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泵的汽蝕試驗
泵的汽體試驗是通過試驗方法，得到被試驗泵將要發(fā)生汽蝕現(xiàn)象時的汽蝕余量NPSH值，此汽蝕余量被稱為臨界汽蝕余量NPSHB (或稱試驗汽蝕余量)。
1.汽蝕試驗方法
試驗時，采用逐漸降低NPSH值，直至在恒定流量下泵的揚程(第一級) 下降達到3%，此時的NPSH值即為臨界汽蝕余量NPSH3,對揚程非常低的泵，揚程下降3%的量很小，試驗有困難，可以商定一個大一些的揚程下降量，建議可用下降量為(3 + )%，式中K為型式數(shù)，見式(8-8)。
NPSH值下降方法，從式(3-5) NPSHA=pg pgPo_Pr-H。-hw知道:可以是①減低液面
的壓力Po; ②提高安裝高度H; ③增加進口管路的阻力損失hen;④提高試驗液體的溫度，增加汽化壓力。表8-7及圖8-3-圖8 5表示了各種NPSH值下降方法的確定臨汽蝕余量NPSH3的方法。
(1)降低吸入液面的壓力Po的方法:即在吸入水罐(又稱汽蝕罐)上端抽真空的方法。如果NPSHBS > 10m時，則是先對液面加壓，然后慢慢降低吸入液面的壓力。這種方法只能用于閉式試驗回路裝置中。用這種方法時，因抽真空會導致試驗液體中氣體含量的改變，影響試驗的正確性，尤其是在NPSH3較小時影響較大，一般 是測得的NPSH3值偏小。
(2)改變進口管路的阻力損失hw :即在吸入管路上加調節(jié)閥門來改變其阻力大小。這種方法最大的優(yōu)點是操作方便，所以在開式試驗回路中得到廣泛使用。但這種方法的缺點是當NPSH3較小時，試驗誤差很大。因為關閉進口閥門增加阻力時，該閥門處的流速會增加很大，使壓力降低，造成了局部汽蝕而產生氣泡，這些氣泡隨液流帶到葉輪進口處，使泵提前發(fā)生汽蝕，所以一般是測得的NPSH3偏大。但當NPSH3≥5m時，影響是很小的。
(3) 提高安裝高度H的方法:即降低吸入液面的方法，又稱降水位法。這種方法誤差最小，但建試驗室時就要考慮能降水位的方法，對大型立式泵只能用這種方法來進行。
(4)提高試驗液體溫度，增加汽化壓力，這種方法很難實現(xiàn)，一般不采用。
2. NPSH的計算
根據式(3-4):
式中P1 ---入口測量截面處的表壓值
Pb----當?shù)禺敃r的大氣壓力值。
NPSH計算時，要注意必須在其基準面上進行，各種結構泵的NPSH基準面如圖8-6所示
如果進口壓力測量儀表的基準不在NPSH的基準上測量，則應加上位差Z。
3.試驗液體
試驗液體同性能試驗的液體，為試驗正確起見， 自由氣體應盡可能在試驗前被除去。4.試驗精度
同性能試驗。
5.試驗轉速和轉速換算
試驗轉速宜在規(guī)定轉速的80%~120%的范圍內進行，然后將試驗得到的NPSH3值換算到規(guī)定轉速下的(NPSH3)T 值，換算方法如下:
6. NPSH的容差系數(shù)及保證值(1)容差系數(shù):
對1級精度和精密級ENEHR= +3%或tNsHR= +0. 15m
渣漿泵對2級精度LNsSHR= +6%或hNPsTR +0.30m
(2)保證證實:
(NPSHR)c + hrsn . (NPSHR)。≥(NPSH3)，(NPSHR)g + (0.15m或0.30m)≥(NPSH3)，
保證的 (安全的) 汽蝕余量。-實測的臨界汽蝕余量。
或
式中( NPSHR)c
(NPSHR3)T一上述兩式取較大值。
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.
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