Install critical installation technology
The key to the installation technology of the pipeline centrifugal pump is to determine the installation height of the centrifugal pump, that is, the suction stroke. This height refers to the vertical distance from the water source to the centerline of the centrifugal pump impeller. It is not to be confused with the allowable vacuum height. The allowable vacuum height indicated on the pump or nameplate refers to the vacuum value on the pump inlet section. The test was carried out under the conditions of 1 standard atmospheric pressure and a water temperature of 20 ° C. It does not consider the flow of water after the suction pipe is matched. The installation height of the pump should be the value of the remaining part that allows the suction vacuum height to be deducted from the loss of the suction pipe. It must overcome the actual terrain water absorption height. The installation height of the pump should not exceed the calculated value, otherwise the centrifugal pump will not be able to pump water. In addition, the magnitude of the influence calculation value is the resistance loss head of the suction pipe. Therefore, it is advisable to use the shortest pipe arrangement and to install accessories such as elbows as little as possible. It is also conceivable to equip the pipe with a larger diameter to reduce the flow rate inside the pipe.
It should be pointed out that when the elevation and water temperature of the installation location of the pipeline centrifugal pump are different from the test conditions, such as the local altitude above 300 meters or the water temperature of the pumped water exceeds 20 °C, the calculated value should be corrected. That is, the atmospheric pressure at different elevations and the saturated vapor pressure above 20 °C. However, when the water temperature is below 20 ° C, the saturated steam pressure is negligible.
From the pipeline installation technology, the suction pipe requires strict sealing, and it cannot leak or leak water. Otherwise, the vacuum at the water inlet of the centrifugal pump will be destroyed, so that the water output of the centrifugal pump will be reduced, and even if it is serious, the water will not be pumped. Therefore, it is necessary to carefully do the interface work of the pipeline to ensure the construction quality of the pipeline connection.
Installation height Hg calculation
The allowable suction vacuum height Hs refers to the maximum vacuum that the pressure p1 at the pump inlet can allow.
The actual allowable vacuum height Hs value is not a value calculated according to the formula, but a value determined experimentally by the pump manufacturer. This value is attached to the pump sample for the user to check. It should be noted that the Hs value given in the pump sample is the value when the water is used as the working medium, the operating condition is 20 ° C and the pressure is 1.013 × 105 Pa. When the operating conditions and working medium are different, the conversion is required.
1 Conveying clean water, but the operating conditions are different from the experimental conditions, and can be converted according to the following formula
Hs1=Hs+Ha-10.33 - HÏ…-0.24
2 Conveying other liquids When the conditions of the liquid to be transported and the villain are different from the experimental conditions, a two-step conversion is required: the first step is based on the Hs1 detected in the pump sample; the second step is converted into Hs1 according to the following formula. H猃
2 NPSH Δh
For the oil pump, calculate the installation height with the NPSH Δh, that is, the vacuum that the pump allows to absorb the liquid, that is, the allowable installation height of the pump, in meters. The NPSH Δh was taken from the oil pump sample and its value was also measured with 20 ° C water. If you are transporting other liquids, you will need to make corrections and check the relevant books.
Suction stroke = standard atmospheric pressure (10.33 m) - NPSH - safety (0.5 m)
The standard atmospheric pressure energy pipeline has a vacuum height of 10.33 meters.
For example: a pump must have a NPSH of 4.0 meters, and ask for a suction Δh?
Solution: Δh=10.33-4.0-0.5=5.83 m
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, the pump's suction port position should be below the tank level.
Example 2-3 A centrifugal pump was found to allow the vacuum height Hs = 5.7 m from the sample. It is known that the total resistance of the suction line is 1.5mH2O, the local atmospheric pressure is 9.81×104Pa, and the dynamic pressure head of the liquid in the suction line is negligible. Trial calculation:
1 Installation of the centrifugal pump when conveying 20°C clean water.
2 Change the installation height of the centrifugal pump when transporting water at 80 °C.
Solution: 1 The installation height of the pump when delivering 20°C clean water.
Known: Hs=5.7m
Hf0-1=1.5m
U12/2g≈0
The local atmospheric pressure is 9.81×104Pa, which is basically consistent with the experimental conditions when the pump is shipped, so the installation height of the pump is Hg=5.7-0-1.5=4.2 m.
2 Pump installation height when transporting 80°C water
When transporting water at 80 °C, the installation height cannot be calculated directly from the Hs value in the pump sample.
Hs1=Hs+Ha-10.33 - HÏ…-0.24
It is known that Ha = 9.81 × 104 Pa ≈ 10 mH 2 O, and the saturated vapor pressure of water at 80 ° C was found to be 47.4 kPa by the appendix.
Hv=47.4×103 Pa=4.83 mH2O
Hs1=5.7+10-10.33-4.83+0.24=0.78m
Substituting the Hs1 value into the formula to obtain the installation height
Hg=Hs1-Hf0-1=0.78-1.5=-0.72m
Hg is negative, indicating that the pump should be installed below the pool level, at least 0.72m below the liquid level.
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