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Air in centrifugal water pump
3

Air in centrifugal water pump

Air in centrifugal water pump

(OP)
Hi all,

I have a bit of an issue with a potable water pump on my site. We have two pumps in parallel, one duty, one standby (A and B). When changing over from A to B the last 3 times, the discharge pressure has dropped to zero. I have opened the casing vent on B pump, and there has been a lot of air in the pump casing. After venting out all the air and priming the pump again, it runs fine, but after extended time being off, it seems to get an air build up.

I am at a loss as to how this could happen. The pump suction valve is always open, with positive suction pressure (tank head about 50kPa), and the pump is primed and ready to go. There is always one pump running, with the discharge header pressure running about 1000kPa. There doesn't seem to be any way to get air ingress into the pump casing. The discharge line check valve doesn't seem to be passing, as the pump is not spinning backwards while it is not running.

The issue doesnt seem to be occuring on the A pump, which is identical in equipment and lineup.

I figured it must be some issue with the actual pump or its seal system...but with constant positive pressure from inside the line, I can't see how air at atmospheric pressure would get in.

Has anyone experienced a problem like this before? Any advice on what might be causing the air ingress?
Any light you may be able to shed on the issue would be useful.

Thank you in advance for any answers.


RE: Air in centrifugal water pump

My guess is that the air is dissolved in the incoming supply. If there is even a small height difference or imbalance in the incoming flow then air will slowly gather in the b pump. Can you sketch the incoming pipe in plan and section.

This is a bit like a domestic central heating system where air always gathers in one radiator rather than another.

If you put a tee with a blind end and an auto vent on this will gather the air before it gets to the pump.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Air in centrifugal water pump

What kind of reducers are installed in pump suction and how are they positioned?

Dejan IVANOVIC
Process Engineer, MSChE

RE: Air in centrifugal water pump

(OP)
Thanks for the replies.

I have uploaded a quick sketch of the pump flow scheme (sorry, I should have done that initially).

The idea of disolved air is intriguing...we have recently switched from desalinated reverse osmosis water, to normal town supply water, which has more minerals and much higher turbidity etc..and this issue didn't happen with the pumps when we were on RO water (not to my knowledge anyway). I wonder if this could have anything to do with the air trapping.

Dejan, the reducers are 80mm>50mm on the suction. Between the suction valve and the reducer, there is a suction strainer also (see drawing for reference).

Cheers,

Joss.

RE: Air in centrifugal water pump

(OP)
Just as an afterthought, a bit of additional info about the system...For the past couple of years the water circuit has been springing leaks all over the place, so there is a bit of internal corrosion (i think due to the old RO water)...so i imagine there might be a bit of redox in the system (although I dont know if that would make any difference).

RE: Air in centrifugal water pump

(OP)
I had a look...the eccentric reducer does have the flat side on the bottom..and the supply line to pump suction is flat - coming straight off the bottom of the water tank at the same elevation.

However, I would think that entrained air would be a once off occurence, no?..and this situation has occured three times now, all after the pump has been offline for a week or so.

RE: Air in centrifugal water pump

A few questions.

On the inlet can you draw an isometric of the actual piping layout

Which pump is which on your plan?

Is one higher than the other?

Is the piping exactly level

Are the reducers eccentric top flat or concentric?

The fact this seems to have happened after changing water supply is a good starting point. I meant air in the water, either dissolved or coming through as discrete bubbles.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Air in centrifugal water pump

There must be some difference in piping layouts and reducer orientation for the two pumps, otherwise it makes no sense to encounter this problem on one pump only. It is impossible to analyze layout based on the hand sketch provided.

As LittleInch and bimr have pointed out, look at these two possibilities - incorrect reducer installation and/or air pocketing in the suction piping of B pump.


Dejan IVANOVIC
Process Engineer, MSChE

RE: Air in centrifugal water pump

(OP)
The layouts are identical for both pumps, and both share a common 'Y' suction and discharge line. I will try and find an isometric drawing, but both pumps are at the exact same level.

I have attached a cross sectional drawing of suction, pump and discharge (identical for both pumps) to give you an idea.

The reducers are eccentric, bottom flat for both pumps.

The only difference in operation, is that I believe the A pump is run more as the duty pump, while B remains the standby (however we need to run B every now and then to stop stagnation of water in the line.

Either pump could be A or B on the plan I drew...as both are identical and share the same features. I just realies though, that I didn't draw the suction and discharge lines in the 'Y' configuration, which is how they are.

RE: Air in centrifugal water pump

(OP)
I had a look in the database...unfortunately no 3D CAD or iso schematics for this particular system (literally the only system on the plant without them). I could freehand draw it if you like, but might not do it much justice. haha.

RE: Air in centrifugal water pump

Maybe that's the (part of) the answer - there is much more time for air pockets to build up in suction piping of pump B than in pump A, given the service time (operating hours) of both pumps.
Why don't you try to inverse the schedule? Let the pump B run longer and then switch to A. At least you can make some additional observations and conclusions.

Although there must be something else. If everything is installed properly, you shouldn't be seeing those bubbles. bugeyed

Dejan IVANOVIC
Process Engineer, MSChE

RE: Air in centrifugal water pump

(OP)
I agree...there has to be something else, and I have been scratching my head trying to figure out what that might be. Unfortunetly, staring at it isn't giving me the answer.

Pump B has been running for about 4 days now, so I might try starting up pump A tomorrow and see if it has any issues as well. If it doesn't, then at least I can narrow it down to being an issue with pump B or its auxilliaries.

RE: Air in centrifugal water pump

Can you double check the minimum recycle line you drew earlier. It looks to me like one pump can end up recirculating through the other pump but the other pump can't. That's why I asked which was A and B on that diagram if it really is how you've drawn it.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Air in centrifugal water pump

(OP)
My apologies..this really isn't my night. I drew that up in a bit of a rush..the minimum flow line from the pump on the right ties in to the common line downstream of the check valve, not upstream as indicated. Sorry.

- Joss

RE: Air in centrifugal water pump

Your reducers are upside down if the flat side is on bottom. Look at the picture and article supplied by Bimr above.

RE: Air in centrifugal water pump

(OP)
I was thinking because the length of horizontal line between the tank and the pump is short, that may not apply...but I have looked at other water pumps in the same module, and I have to concur, since they also have the reducer positioned flat side on top. These pumps seem to be the only ones around that have the reducers positioned flat side bottom. Interesting...could be improper installation by the sound of things..

RE: Air in centrifugal water pump

Try closing off the discharge block valve on the pump that is not running and see if this air still builds up in this pump casing when you start it next time.

RE: Air in centrifugal water pump

jlbish,

I think you have some things to go on here - my advice would be to cycle your pumps at shorter intervals and for longer. If you swap A for B, does the same thing happen?

The min flow re-cycle should also be checked, the RO might be creating air bubble which could be slowly re-circulating via a not fully closed NRV on pump B, but the inverted reducer is probably prime suspect at the moment.

Options to me are:
Start pump B on a shorter time interval
Put tee and blank end with an auto air valve on the inlet line to catch the small air bubbles
check the min cycle NRV or maybe pipe each one individually into the tank
modify pump inlet pipework to be top flat reducers

Let us know how you get on.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Air in centrifugal water pump

Perhaps Pump B is slightly higher than Pump A, causing air to flow to Pump B.

One additional item to check is the depth of the suction in the tank. One would expect that it is more likely that you are pulling air in rather than releasing dissolved air.

If you turned the eccentric reducers around, the problem would go away.

RE: Air in centrifugal water pump

jlbish,

Re-reading soe of the posts, it is not clear in your original post and subsequent ones how you know what the running head / pressure is at the pump inlet? Tank head might be 0.5 bar or around 5m when static, but it's not clear what the flow rate is or how long the inlet line is or what temp you're operating at. Unless you have a guage right there, you might be operating below atmospheric?? especially if your tank gets low.

As ever a complete profile sketch with flows, dimensions, heights (min / max) and pipe lengths would let us see what you can see. Otherwise it's all a bit of a fog....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Air in centrifugal water pump

(OP)
Thanks for all the input gents..I think the reducer theory is the best thing to go on so far, but our RE is on leave, so will have to wait to see what he thinks.

LittleInch, i took the head pressure from the tank height. The tank is always full, and is 5m high and open to atmosphere. The inlet line to the pump is maybe 4m of pipe, all level between the tank outlet and the pump suction, operating at ambient temperature (water is probably about 20-25 degrees C).

There is very little (if any) demand for potable water at the moment, so the discharge PCV is closed. Pump minimum flow through kickback line is 4.45m^3/hr.

here is the pump data:


Detail Potable Water Pump
Driver Electric
Motor Power 18.5kW
Motor Speed 3000rpm
Motor Type Squirrel Cage Induction
Pump Type Centrifugal
Suctions 1
Stages 1
Service & Potable Water
Seals Flowserve Type A
Fluid Pumped Water
Temperature 40°C
Density 992kg/m³
Suction Pressure Atm
Discharge Pressure at Rated Flow 871kPag
Rated Capacity 20m³/hr
Minimum Flow 4.45m³/hr

No worries, I will do up a bit of a sketch when I get some time and post it.

Cheers,

- Joss

RE: Air in centrifugal water pump

Ok, I'll look at that later, but seems unlikely on its own to be the issue.

Next question - what is the filter element in the inlet strainer and how often do you clean it? / is there a differential pressure indicator on them?

Having strainers in low peruse inlet line sis usually a no no as the DP at full flow can be quite high and end up causing issues. It could easily drop 0.5 bar if it got clogged even a small amount if there is any debris there, sufficient to lower the inlet pressure to < atmospheric and hence draw air in that way.

If you can, drop the filter out of bot of them and see what is in there if it's not a regular maintenance task. If you've had issue of internal corrosion could be you've got some debris in there.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Air in centrifugal water pump

(OP)
Ok,I have attached a quick sketch I did (please excuse its crudeness, I could not find any isometric paper and have been a bit rushed tonight).

A bit of info about it:
- the pumps are at the same level
- the inlet line is at the same level from the tank to the pumps
- the pump discharge and kickback lines go beneath the level of the pump before coming back up and heading off to distribution or back to top of the tank.

I am not sure about the suction strainer (what type), but i'll try to find out. There is no DP indicator across them. However, while the pump is running, it is fine and makes required discharge pressure and flow rates...it is while the pump is shut down that the air seems to build up. I'm not sure what the maintenance/change out intervals are for the strainers.

I had the pumps changed over from B to A a couple of hours ago, and the A pump started with no issue and made full discharge pressure. I will start the B pump again in an hour or so and see if it has any issues (that will at least tell me if it is something that happens over a short or longer time frame).

RE: Air in centrifugal water pump

In both the discharge and the min flow recycle, any air entrained will gather in the piping of pump b and could easily bypass the nrvs. This won't happen with pump a as the piping won't allow it. Pump a flows past the vertical legs of pump b making it an ideal air trap.

The inlet pressure when pumping using pump a would only need to be just below atmospheric to allow air in and wouldn't be noticed by the pump performance.

I think the RO on the min flow is more likely to liberate air than sucking it in due to a partially blocked filter, but might be a mixture of things.

Are the other pumps set up like this as well?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Air in centrifugal water pump

(OP)
I see what you are saying...although wouldn't it be able to be noticed if air was getting through the NRV? Would I be seeing water pass through the NRV's as well?

I just switched pumps back from A to B, and didnt have an issue (but B was only off for a couple of hours). One thing I noticed was that the discharge pressure on A pump was about 1000kPa, while the discharge pressure on the B pump is about 1200kPa.

I'm still at a loss as to how air could be entrained in the water to begin with...the pumps are fully primed, the inlet line comes off the bottom of the tank and the tank is always full. There is also a vent on top of the tank open to atmosphere.

I'm stumped. I am going to keep rolling with the reducer theory, and see if we can get them swapped around. I will also mention the possibility of a blocked strainer, although I don't think this is the issue.

The other pumps are fairly similar in set up..although they are slightly bigger (150mm line) and have a FCV in lieu of an RO. They also have a much bigger tank (20m high), with the suction line coming off from half way up the tank...the suction line come down from above the pump, and has a horizontal run of around 3m before pump suction (and has a top side flat reducer, as stated before).

RE: Air in centrifugal water pump

You wouldn't notice a small amount of water ( a few drops a minute) but few nrvs are gas tight.

Even if the strainers are clean they probably account for 2 to 3 m equivalent. Add a bit if debris and some friction losses and you're below atmospheric. Valve stems or pump seals will let a small amount of air in which accumulates over several hours or days.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Air in centrifugal water pump

Thanks for posting the sketch which resolves the mystery.

The piping is trapping air with the downward leg after the pumps.

The problem is that the pumped flow moving downward (you show 10 meters down) does not have adequate velocity to push the air downward and flush the air out of the piping. You probably need a velocity of at least 1 m/s. See the attachment.

Installation of an air release valve and maintaining pressure in the system will cure the problem.

RE: Air in centrifugal water pump

Suspect you have a backup supply line from towns water directly downstream of the pumps which feeds into the distribution header? Entrained air in this supply is gurgling back into the standby pump (countercurrent to water flow) and trickling back into the pump casing?

RE: Air in centrifugal water pump

(OP)
Sorry, there is a decimal point in there...the downward leg is about 1.0m. It then goes up about 6.0m to the top of the tank (for the kickback line) and 15.0m or so for the main distribution up to the pipe rack).

Would the fluid velocity be less than 1m/s? I feel like it would be more than that. I mean the rotational velocity of the pump must be at least 20m/s.

Thanks for the response georgeverghese...however, there is no backup supply line downstream of the pumps...downstream of the pumps is a pressure control valve, 2 RV's and the distribution header.

RE: Air in centrifugal water pump

Flow velocity in your 80mmND pipe is a maximum of 1 m/sec assuming the minimum flow goes via the recycle line.

I don't think that's your problem (not blowing air out of the pipe), I think it's much more like the RO is creating bubbles which slowly fill up the min flow line on the B pump with air and slowly bypass the NRV. I could be wrong and it might be filling up the discharge line and doing the same thing, but that's my current hypothesis.

one more thing to check though - Is the RO on pump A still the same size it was before? If you have been running this continuously and much more than pump B and been having corrosion issues, you might find your RO is somewhat larger or more jagged than it was when it went in which is possibly a reason why pump A creates less pressure / head than pump B.

So:
Check out the filters,
check out the RO

The easy way out is just to install an autovent on the pipework.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Air in centrifugal water pump

Troubleshooting is a beauty. OP, please let us know what solves the problem.

Dejan IVANOVIC
Process Engineer, MSChE

RE: Air in centrifugal water pump

(OP)
Don't know what I was thinking there..I did the calculation, flow velocity is about 0.5m/s when going through the minimum flow line with discharge PCV closed.

I have conferred with some colleagues, and the general consensus is that there is enough head from the water level in the tank to flood the pump suction piping and casing, with the orientation of the eccentric reducer having negligible effect (not enough to get the amount of air build up we are seeing).

I do like your theory about the RO on pump A creating bubbles. It is definietly an issue that is occuring over time, so migration of air across the check valves seems like a possibility.

I couldn't tell you about the RO size on pump A...I dont know when or if it has been checked. I will mention these issues to the oncoming shift..

Installing an autovent on the pipework may be a good and logical fix, but I would be waiting around for a decent 12months to get the technical deviation approved. Haha.

Anyhow, I am off for 4 weeks as of tomorrow, so won't have anything to do with it for a while...but I will make sure I post any findings or fixes on this thread when I come back.

Thanks for all your advice everyone.

Regards,

Joss

RE: Air in centrifugal water pump

Don't forget to ask the next shift to clean the strainers out as well!

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Air in centrifugal water pump

(OP)
No worries, will do. ;)

RE: Air in centrifugal water pump

(OP)
That is very interesting. .we have been dosing the system almost daily with sodium hypochlorite, keeping it up above 1ppm. Although the hypo is tested before dosing to ensure its quality, that doesn't factor in possible suspended metal particulate already inside the system...I will pass that along as well. Thanks for that one.

Joss Bishop
Process/Operations Tech
Oil and Gas (LNG)

RE: Air in centrifugal water pump

The hypochlorite dosage is so small that any affect on the water supply will be inconsequential. Off-gassing and air binding is a problem when you are pumping 12% (12,000 mg/L) hypochlorite, not 1 mg/L hypochlorite.

RE: Air in centrifugal water pump


jlbish,

as a result of the configuration between the minimum flow lines from pump 'a' and pump 'b', is there a possibility of a venturi effect where the pump 'a' minimum flow line when operating is sucking fluid from the pump 'b' minimum flow line? after some time the water will be vacated from pump 'b' and air introduced through the pump 'b' seals. this would not be the case when 'b' is running and 'a' is on standby.

arthur

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