1. Installation height of the variable frequency pump without negative pressure water supply equipment Hg
Allowing the suction vacuum height Hs means that the pressure p1 at the pump inlet can be allowed to reach *** large vacuum
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. The position should be noted in the pump sample. The Hs value is the value when the water is used as the working medium, the operating conditions are 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: ***step according to the above formula will be detected by the pump sample Hs1; the second step will be as follows Hs1 is converted to H's
Cavitation allowance Δh
For the variable frequency pump , calculate the installation height with the NPSH Δh, that is, use the NPSH Δh to be taken from the pump sample, and the value is also measured with 20 ° C water. If other liquids are transported, correction is also required. Check out the relevant books.
From the safety point of view, the actual installation height of the pump should be less than the calculated value. Also, when the calculated Hg is negative, the pump inlet position should be below the tank level.
For example, a non-negative pressure water supply equipment variable frequency pump is checked from the sample to allow the vacuum height Hs=5.7m. The total resistance of the known suction line is 1.5mH2O, the local atmospheric pressure is 9.81×104Pa, and the liquid is in the suction line. The dynamic head can be ignored. Try to calculate:
(1) Installation of the pump when conveying 20°C clean water;
(2) The installation height of the pump when the water is transported at 80 °C.
Solution: (1) Installation height of the pump when conveying 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 leaves the factory, so the installation height of the pump is Hg=5.7-0-1.5=4.2m.
(2) Mounting height of the pump 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. It is necessary to convert the Hs according to the following formula, ie Hs1=Hs+(Ha-10.33)-(Hυ-0.24)
It is known that Ha=9.81×104Pa≈10mH2O, and the saturated vapor pressure of 80°C water is 47.4kPa.
Hv=47.4×103Pa=4.83mH2O
Hs1=5.7 10-10.33-4.83 0.24=0.78m
2. Cavitation of variable frequency pump without negative pressure water supply equipment
The cavitation phenomenon of the variable frequency pump means that the liquid to be delivered is partially vaporized due to the pressure of the saturated vapor at the delivery temperature being equal to or lower than the pressure at the inlet of the pump (actually at the inlet of the blade), causing noise and vibration of the pump. The flow rate, head pressure and efficiency of the pump are significantly reduced. Obviously, the cavitation phenomenon is not allowed in the normal operation of the variable frequency pump without negative pressure water supply equipment.
The key to avoiding cavitation is that the pump should be installed at the correct height, especially when transporting volatile liquids with high temperatures.
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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