2. Calculation of installation height Hg of sanitary centrifugal pump
The allowable vacuum height Hs refers to the maximum vacuum that can be reached by the pressure p1 at the inlet of the sanitary pump .
The actual allowable suction vacuum height Hs value is not a value calculated according to the formula, but a value determined experimentally by the manufacturer of the sanitary negative pressure pump . This value is attached to the pump sample for users to check. It should be noted that the Hs value given in the pump sample is the value when clean water is used as the working medium, and the operating condition is 20℃ and the pressure is 1.013×105Pa. When the operating conditions and working medium are different, conversion is required. 1 Transport clean water, but the operating conditions are different from the experimental conditions, can be converted according to the following formula
Hs1＝Hs＋Ha－10.33 －Hυ－0.24 2 When conveying other liquids, when the conditions of the conveyed liquid and the villain are different from the experimental conditions, a two-step conversion is required: the first step will be the Hs1 found in the pump sample according to the above formula; In two steps, convert Hs1 to H?s 2 NPSH Δh according to the following formula
For oil pumps, the cavitation allowance Δh is used when calculating the installation height, that is, the vacuum degree of the pump allowed to suck liquid, that is, the allowable installation height of the pump, in meters. Use the NPSH Δh to check from the oil pump sample, and its value is also measured with 20°C clean water. If other liquids are to be transported, calibration is also required, and the relevant books should be checked in detail.
Suction range = standard atmospheric pressure (10.33 meters)-cavitation margin-safety amount (0.5 meters)
The vacuum height of the standard atmospheric pressure energy pressure pipeline is 10.33 meters.
For example: a certain pump must have a NPSH of 4.0 meters, what is the suction stroke Δh?
Solution: Δh=10.33-4.0-0.5=5.83 meters
From a safety point of view, the actual installation height of the pump should be less than the calculated value. When the calculated Hg is negative, it means that the suction port of the pump should be below the liquid level of the storage tank.
Example 2-3 A centrifugal pump found from the sample that the allowable suction vacuum height is Hs=5.7m. It is known that the total resistance of the suction pipeline is 1.5mH2O, the local atmospheric pressure is 9.81×104Pa, and the dynamic pressure head of the liquid in the suction pipeline is negligible. Try to calculate:
1 Installation of centrifugal pump when delivering 20℃ clean water.
2 Change to the installation height of the centrifugal pump when delivering 80℃ water.
Solution: 1 The installation height of the pump when delivering 20°C clean water.
The local atmospheric pressure is 9.81×104Pa, which is basically in line with the experimental conditions when the pump leaves the factory, so the installation height of the pump is Hg=5.7-0-1.5=4.2 m.
2 The installation height of the pump when delivering 80℃ water
When delivering 80°C water, the Hs value in the pump sample cannot be used directly to calculate the installation height. The Hs time line must be converted by the following formula, namely
It is known that Ha=9.81×104Pa≈10mH2O, and the saturated vapor pressure of water at 80℃ is 47.4kPa found from the appendix.
Hv=47.4×103 Pa=4.83 mH2OHs1=5.7+10－10.33－4.83+0.24=0.78m
Substitute the value of Hs1 into the formula to obtain the installation height
Hg is a negative value, which means that the pump should be installed below the liquid level of the pool, at least 0.72m lower than the liquid level. The efficiency can be nearly 20% higher than that of the original water pump.
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