Avoid pump cavitation methods
（一） pump type and principles
一. The definition of the pump: usually ascending liquid, conveying liquids or causes the liquid pressure increases, namely change the prime mover mechanical energy into the liquid energy so as to achieve the purpose of pumping the liquid machine referred to as pump.
二. working principles
1 Positive displacement pump- using working chamber volume periodic change to convey liquid.
2 Vane pump - using leaf and liquid interact to transport the liquid
三 Pump specific usage: pump different usage, different transport liquid medium, different flow and head range, of course pump structure type is different, material is different, summarizing, can generally be divided into:
1, urban water supply, 2 sewage system 3 , construction system 4.agricultural water conservancy system 5, power systems 6.chemical system, 7.petroleum industry system 8, mining metallurgical system 9, light industry system 10, Marine systems
(二) cavitation phenomenon
When under certain temperatures, reduce the pressure to the temperature vaporization pressure, the liquid produces bubbles. Take this bubbles phenomenon as cavitation. Cavitation occurred bubbles, flows to place of high pressure, its volume reduced and burst. This because pressure rising to cause bubble disappeared in liquid phenomenon known as cavitation crush.
Water pump in operation, if its over-current part local area (usually impeller blades inlet back somewhere) for some reasons, pumping liquid absolute pressure decreased to the liquid vaporization pressure at that temperature, the liquid starts vaporization at this spot, produce the massive steam, form bubbles, when contains a lot of bubbles in the impeller liquid forward high-pressure zone, the high pressure fluid around bubble causes bubbles have dramatically narrow and fracture. At the same time of bubble condensation rupture, liquid mass point fills the caviation with high speed, in this moment produce very strong surge function, and with the impact of high frequency strike metal surface impact stress can amount to several hundred to thousands of atmospheric pressure, impact frequency can reach tens of thousands times per second, when serious, will breakdown wall thickness.
The water pump produces bubbles and bubbles break make the flow parts suffering damage process is the water pump cavitation process. After water pump produced cavitation, except damage to flow parts, it also produces noise and vibration, and lead to pump performance drop, when serious, will break the liquid in the pump, and can't work normally.
(三) pump cavitation basic relation
Water pump cavitation condition is decided by pump itself and the suction devices two aspects. Therefore, the study of cavitation occurred conditions, should be considered both sides, and the basic relation is
NPSHc≤NPSHr≤[NPSH]≤NPSHa
NPSHa=NPSHrNPSHc—— pump begin to cavitate
NPSHa NPSHa>NPSHrNPSHc—— pump without cavitation
in formula
NPSHa—— Device NPSH is also called effective NPSH, bigger is harder to cavitate
NPSHr—— Water pump NPSH, also called the necessary NPSH or pump inlet dynamic pressure drop, the small cavitation resistance, the better performance
NPSHc—— Critical NPSH, refers to the corresponding pump performance drop certain value NPSH.
[NPSH]—— The allowable NPSH is determined pump using conditions NPSH, usually take[NPSH] = (1.1 ~ 1.5) NPSHc
(四) prevent cavitation measures
To prevent the occurrence of cavitation must improve NPSHa, make NPSHa > NPSHr can prevent cavitation measures as follows:
1 ． Minimize geometrical suction up height hg (or increase geometric flow backward height),
2 ． Reduce inhaled loss hc, therefore can try to increase diameters, try best to minimize pipe length, elbow and accessories, etc
3 ． Prevent long time operating under big flow,
4 ． under same speed and flow, adopting double suction pump, because reducing inlet flow, pump is hard to occur cavitation,
5 ． When water pump cavitates, should reduce flow or slow down operating speed,
6 ． The pump suction pool situation has important influence to pump cavitation,
7 ． To the pump operated under rigorous conditions, to avoid cavitation damage, can use cavitation resistant materials
(五) pump selection
Design institute need to make sure pump usage and performance and select type when designing equipment. This choice must first start from the selection of pump type and form, then what is principle to choose pump? According to what ?
一 、selection principle
1、 make the selected pump type and performance confirms to equipment flow, head, pressure, temperature, cavitation flow, suction and other parameters requirements.
2、 must satisfy medium characteristic request. To transport inflammable, explosive toxic or precious liquids pump, requiring dependable shaft sealing or use pump without leakage, such as magnetic drive pump, diaphragm pump, shielding pump conveying corrosive medium pump, request flow parts adopting corrosive resistant materials, such as AFB type stainless steel centrifugal pump, CQB-F type fluorine plastic magnetic pump. For conveying solid particles medium pumps, request flow parts adopting wear-resistant materials, when necessary, shaft seal washed by clean liquid.
3、 High reliability, low noise, small vibration in machinery.
4、 economically we need to comprehensive considering the lowest total cost of equipment cost, operation fee, maintenance fee and the management fee.
5、 centrifugal pump has high speed, small volume, light weight, high efficiency, large flow, simple structure, no-pulsation conveying liquid, stable function, easy operation and maintenance is convenient characteristics. Therefore except the following circumstance, choose centrifugal pump as far as possible:
Have measurement requirement, the selection of metering pump head requires high, small flow and without proper small flow high head pump centrifugal pump, you can choose reciprocating pump, if cavitation requires not high, you can also choose vortex pumps. Head is very low, flow is very large, can choose axial-flow pump and mixed flow pump. When the medium viscosity is larger (more than 6.5 ~ 1000mm2 / s), can consider to choose rotor pumps or reciprocating pump (gear pump, single screw pump). When medium containing air capacity is 75, flow is smaller and viscosity less than 37.4mm2/s, can choose vortex pumps. For frequent restart or irrigation pump inconvenient occasions, should choose pump with self-priming performance, such as self-suction centrifugal pump, self-suction vortex pumps, pneumatic (electric) diaphragm pumps.
(六) selection general procedure
1.According to the device arrangement, topography condition, water conditions, and operating conditions, economic scheme comparison and other elements 2, consider to choose horizontal centrifugal pump, vertical centrifugal pumps and other types (pipeline centrifugal pump, right angle centrifugal pump, change angle centrifugal pump, corner type centrifugal pump, parallel type centrifugal pump, vertical centrifugal pump, upright type sewage pump, submerged type sewage pump, and easy disassembly type sewage pump, submerged type sewage pump and non-clogged type sewage pump, self-suction sewage pump, gear type oil pumps, oil filled type submerged pumps,). Horizontal pump disassembly is convenient, 3, easy management, but with big volume, 4,price is more expensive 5, need large floor area; 6, vertical pump impeller is under water in many cases; 7,can be started anytime 8, facilitating automatic or remote control 9, compact,10, small installation area 11. the price is cheaper.
3, according to the medium nature, to determine the clean water pump, hot water oil pump, chemical pump or anti-corrosive pumps or impurity pump, or uses non-block sewage pump. Pump innstalled in the explosion area, should be based on the explosion regional grade, adopts explosion-proof motors. 4, vibration quantity is divided into: pneumatic, electric (electric divided into 220v voltage and 380v voltage). 5, according to the flow, select signle or double stage suction pump: according to head height, select signle stage suction pump or multi-stage suction pump, high speed pump or low speed pump (air conditioning pump), the multistage pumps efficiency is lower than single stage pump, when single stage pump and multistage pumps can be both used, it is appropriate to chooses single stage pump. 6, determine pump specific type, adopt pump series selection, according to the maximum flow, after amplify 5-10 allowance head two major parameters, in diagram or series of characteristic curves to determine specific models. Using pump characteristic curve, on abscissa to find the required flow value, on y-coordinate to find the required head value, from two values respectively upward and rightward lead vertical or horizontal line, two line intersection exactly lands on characteristic curve, the pump is the selected pump, but this kind of ideal situation generally is few, usually meet the following several circumstances:
A 、 First: the intersection is above characteristic curve, this indicates flow requirements, but head is not enough, at this time, if the head is similar, or differ 5 or so, still can be choosen, if the head is quite different, then choose higher head pump. Or try to reduce the pipeline resistance losses.
B 、 Second, the intersection below characteristic curve, in pump characteristic curve fanlike trapezoid range, preliminary set this model, then according to the head difference, to determine whether cutting the impeller diameter, if the head difference is small, will not cutting, if head is quite different, according to the required Q, H,, according to its ns and cutting formula, cutting impeller diameter, if intersection does not fall in fanlike trapezoid range, should select smaller head pumps. When choosing pumps, sometimes must consider production process requirement, choose different shapes Q-H characteristic curve.
Such as: transport liquid level to contain that must maintain certain liquid height, then hope quantity change a lot, and head changes little, therefore should choose plain H-O curve pump. Such as: transport oil to tubular furnace, if flow changes small in working, then the furnace pipe is easy produce coked phenomenon. To avoid this circumstance
