hot days cause water feedline to boil before pump?!

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1

I want to know if I am thinking of a feasible solution to my problem...I have been able to build a solar charger to keep a waterpump filling our 5000 gal house supply tank. We are having one of our worst draughts in memory.

Using good advice here and some similar projects found online my simple system design seems to work EXCEPT on hot sunny days (when we need it most of course).

I watched it yesterday almost continuously from about 10 am when the temperature was still moderate...it ran fine until about 11 ish. Then the pump would sputter air. There are no air leaks in the supply line. BUT that line is some 600 meters long and 20 mm (3/4 in) dia. Water will actually dribble to the pump by syphon action but it is too slow to be useful.

If there is no air leaking into the line AND it only happens when hot is the reduced pressure (this pump has a flow rate of about 200 ml per minute and can lift water to fill and empty line ... i'm guess but say about 2 meters.) so is it possible/likely that the hot water is boiling under reduced pressure?

Although the pump can lift water when it hits this problem all flow stops. Maybe water vapor is continuously boiling off and that vapor is all the pump gets a bite of?

OK if youre still with me here is my question: Is there a way to sense when water in the line is boiling or preferably just about to boil? My thought is to use a sensor to control the pump speed (24 - 90 VDC, ~0.4 amp).

If i don't do something I believe i will destroy the pump, it is an FMI lab pump that I've used for ages in my little home lab...now it is keeping us wet. a vid here shows the clever no-checkvalve piston action if interested: http://fluidmetering.com/

thanks in advance

3

Draw a pic of the system. Where is the pump in the system. A pump "sucking" can only do 30ft/10m at STP, after that you get a gas bubble (well really a vacuum but ..) your pump needs to be as far down (near the source) as possible so that it does as much pushing of the water and as little sucking as poassible.

Mark

4

Nobody here can tell, but can't you put your finger in the outlet and see how hot the. water is? Then make plans. While consistent boiling is hardly possible, I have seen seriously hot water coming out of black rural pipe, which is what you might be using, and there is a lot of it.

5

3/4" poly will get hot on the sun, it will also get soft due to the thin wall. It is possible that the pipe is getting soft and allowing air to enter.

Try protecting the pipe for a few meters and clamp the pipe well.

Weedpharma

6

what you are saying is true, it is called cavatation

sounds like you have your pump close to the discharge end, can you move the pump closer to the source or somewhere in the middle?

if you can make a relation to the outside temp when the pump starts to stop pumping, you could use that

also you could add a check valve just up stream of the pump and use a pressure sensor to test when the pressure starts to go

you dont want the pump to loose prime

7

Greensprings:
what you are saying is true, it is called cavatation

sounds like you have your pump close to the discharge end, can you move the pump closer to the source or somewhere in the middle?

if you can make a relation to the outside temp when the pump starts to stop pumping, you could use that

also you could add a check valve just up stream of the pump and use a pressure sensor to test when the pressure starts to go

you dont want the pump to loose prime

Thank you Greensprings. I have a mechanical gauge on the output side...it is not very accurate but there is a clearly detectable decrease in pressure when flow stops...brilliant. BTW/ I am forced to draw water over this long distance because, although the owner was kind enough to give me access to his spring water, he doesnt want a solar panel and pumphouse in his paddock which is entirely understandable.

You might have a solution! if I can get the data I could model the function.

Then I could start collecting flux data and get an idea of when to reduce the pump speed to avoid sucking fumes.

Do you know of such a sensor I could put in line? I have a 1/2" NPT "TEE" for the mechanical gauge. ideally I would look for a similar fitting sensor that could work with a 5 volt input from the v reg that supplies my micro.

much obliged, that sounds like the simple answer to me!

8

not sure if it is actually cavitation (phase change caused by low pressure) or air getting into the line.
regardless, the pump is not functioning within desired parameters. but since it happens with the water is hotter, less vicious , it seems a legitimate possibility.

for cavitation to occur, your impeller has to be shaped such that it can allow the bubbles to form. one possibility is to get a different impeller, possibly a different type of pump.

the rotating piston pump is not something I am familiar with. centrifugal pumps will destroy themselves as the formation of the bubbles is explosive and will tear apart the metal. the amount of energy released at a phase change is quite astounding.

any centrifugal pump will have such a problem if it looses prime. or if air enters the water at some point.

is the feed purely suction ? or is gravity part of the process ?

one of the simpler ways might be to put the pump lower so that there is a reservoir above the inlet.
then monitor the reservoir level. if it drops below some set level, you are running low on water and need to either slow the pump, stop the pump or possibly have some of the discharge flow back into the inlet reservoir.

you could actually have a float that blocks the water from recirculating from the discharge side, and when the float drops, it could open the valve. just look inside of your toilet.

if it is cavitation, then a pressure sensor near the pump inlet could monitor the pressure and you could slow the pump when it gets into the realm of where your experience the phenomena.

as you already know both pressure and temperature seem to be part of the equation.

9

look for absolute pressure gauges/sensors

keeping the pump operating in the most efficient part of the pump curve will allow it to run maintenance free for a long time.

I would check the application of your pump with the manufacturer. they might offer some other solutions.

10

Bury the pipe six inches down to keep it cool.

11

Hi,

Lot of good comments. I'm in the camp of air at the impeller (it's a centrifugal pump, correct?). The time-of -day correlation (heating the water) supports this. If true, then the fix is a 'blow-off' valve that you open while pump is running and 'air-locked'. Need to locate the valve as near as possible to impeller and at the highest point (where the air is). Manually (for now) open the valve, air will sputter out, then close as it becomes a water stream.

In a more drastic case you could partially close (force) a downstream check valve, simultaneously.

Best

12

Hi,
can you post the specs or model of your pump please.
I think most pumps are designed to PUMP not SUCK.

holmes4:
Draw a pic of the system. Where is the pump in the system. A pump "sucking" can only do 30ft/10m at STP, after that you get a gas bubble (well really a vacuum but ..) your pump needs to be as far down (near the source) as possible so that it does as much pushing of the water and as little sucking as poassible.

Mark

Also what colour is your pipe and is it under ground?
If the pump can pump cool water under the present conditions, then bury the pipe in the cool earth, at least 40cm.
Tom...... :slight_smile:

www.handymath.com/cgi-bin/cylinder.cgi?submit=Entry

This site tells me that the pipe 20mm diam and 600m long has 188.5 litre or 188kg of water in it.

13

Procyan:
Do you know of such a sensor I could put in line?

The first thing you would recognize with a pump going into cavatation conditions is the increased sound level.

While pumping at cavatation conditions a pump gets very loud compared to normal pumping conditions.

This is caused by imploding of the generated bubbles, see picture.

This creates a loud sound, so maybe a microphone/sound sensor will be able to detect the increased sound level when cavitation occurs.

Be careful: Cavitation will also be able to damage the impeller of your pump within a few hours, pump impellers are usually NOT SAFE for operation in cavitation conditions and expensive repairs might follow.

The second thing you would recognize is a reduced flow rate. So also a flow rate sensor may help detection of cavitation: If the rotation speed stays the same but flow rate decreases, cavitation occurs.

The main reason for cavitations is the pressure ratio at the water sucking side of the pump: The lower the pressure falls, the higher the risk for cavitation. Also it depends on the construction of the pump (impeller pump vs. plunger pump). The higher the pressure at the water sucking side, the better.

So the risk of cavitation becomes smaller with:

  • lower flow rate
  • bigger hose diameter
  • shorter distance on the water sucking side, longer distance on the pressing side
  • different pump construction
  • using two or more distributed pumps (one pump at half pumping distance)

A good sensor to avoid cavitation may be a pressure sensor at the water sucking side of the pump.

In the datasheet of your impeller pump you should find the NPSHR value (Net Positive Suction Head Required).

The NPSHR value of the pump will be the minimum pressure value at the sucking side, guaranteed by the manufacturer of the pump, for operating the pump with defined cavitation conditions. If the pressure falls below on the sucking side, cavitation may occur. Maybe you can find a datasheet for your pump with an NPSHR value.

Edit:
When reading your posting a second time and read about "water will actually dribble to the pump by syphon action but it is too slow to be useful" as well as about the very low flow rate "this pump has a flow rate of about 200 ml per minute", then I doubt, that cavitation is your problem.

Instead I think the warming water is the problem, because gases such like oxygen will not stay soluted in the water, but will outgas from the water to be actual gas.

Just look up about solution of oxygen in water, dependent from temperature:
5°C ==> 12,7 mg/l
15°C ==> 10,1 mg/l
25°C ==> 8,2 mg/l
40°C ==> 6,4 mg/l

So if oxygen rich water is warmed from 15°C to 40°C, there will be (10.1-6.4) mg/l Oxygen that cannot stay soluted in water. If your pipe contains 188 litres of water and you will get 188*3.7 mg oxygen, this is 0,695 grams of oxygen. Oxygen weights something like 1400 grams per 1000 litres, so 0,695 grams of oxygen = ca. 0.5 litres. As gas is lighter than water, the gas will collect itself at the highest point of the pipe. And your pump seems a bit too weak to suck the 0.5 litre gas buble through the pipe easily, so the flow stops.

So perhaps the best idea would be: Keep the pipe cool, prevent the water from warming and developing a gas bubble that stops the flow.

14

have a look at automationdirect.com

also, burrying the pipe in the ground or some other cover was also a good idea

also replacing the supply pipe with a larger size would work too

moving the pump closer to the supply would work