The use and maintenance of beer equipment-centrifugal pump suction height and the cause of cavitation phenomenon To prevent the pump from cavitation, the unit weight of the liquid at the inlet of the pump impeller must exceed the surplus 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 saturated vapor pressure of the liquid, the liquid will boil and vaporize at the pump inlet under this environment, forming a full steam in the pump casing As the pump rotates, the bubbles enter the high-pressure zone. Due to the pressure difference, the bubbles collapse under pressure and re-condensate. At the moment of condensation, the particles collide with each other, resulting in a high local pressure.
If these bubbles rupture and condense near the metal surface, the liquid particles are like countless small bullets, continuously hitting the metal surface, causing cracks on the metal surface, and even partial spalling, making the surface of the impeller honeycomb. Some of the active gases, such as oxygen, enter the cracks on the metal surface. With the help of the heat released when the bubbles are condensed, the metal is subjected to chemical corrosion. The above phenomenon is cavitation. The cavitation phenomenon of a centrifugal pump refers to the partial vaporization of the liquid to be transported due to the saturated vapor pressure 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 the pump to produce noise and vibration. In severe cases, The flow rate, pressure head and efficiency of the pump drop significantly. Obviously, cavitation is not allowed in the normal operation of the centrifugal pump.
The key to avoiding cavitation phenomenon is to install the pump at the correct height, especially when transporting volatile liquids with higher temperatures. Substitute the value of Hs1 into the formula to obtain the installation heightHg=Hs1-Hf0-1=0.78-1.5=-0.72mHg 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. When cavitation occurs, the pump will produce noise and vibration, which will cause the pump’s head, flow, and efficiency performance to drop sharply. At the same time, it will accelerate the damage of materials and shorten the service life of the parts. Therefore, the suction height of the pump must be limited. Prevent the liquid from vaporizing in a large amount to avoid cavitation.
The height between the center of the suction port of the pump and the liquid level of the storage tank is called the suction height. It is assumed that the impeller inlet is absolute vacuum, the suction pipe resistance is zero, and the liquid surface is a standard atmospheric pressure. Then the theoretical geometric height is 10.33 meters, however, due to various resistance losses in the suction pipe of the pump, and unfavorable factors such as the impeller inlet of the pump cannot reach complete vacuum, plus the necessary cavitation allowance at the inlet of the pump, the suction height of the general centrifugal water pump is not More than 4-5 meters. The allowable vacuum height Hs refers to the maximum vacuum that can be reached by the pressure p1 at the inlet of the 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 pump manufacturer. 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 formulaHs1=Hs＋(Ha-10.33)-(Hυ-0.24) 2) Transporting 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 will be the Hs1 found in the pump sample according to the above formula; the second step will be the conversion of Hs1 according to the following formula Into H’s NPSH ΔhFor oil pumps, the cavitation allowance Δh is used to calculate the installation height, that is, the cavitation allowance Δh is checked from the oil pump sample, and its value is also measured with 20 ℃ clean water. If other liquids are to be transported, calibration is also required. Check the relevant books. 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. Also, 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. For example, a centrifugal pump found from the sample that the allowable vacuum height 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 can be ignored. Try to calculate: 1) Installation of the pump when delivering 20℃ clean water; 2) Change to the installation height of the pump when delivering 80℃ water. Solution: The installation height of the pump when delivering 20℃ clean waterKnown: Hs=5.7mHf0-1=1.5mu12/2g≈0The 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=4.2m. 3) The installation height of the pump when delivering 80℃ water When delivering 80℃ water, the Hs value in the pump sample cannot be used to calculate the installation height directly. The Hs time must be converted by the following formula, namely Hs1=Hs＋(Ha-10.33)-(Hυ-0.24)It is known that Ha=9.81×104Pa≈10mH2O, and the saturated vapor pressure of water at 80℃ is found to be 47.4kPa from the appendix.Hv=47.4×103Pa=4.83mH2OHs1=5.7+10-10.33-4.83+0.24=0.78m