The working principle of the centrifugal pump is that the centrifugal pump can send water out due to the centrifugal force. Before the pump is working, the pump body and the inlet pipe must be filled with water to form a vacuum state. When the impeller rotates rapidly, the blade causes the water to rotate rapidly. The rotating water flies away from the impeller under the action of centrifugal force, and the water in the pump is After being thrown, the central portion of the impeller forms a vacuum region. The water from the water source is pressed into the inlet pipe through the pipe network under the action of atmospheric pressure (or water pressure). This cycle is not enough, you can achieve continuous pumping. It is worth mentioning here: before the centrifugal pump starts, it must be filled with water in the pump casing before it can be started. Otherwise, the pump body will not be able to complete the liquid absorption, causing the pump body to heat up, vibrate, and not produce water, resulting in “idle”. Damage to the pump (referred to as "gas binding") causes equipment accidents.
First, the installation height of the centrifugal pump Hg
Allowable suction vacuum height Hs is the maximum vacuum that can be reached by the pressure p1 at the pump inlet.
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 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
(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 as follows: Hs1 Converted to H's
Cavitation allowance Δh
For the oil pump, calculate the installation height with the NPSH Δh, that is, use the NPSH Δh to be taken from the oil pump sample, and the value is also measured with 20 ° C water. If other liquids are transported, corrections are also required. Check the relevant books.
From a 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, it indicates that the suction port position of the pump should be below the liquid level of the sump.
For example, a centrifugal pump is found on the sample to allow the vacuum height Hs = 5.7 m. It is known that the total resistance of the suction line is 1.5 mH2O, the local atmospheric pressure is 9.81 x 104 Pa, and the dynamic head of the liquid in the suction line is negligible. Trial calculation:
(1) Installation of the pump when conveying water at 20 ° C;
(2) The installation height of the pump when the water is transported at 80 °C.
Solution: (1) Installation height of pump when conveying 20°C clean water
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=4.2m.
(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 × 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.
Second, the cavitation phenomenon of centrifugal pumps
The cavitation phenomenon of the centrifugal 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, cavitation is not allowed in the normal operation of the centrifugal pump.
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 is a negative value, indicating that the pump should be installed below the pool level, at least 0.72m below the liquid level.
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