泵的汽蚀试验

泵的汽体试验是通过试验方法，得到被试验泵将要发生汽蚀现象时的汽蚀余量NPSH值，此汽蚀余量被称为临界汽蚀余量NPSHB (或称试验汽蚀余量)。  
1.汽蚀试验方法
    试验时，采用逐渐降低NPSH值，直至在恒定流量下泵的扬程(第一级) 下降达到3%，此时的NPSH值即为临界汽蚀余量NPSH3,对扬程非常低的泵，扬程下降3%的量很小，试验有困难，可以商定一个大一些的扬程下降量，建议可用下降量为(3 +  )%，式中K为型式数，见式(8-8)。
    NPSH值下降方法，从式(3-5) NPSHA=pg pgPo_Pr-H。-hw知道:可以是①减低液面
的压力Po; ②提高安装高度H; ③增加进口管路的阻力损失hen;④提高试验液体的温度，增加汽化压力。表8-7及图8-3-图8 5表示了各种NPSH值下降方法的确定临汽蚀余量NPSH3的方法。  

(1)降低吸入液面的压力Po的方法:即在吸入水罐(又称汽蚀罐)上端抽真空的方法。如果NPSHBS > 10m时，则是先对液面加压，然后慢慢降低吸入液面的压力。这种方法只能用于闭式试验回路装置中。用这种方法时，因抽真空会导致试验液体中气体含量的改变，影响试验的正确性，尤其是在NPSH3较小时影响较大，一般 是测得的NPSH3值偏小。

(2)改变进口管路的阻力损失hw :即在吸入管路上加调节阀门来改变其阻力大小。这种方法最大的优点是操作方便，所以在开式试验回路中得到广泛使用。但这种方法的缺点是当NPSH3较小时，试验误差很大。因为关闭进口阀门增加阻力时，该阀门处的流速会增加很大，使压力降低，造成了局部汽蚀而产生气泡，这些气泡随液流带到叶轮进口处，使泵提前发生汽蚀，所以一般是测得的NPSH3偏大。但当NPSH3≥5m时，影响是很小的。

(3) 提高安装高度H的方法:即降低吸入液面的方法，又称降水位法。这种方法误差最小，但建试验室时就要考虑能降水位的方法，对大型立式泵只能用这种方法来进行。

(4)提高试验液体温度，增加汽化压力，这种方法很难实现，一般不采用。

2. NPSH的计算

根据式(3-4):

      式中P1  ---入口测量截面处的表压值

Pb----当地当时的大气压力值。

NPSH计算时，要注意必须在其基准面上进行，各种结构泵的NPSH基准面如图8-6所示

如果进口压力测量仪表的基准不在NPSH的基准上测量，则应加上位差Z。

3.试验液体

试验液体同性能试验的液体，为试验正确起见， 自由气体应尽可能在试验前被除去。4.试验精度

同性能试验。
5.试验转速和转速换算
    试验转速宜在规定转速的80%~120%的范围内进行，然后将试验得到的NPSH3值换算到规定转速下的(NPSH3)T 值，换算方法如下:
6. NPSH的容差系数及保证值(1)容差系数:
    对1级精度和精密级ENEHR= +3%或tNsHR= +0. 15m

渣浆泵对2级精度LNsSHR= +6%或hNPsTR  +0.30m

(2)保证证实:
(NPSHR)c + hrsn . (NPSHR)。≥(NPSH3)，(NPSHR)g + (0.15m或0.30m)≥(NPSH3)，
保证的 (安全的) 汽蚀余量。-实测的临界汽蚀余量。
或
式中( NPSHR)c
(NPSHR3)T一上述两式取较大值。

Cavitation test of pump

Through the test method, the NPSH value of the NPSH of the pump to be tested when the NPSH will occur is obtained. This NPSH value is called critical npshb (or test NPSH).

1. Cavitation test method

During the test, gradually reduce the NPSH value until the head (first stage) of the pump decreases to 3% at a constant flow rate. At this time, the NPSH value is the critical NPSH 3. For the pump with a very low head, the head decrease of 3% is very small. It is difficult to test. A larger head decrease can be agreed. It is recommended that the available decrease is (3 +)%, where k is the type number, see formula (8-8).

NPSH value reduction method, from formula (3-5) npsha = PG pgpo_pr-h. -HW knows: it can be ① reduce the liquid level

(2) increase the installation height h; (3) increase the resistance loss of the inlet pipeline hen; (4) increase the temperature of the test liquid and the vaporization pressure. Table 8-7 and figure 8-3-figure 8 5 show the methods for determining NPSH 3 of various NPSH value reduction methods.

(1) method to reduce the pressure Po of suction liquid level: that is, the method of vacuumizing at the upper end of suction water tank (also known as cavitation tank). If npshbs > 10m, first pressurize the liquid level, then slowly reduce the pressure of suction liquid level. This method can only be used in closed test loop device. When using this method, vacuumizing will lead to the change of gas content in the test liquid and affect the correctness of the test, especially when npsh3 is small, generally the measured npsh3 value is small.

(2) change the resistance loss HW of the inlet pipeline: that is, add a regulating valve to the suction pipeline to change its resistance. The biggest advantage of this method is convenient operation, so it is widely used in open test circuit. But the disadvantage of this method is that when npsh3 is small, the test error is large. Because when closing the inlet valve to increase the resistance, the flow velocity at the valve will increase greatly, reducing the pressure, causing local cavitation and generating bubbles. These bubbles will be brought to the inlet of the impeller along with the liquid flow, leading to cavitation of the pump in advance, so the npsh3 measured is generally larger. But when npsh3 ≥ 5m, the effect is very small.

(3) the method of increasing the installation height H: that is, the method of reducing the suction liquid level, also known as the method of lowering the water level. The error of this method is the smallest, but the method of lowering water level should be considered when the laboratory is built.

(4) it is difficult to raise the temperature of the test liquid and increase the vaporization pressure, which is generally not used.

2. Calculation of NPSH

According to formula (3-4):

Where P1 - gauge pressure at the entrance measurement section

Pb - local atmospheric pressure at that time.

When calculating NPSH, it should be noted that it must be carried out on its datum plane, and the datum plane of NPSH of various structural pumps is shown in Figure 8-6

If the benchmark of the inlet pressure measuring instrument is not measured on the benchmark of NPSH, the upper difference Z shall be added.

3. Test fluid

The free gas shall be removed as far as possible before the test for the sake of test correctness. 4. Test accuracy

Same energy test.

5. Test speed and speed conversion

The test speed should be in the range of 80% - 120% of the specified speed, and then the npsh3 value obtained from the test is converted to the (npsh3) t value under the specified speed, and the conversion method is as follows:

6. NPSH tolerance coefficient and guarantee value (1) tolerance coefficient:

For level 1 accuracy and precision level enehr = + 3% or tnshr = + 0.15m

Slurry pump for class 2 accuracy lnshr = + 6% or hnpstr + 0.30m

(2) guarantee confirmation:

(NPSHR)c + hrsn . (NPSHR)。 ≥ (npsh3), (NPSHr) G + (0.15m or 0.30m) ≥ (npsh3),

Guaranteed (SAFE) NPSH. -Measured critical NPSH.

or

Where (NPSHr) C

(npshr3) T - the greater of the above two formulas.