Cavitation question
- Thread starter amin36
- Start date
If it is true cavitation, not to be confused with a clogged impeller, clogged suction pipe. Clogged basket. Suction valve set wrong. Pipe too small for size or rpm of pump. Not enough straight pipe in front of pump, usually should be about 5X the I.D. of the pipe. It can also happen if there is restriction at the output of the pump or even downstream of the filter.
How Pool Pump Cavitation Sucks!
I'm afraid there's not going to be anything pretty about this one. At least not with the diagrams. I want to talk about Why Pool Pump Cavitation Sucks and
cpoclass.com
Last edited:
- Jun 22, 2014
- 52,030
- Pool Size
- 17888
- Surface
- Fiberglass
- Chlorine
- Salt Water Generator
- SWG Type
- CircuPool RJ-45 Plus
Here are some additional comments about cavitation (what it is and is not) from our hydraulics expert @mas985:
- Cavitation occurs when the water pressure gets so low, the water boils.
- Cavitation can be caused by a suction side blockage (also see items noted above in Post 2)
- Cavitation vapor bubbles collapse almost as soon as they are created so they do not make it out of the impeller.
- For residential pool pumps, cavitation only occurs in the impeller and you won't see it but you will hear it. It sounds like gravel is going through the pump.
- Air in the pump basket is not the same as cavitation
- Cavitation does not cause air bubbles to go out of the returns.
- Cavitation occurs when the water pressure gets so low, the water boils.
- Cavitation can be caused by a suction side blockage (also see items noted above in Post 2)
- Cavitation vapor bubbles collapse almost as soon as they are created so they do not make it out of the impeller.
- For residential pool pumps, cavitation only occurs in the impeller and you won't see it but you will hear it. It sounds like gravel is going through the pump.
- Air in the pump basket is not the same as cavitation
- Cavitation does not cause air bubbles to go out of the returns.
The air pressure at sea level is about 14.7 psi, which is about 34 feet.
When the impeller spins, the pressure in the suction line goes below 14.7 psi and the air pressure pushes water towards the impeller.
The pressure that is in the suction line is considered to be zero relative to the air pressure before the pump is on, but it is 14.7 psi or 34 feet absolute.
When the pump is on, the suction is considered to be negative relative to air pressure.
This is also called a vacuum, but the absolute pressure is always positive.
The remaining positive absolute pressure is called net positive suction head or NPSH.
Every pump has a specification for NPSH that is required to prevent cavitation.
To figure out the NPSH, calculate or measure the head loss and subtract it from 34 feet at sea level or the local air pressure at higher altitudes.
Make sure that the NPSH is always greater than the NPSH specified by the pump manufacturer.
For most pool pumps, use 17 feet as the NPSH unless you have the actual number.
If you keep the suction flow velocity below 6 ft/sec, you will usually be fine for pumps at ground level for plumbing under 100 feet in length.
For pumps above the pool, subtract the height.
For pumps over 100 ft away, use a head loss calculator or chart to calculate the head loss.
For existing systems, you can use a vacuum gauge to see what the vacuum pressure is to see if it is excessive.
Bill1974
Well-known member
- Jun 18, 2014
- 662
- Pool Size
- 32000
- Surface
- Vinyl
- Chlorine
- Salt Water Generator
- SWG Type
- CircuPool RJ-60 Plus
Cavitation can also occur when the flow is to fast for the piping/fittings. If the velocity of the liquid (water) through a fitting or pipe is to high it can cause areas where the water turns to a gas (steam, even at room temperatures) for a fraction of second and then back to a liquid. This change of states from liquid to gas occurs when the pressure in the fluid drops below the vapor pressure as it goes through fittings. The change from liquid to gas and back to liquid is extremely voilent and will erode the inside of pipes, valves, fittings and the backside of the blades/vanes of impellers. As liquid water changes to gaseous water (steam) it expands around 1600 times is size for the same mass of water. Then it changes back to liquid water and contracts 1600 times in size. The velocity of the molecules at the interface of the liquid and gas is incredible and as the fluid goes between phase changes on the surface of another material (inner wall of pipe or fitting) any tiny imperfection in the pipe or fittng has to endure the abbrasion from these phase changing water molecules. It's not just an issue on plastic pipes, it will equally erode steel as well.
Toxophilite
Silver Supporter
It can be caused by anything that sets up a disruption within flow and circulation, even vibrations can induce cavitation damage. Was a major issue back when I serviced wet liner diesel engines where cavitation would eventually eat holes through cylinder liners. There were additives to coolant to prevent such, and was almost mandatory to use. I don't thinks such applies to our colloquial use of a pump cavitating.
Thanks @JamesW it was a great explain but how does the suction flow velocity below 6 ft/sec help with avoiding cavitation?
For a given pipe size, the higher velocity requires a lower pressure at the pump.
For a 2" PVC pipe at 60 gpm, the velocity is 5.9 feet per second and the head loss is 5.8 feet (per 100 feet of pipe).
For a 2" PVC pipe at 90 gpm, the velocity is 8.8 feet per second and the head loss is 12.4 feet (per 100 feet of pipe).
For a 2" PVC pipe at 90 gpm, the velocity is 8.8 feet per second and the head loss is 12.4 feet (per 100 feet of pipe).
