×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Reading Pump Curve
6

Reading Pump Curve

Reading Pump Curve

(OP)
Hello all,
When looking at a pump curve, how do you determine if it's safe to run at a certain GPM without causing harm to the pump internals (seals, bearings, impeller).

For example, FLYGT 3300.181 : CLICK FOR SPECS

If I ran my system at 90 GPM, would this be okay and why (please feed me some basics)? If not, what can be the damage? Anything to recommend or change in the system besides picking a new pump?

Your help is greatly appreciated. I'm a new engineer; hopefully I'll be able to help someone later on!!!

RE: Reading Pump Curve

2
Attached is a collection of articles and diagrams explaining what you can expect when you try operating off BEP. Generally you need to be within 80 and 105% of BEP for best performance and minimum maintenance. Here's why.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

RE: Reading Pump Curve

BigInch…that's a nice set of articles. Thanks.

resilientsoma…when in doubt, talk to the manufacturer or one of their reps (the good ones are engineers themselves). The pump you reference will NOT be happy at 90 gpm and neither will you or your client. That pump is far too large for 90 gpm.

==========
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

RE: Reading Pump Curve

Burn baby, burn!

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

RE: Reading Pump Curve

(OP)
haha, yeah I had a itch that it wouldn't work with that GPM range. Some of our processes require only 90 GPM, so maybe putting a drain pipe at the header to dump back into the tank will help. Say if you put it up to 250 GPM, water can exit to the tank as well as supply the 90 GPM needed without harm to the pump!


THANKS A LOT everyone, the articles are a great help - NOW I NEED to figure out how to plot the BEP curve for the pump and process requirements. I will have to use your information for this tomorrow!

How were you able to tell by looking at the curve in my link provided that 90 GPM is too little for this pump though? It's a 460V Flygt 60HZ, and kW I believe ranges from 28-51.

RE: Reading Pump Curve

Less than 20 % BEP gets critical. 10% BEP flow starts cooking up the soup.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

RE: Reading Pump Curve

How were you able to tell by looking at the curve in my link provided that 90 GPM is too little for this pump though? It's a 460V Flygt 60HZ, and kW I believe ranges from 28-51


I'm not sure where that information is available on the chart you attached. When the flows start at 1,000 gpm and go up from there for the various pumps, 90 gpm sounds a little small.

RE: Reading Pump Curve

(OP)
I'm not sure where that information is available on the chart you attached. When the flows start at 1,000 gpm and go up from there for the various pumps, 90 gpm sounds a little small

Okay, that makes sense, but just wondering the basics of why; is it because it will dead-head, causing cavitation and become very hot?? If you put a VFD on a pump, can you just turn down the load, protecting the pump to run at lower GPM's than it's designed for?

RE: Reading Pump Curve

if you read the documents from BigInch, you may find some of your questions will be answered.

RE: Reading Pump Curve

Question: Why would you want to run this pump at 90 GPM?

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.)

RE: Reading Pump Curve

(OP)
@ nfinit : I have been reading his articles; I'm not quite done yet, but they are very helpful so far. I'm just trying to stay updated with this thread.

@Artisi : The pump has been in place for many years, and going through a lot of rebuilds, most likely to operating out of its operating curve. 90 GPM is needed for certain process requirements (manufacturing). It isn't always at this GPM range, but sometimes it's required.

I'll am really trying to study this more and try to understand as much as possible before posting things to waste your time; sorry.

RE: Reading Pump Curve

3
Install a second smaller pump for the 90GPM duty, probably a lot cheaper in the long-run.

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.)

RE: Reading Pump Curve

(OP)
@ Artisi : that was thought about for sure. I'm just trying to work with what we have; of course not trying to go through many rebuilds as we do now though. We do have VFD for the pumps, but are VFD's a good solution for pump protection... for low GPM, turn setpoint % down from the 60Hz ... still trying to look into this/

RE: Reading Pump Curve

You could easily have problems with the motor running at that slow a speed as well.

1.) Motors lose a heck of a lot efficiency when they are at less than 50% load.
2.) Motors must have special windings to use a VFD. A VFD on a motor not suitable for VFD service could easily overheat and burn.
3.) A new motor suitable for this service will be a major, major cost item.
4.) If you can afford that option, then you can EASILY afford a new special purpose pump for 90 gpm.

To put it bluntly, don't be dumb about this. You are a new engineer. There's far too much to say about this subject than we have time to teach you on the internet. Please listen to Artisi. Take and make his advice work to your advantage. Don't be stubborn. Buy another pump for those times you need 90 gpm!

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

RE: Reading Pump Curve

(OP)
@ BigInch :
There already is a VFD for our pump, but we currently do not use it, only a set-point can be used, but is maintained at close to 95% of its 60 HZ. We have PLC control, just need to make it automatic w/ PID integration. I know there are certain concerns w/ running a VFD from your articles, but these pumps can handle it for the most part.

My main concern is the low GPM obviously, so a working VFD and/or a system bypass is necessary - bingo.

My other concern is just understanding the simple basics of what happens when the head is at MIN and what happens when the head is at MAX on the pump curve vs GPM.

I'm not trying to be dumb, just trying to see if I can get some expert opinions and input, which I have. I want to keep an open mind and try to learn everything I can. Someday I will be more experienced and can help the next struggling new engineer. I have many areas I work in; so please excuse me.

Thanks again.


RE: Reading Pump Curve

That's quite a different set up than what you mentioned on 9 Oct.

But it still doesn't matter that much. With a VFD running the pump at 10% BEP, you've still got the efficiency degrade of both the motor and the VFD, each running at 10% of their rated load.
You might be able to do it, if the motor is rated for VFD service, but you'll still pay for it, bypass or not. At 10% of BEP, probably both the VFD and motor will have about the same 10% efficiency, giving a subtotal net 5% eff and you still have the pump efficiency to multiply so, say 0.65 or so for that. Hey, that's about 3% net efficiency x 0.9 or so for power line loss, maybe you'll get 2.5% out of it. You might be able to do it, but I wouldn't even want to do that during emergency upset conditions. The equipment might work, but you won't be able to pay for it.

Are we getting any closer to solving this your way?
Artisi's solution still looks pretty damned good to me.
You can't fool Mother Nature!

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

RE: Reading Pump Curve

At a flow of 90GPM - do you know what the discharge head is? The pump speed will change as the square of the speed change - reducing the speed to get your 90GPM - (without spending the time to calculate it) the head developed will probably be only a few metres.
A good exercise for you to calculate the new system curve and pump curve at the reduced speed / flow.

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.)

RE: Reading Pump Curve

Or instead of calculating it, estimate and just draw a small circle around (0,0).

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

RE: Reading Pump Curve

(OP)
@ BigInch & Artisi :

From more researching and getting your inputs, I'm getting to where I need to be even more. The efficiency w/ VFD control does seem problematic. Our pumps are rated for VFD's so that is what I will look into more. I can understand your explanations and will keep this in mind. I will try to talk my team into a smaller pump for these low GPM conditions. So the discharge head can be assumed to be around zero (0) at 90 GPM, based on looking at the curve.

I did some measurements with our setup; running from 90 GPM to 1530 GPM (blue line). The two curves 3300, 3301 represent different model pump curves. How would you determine the BEP, or as you said before, would you have to get this from the vendor? Anything else suspicious w/ my findings that you can see? It looks like the pump will be happy at 1200 GPM w/ Pump 3301 and 1650 GPM with Pump 3300. So how do you base your BEP off of that? I really do appreciate your time and efforts. The 72.4% is for Pump 3301, which I got from the pump curve. I assume that is the EFF at the BEP?

RE: Reading Pump Curve

BEP is found at the flat spot on the power curve. I've marked approximate bep for 4 of the pumps on the data you've given above. see attch. You can measure power at varying flowrates while holding the same rpm, then connect the dots.
Change the speed to a new rpm and measure again.

The bep should, more or less, show a direct relationship to speed as you as you move from rpm to rpm. If the bep flow is 1600 gpm at full rated speed, it will be around 800 gpm at half rated speed. Head at half speed will be (1/2)^2 = 1/4th that of the head at rated speed.

Pump efficiency stays roughly the same for all BEP points for each rpm. In other words, if your blue line intersection with the pump curve at rated speed is indeed BEP, then the BEP point at each pump speed would follow the blue line as you changed rpm. The same pump efficiency at rated speed would then be used at the BEP point for each rpm.

Motor efficiency drops with load.
I usually assume two straight lines
(0% load,0% eff) to (50% load, 75% eff)
(50% load, 75% eff) to (Max Eff, 100% load)

Don't forget VFD efficiency drops with load too.

Calculate the power cost at 10% motor eff and 10% VFD eff and 65% pump eff and 90% power line efficiency. $/gallon pumped will be quite high. That should force the team to consider buying a secondary pump for small flows. The more time you spend at 10% flow, the more money it will cost.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

RE: Reading Pump Curve

I've found that a typical VFD and pump setup (not closed circuit piping) only works well between 75 to 85% of rated full speed BEP flowrate. Above 85% you should use a control valve, below that, either too low a head output, or power costs due to multiple inefficiency hits runs the bill up too high to make pumping for profit feasible. If you must have it for some reason, for very short-time flows, I guess you must, but don't try to pump those rates and make a living at it.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

RE: Reading Pump Curve

(OP)
@ Big Inch : Wow, what a wealth of knowledge; I truly appreciate your time.

I'm feeling stupid again: I can't seem to match my pump with your 4 highlighted points. The most kW I've seen was 71.8, 115.5 A, and 59.2 Hz. The kW in the PDF are much higher.

I am starting to wake up to reality to see a smaller pump would be much easier and more cost-effective.

I will try to change the RPM this week and re-measure. Have some other things to look into first.

Also, even with running low GPM out of the BEP range w/ VFD, you will still experience non-uniform radial thrust? Running a VFD lowers the efficiency, so you have to "pay to play." Pumps are more difficult than I thought.

RE: Reading Pump Curve

On these straight line pump curves it's sometimes hard to tell. Estimate it at about where 75% to 85% of motor power rating is being consumed, estimating the efficiency of the pump at that point of around 75%. It might be a long shot, but it's the only shot you've got without the real curve.

"The kW in the PDF are much higher" I forget exactly, but I think starting currents can be maybe 2-4 times higher than run currents. EE is my weak suit.

Off-BEP ops with a VFD isn't the same problem for a pump varying flow while running at constant speed at a fixed BEP, as a lot of the forces and pressures retain balance as the BEP moves more closely with the change in flowrate. The problems encountered with VFDs manifest themselves on the power side.

In engineering, if you've really got a handle on what you are doing, you will always realize that trade-offs are being made and that the trade-offs come at a definate cost... somewhere. You've got to figure out where the penalties are being levied and how big they are. "No free lunches" and "You can't fool Mother Nature" are the reoccuring themes.

At 100% BEP, a control valve is probably fully open and no efficiency is lost there, but at 100% flow with the VFD on, you're paying maybe 5% eff loss just to have it there. That's why I say 85-100% BEP flow is better done with a control valve, as actual power losses are less than, or very close to, the VFD's 5% loss, - possible problems with power line harmonics and bearing corrosion. From 50% to 85% is VFD territory, provided you have available head when speed is lower (not for example as for Boiler Feed Water systems, etc. where head requirements remain high even for small flows, or for longer pipelines with + elevation changes). For a VFD to be economical, you have to spend some major percentage of pump time at 50-85% flowrates, which in that case may mean that you've set your system flowrate target too high and it probably should be lowered, making the control valve the more optimal solution once again. Lower than 50% BEP many system start to fall apart due to multiple efficiency reductions and not many systems work well at those flowrates at all, aside from cooling water closed circuits, etc. See my drift? A VFD on a submersible well pump is a real waste of money, as the high constant head required to reach the surface, even at low flows, will blow that idea. But I'm sure that somewhere they can be useful, I just don't often see too many of those cases in my pipeline work. If I need low to no flow, I'll pump to a tank at BEP flowrate, then shut the pipeline down until the tank starts getting low enough to pump full BEP flowrate into it again. That's he best way to save money. In any case, my usual objective is to pump as much as possible from point A to point B and actually neither a control valve or a VFD helps me out much there. I've generally lived a pretty long life without much use for them.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

RE: Reading Pump Curve

(OP)
Sorry for the delays! I understand the trade-offs, and VFD's aren't the most ideal scenario. I will take all of this into consideration for sure. We use the pumps for constant flow for process requirements, so it's either a smaller pump, or run w/ VFD and/or bypasses. A smaller pump to handle the low flow isn't favored due to not being able to have a back-up pump in the tank if one fails that is used for larger flows. A bypass can be created at the manifold, but will not be any cheaper unfortunately. Again, I appreciate all the insight and realize I have much to learn and experience. Thank you.

RE: Reading Pump Curve

At least you are making a somewhat more "informed" decision.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

RE: Reading Pump Curve

(OP)
@ BigInch :

Yes sir, I'm trying. Thanks for your time and patience!

For an update: I'm working on nozzle selection, looking at PSI needed for area coverage. Next will be bypass constructions w/ PLC control. Fun stuff.

RE: Reading Pump Curve

Have fun while it lasts.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

RE: Reading Pump Curve


"resilientsoma (Mechanical)
..................... Pumps are more difficult than I thought."

You can't get anything much simpler than a centrifugal pump, it's all in the way you employ / install them.

You simple solution is to install a smaller pump dedicated to the 90 GPM application.

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.)

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources