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]

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.

 

[jlbish]

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

 

[LittleInch]

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.

 

[jlbish]

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).

 

[LittleInch]

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.

 

[jlbish]

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).

 

[LittleInch]

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.

 

[bimr]

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.

 

[georgeverghese]

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?

 

[jlbish]

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.

 

[LittleInch]

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.

 

[EmmanuelTop]

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

Dejan IVANOVIC
Process Engineer, MSChE

 

[jlbish]

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

 

[LittleInch]

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.

 

[jlbish]

No worries, will do.

 

[georgeverghese]

Towns water would contain hypochlorite,yes ?

Have a read of section 2.4.3 in this link - oxygen lock in pump casings

http://www.omegachem.com.au/docs/mega_handbook.pdf

 

[jlbish]

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)

 

[bimr]

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.

 

[Softedge88]

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