pump cavitation and net positive suction head
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- Опубликовано: 5 мар 2021
- If you are dealing with a centrifugal pump. You have heard of net positive suction head and cavitation. This video will explain what is npsh. Why Net positive suction head is so important for pumps and how to determine the net positive suction head is also explained in this video.
As per hydraulic institute standard, npsh is the liquid energy, above the vapor pressure, at the pump inlet. Each pump requires some minimum absolute pressure at the pump inlet to avoid cavitation in the pump.
This minimum absolute pressure or absolute suction head in terms of the height of the liquid column, at the pump inlet, is known as net positive suction head. if the suction head at pump inlet drops below vapor pressure. The fluid will evaporate and generates small vapor bubbles. These bubbles carried along with the fluid, and collapse instantly when they get into areas of higher pressure. This will result in excessive noise and vibration, due to the collapse of vapor bubbles. it will damage the pump impeller and pump casing due to erosion. This whole phenomenon is known as cavitation. This minimum absolute suction head, at the pump inlet, which is above the vapor pressure represents the available net positive suction head.
To avoid cavitation, this absolute head at the pump inlet must be greater than vapor pressure. So if we subtract vapor pressure from this absolute inlet head. It must be always a positive term. And known as Net Positive Suction Head Available.
Till now, we have understood what is net positive suction head available. But how much net positive suction head required for a pump is calculated by testing a pump.
The most common method. Often known as the constant flow method is used to determine the npshr. In this method, the pump discharge valve is kept at the fixed open position. so that pump flow remains constant. However, the pump suction valve is throttled. Due to throttling of the suction valve, pump inlet pressure reduces. But, the pump developed head, and flow remains constant. Until a point where inlet pressure reduces to vapor pressure.
After each throttling of the suction valve, the Pump manufacturer notes down the Total head developed by the pump, and Net Positive Suction Head Available. The Pump manufacturer repeats this step, Until a point where the total developed head decreases by at least 3 percent. The net positive suction head corresponding to the 3% drop in pump developed head is known as NPSH3 or net positive suction head required.
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npsh margin 6:15 to 6:35
introduction 0:02 to 0:34
npsh definition 0:34 to 1:38
what is cavitation 1:40 to 2:15
pump developed head 1:02 to 1:39
how to calculate net positive suction head available 2:29 to 4:33
how to calculate net positive suction head required 4:39 to 6:12
NPSH3 4:39 to 6:12
how to perform npsh test 4:46 to 6:10
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best video. i seen on npsh explanation .
Best Video on NPSH
The best explanation..........
Crystal clear concept clarification about NPSH. Excellent👏👏👏 video.
Make video on Capitation
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i am thaking you so much , the first time i understand what the NPSH is !
a very valuable and proffesional explanation ,
what can I say...complete presentation for beginners
Best explanation. Thanks
Excellent, nailed it!
Very creative ..thanks a lot
Thank you sir
Superb explanation 👌
Good explanation, but boiling temperature should be mentioned to complete the picture.
We know water boils at 100degC in our kitchens because we are typically at standard sea level atmospheric pressure and saturation temperature.is 100degC.
However at pump inlet, we have a vacuum and pressures could be much.lower than the standard sea level atmos pressure and in some regions the absolute pressure is.lower than the vapour pressure threshold, this.means its boiling temperature is near to the room temperature and the water starts boil and then cavotation.process continues.
6:43 Once you close the valve at suction pipe, the head of the pump does no remains constant. By closing the valve you are increasing the piping resistance and therefore the behavior expect for the pump is to reduce the flow rate and increase its head. One of the methods to measure NPSH required uses a closed suction tank.
Grt explanation
thanks
In the section on measuring NPSHR using a constant flow test (constant Q) you state @4:56 that outlet valve is kept at a "fixed open position" to maintain constant flow. However, if you throttle down the suction valve to increase flow resistance and decrease NPSHR then you have to adjust the outlet valve to maintain constant flow.
Super
Nice
hi, why frictional loses increase when we throttle? great vid by the way thx.
👍
👏🏻
Thnk you
You’re welcome 😊
what happen if the NPSHa is 3,6 bar but NPSHr 4,6 bar?
known vapor pressure 3,1 bar
Kya baat hai?
The velocity is a decrease in pressure
Where are you from
Ķ
Great presentation, horrible ai voice.
thanks