Q(m3/h) 6 20 60 100 150 200 300 >300 C refers to (n=2900) 400~450 550~600 750~800 900~1000 1000~1100 1100~1200 1200~1300 1250~1350 C value (n=1450) / / / 550~600 650~700 700~750 750~850 850~1000 Specific pump speed ns 50~70 71~80 81~150 151~200 C value 600~750 800 800~1000 1000~1200 Single Screwdriver,Key Tool Screwdriver,Single D Screwdriver,Plastic Handle Screwdriver henan horn tools co.,ltd. , https://www.horntoolsltd.com
To understand the phenomenon of cavitation, first of all, talk about water changes in our daily life.
Normally, when we heated the water from 20°C to 100°C under an atmospheric pressure, there was a large amount of air bubbles overflowing from the water to form a boiling phenomenon. If the pressure is reduced to 0.024 atmospheres at 20°C, the water can also boil. Therefore, water and steam can be transformed into each other. The conditions for conversion are temperature and pressure. Not only water but other liquids also have such properties.
At a certain temperature, the critical pressure at which the liquid begins to vaporize is called the vaporization pressure of the liquid, expressed as PV.
After knowing the physical properties of the liquid itself, we will analyze the cause of the cavitation in the pump.
In general, the impeller inlet of a centrifugal pump is the place where the pressure is lowest. If the pressure of the liquid at this location is equal to or lower than the vaporization pressure Pv of the liquid at this temperature, there will be a large amount of vapor and gas dissolved in the liquid overflowing from the liquid, forming many small bubbles of vapor mixed with the gas. When these small air bubbles flow with the liquid to the high pressure region, the pressure around the air bubbles is higher than the pressure inside the air bubbles, and the air bubbles are broken by pressure (condensation). The liquid particles, like numerous small zi bullets, hit the metal surface continuously, causing the metal surface to be destroyed. This is the cavitation of the pump. When the centrifugal pump is operated under severe cavitation conditions, the site where cavitation occurs will quickly become honeycomb or spongy.
When the centrifugal pump starts cavitation, the cavitation area is small, and there is no obvious influence on the normal operation of the pump. However, when cavitation develops to a certain extent, a large number of cavitation bubbles are generated, affecting the normal flow of the liquid and even causing the flow to break. At the same time with noise, vibration, and the pump's flow, head, efficiency have significantly reduced. Therefore, cavitation should be avoided as much as possible.
Ways to prevent cavitation:
Use side: 1. Reduce the installation height of the pump; 2. Reduce the suction pipeline resistance; 3. Reduce the temperature of the transport liquid to reduce the vaporization pressure; 4. To avoid the use of valve throttling on the inlet line;
Manufacturer: 1. Increase the degree of cleanliness of the pump flow path; 2. Increase the diameter of the impeller inlet; 3. Reduce the pump speed to reduce the internal pressure of the pump; 4. Add inducer at the pump inlet.
Second, the concept of NPSH pumps <br> <br> why pumping Sheung Shui? Because when the impeller rotates to throw water out, the liquid pressure at the inlet of the impeller drops, and the liquid in the reservoir enters the pump through the suction pipe under the action of atmospheric pressure (open tank).
Pumping with a centrifugal pump, assuming that the pump inlet is absolute vacuum, the pipeline resistance is zero, and the liquid surface pressure is a standard atmospheric pressure, then the water should be able to rise 10.33 meters along the suction pipeline. However, it is impossible for the pump inlet to reach absolute vacuum. When the pressure at the suction inlet of the impeller approaches the vaporization pressure of water, cavitation will occur and the water will not be sucked. Furthermore, the suction pipeline has a certain resistance, so the pump is not It may pump 10.33 meters.
In order not to cause cavitation in the pump, the cavitation performance of the pump must be characterized by the amount of cavitation that must be used.
1, must vacancy amount - NPSHR
The scavenging margin must be the excess energy that the pump must have to produce cavitation at work, expressed in NPSHR in meters. Internationally known as the net positive suction head, the smaller the NPSH margin, the better the anti-cavitation performance of the pump.
The NPSH must be determined experimentally. For any newly designed pump, the manufacturer must obtain a test NPSH through the cavitation test, and then add the 0.3 meter safety margin to obtain the cavitation must be cavitation. margin. Of course, in the new design, some empirical formulas can be used to estimate, and after the completion of manufacturing, the test finger can be used for calibration.
Estimation of necessary NPSH - NPSHR(m)
(1)
Where: n - the pump shaft speed Q - liquid flow rate at the delivery temperature, m3 / s (double suction pump 1/2Q)
C——Cavitation ratio (approximate according to Table 13-2)
Centrifugal pump cavitation than dietary C value
(2) NPSHR=10.33-hs
Where: hs - suction vacuum height (m)
2, the device cavitation margin - NPSHA (m)
Suction device:
Inverting device:
Where: p1 - suction surface pressure (pa);
HV1 - suction pipe resistance loss (m);
H—installation height of the pump centerline to the liquid level (m);
Pv - Vaporization pressure of the liquid at this temperature (Pa)
Cavitation-free conditions NPSHA>NPSHRNPSHA-NPSHR>S (S: safety margin 0.6-1 m).
A, a centrifugal pump cavitation in the pressure in the <br> <br> single centrifugal pumping liquid at lower than the saturation vapor pressure at this temperature, the liquid will have bubbles, cavitation occurs.