Hydraulic Flow Head Loss in AF Foam Pump

The vane pump includes the theoretical head of the pump for pumping abrasive solid-liquid mixtures. The test method can be used to determine the pump according to the special equilibrium test results (first C.C. Rudenev research test method, and then C.A. Baughnitskaya carried out the pump test).

In order to obtain the theoretical head of equilibrium test, the following methods can be used.

According to the pre-test results, considering the disc friction loss, the power consumed in the mechanical loss is determined, the quantity Q of liquid overflowing through the impeller inlet seal is measured, and the leakage power loss is estimated. Thereafter, the power of the pump and the head of the pump related to the flow rate are measured.

The amount of liquid flowing through the impeller is equal to the sum of the flow rate and the overflow Q of the pump. The power transmitted to the fluid flowing through the impeller is equal to the difference between pump power and power consumed in terms of mechanical loss (considering disc friction) and volume loss (determined in advance by test). Thus, the theoretical lift of the pump is the work that is transmitted to the liquid per unit weight flowing through the impeller.

The flow rate and power of the pump can be measured with a given accuracy, so the method of determining the theoretical head of the pump is widely used. According to Euler equation, the tangential velocity c2u at the impeller outlet can be calculated by knowing the theoretical head of the pump at different flow rates.

It should be noted that the theoretical head close to or greater than the optimal flow rate can be obtained with sufficient accuracy by using the method. The reasons are as follows: when the flow rate is less than the optimal flow rate, the relation N/Q+q, so the theoretical head is related to the value Q+q.

It increases rapidly and becomes a non-linear relationship (Fig. 3-2-9). It should be noted that at zero flow rate, the flow through impeller in form is only equal to leakage value q, which is very small compared with flow Q, so that the pump power has the final value. Therefore, the theoretical Yang Cheng determined by the above method and the calculated velocity circulation at the impeller outlet based on its value are quite large. At the same time, theoretical head is not a linear function of flow rate.

However, according to the theory of rotating annular cascade, the theoretical head generated by impeller is a linear function of flow rate, so the straight line of linear part of relation Ht=f(Q) can be extrapolated to low flow area (straight line Hr.cam) when the optimal and large flow state is proved by balance test.

If the theoretical head (Ht.1cm) obtained by the extrapolation method is multiplied by the corresponding values (Q+q) and pg, the hydraulic power obtained will be significantly lower than the measured power of the pump (considering the power consumed in terms of mechanical loss). The difference between the hydraulic power obtained by the test method and the calculated value by the above method is called braking power. When the flow rate is small, the braking power increases sharply. The zero flow rate of the pump corresponds to the state in which all hydraulic power is completely converted into braking power.

The occurrence of braking power can be explained by the closed circulation of liquid between impeller and water chamber, that is, the liquid flows into the water chamber with the reserve energy transferred from impeller. This part of the liquid returns to the impeller from the pressure chamber, but has the reserve energy corresponding to the pressure chamber. This process is related to the additional consumption of work, which is not used to improve the liquid lift, but is converted into loss.

2. Flow Conditions of Liquids in Pumps

The following studies the flow conditions of liquid in the pump impeller, under which the liquid may exchange between the impeller and the pressurized water chamber. When the flow rate of the pump is enough to make the outlet section of the impeller fully filled with flowing liquid, the flow around the blade can be guaranteed. According to cascade hydrodynamics theory, the relationship between theoretical head and pump flow rate is linear.

When the flow rate is small, the potential velocity decreases and the blade profile angle of attack increases, resulting in partial hydraulic braking attached to the back of the blade. In these cases, the flow around impeller blades can not be regarded as no-flow-out motion. At this time, the relationship between theoretical head and pump flow may be non-linear. Therefore, the method of extrapolating the linear law between theoretical head and flow rate can not be considered to have sufficient basis when the pump is in a small flow state.

In the first section of this chapter, according to Stodora's method, the formula of theoretical head H for finite-time slice teaching is obtained. The existence of large detachment zones will lead to a distinct difference in tangential velocity C2o of the impeller outlet absolute velocity when considering the detachment zone [Formula (3-28) and not considering the strand zone [Formula (3-2-7).

Comparing the C e­xpression with and without considering the stripping zone, it is pointed out that the modified formula of finite number of sheets is based on Stodora's method.

When deducing the relation (3-2-12), it was used to produce liquid flow off along the straight EA, but in fact along the straight DA.

The cascade curve is approximately replaced by a straight line on Fig. 3-2-10.

The straight line AE is parallel to the blade and divides the blade pitch into two segments

It is the projection of the surface of the liquid flow outflow in the space between the blades. It is assumed that at the intersection point e, the angle formed by the lines de and AE is o, and the curve De is equal to each other. Because the normal line of the streamline is the equipartite of the flow angle, the direction of the straight line Oc (point O is the intersection of the normal line and the eddy streamline, which is point B in Figure 3-2-7) remains unchanged, while the straight line O e occupies the position of O'e. In triangle Tee, the angles are equal to: A (top angle C), respectively.

In this case, the circulation of ABC along the closed line (Figure 3-2-7) should be determined according to the triangular O'ce area (see Figure 3-2-10). In order to find this area, the vertical line (straight line cO) is made from point C to bisector eO', and the triangle O'ce area is used to replace the triangle O'ec area to a certain degree of approximation. In triangle O'0c, angle O"cO equals. According to the above calculation, in the impeller of slurry pump, the horn changes between 3*~8, so the triangle 0'o "c area is 10% smaller than the triangle Oec area. In this case, it is decided to follow the closed line.