Next, the influence of the shape of the pressure chamber, i.e. the relative flow coefficient a, on the hydraulic loss is studied in detail. For this reason, the relationship between the loss in the pressurized water chamber and the flow velocity Ur is studied. For the best working condition, the loss in the pressurized water chamber is as follows

That is to say, the loss is related to the tangential velocity of liquid flow at the outlet of impeller and the throat velocity of pressure short pipe. The throat fluid velocity is variable, and the pressure chamber loss is the minimum when ur equals 0.75 cz. In the pressurized water chamber, the minimum loss is guaranteed when the optimum flow rate is selected. According to formula (3-30-22), hors = 0.44c. /2g.

In the throat of pressure short pipe, generally u-<0.75c2a, so when Ur decreases, the loss in the pressure chamber increases with the increase of coefficient a. The maximum loss in ensuring the optimal flow rate will be in the annular water pressure chamber.

It should be considered to study the best working condition. When other flow rates are large or small, the optimal loss in the pressurized water chamber of the selected size will increase.

From the above situation, it can be seen that the increase of a, that is, the shape of the water chamber gradually approaches the annular shape, on the one hand leads to the decrease of the liquid flow velocity on the calculated section of the water chamber, that is, the increase of its life; on the other hand, leads to the increase of hydraulic loss.

Compared with the calculated section (coefficient k), the section area of the throat of the pressure short pipe increases, which will result in the annular pressure chamber when the value of a is the smallest. Therefore, both the parameters A and the calculated cross-section area of the water chamber are reduced. Because the K value increases and the calculated cross-section area decreases, the change of throat cross-section area of pressure short pipe should be very small. Therefore, coefficient a has little effect on the loss of the pressurized water chamber.

When the coefficient a is different, it is possible to keep the constant value of the maximum flow rate. At the same time, the hydraulic loss of the pressure chamber increases with the increase of the pressure chamber. At this time, the theoretical head HT will be constant. The pump head is equal to the theoretical head and the loss of HX in the impeller, and the pressure water is the impeller efficiency. Hence, the difference of Hun loss in pump head chamber. The loss in impeller is HX = Hr (1-x), in which H = xH_ - hors is added.

When the optimum flow rate is maintained, the system coefficient a increases, i.e. gradually turns to the annular pressure chamber, which results in the decrease of pump head H. For example, if a pressurized chamber is selected for a high-lift pump with theoretical head H-=96m, the variation of the head can be seen in Table 3-3-5 at different throat velocity ur values of the short-pipe under different pressures.

When determining the pump head, such as the tangential velocity equal to 30m/s, the loss in the impeller is 7% of the theoretical head (when z = 4), the impeller speed n = 600r/min, and the impeller diameter D = 1m.

From the calculation results, it can be seen that the loss in the pressurized water chamber increases with the decrease of the throat velocity of the pressure short pipe. In this case, the change of throat velocity of pressure short pipe is 50%, which will cause the head of pump to decrease 8% in good working condition and the hydraulic efficiency to decrease from 6% to 53%, which is the influence of the change of liquid flow velocity on the loss of pressure chamber in low-lift system. For example, the change of liquid flow velocity in the throat of pressure short pipe is U2 = 19m/s, the theoretical head of pump is Hr = 31m, and the loss in impeller is 7% of the theoretical head, i.e. H = 2. lm, and the tangential velocity of impeller outlet is C2u 16m/s.

The change of liquid flow velocity in throat of pressure short pipe leads to the change of pump head as shown in Table 3-3-6.

For low-lift pumps, when the throat fluid velocity of pressure short pipe doubles, the pump head drops to about 4%.

The example above shows that the change of throat section area of pressure short pipe has little effect on the loss of pressure chamber, that is, the change of pump head is very small when the flow rate is given. This allows the selection of coefficient a, i.e. the pump efficiency value is quite high when the life of the pressurized water chamber is increased as much as possible. At the same time, for a given value, the change of pump head will be within the allowable range.