PDA

View Full Version : cavitation



karm0909
05-28-2011, 07:52 AM
I have an artesian well approximately 505 ft deep. It has 15 to 18 positive psi on the pump. The pump is a 1hp hne sta-rite with a 30/50 pressure switch with a 1 1/4" check valve at the intake. The tank pressure is 28 psi. At start up I am experience 5 to 6 seconds of cavitation sometimes longer. I have tried adjusting the tank pressure and I have checked the check valve, but I have had no success. Any suggestions?
Thanks Mark

LLigetfa
05-28-2011, 07:59 AM
A frequent mistake made is to assume the air pressure gauge used to measure the bladder pressure reads the same as the water pressure gauge used to set the switch limits. Two PSI differential leaves a lot of room for error. If the bladder tank completely empties when the pump starts, the pump lacks the pressure it needs to make pressure.

P.S. I'm not going to get into the pump does not make pressure debate.

hj
05-28-2011, 08:05 AM
Cavitation is usually caused by inadequate flow TO the impeller when the pump is running, which should NOT be a factor with a submerged pump, and definitely not with an artesian well. If the pump is above ground, then you have a problem with the intake pipe, NOT the tank's pressure. The tank pressure just determines the MINIMUM pressure you will have in the system before you run out of water, and how much water you can use before the pump starts. What "the pump does not make pressure debate. "? Of course a pump creates pressure, if you do not believe it, put a pressure gauge on the outlet and turn the pump on.

LLigetfa
05-28-2011, 08:29 AM
The OP's pump is an above ground shallow well jet pump. An easy way to test the minumum pressure start issue is to pull the cover on the pressure switch and manually start the pump at a slightly higher pressure to see if it still cavitates. A symptom of having too much precharge is that the pressure drops momentarily when the pump starts. As I said, a jet pump uses pressure to make pressure and it should not be let to start often at too low a pressure.

The check valve could also be the culprit.

I wonder why the OP lets the pressure drop to 30 PSI. Personally, I would prefer 40-60 as a working range if not for my micronizer.

LLigetfa
05-28-2011, 08:35 AM
I have checked the check valve...

How did you check it? Is it a swinging gate or spring loaded? Is it at the well or at the pump?

Also, how does your well give you a positive pressure at the pump? Is the pump down hill from the well or is the wellhead sealed to build head?

ballvalve
05-28-2011, 02:00 PM
http://www.surpluscenter.com/item.asp?item=20-1152&catname=water

had several of these working for 10 years without a hiccup. top quality interior, way better than any brass well type.

I hate to reveal this incredible valve - its 60 bucks everywhere else. My 12 stage 1.5 hp booster has inlet psi of 8 psi and no signs of cavitation.

The only way to tell us you have really have cavitation is to dissasemble the pump and inspect the impeller surfaces.

jimbo
05-28-2011, 02:49 PM
Well, if you use words that jog the old memory bank, you can expect to hear about it!!!!!

Cavitation is a bad word on a nuclear submarine....because if the propellor cavitates, the bad guys can hear that! It is a function of depth...below a certain depth, the bubbles just wont form, also rpm, and also changing RPM. When increasing speed, the throttleman must do so gradually. If he cavitates the prop, he can expect to be smacked upside the head!!! And in a casualty or other situation which require SPEED>>>NOW....the order is "ahead flank..cavitate" meaning get the bleeping turns on because we are beyond worrying whether someone hears us.

Sorry, just couldn't resist, for old time sake.

karm0909
05-31-2011, 05:12 AM
Thanks for the info. I installed a 40/60 switch increased the tank pressure to 38 and that fixed it. Again thanks a lot.
Mark

LLigetfa
05-31-2011, 09:03 AM
Good to hear. Enjoy your water pressure.

DonL
05-31-2011, 09:12 AM
I Love when a plan comes together !!!

Yersmay
06-01-2011, 07:22 AM
In my limited study of how to install an ejector pump, I came across the subject of cavitation. It seems to be a function of not enough 'suction head' or, in other words, not enough water flowing into the pump's intake. That all makes sense. But HJ said that 'usually' cavitation is not a problem with a submerged pump. But then an ex-nuclear submarine crew member said that, indeed, cavitation is a problem with submarines. How could a submerged submarine not have enough 'suction head'? Or any other submerged pump? How would cavitation happen even with a submerged pump? Thanks in advance!

jadnashua
06-01-2011, 03:35 PM
When an impeller (propeller) turns faster than the fluid can actually accellerate or flow, there will be low pressure areas which can actually cause the liquid to boil in the vacuum (cavitation). It's all a function of start up speed and the ability of the material to flow smoothly. Just like a powerful car can break traction with the road, an impeller can try to move faster than the liquid it is trying to move. The state change of the material can release a lot of energy and cause some significant damage under just the right conditions.

jimbo
06-01-2011, 05:47 PM
What jad said.

On the boat, the depth ( pressure of the water ) was a facor in whether or not at a certain rpm you would cavitate.

ballvalve
06-01-2011, 11:28 PM
Read this and become the king of cavitation. Notice that trees and dolphins suffer from cavitation [!]

http://en.wikipedia.org/wiki/Cavitation

jimbo
06-02-2011, 06:11 AM
last year I had cavitation in a back molar,,,,is that the same thing??