An analysis of literature and experience draining installations showed that when the main draining installations have occurred cavitation regimes associated with uncontrolled sensitive parameters with existing automation systems. Therefore, the study cavitation phenomena in the supply pipeline and development of the automatic protection system of cavitation shaft pumps are relevant. In doing so, the following tasks:
- definition of account dependencies cavitation characteristics of the pumping installation;
- definition of range of input values.
To solve the tasks the analytical-tabular method with the subsequent processing of the results of research on the method of least squares is adopted.
In centrifugal pumps at a constant speed of rotation there is a determined dependence pressure, power and efficiency of the pumping, portrayed graphically in the form of individual characteristics. The emergence of cavitation modifies the form of these curves, limits the possibility of a pump, it complicates the operation and reduces the durability of parts of the machine where water is runing.
The schedule of characteristics change of pressure pump and pipeline network is shown in Figure 1, where Pc - characteristic of the network; Pì - characteristic of the pump; Q0, Q1, Q2 – respectively working minimum and maximum pump supply in the working zone; P0, P1, P2 - respectively working minimum and maximum pump pressure in the working zone; Pã - geometric water-raising pressure. Ð0ÂÃ, Ð1ÂÃ, Ð2ÂÃ - respectively working minimum and maximum geometric suction pressure. Ðäîïâàê - allowable vacuum suction.
Figure 1 - The schedule of changes of pressure characteristics of the pump and pipeline network
To accommodate the possibility of cavitation we must have available the supply pipeline characteristics, as well as a pump cavitation testimonial, which can be accepted as a passport for this type of pump. However passport characteristics provides for a narrow range innings pump, which is defined as a zone of industrial use.
However, there is need to identify this characteristics in a wider range.
As a result of studies following dependency has been received:
where Hâàêäîï - allowable height of vacuum suction, m;
ν - average speed of water in the supply pipeline, m/s;
∆hêð - a critical cavitation stock;
k – safety coefficient equal to 1,2-1,3, accepted 1,25.
To ensure uncavitation work of the pump you need the following conditions to be met:
If you equate among themselves Hâñ.òð = Hâñ + aâñ · Q2 and (2) and decide on Hâñ, you can obtain the following dependence:
Hâñ = 10 - 8,97 · Q2 · (48,39 · λ + 4,63) - k · ∆hêð.
This dependence is obtained for the pumping plants, equipped with type CNS 300-120-600 pumps, which provides water rising to a height of H = 560 m. The diameter of the supply pipeline ditch is 0.31 m.
Critical cavitation margin is calculated by the formula Rudneva:
where n - working wheel speed pump, 1475 rpm;
c - the rate of high-speed pump cavitation, which can be computed on an empirical formula, min-1:
ñ = 600 + 18,433 · (ns - 50)0,676 .
where ns
- specific pump high-speed , which is defined by the following dependence, min-1:
where Qí, Hí - supply respectively (300 m3/h) and head (60 m), which are develop at one stage if the pump-type CNS 300-120-600 in normally regime.
After all the permutations we get the following formula:
Hâñ = 10 - 54,639 · Q2 - 30,604 · Q2/3.
Above listed dependences are the basis for establishment of protection of pumping installation against cavitation. To this end we have control to the following parameters: the current level of water in the reception well; current pump supply at the working point.