What is Pump Cavitation?
When the pressure of the suction line drops such that it is lesser than the vapor pressure of the fluid flowing inside the suction line, that causes to form vapor bubbles, which in turn collapses at the eye of the impeller results into cavitation.
(Besides them, another person was drinking a soda water from a bottle. He saw him and added)
To give you a simple example, the gas in the container is pressurized, and hence, nothing happens until it remains closed, but the moment you open the cap of a bottle what happens?
The bottle is suddenly open to the atmospheric pressure consequentially you observe that the entrained carbon dioxide starts releasing, this is because the atmospheric pressure is less than that of vapor pressure.
Why does the pump cavitation happen in the first place?
If the pressure in the suction line of the pump is less than the vapor pressure of the fluid, it results in the formation of bubbles and collapsing of those bubbles at the ‘eye of the impeller’ and that is termed as cavitation.
The pressure in the suction line can drop due to serval reasons.
● The suction line has too many turns and bends (avoid that, talk to layout and piping engineers)
● Plugged strainer or filter in the suction line (ensure the line is clean, maintenance is necessary)
● Rusted Pipe results in friction losses (keep an eye on those and change if necessary)
● Low level in the suction vessel (make sure that it operates at operating set point or close to it)
● Damaged fittings over a period and so forth
What about NPSH?
Net Positive Suction Head? I will explain it to you in a minute.
Now, listen to me, this is important, the units of NPSH are the meter of liquid head.
There are two terms one is called NPSH R and the other NPSH A.
R stands for required, it is given by the manufacturer and is determined based on the experimental data.
A is for available and is calculated and cross verified by smart engineers like you.
But why do we need this data? For obvious reasons as to avoid the cavitation by making sure that at a given flow rate with a particular impeller dia size, yourNPSH A is greater than the NPSH R.
● For every flow capacity, there’s a different NPSH R, and it increases with a square of a capacity.
● Pump manufacturers make sure that for each flow capacity, there’s anNPSH, which is found experimentally.
● Flow capacity is an independent variable when plotted flow, head, efficiency, and NPSH. This NPSH that is found is called NPSH R.
● The pump manufacturer set up a system where the fluid, water, is passed through the system.
● The flow rate and speed are kept constant while the pressure of the suction line is reduced (as shown in the video above).
● As the pressure is keep reducing there will be a point when the pressure in the suction line reduces to vapor pressure and ultimately lower than that of vapor pressure. This pressure, meter of liquid head, is called NPSH R.
1. As the flow capacity is increased, head reduces, and the NPSH R increases.
2. There are other factors also like, impeller size, at a given flow rate theNPSH R increases with the decrease in the dia of an impeller.
3. The curve that is developed is somewhat in a “U” shape; that implies that it’s minimum at the mid-range capacity flow.
The tank was at 6 m above the pump suction nozzle. The pressure inside the tank was 10 psi g, and the liquid in the tank was at bubble point.
Specific gravity 0.58 ; Density of the fluid = 1000 kg/m3 x 0.58 = 580 kg/m3,
implies P = ρgh = 5 psi g and hence, the pressure measured at the suction line was 15 psi g. The total pressure of (15 psi g + 14.7 psi a) 29.7 psi a.
The vapor pressure of the fluid = The total pressure in absolute = 24.7 psi a (10 psi g + 14.7 psi a)
Now let’s calculate the NPSH available (NPSH A) =
Total pressure at the suction – Vapor pressure of the liquid – Pressure drop across the suction line
If for the time being you neglect the pressure drop across the line and fittings the resultant would be:
29.7 – 24.7 = 5.0 psi a
So far you haven’t minus the pressure drop that is inside the line and fittings.
If you convert this 5 psi a, into meter liquid head it would be near about 6 m.
Additionally, you haven’t deducted the pressure drop across the suction line, but if you do it will result in NPSH A as 5 m or lesser than that.
Okay, I see the problem, there, NPSH A was less than that of NPSH R.
Yes. But there’s something more. There’s a twist.
The problem was that calculated NPSH available (NPSH A) was 5 psi a without the pressure drop in the suction line. That means you need additional head to avoid cavitation.
But one thing that nobody told you about pump cavitation and perhaps no one will ever, Starting NPSH.
Starting NPSH?
Yes, the NPSH R that we are talking about is called running NPSH, something like running torque and starting NPSH as starting torque.
That means you need additional NPSH to start the pump than that of required for running pump. That means, today’s case,
● The NPSH R for 5 m3/hr was 6 m
● The NPSH A that we calculated without friction loss was 6 m
● Moreover, you need additional head, when you are starting the pump, called starting NPSH
So the actual value of NPSH A, which we just calculated, was not sufficient enough to the addition of
1. running NPSH,
2. the frictional loss in the line,
3. starting NPSH.
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