Air in centrifugal water pump 3

[jlbish]

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.

 

[LittleInch]

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.

 

[EmmanuelTop]

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

Dejan IVANOVIC
Process Engineer, MSChE

 

[jlbish]

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.

 

[jlbish]

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).

 

[bimr]

You probably are dealing with entrained air rather than dissolved air Check your piping to see if you have the correct reducers. See the article:

http://www.hydrocarbonprocessing.co...nd-straight-runs-of-pipe-at-pump-suction.html

 

[jlbish]

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.

 

[LittleInch]

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.

 

[EmmanuelTop]

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

 

[jlbish]

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.

 

[jlbish]

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.

 

[EmmanuelTop]

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.

Dejan IVANOVIC
Process Engineer, MSChE

 

[jlbish]

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.

 

[LittleInch]

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.

 

[jlbish]

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

 

[DubMac]

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

 

[jlbish]

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..

 

[georgeverghese]

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.

 

[LittleInch]

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.

 

[bimr]

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.