- Cavitation of centrifugal pumps
In order for the pump to not cavitation, the unit weight of liquid at the inlet of the pump impeller must exceed the excess energy of the vaporization pressure. Please see the following explanation:
When the suction height of the centrifugal pump is too large and the liquid temperature is relatively high, causing the suction pressure to be less than or equal to the liquid saturated vapor pressure, the liquid will boil at the pump inlet and form a full steam in the pump casing. The space, as the pump rotates, the bubble enters the high pressure zone. Due to the pressure difference, the bubble is ruptured and recondensed. At the moment of condensation, the particles collide with each other, resulting in a high partial pressure if the bubbles are in the metal. When the surface is cracked and condensed, the liquid particles are like a small number of small warheads, which are continuously hit on the metal surface, causing cracks on the metal surface, and even local spalling, making the surface of the impeller honeycomb, and some active gases in the bubbles. If oxygen enters the crack on the metal surface, the metal is subjected to chemical corrosion by the heat released when the bubble is condensed, and the above phenomenon is cavitation.
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=Hs1-Hf0-1=0.78-1.5=-0.72m
Hg is a negative value, indicating that the pump should be installed below the pool level, at least 0.72m below the liquid level.
When cavitation occurs, the pump will generate noise and vibration, which will greatly reduce the performance of the pump’s head, flow and efficiency. At the same time, it will accelerate the damage of the material and shorten the service life of the machine. Therefore, the suction height of the pump must be limited. Prevent large amounts of liquid vaporization to avoid cavitation.
2, the suction height of the centrifugal pump
The height between the center of the suction port of the pump and the liquid level of the reservoir is called the suction height. Imagine that the impeller inlet is absolutely vacuum, the suction line resistance is zero, and the liquid level is a standard atmospheric pressure. The theoretical geometric height is 10.33 meters, however, due to various resistance losses in the pump suction pipe, and the pump impeller inlet is unlikely to reach the full vacuum and other unfavorable factors, plus the necessary NPSH at the pump inlet, the suction height of the general centrifugal pump is not More than 4-5 meters.
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)-(Hυ-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 according to the following formula: Hs1 H’s
Cavitation allowance Δh
For the oil pump, the calculation of the installation height is calculated by using the NPSH Δh, that is, the NPSH is taken from the oil pump sample, and the value is 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. 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 1.5mH2O, 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 pump when conveying 20°C clean water;
2) The installation height of the pump when transporting water at 80 °C.
Solution: 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 is shipped, so the installation height of the pump is Hg=5.7-0-1.5=4.2m.
3) Installation height of the pump when transporting water at 80 ° C
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.
Hv=47.4×103Pa=4.83mH2O
Hs1=5.7+10-10.33-4.83+0.24=0.78m