Pumps in Series 2

[ivanhoe374]

We have a reverse osmosis system that has a small feed pump which then feeds the larger high presu pump. The sum of the concentrate and permeate from the RO is 1.7m3/hr so the feed pump should be running at the same rate. According to the curve this should correspond to a pressure of 30m. However the discharge pressure is only 20m, and this is resulting in the RO cutting out as it has a low pressure switch set at 20m (which cant be changed).

All that is between the pump discharge and the RO is a few elbows, a check valve and about 1.5m of pipe (20mm).

Any reason why this pump isnt running on the curve? besides the obvious reasons of a line blockage, insuffuciet power supply?

I wasnt sure if having the RO high pressure pump in the same line in series could cause this? wouldnt it just add to the pressure fed from the feed pump?

 

[quark]

Your feed pump may be running on the curve but not at the design point. The pressure drop, from your description, seems to be not more than 2meters unless you forgot to mention something. I would suggest the following things.

1. Check the pressure immediately after pump discharge.
2. The head mentioned on the pump curve is differential head and if you have to count it if you don't have positive suction.
3. Ensure 1.7cu.mtr/hr flow. If the resistance is low then pump will run to the right of the design point and thus head developed will be low.
4. Check for proper operation of check valve.
5. Also check the performance of HP pump.
6. Check the pressure switch. Further, 20m seems to be quite high to me. We just run with 0.5kg/cm² which is about 5m.

Regards,

 

[TD2K]

I'm going to assume that the first pump is a centrifugal that is intended to supply the second pump (likely a PD) with sufficient NPSHA. Please correct me if I'm wrong.

Quark is on the money. Check the suction pressure and discharge pressure of the first pump. Double check your flow rate. 1.7 m3/hr is only about 7 gpm so you can do a rough check of your flow by sending it to a container and timing it (make sure you keep the same pressures on the pump when doing this to get the most revelant data). Verify your data (eg. don't just use the field pressure gauge, use one you've checked and calibrated in the instrument shop at least to the point you trust the numbers). In other words, be paranoid.

Check that the flow you think you have (and hopefully verified) and the differential pressure/head falls on the curve. Compare that to the design point. If the head and flow this pump is operating at isn't on the curve you either have internal pump damage OR you data isn't right. Time to start recheck your field data and ask yourself 'what could be wrong'.

1.7 m3/hr is only about 7 gpm so you can do a rough check of your flow by sending it to a container and timing it (make sure you keep the same pressures on the pump when doing this to get the more revelant data)

If you have a suction strainer, check the pressure upstream and downstream of it if possible as I've seen more than a few of these plugged causing your pump to cavitate.

Finally, did the pump ever used to work right? If so, when? Have you made any changes to the system or capacity since that time?

 

[ivanhoe374]

This is a new system. It is a grundfos pump (CH2-40) im not sure if they test them inshop or not? The HP pump is a Tonkaflo multistage.

There is no strainer on the suction and the feed tank is fed with water produced from a UF system(so im assuming it is quite clean)

I will check the flow, however the RO system (Osmonics) is producing the design flow/recovery. The reason i asked was because we have experienced this exact problem with a similar setup.....after checking all the above we left the problem in the pump manufacturers hands and never really got a solution!

 

[quark]

I checked the performance curve and found no problems. I may be silly, but have you checked the direction of rotation of feed pump? Your feed pump is also a multistage pump. Just check for any loose impellers.

Just check power consumption by the feed pump and HP Pump and compare it with that on the pump curve.

Is the feed pump single phase or 3 phase?

What is the discharge pressure gauge reading for feed pump and HP pump? What is the suction pressure gauge reading for HP pump. What is actual power consumption for feed and HP pumps?

Regards,

 

[ivanhoe374]

"I checked the performance curve and found no problems"

1.7m3/hr @20m?

I have checked the rotation of the pump.
Discharge Pressures are:
Feed pump 30psi
HP pump 140psi

Will check power consumptions. Feed pump is single phase.

HP suction pressure is 30psi

The main reason i posted was because i thought i may be missing something associated with the pumps being in series, which could cause the problem....like the feed pump not be able to keep up with the HP pump....but then i know the outlet flowrates so that cant be right.....

 

[quark]

1.7cu,mtr/hr at 30m! is what the curve shows.

But the curve says it is for a 3 phase pump. You should speak to the manufacturer for the actual curve(with single phase power supply).

I don't see any other problems.

Regards,