Guidance on pump selection

[colwem]

Hi, I was recently hired for something I'm not even remotely qualified for. I have to help figure out a solution to a pumping issue.

What we want to do is see if we can use one pump as a booster to a slow high pressure pump feeding a high pressure system. Then we want to use the same pump to flush the system with hi flow. But I'm quickly realizing this is hard to do.

The flow will be severely restricted when it is in booster mode. Pretty much every pump I've looked at that can deliver 30gpm doesn't seem to be able to do 1gpm. I get the impression that you can operate it at that rate but it will hurt the pump.

Can anyone give me some guidance on how to think about this problem? We can get a centrifugal pump with a vfd. The vfd can lower the H/Q curve when running as a booster, but even then it looks like it's physics problem and you just can't get 2 or 3 bars of pressure with a pump designed to deliver 30gpm when it's being restricted to 1gpm. So I was thinking we can install a control valve that will just recirculate the excess flow so we can operate it in the safe range? But even then most of the pumps I'm looking at we will be wasting like 90% of the water we are pumping during boosting. But if we go this route how do we appropriately size it such that we waste as little energy as possible? How do I think about things like impeller size and rated rpm? Or what are the things to look for that actually make a difference? Sorry, did I already say I'm in way over my head here?

I've been looking at curves of many pumps and they are often so different in shape, with some basically flat for most of the low end range and some look like they are more continuously sloped (lower head for higher flow). I don't understand pumps well enough to have any idea why this happens or which shape curve will work best.

I know there are multistage pumps. will that solve our problem? Should I just tell my boss we need two pumps? One won't work?

 

[LittleInch]

Can you identify the two flows and pressure / head requirements?

The way to do this is probably recirculate the flow, but you should be able to minimise the flow through the pump to about 30% of max flow. What the pump calls the Minimum continuous flow. Not all pump curves show this, but if you take a figure of 35% of rated flow you won't be far off and hence the power demand / loss is probably about 40% as the efficiency is a bit lower.

You can do this via a fancy control system or look up Automatic Recirculation Valves or ARVC valves, which need no power.

The better pumps are probably flatish curves for your duty.

Multi stage is for high pressure lower flow which doesn't meet your needs.

Just for this recirculation flow you might need a cooler somewhere if this goes on for a long time as that power loss needs to go somewhere. So at least go back into a large volume / tank and not just round the pump.

Does that help?

 

[colwem]

Can you identify the two flows and pressure / head requirements?

The way to do this is probably recirculate the flow, but you should be able to minimise the flow through the pump to about 30% of max flow. What the pump calls the Minimum continuous flow. Not all pump curves show this, but if you take a figure of 35% of rated flow you won't be far off and hence the power demand / loss is probably about 40% as the efficiency is a bit lower.

You can do this via a fancy control system or look up Automatic Recirculation Valves or ARVC valves, which need no power.

The better pumps are probably flatish curves for your duty.

Multi stage is for high pressure lower flow which doesn't meet your needs.

Just for this recirculation flow you might need a cooler somewhere if this goes on for a long time as that power loss needs to go somewhere. So at least go back into a large volume / tank and not just round the pump.

Does that help?

Yes, this helps greatly. I think we settled on a multistage pump because the pressure / head curve seemed correct. We needed 30gpm through a system with about 2 bar of back pressure. This seemed to be too high a pressure for the single stage pumps we looked at. Does this make sense to you or do you think we are making a mistake or misunderstanding?

Our system is cycling about once every 3 minutes between a slow flow high pressure stage and then to a low pressure system flush stage. So I really don't think thermal build up should be an issue.

The ARVC would help a lot here. Thank you.

 

[LittleInch]

I think you're looking int he wrong places for pumps.

30GPM at 2 bar differential should be easily available in single stage.

Don't know where you're looking, but try Goulds, Grundfos, KSB, FLowserve.

Many others should offer this sort of unit.

But first sort your system details out.

 

[colwem]

We were looking at Grundfos only. I guess we can look at Goulds. We selected the CR5-3 which is a 3 stage pump. We looked at their single stage TP models and found that to get get 2 bar you had to go really far to the left on the curves. Like we have to get something that has a nominal flow rate of like 100 gpm and run it at 30 gpm (I am just making those numbers up, its intended to be illustrative of why we made this decision). Then the NB line is all too powerful for us.

 

[LittleInch]

TBH, 2 bar is pretty low. You might need to get a higher diff pressure and throttle it off.

I still don't really understand your flow versus differential head requirements for your two modes of operation

 

[colwem]

Yeah, it's possible I'm misunderstanding something. Our requirements are to provide between 1 and 5 bar at 0.6gpm and to provide approximately 2 bar at 30gpm. To be honest I do not know if that includes the system inlet pressure or what the pressure might be? it's fed by a tank that is not very elevated so I assume there is no significant static head. But maybe if I subtract that from the 2bar requirement then it could be lower.

Reading this document https://api.grundfos.com/literature/Grundfosliterature-836.pdf for instance. This is one of their single stage lines. On pages 9 through 13 they have the graphs. Our operating point is going to be around 20m of head right? That's about 2 bar? And our flow is around 7 m^3/h. I believe that's the conversion for 30 gpm. Looking at those graphs our operating point does not seem to be a good match.

Likewise https://www.lenntech.com/Data-sheets/Grundfosliterature-NB-NBE-NK-NKE-GB-L.pdf Nothing seems to fit well

 

[pierreick]

Hi,
Better to write a specification sheet and submit it to vendors to get a meaningful answer.
At least write a scope and submit to them. This will help you to clarify your needs.
My view
Pierre

 

[LittleInch]

Page 11, a 50-250 or 50 - 300 looks ok to me. TPD pump.

Your flow range is such that you'll need to run a recirculation line at your low flow, prob at about 3 m3/hr.

 

[colwem]

Page 11, a 50-250 or 50 - 300 looks ok to me. TPD pump.

Your flow range is such that you'll need to run a recirculation line at your low flow, prob at about 3 m3/hr.

Thank you but I believe i confused you with the units. those look to me like they are good matches for 20m head and 30 or so m^3/h. The graphs have flow measured in m^3/h in this case. However our requirements are 30 gpm and 30 gpm is about 7 m^3/h.

 

[LittleInch]

Sorry, I was looking at l/sec.

A 32-330 on the same page will be good.

 

[colwem]

Sorry, I was looking at l/sec.

A 32-330 on the same page will be good.

Thank you, yes that does seem correct. I think I ignored that because that line will not work for us for other reasons. They seem to have TP versions with a fixed speed motor but they do not have a 32-330 in the TPE line that has the variable speed motor. And more importantly it only seems to come in cast iron and we need corrosion resistance of 316 or better because this will be >35g/l concentration of salt water.

But I have decided to follow the other persons advice in this thread and have found a number of distributors sell a variety of brands and have been sending out our specifications and asking for their recommendation. Hopefully this solves it for us.

 

[LittleInch]

Fair enough on materials but why do you want a VFD motor? For your usage this is not going to work. remember head falls as the square of the speed and if at full speed you are only getting 2 ba-3 bar, then you're not going to get anything worth having at half speed.

 

[colwem]

Fair enough on materials but why do you want a VFD motor? For your usage this is not going to work. remember head falls as the square of the speed and if at full speed you are only getting 2 ba-3 bar, then you're not going to get anything worth having at half speed.

Well we were thinking this was going to be the only way to get close to the low flow 0.6 gpm condition and avoid wasting too much energy recirculating water. Yes the head would drop but for instance in the CR5-3 it looked like we would still get enough pressure when we drop the speed to it's lowest rated level. But the CR5-3 is a multistage pump.

Because yes the head drops when you lower the speed but the head increases when you restrict flow. Energy efficiency and reliability are the two determining factors for project success. This is why we want to use a VFD on all the pumps we are using. Though we are not expecting this pump to make up a significant portion of our energy usage regardless of what we do.

I feel like I was being sloppy in explaining the situation. We need to make sure we can provide 0.5 barg (7.5psig) of pressure to the high pressure pump but not more than 5barg (72psig). So maybe the pressure would be lower than 2 barg but that's ok. The system will be tank fed and the tank itself might provide enough pressure but we do not have control over the tank level so we do not know.

 

[georgeverghese]

LP flush mode : 2bar dp at 30gpm
HP booster mode : 2bar at 1gpm
This sounds like a pd pump to me. You could run the pump at reduced speed (on VFD if this is preferred) for HP mode.

If this brackish water is clean enough, use a screw or vane or gear pump suited to brine service. For screw pumps, ask for a pump with larger rotor to stator clearances. You may have to flush out this entire pumping circuit with clean, low chloride water when not in service to prevent salt breakout.