suction head.
SPECIFIC SPEED
Specific speed may be defined as the speed of the pump which will produce unit flow Q against
unit head h under conditions of maximum efficiency. This is a parameter to be applied to all types
of pumps in selection. It is given by:
𝑁
𝑠
=
(𝐴)(𝑁)(𝑞
0.5
)
𝐻
0.75
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