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Bubble & Pump Impeller

10/27/2012 1:12 PM

Question:

1. Can the inflow of air bubbles through suction pipe cause any

damages at pump impeller? Or the bubbles can be disappeared before arriving

at the pump impller?

2. Please advise what we should consider to prevent any

problems might be caused by bubbles.

* The elevation of suction pipe & pump suctions is the same.

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#1

Re: Bubble & Pump Impeller

10/27/2012 1:37 PM

I think cavitation is worse if the bubbles are vapor of the liquid being pumped. Air bubbles could still be a concern, but not as severe.

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#2

Re: Bubble & Pump Impeller

10/27/2012 1:59 PM

1. If the air bubble gets big it can reduce pump performance & cause damage due to effects of cavitation. 2. Mariners use specially designed pumps, to overcome this problem. In your case, can the pump suction level be below the diffusers?

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#3
In reply to #2

Re: Bubble & Pump Impeller

10/27/2012 2:48 PM

As shown on the picture above, suction pipe & diffusers were already installed. So the pump suction level can not be changed.

Which conditions are the bubbles are disappeared before arriving at the pump suction?

Thanks

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#6
In reply to #3

Re: Bubble & Pump Impeller

10/27/2012 3:16 PM

If you are prepared for some experiments, one you can explore is that of insertion of a properly sized vacuum chamber, with a suitable vacuum pump, between the suction line & the pump.

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#8
In reply to #3

Re: Bubble & Pump Impeller

10/27/2012 4:15 PM

Try this simple modification. Make a semicircular baffle around the suction head to prevent ingress of bubbles into the suction pipe.

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#4

Re: Bubble & Pump Impeller

10/27/2012 3:10 PM

Part of the last chamber will need a divider so there's some clean water with no more air bubbling there.

You'll have to find a way to make some calm water for the intake.

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#5

Re: Bubble & Pump Impeller

10/27/2012 3:12 PM

1a. yes

1b. no

2. Reverse the pump and take the suction form the still chamber and return it to the chambers with the bubbles

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#7
In reply to #5

Re: Bubble & Pump Impeller

10/27/2012 3:18 PM

If doable, yours is easier to do. Not sure what's going on in that tank.

The bulkhead fitting looks lower on the clear chamber, too which won't hurt.

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#9

Re: Bubble & Pump Impeller

10/27/2012 4:20 PM

Bubbles would resit going down and may consolidate and impede the flow in the down pipe.

You could install an air trap and a air release valve at the bend going down. Replace the bend with a T and a short length of pipe, Or even discharge the air back into the tank.

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#10

Re: Bubble & Pump Impeller

10/27/2012 5:38 PM

Once the air content gets to a certain level it will have a severe impact on pump performance. You also have an increased risk of air-lock in the system.

If you have problems you could consider a pump type that is better for handling air-laden/mixed-phase fluids. For centrifs this includes triple-throat volute pumps, pumps with concentric casings and fully open impellers, and self-priming pumps.

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#11

Re: Bubble & Pump Impeller

10/27/2012 6:02 PM

If this is a sewage treatment plant, you are begging for trouble. Every tampon and male contraceptive is going to end up in your pump. Some always get by the screens. Why not just pump from the other tank and let gravity do the job? That is the usual system. A bottom return from the still tank will send anything that settles out back to the bubbler. That makes the still tank lower, and gravity will pull water, bubbles and all, from the bubbler tank. The bubbles will just go away in the still tank.

Why do you want to pump bubbled water to the still tank?

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#12
In reply to #11

Re: Bubble & Pump Impeller

10/27/2012 11:38 PM

The waste water contained in biological tank is coming from refinery plant and will be screened with strainer (mesh 40). So there is no tampon and male contraceptive.

Thanks

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#13

Re: Bubble & Pump Impeller

10/28/2012 12:01 AM

Yes, bubbles can destroy an impeller.Each bubble creates a microscopic explosion when it collapses.This is called cavitation erosion.

Put a "J" tube on suction side, facing up.Even if you have to elbow down first,like a trap on a sink drain.This is used in gaseous wells to prevent gas in water.

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#17
In reply to #13

Re: Bubble & Pump Impeller

10/28/2012 5:01 AM

Hi TekRedNek,

I agree with your reply regarding the damage to the impeller caused by air bubbles. When carrying out normal preventative maintenance on a centrifugal pump it is prudent to examine the impeller carefully. The damage to the impeller caused by air bubbles will appear as 'pitting' (little crater like damage) especially around the leading edge of the impeller. Left unresolved this damage will lead to the premature failure of the impeller.

As far as the OP's question about eliminating the bubbles from the pump suction line - why not just install another small, open-topped vessel in-line which would allow the bubbles to escape and not be drawn into the pump suction. The OP gives no indication as to the volume of water being pumped so this would have to be taken into consideration when deciding on the size of this tank and the location of the pump suction line to the tank.

Hope this assists!

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#19
In reply to #17

Re: Bubble & Pump Impeller

10/28/2012 10:22 AM

While we agree about an additional chamber to allow bubbles to dissipate, I'm not sure that pumping a fluid that has entrapped air bubbles in it will caused the same type of impeller damage that true cavitation will cause. Cavitation is caused by applying excess suction pressure at the pump inlet. This causes the liquid to "boil" creating pockets of gas that collapse as the pressure on the fluid goes from negative to positive inside the pump.

Here, the air is already in the fluid at atmospheric pressure.

Different animal, I think.

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#21
In reply to #13

Re: Bubble & Pump Impeller

10/28/2012 8:12 PM

Each bubble creates a microscopic explosion when it collapses.This is called cavitation erosion.

Actually, cavitation erosion is not caused by air bubbles. Nor is there an explosion. It is caused by the violent collapse (implosion) of vapor bubbles. These vapor bubbles form wherever the local pressure is low enough to cause boiling of the pumped fluid. These vapor bubbles then condense from a relatively large volume to an extremely small volume (the vapor:liquid volumes being about 1600:1 in water).

Air bubbles are quite different. As they move from lower to higher pressure regions, they change in size in inverse relation to the pressure difference. In in a system like the one diagrammed, the air bubble may only change in volume by a 2:1 or 3:1 factor, depending upon system pressures. They never collapse, but will generally coalesce into larger bubbles. Such air bubbles do not generally cause pitting (but can cause some vibration of equipment).

Aeration and cavitation are completely different phenomena, with much different effects on equipment.

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#14

Re: Bubble & Pump Impeller

10/28/2012 12:11 AM

Hendrik and RedNek have good recommendations for getting rid of the air bubbles before the pump. Find some way to place an air separator before the pump suction to get rid of the air bubbles before they hit the pump. Bell & Gossett (now part of Xylem) is one of several manufacturers that produce commercial versions of air separators.

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#15

Re: Bubble & Pump Impeller

10/28/2012 12:31 AM

Dear Mr. JBK,

1. Answer for your Question No.1 is Yes. It will cause damage to the impeller, especially when the temp. of liquid is slightly high with Negative Suction and affect performance also. Turbulent Approach of liquid (to be pumped) to the Suction is the main source of such problem

2. Answer for your Question No.2 is to solve the problem there should be a minimum level above the strainer to avoid SWIRL and ingress of air/bubble in to the pump, perhaps a minimum of 0.5 to 1.0 Metre of Liquid Level above foot valve will help to control/solve the problem.

Thanks,

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#16

Re: Bubble & Pump Impeller

10/28/2012 1:40 AM

Do you have to use a centrifugal pump or can you use another type?

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#18
In reply to #16

Re: Bubble & Pump Impeller

10/28/2012 6:28 AM

We use a centrifugal pump (BHP 34.6 kW, Capacity 7000 USGPM).

Thanks.

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#20
In reply to #18

Re: Bubble & Pump Impeller

10/28/2012 12:10 PM

so are you saying you CANNOT use another type of pump?

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#23
In reply to #20

Re: Bubble & Pump Impeller

10/29/2012 3:03 AM

So far we've not considered replacing by new another type pumps.

We look for the cheapest solution.

Thanks

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#25
In reply to #23

Re: Bubble & Pump Impeller

10/29/2012 7:19 AM

This comes as no surprise to me.

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#26
In reply to #23

Re: Bubble & Pump Impeller

10/29/2012 9:10 AM

Don't buy the cheapest solution. Understand your process and pick an optimal solution to improve the desired process result!

You have several decent options to consider for the stated issue of bubbles in the water, and certain concerns stated about your process flow. Start throwing these against the wall and see which ones stick. Bring in a consultant or talk with your process operators to see which ones make sense for your operation.

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#22

Re: Bubble & Pump Impeller

10/28/2012 10:18 PM

There seem to be some fundamental flaws in your waste treatment system and pumping bubbles is just one of them.

If the diagram is close to correct it implies that you are/have modified a linear reaction process (Intermittant Decant Aeration) into a continuous flow process and need the pumps to circulate the mix. Pumping from the aerated section puts your pumps at risk. The aerated fluid density is typically around 80% of water, so even the short path to your pump will enable very large bubbles to form, leading to vibration.

You also seem to be adding the fluid back into the bottom of the anoxic/anaerobic zone. The oxygen rich material arriving there will potentially poison your anaerobic cell. (And the constant flow of bubbles will stir the tank, effectively making it into an aerobic tank.)

At 7000 GPM, it's not a small system and the organisation SHOULD seek paid professional advice on how to construct and tune the process.

Alternatively, pumping from the bottom of the still tank will draw the sludge out and inject it into the aeration section for decomposition. This may have been the original intention with the pump installed incorrectly.

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#27
In reply to #22

Re: Bubble & Pump Impeller

10/29/2012 9:14 AM

Being from Aussie, are you sure you don't have the drawing up-side down?

Seriously though, you do raise a valid point on what the process requirement is, and as I noted earlier regarding process review and optimization. Thanks for the great comment!

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#28
In reply to #27

Re: Bubble & Pump Impeller

10/30/2012 7:40 PM

Regardless of where we are in the world, it still all flows "downhill".

Thanks for the compliment.

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#24

Re: Bubble & Pump Impeller

10/29/2012 4:39 AM

The easiest way of ensuring that bubbles are not drawn into the pump so as to risk damage, is to ensure that the pipe feeding the inlet to the pump is taken from a location in the tank where there are no bubbles present.

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