The issue of NPSH ratio for centrifugal pumps
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RT, is this also called the suction specific speed? I think it’s specified in API610, but I can’t remember the details; it seems that the smaller it is, the better? Let’s discuss the role of this in actual selection processes The formula is: nq = N * Q1 / 2 / NPSHr^(3/4). My experience shows that the calculated value should be below 12,000
This issue is a bit complex; generally, there is an approximate value corresponding to pumps such as OH2, BB2, BB3, etc., though this is not specified in the APi standard. In simple terms, this value is related to the NPSHr of the pump itself; currently, for pumps such as OH2 and BB2, it is often required that the Nss be less than 11,000–13,000
I’m sorry, the moderator upstairs – what are the concepts of OH2, BB2, and BB3? . . . Could you please explain it? I don’t quite understand. . . .
API 610 specifies that OH2 is a radially split, centerline-mounted pump; BB2 is a radially split single-stage or two-stage pump; BB3 is an axially split multi-stage pump
The common standard specifications such as API610 and 682 suggest that the poster should study them carefully first~! This thing is the foundation of foundations
Firstly, the lower the NPSHr, the greater the capacity to resist cavitation. As can be seen from the formula, when the flow rate and speed remain constant, the higher the specific speed, the lower the NPSHr and thus the greater the resistance to cavitation. Therefore, users generally prefer a high specific speed, but it’s not true that the higher the better. Currently, both manufacturers and users rely on empirical data. Some literature indicates that for general cold water and ordinary applications, a value of ≯8500 is sufficient ； Boiler feedwater or condensate, range: 8500–11000 ； >12000 is for special purposes.
What are the consequences of a too high cavitation specific speed? Is there also a difference between the cavitation specific speed and the specific speed? Thank you
As mentioned above, it’s not true that the NPSH ratio the higher, the better; however, it wasn’t explained why this is the case. The reason is that the NPSH ratio is a measure of the operating flexibility of a pump, or in other words, its operating range. The higher this ratio, the narrower the pump’s operating range. Therefore, the NPSH ratio needs to be kept within certain limits. There are standards for this operating range, and people can refer to Grundfos’ training materials, which contain graphs along with relevant data