Can someone help me figure out what is going in this pumping system? 5

[jimmy001]

Hi all,

I am an intern working on this project, I am wondering if you can help me out with figuring out this pumping system?
Here is a sketch for the system: 2 identical pumps connected in parallel drawing from an atmospheric pressure storage tank, pumping to two discharge locations, and there is another 2'' re-circulation line with a around 60% closed manual valve at the end of the line.

There is only one pressure gage on one of the discharge lines. The pressure reading is 80 psi. This one pressure gauge was newly put in so it shouldn't be defect.

Here is the pump curve for the 2 pumps. (Both have 8'' impeller diameter)

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1501764838/tips/2K4x3-10_Curve_at_3550_rpm_esa28a.pdf[/url]

I added the flows together and made a pump curve with 2 pumps in parallel here.

Here is what's weird. The two discharge locations are only taking in on average 400 gpm of flow, maximum 500 gpm. The discharge pressure is 80 psi, although there is no suction side pressure gauge, but since the suction side comes from an atmospheric storage tank,the total pump head should be around 80 psi, which corresponds to around 1250 gpm. However I doubt that that the 2'' recirculation lines with a 60% closed valve is carrying the rest of the 750 gpm. I explained this to a pump mechanic at the plant, and he told me that water might be recirculating inside the pumps. I looked up suction and discharge recirculation within pumps, and found out that it's not good lol. However I'm not sure if 750 gpm could just recirculate within the two pumps.

I know that's not a lot of information to really fully comprehend the issue, but since I am just an intern I can't ask for more pressure gauges or flow meters. Wondering if you can help me figure out what is happening here; are the pumps performing below expectation, is 750 gpm just being recirculated through the 2'' line or is 750 gpm recirculating inside the pumps? Which scenario is more likely?

Thank you all in advance.

 

[pumpingtips]

Jimmy, you are a good intern who is trying his best to provide good field data. Well done. This is an interesting brain teaser, particularly when you keep feeding us new info. Beware of plant technicians input. It seems that this system should/could be running using one pump and 2" valve 100% open. But there seems to be a pumping output pressure issue. It can only be lower pump rpm, or smaller impeller diameters than you believe, or were told!! Maybe they have been cut down to 6" at some moment. And then the guys decided they didn't have enough pressure so now run two pumps. Anyone's guess what plant maintenance have been changing over the years. Always good to strobe the pump shaft to prove up rpm. Ignore comments like recirculation. That's what pump efficiencies are all about. They all churn. The plant techs are probably seeing low flow (restricted flow) situations and think pumps are not performing.

 

[LittleInch]

Catching up a bit here.

Jimmy, what you have here is what we technically call a "mess".

I still don't have a real idea of what the system is, how it works or what is going on, but some commentary on the new information and a bit of speculation.

Your dead head figure and single pump operation seem to show that the pumps are working well below their original pump curve. It is also interesting that the normal operating point is now much closer to the dead head pressure than the pump curves suggest so it might well be that one pump is actually doing 75% of the work and the difference in the head versus flow on the flat part of the curve means that it is very difficult to balance parallel pumps. E.g. if the difference of say 2 psi means a flow difference of say 100 to 250 gpm you can see how a small pressure/head difference can make. At a steeper part of the pump curve a difference of 2 psi might only make a flow difference of say 350 to 360 gpm. Get the picture?

Back to your pumps. First - your power figure is not correct. I am assuming the 460V is a 3 phase Line to line voltage (at 460V this would be normal). Thus actual power is V X A X sqroot 3 x power factor. Guessing a PF of 0.85. you get 40kW = 55 bhp. Now that's motor power in - shaft power out say 0.96 efficient motor = 52 bhp. So that on your pump curve puts you at the far end of the curve or else your pump is now horribly inefficient compared to its pump curve data.

So possible causes
1) pump going round wrong way - sometimes happens but to do it for both pumps seems a bit unlikely. Alsways worth checking if you can start and stop a pump and observe the motor fan or shaft and also find a direction arrow on the motor or pump
2) motor running at a different speed - the pump curve is for 3550 rpm - basically a two pole motor running at 60 htz. If you ran the same motor/pump combination at 50 htz you would get an approx 70% reduction in head.
3) Most likely IMO - your pumps are knackered. From your description they are 30 years old and run 24/7 with a very variable flow. My guess is that they were never designed with a minimum flow loop and only after a while did anyone try to do a very basic min flow with that 2" valve, which is better than nothing, but very hard to get right. They really should either use a fixed orifice type system or an ARV - automatic recirculating valve which diverts flow back to the tank when outflow reduces below your min flow (usually about 25-30% of max flow).

What next?
Ask the maintenance crew when they last took a pump apart.
Try a different pressure guage
Try it when pumping without the bypass line in operation and see what happens
Repeat your dead head flow with the other pump
Check power in to see if it is a 3 phase motor
Check rotation and pump speed

At that power you're burning a lot of electricity that is being wasted so worth investigating a bit more.

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

 

[Artisi]

Mess is an understatement.
OP, you need to run each pump separately, but without some flow measurement it is guess work.
Can you hire an ultrasonic flow meter, run each pump either on the bypass line or directly onto the system and take some measurements together with pressure and power input.
Plenty of well qualified people here to help but we aren't crystal ball experts (unfortunately).

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[jimmy001]

Thank you guys all for the responses! Really helped me out.

 

[Artisi]

A thank you is always appreciated especially if followed up by the solution and outcome to the problem.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[LittleInch]

I agree with artisi - after this much response, we would like a bit more feedback / next step / what you found out.

From the data you've supplied (which needs verification) it seems your system is operating very inefficiently and costing a lot more to operate than it should. Money talks so present it as an improvement which could save money but needs a bit more investigation - ideal for an intern.

don't neglect the going round the wrong way bit though - I've know systems installed for 25 years which "never worked properly" which were revolutionised when the pump was made to go the right way around....

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