Excellent engineering design, proper pump selection, and proper application of pumping technology cannot overcome the hazards that improper use may bring to the pump system, here are 8 easy to make mistakes in the use of centrifugal pumps.
Many people believe that the lower the pump head, the smaller the motor load. Under the misconception of this misconception, when purchasing a pump, the head of the pump is often chosen to be very high.
In fact, for centrifugal pumps, when the pump model is determined, the size of its power consumption is proportional to the actual flow of the pump. The flow of the pump will decrease with the increase of head, so the higher the head, the smaller the flow, the smaller the power consumption. Conversely, the lower the head, the greater the flow, the greater the power consumption. Therefore, in order to prevent motor overload, the actual head of the pump is usually required to be no less than 60% of the rated head. So when the high head pump is used for low head pumping, the motor is easy to overload and get hot, and the motor will be burned up seriously.
If emergency use, it is necessary to install a gate valve on the outlet pipe for regulating the water output to reduce the flow rate and prevent the motor from overload, pay attention to the motor temperature rise, and if the motor is found to be overheated, the outlet flow rate should be shut down or the pump should be stopped in time.
Many people think that a large diameter pump with a small diameter water pipe can increase the actual head, in fact, the actual head of the pump = total head - loss of head. When the pump model is determined, the total head is certain; the lost head mainly comes from the pipe resistance, the smaller the pipe diameter, obviously the greater the resistance, and therefore the greater the lost head, so after reducing the pipe diameter, the actual head of the pump will not be increased, but will be reduced, resulting in a decline in pump efficiency. Similarly, when a small diameter water pump uses a large diameter water pipe to pump water, it will not reduce the actual head of the pump but will reduce the loss of head due to the reduced resistance of the pipeline, so that the actual head is increased. Some people think that when the small diameter water pump uses the big water pipe to pump water, it will certainly increase the motor load greatly, they think that after the diameter of the pipe increases, the pressure of the water in the outlet pipe to the impeller of the pump is big, so it will increase the motor load greatly. In fact, the size of the liquid pressure is only related to the height of the head and has nothing to do with the size of the cross-sectional area of the water pipe. As long as the head is certain, the impeller size of the pump is unchanged, no matter how big the pipe diameter is, the pressure acting on the impeller is certain. Only after the diameter of the pipe increases, the resistance of water flow will be reduced, and the flow rate will be increased, and the power consumption will be increased appropriately. But as long as in the rated head range, no matter how to increase the diameter of the pump is normal to work, and can also reduce the pipeline loss, improve the efficiency of the pump.
When installing the water inlet pipe, the horizontal section will gather air in the water inlet pipe and reduce the vacuum of the water pipe and pump, which will lower the suction head of the pump and reduce the water output. The correct practice is: the horizontal section should be slightly inclined to the direction of the water source
If more elbows are used in the inlet pipe, it will increase the local water flow resistance. And the elbow should turn in the vertical direction, not allowed to turn in the horizontal direction, so as not to gather air
Centrifugal pump inlet and elbow directly connected will make the water flow through the elbow into the impeller when the distribution is uneven. When the diameter of the inlet pipe is larger than the water inlet of the pump, an eccentric reducer should be installed. The flat part of the eccentric reducer should be installed on the top and the beveled part should be installed on the bottom. Otherwise, the air will be gathered, the water output will be reduced or the water will not be pumped, and there will be a crashing sound, etc. If the inlet pipe is equal to the diameter of the water pump inlet, a pipe should be added between the water pump inlet and the elbow, and the length of the straight pipe should not be less than 2~3 times the diameter of the water pipe.
If the bottom valve is equipped with the next section of the inlet pipe is not vertical, it will make the valve can not be closed by itself, resulting in water leakage. The correct installation method is the inlet pipe with bottom valve, the next section is best vertical, such as due to terrain conditions can not be installed vertically, the axis of the water pipe and the horizontal plane angle should be more than 60 °.
The inlet of the water pipe from the inlet pool bottom and pool wall distance is less than the inlet diameter. If there is mud and sand and other dirt at the bottom of the pool, the distance between the inlet and the bottom of the pool is less than 1.5 times the diameter of the inlet, which will cause poor water intake when pumping or suck in mud and sand debris, blocking the inlet.
When the water inlet of the water pipe is not deep enough, this will cause the water surface around the inlet pipe to produce a whirlpool, affecting the water inlet and reducing the water output. The correct installation method is: the water inlet depth of small and medium-sized water pumps shall not be less than 300~600mm, and large water pumps shall not be less than 600~1000mm
If the outlet is above the normal water level of the pool, although the pump head is increased, but the flow rate is reduced. If the water outlet must be higher than the water level of the outlet pool due to the topographic conditions, the elbow and short pipe should be installed at the mouth of the pipe to make the water pipe become siphon type and reduce the height of the outlet.
Table of content
What is cavitation
How does cavitation occur
What is the suction range?
What is NPSH(net positive suction head)
NPSH(net positive suction head) Related Terms
What is the difference between NPSHr and NPSHa
The result of cavitation in the centrifugal pump
Measures to prevent cavitation
Cavitation is a phenomenon in which vapor bubbles are generated in a flowing fluid due to local pressure reduction. When cavitation occurs in a pump, it causes erosion of the machine parts at the cavitation site, and further development will cause a drop in head and vibration noise.
The pressure inside the pump at a certain place drops below the saturation vapor pressure at that temperature, then there are bubbles generated in the liquid, while the gas dissolved in water will also precipitate out, at this time the separated vapor and gas bubbles, sent by the flow of liquid to the high-pressure area, where the rapid breakage (at this time the continuation of time T ≤ 0.001s), accompanied by excessive pressure rise so that this process has all the characteristics of the breakage This produces the impact phenomenon quickly spread in turn, so that the vapor bubbles immediately adjacent to the eroded surface, as well as in the pores (micro-pores, fractures, cracks, etc.) are broken so that the lower strength of the place will produce cracks. The bombardment of the surface caused by bubble breakage causes vibration and even damage to the material layer immediately adjacent to the cavitation zone
The suction range is the necessary cavitation margin Δh: that is, the vacuum degree of the pump allowed to suck the liquid, which is also the installation height allowed by the pump, in meters. Suction range = standard atmospheric pressure (10.33m) - NPSH margin - safety amount (0.5m) The standard atmospheric pressure can press the vacuum height of the pipeline 10.33m.
When the pump is working, the liquid at the inlet of the impeller will produce vapor under certain vacuum pressure, the vaporized bubbles will be under the impact movement of the liquid masses, which will exfoliate the impeller and other metal surfaces, thus destroying the impeller and other metals, the vacuum pressure is called vaporization pressure, the vapor corrosion margin is the surplus energy per unit weight of liquid at the suction inlet of the pump over the vaporization pressure, the unit is marked with meters, with (NPSH)r.
NPSHa: available net positive suction head, the larger the less likely to cavitate
NPSHc: critical net positive suction head, which is the NPSH corresponding to a certain value of pump performance reduction.
NPSHr: pump NPSH, also known as the required NPSH or pump inlet dynamic pressure drop, the smaller the better anti-cavitation performance
[NPSH]: allowable NPSH, is to determine the NPSH for pump use conditions, usually take [NPSH] = (1.1 ~ 1.5) NPSHc.
NPSHr is the characteristics of the pump, determined by the design, and NPSHa is determined by the process of piping
NPSHr and the internal flow of the pump are determined by the head of the pump itself, its physical meaning is to indicate the degree of pressure drop in the inlet port of the pump, that is, in order to ensure that the pump does not occur cavitation, the requirements of the pump inlet unit weight of liquid has more than the surplus energy of the vaporization pressure head. and flow rate is determined by the geometric parameters. This means that the NPSHr is determined by the pump itself (geometric parameters of the suction chamber and impeller inlet section). For the established pump, no matter what kind of media (viscous media due to the impact of the velocity distribution, except), in a certain speed and flow rate through the pump inlet, because the velocity size is the same, so there is the same pressure drop, that is, the same NPSHr. The smaller the NPSHr, the smaller the pressure drop, the smaller the NPSHa required to be provided by the device, and thus the better the pump's resistance to cavitation. Therefore:r represents the required necessary, determined by the pump body, specifically related to the speed, impeller form, etc.
v0, absolute velocity before blade inlet
w0, relative velocity before blade inlet
σ, pressure drop coefficient of blade inlet
g, acceleration of gravity
NPSHa is the NPSH determined by the pump installation conditions, is provided by the suction device at the pump inlet unit weight of liquid has more than the surplus energy of the vaporization pressure head. the larger the NPSHa, the pump is less likely to cavitation. the size of NPSHa and device parameters and liquid properties related. Because the hydraulic loss of the suction device is proportional to the square of the flow rate, NPSHa decreases with the increase of the flow rate. Therefore: A stands for available effective and available, this is determined by the system and piping and must be rigorously calculated.
Pc, absolute inlet pool liquid level pressure
Pv, liquid saturation vapor pressure
hg, pump installation height, the maximum value is the suction range of the pump.
hc, inlet pipeline head loss
ρ, the capacitive weight of water
g, acceleration of gravity
To ensure that the pump does not cavitate, NPSHa must be greater than NPSHr. How much larger, a variety of different forms of pumps have empirical values, generally the pump must increase the cavitation margin of 0.5-1m surplus energy head as the allowable NPSHa.
As the pump cavitation, the bubble in the high-pressure area continuously sudden ruptures, as well as the accompanying strong water shock, and the generation of noise and vibration, you can hear the crackling sound like popping beans. The initial nature of cavitation can be detected according to the noise, but it is difficult to distinguish this cavitation noise from the surrounding noise and machine noise in the surrounding environment, so it is very difficult to determine its length quantitatively.
When cavitation occurs, the long-term effect of mechanical erosion and chemical corrosion will make the inlet and outlet of the first impeller, the inlet of the guide vane or the snail casing become rough and porous, producing microscopic cracks, and in serious cases, honeycomb erosion and even the formation of cavities, thus shortening the service life of the pump
The energy exchange in the impeller is disturbed and destroyed when pump cavitation occurs, which is manifested by Q-H curve, Q-P, Q-η curve drop in external characteristics, which will seriously interrupt the liquid flow in the pump and make it unable to work.
For low speed, due to the narrow and long flow channel between the vanes, once cavitation occurs, bubbles fill the entire flow channel, the performance curve will suddenly drop. For medium to high speed, the flow channel is short and wide, so the bubble from the development to fill the entire flow channel needs a transition process, the corresponding performance curve is slowly declining at the beginning, and then increased to a certain flow rate before a sharp decline.
Improve the structural design of the pump from the suction port to the vicinity of the impeller. Increase the overflow area; increase the radius of curvature of the impeller cover inlet section to reduce the rapid acceleration and decompression of liquid flow; appropriately reduce the thickness of the blade inlet and round the blade inlet to make it close to streamline, which can also reduce the acceleration and decompression of the winding blade head; improve the surface finish of the impeller and blade inlet section to reduce drag loss; extend the blade inlet side to the impeller inlet to make the liquid flow accept work in advance. Increase the pressure
Adopt the front induction wheel to make the liquid flow work in advance in the front induction wheel to improve the liquid flow pressure
The use of double suction impeller, so that the liquid flow from both sides of the impeller into the impeller at the same time, the inlet cross-section doubled, the inlet flow rate can be reduced twice
Design conditions using a slightly larger positive impulse angle, in order to increase the blade inlet angle, reduce the bending of the blade inlet, reduce blade blockage, in order to increase the inlet area; improve the working conditions under the large flow rate, in order to reduce the flow loss. But the positive impulse angle should not be too large or affect the efficiency.
The use of cavitation-resistant materials. Practice shows that the higher the strength, hardness, and toughness of the material, the better the chemical stability, the stronger the performance of cavitation resistance
Increase the pressure of the liquid level in the storage tank before the pump
Reduce the installation height of the pump of the suction-up device.
Change the upper suction device to a backflow device
Reduce the flow loss on the pipeline before the pump. Such as in the required range as short as possible to shorten the pipeline, reduce the flow rate in the pipeline, reduce the bend and valve, try to increase the valve opening, etc.
Reduce the pump inlet medium temperature (when the transported medium is close to the saturation temperature)