CHAPTER 6
VELOCIS SOLUTIONS @2018
9
Suction specific speed
Cavitation can happen even NPSHA is more than NPSHR. This led to development of suction specific
speed concept.
𝑁
𝑠𝑠
=
(𝐴)(𝑁)(𝑞
0.5
)
𝑁𝑃𝑆𝐻
0.75
Where:
𝐴 − conversion constant (for metric system is equal to 1)
𝑁 − revolutions per minute [rad/s]
𝑞 − flow rate [ m
3
/s]
𝑁𝑃𝑆𝐻 − required NPSH [m]
Cavitation may occur at low flow condition, if N
ss
is greater than 120. It is experienced that even
NPSHA is 2 to 2.5 times of NPSH
R
in water service, cavitation can occur at 30-40% of best efficiency
flow. The reason is internal recirculation in the impeller. This problem is not usual in hydrocarbon
system, but generally it is recommended to avoid pump with N
ss
more than 200.
Dostları ilə paylaş: