Pump Curve Question 4

[rhatcher]

Can anyone tell me what the triangles on the attached pump curve represent? I am thinking that they represent the 'design requirements' for the application that the pump is being tested for. The application in this case is a dry dock located on a tidal river.

 

[Artisi]

You reported that the motor speed was measured at 270RPM, was this loaded or unloaded?

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.)

 

[rhatcher]

Thanks to pumpsonly and dubmac for the answers further explaining the meaning of the triangles and what they represent in the design process.

I have suspected that the pumps (two) are running off of the right side of the x-axis at start (near zero foot heat) and that they are at the 40ft head point of the curve (not the point represented by the 40ft head triangle) when empty.

The amp readings that have been taken show little change from full to empty and this makes sense seeing how flat the power curve is at the right side of the pump curve where I think we are operating.

My thinking is that the motor nameplate speeds are probably not exact. This is the part of the problem that falls directly in my experience. My post began because I do not see a problem here and I am looking elsewhere.

Anyway...the motors have proper balanced voltage and are both pulling overcurrent approximately equal to my calculation of pump overload due to speed (ie 12-13%). There are no defects in the motors or in the controllers that would cause overcurrent. One of the motors was checked with a tachometer and read 270rpm. My assumption is that the nameplates are off by a few percent rpm. That shouldn't be a big deal unless you rate the pump load to be exactly equal to motor load and then get the speed wrong. Oops!

As a note, the impeller on one pump has been cut 3 inches in diameter during the course of discussion on this thread. This places the calculated pump performance slightly below the original pump curve but well above the design point for 40' head.

For those who check calculations, 2" would put us back at the original pump curve but the desire was to go a little further because of the age of the motors.

My interest in the triangles was because if they represented the design points then I could calculatute the minimum impeller diameter that would meet the application head requirement to be sure that we did not cut too much from the impeller.

I think we are safe since we took a conservative cut. I just hope that the design points are not in error as much as the pump speed/power part of the design (??!!). Obviously I can not figure this out nor could anyone here help me without much more information about the application, piping, etc.

Since the cut is made we will see what happens.

 

[Artisi]

Further trim to the impeller might not have been a good move as there is a minimum diameter that you can trim to.

Looking at the original curve it is marked as 55.75" - 42", it is possible,without having all data to hand that this (42") is the minimum trim for that impeller - which might explain why the pump test showed a flow/head over the duty points marked on the curve.

But who knows, as all data is unknown and we are just guessing, but it will be interesting to see what the result is.

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.)

 

[ports394]

Here's another thought. How do those motors start? Is there just a regular On/Off starter? A slow start?
What about putting in a VFD.
I like Dubmac's theory of the pump just running far too hard and going to the right side of the curve. Not sure about a throttle valve in this case with such a large flow. But a VFD could slow you down, and long term it would save electricity too.

 

[rhatcher]

artisi, the impeller is tapered with a maximum diameter (top) of 55.75 and a minimum diameter (bottom) of 42". The measurements as shown on the pump curve data does correspond to the actual dimensions (minus normal erosion/wear).

ports394, this is a wound rotor motor with about four steps used only for starting. The stator is a 2300V across-the-line start. The application is one that requires maximum flow and minimum time. We are simply pumping water out of a hole in the ground as fast as possible. There is no economical benefit that I can see to variable frequency and variable flow from a VFD. I am thinking that once the trim is correct, the pump can maintain maximum flow from start to finish.

However, you have good may ideas that I am not thinking of. If so, elaborate.

 

[Artisi]

rhatcher, OK - understand the impeller sizing now.
Waiting to hear the outcome.

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.)

 

[DubMac]

Hmmmm, veeeery interesting.....several thoughts about the situation:

What did the impeller look like when pulled? I am curious as to the material for seawater, Nickel Aluminum Bronze???

Are the pumps piped in parallel?? If so, then make sure both impellers are cut to the same diameter or the one with the larger impeller will pump back through the other one. This will cause mucho problems too lengthy to get into here.

Lets just consider the motor is what it is and work the pump side into not creating overload. I don't think any VFD's, soft starts, staging will have much effect at all with your problem.

Still being very curious about the overall condition of these 20 something yr old pumps, the pump efficiency comes into question. If my math is right, every dropped point in efficiency would be about an 8HP burden added to the motor. The original 89% efficiency is EXTREMELY high for any centrifugal PERIOD. If you've lost 10 points over time, which is very likely, you're closing in on 100 HP added. Just thinkin.......

Have you done a single point check and backed an efficiency out of the HP equation?? Might be surprised with what you find.

The farther out to the right you let the pump operate on the curve, the higher the tendency to overload the motor. Once you get to a certain point, the power required will jump through the roof. Forget about trying to run out past 60K gpm and throttle the pump back by pinching a discharge valve until you have some head built up on the discharge side of the system (dock level is low).

Just my 2 cents.

 

[Artisi]

DubMac,
Don't agree entirely with your comment of efficiency, sure the impeller is more than likely worn and if it is the flow and head will reduce as will the power requirement - less work being done - but being worn the hydraulic efficiency will probably be reduced which will increase power requirements for what work is being done in the worn state. Normally if not always as wear takes flow and head drop as does power.
Without measuring flow / head and power and comparing it against the "as new" installed performance it is all guess work.

rhatcher,
What started out as a question of the "triangles" marked on the curve has certainly moved a long way from the original posting.
So I have a few question;
1)are the pumps performing the work required - ie,. pumping out the dock?
2)is the motor overload a problem?
3)has it always been like this?
4)other than not understanding (initially)what the triangle meant, what is 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.)

 

[DubMac]

Artisi, Thanks for the imput, first forum deal I've ever done. Like the mental exercise; especially when it isn't costing me loads of $$ like it may be to RHatcher.

Couldn't agree more on the need to define the actual problem, was it the motor overheating?? If so then it most likely is a PUMP problem.

Must disagree on the efficiency point however.
Internal inneficiencies, worn/leaking impellers, wear rings, volute cutwater, will increase the work necessary for the pump to get to the same head as it would if brand new.

Must keep in mind that a centrifugal pump is pretty dumb, it only knows and responds to the pressure it sees at the discharge flange. If discharge is open to the air, then it will keep pumping more and more volume (run out to the right on the curve) until either HP or NPSH limits are exceeded and then will overamp motor or cavitate, respectively.

Centrifugals always operate at the intersection of the pump's capacity curve and the system curve. ALWAYS. In this case, the system curve is the same, the pump curve however can be modified due to wear.

So in this case, the pump, even though worn, will not see deterioration in head produced, it will still meet the system head, just take more energy to so it. Efficiency curve on original pump curve is no longer applicable. Only a field performance test will enable Rhatcher to generate a true curve and back calculate power used/required.

 

[Pumpsonly]

it is very interesting that what started with a simple question about 3 triangles now evolved into a very technical thread.

ratchet, your below statement is not true
""When the dock is full, the static pump head is essentially zero because the level of the water inside the dock is equal to that of the river outside of the dock ".

because the water will be discharged over the dock at a point higher than the maximum possible water level in the river to prevent a back flow.

You have also not answer the question about the operation history of the pumps,when did the overloading started.

As for the performance of a worn out pump such as enlarged wear ring clearance or worn impeller, the first indication will be the drop in flow and head. You will not see a input power increase.
The pump efficiency is drop because the hydraulic power (Q x H )generated by the pump is reduced but the input power is unchanged.

Another possible cause of higher power input is the corrosion in the casing and impeller and also the marine growth in side the casing causing a lot of friction in the flow passage.

 

[Artisi]

DubMac,
If the pump is worn, it would be incapable of producing the same flow at the original head, therefore power will at worst remain the same however, it will probably reduce.

As for the total head the pump must achieve, even if the head is all static, as in the case of a full dock the performance will shift to the left until it intersects the "worn" H/Q curve.

You made this comment yourself in your earlier post "Centrifugals always operate at the intersection of the pump's capacity curve and the system curve. ALWAYS. In this case, the system curve is the same, the pump curve however can be modified due to wear."



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.)

 

[DubMac]

Back in town and thrilled to see this discussion ongoing.

Have attached what I believe to be a VERY crude sketch of the system and H-Q curve both in original condition and worn states.

Rhatcher, pls confirm system if you're still around. I have only shown for one pump, but no difference in terms of our discussion for 2 pumps.

To original point on the triangles, I would assume Pt 1 is for full dock (all friction and no static head). As docked is pumped lower, static head will rise to Pt 2 @ 35' below surface, and Pt 3 @ 40' below surface.

Over the course of the pumps life, due to wear, the H-Q curve will degenerate as shown. This movement is caused by internal leakage across the wear rings, stuffing box. The pump is still "doing work" on this water, it just isn't coming out of the discharge flange, it is being recirculated internally.

In its worn condition for say Pt 2, the pump will still see the same pressure at its discharge flange of Pt 2, since the system curve is only a function of the system and not of the pump.

However due to internal losses, the pump's output capacity will be that of Pt. 2' (2 prime on worn curve). That is, the pump is still doing the work for Pt.2 but only outputting capacity of Pt 2'. Therefore, when performing a HP calculation for worn condition, the original capacity must be used even though it is not moving out the discharge.

Remembering that: Efficiency =

water HP / (water HP + hyd. losses + disk HP + leakage losses + mechanical losses)

the efficiency curve will be lowered as shown with wear. This loss in efficiency is what will cause HP increase.

All of this is just pump theory and there may be many other factors leading to the overamping of Rhatcher's motors. Again, if it were my project, I would make sure impellers were "re-ringed" and those fits brought to original spec and make sure diamters of bothe impellers were the same so as not to have one pump pumping back through the other.

 

[gadkinsj]

I'm going to say that those triangles are the requirements of the test. If the performance test curve dropped below any of those 3 triangles then it would have been a fail. So in essence they are design points, but they are on the sheet because they are the Pass/Fail requirements for the acceptance test.

Big Centrifugal pumps can get very Efficient, its the little 1/2 HP jobbers that are in the 30-40% range.

 

[Artisi]

Big centrifugals can run into the 90% without any problems.

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.)

 

[Artisi]

DubMac

Sorry can't agree you analysis of a worn pump. You seem to have based your argument that a worn pump will produce the same head with only a shift in flow, in fact the whole H/Q curve will be depressed and the closer it gets to CV the greater will be the droop in the curve.



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.)

 

[rhatcher]

About the question of "what is the problem and what is the question?"

With regard to the application, the pumps (two identical units) do successfully remove water from the drydock. The problem is that the motors draw overcurrent and run hot. The application and the associated problem have obviously existed for quite some time..... This is the reason that I want to know what the triangle points represent. If they represent the application design points, then this is, in my opinion, further evidence of a total application/pump/motor mismatch.

My interest in the triangles was because if they represented the design points then I could calculatute the minimum impeller diameter that would meet the application head requirement to be sure that we did not cut too much from the impeller.

Also, with respect to another comment; the outlet of the pumps is below the surface of the river into which they are pumping. Presumably this is because this amount of water dumping into the river from an elevated pipe at the edge of the docks would cause some surface turbulence and/or otherwise hinder adjacent boat operations.

My understanding is that the difference in head pressure is based on the difference in surface level betwen the 'well' being pumped and the reservoir into which it is pumped. The location of the piping at the bottom or the top of the reservior does not matter, it is the difference in the surface levels. Specifically, if the level of water at the bottom of the dock is 40 feet below the level of the river into which it is pumped, then the static head (not including friction loss) would be 40 feet regardless of whether the pump outlet was at the bottom of the river (40 feet deep) or at the surface.

I would add that if you piped the outlet above the river surface the then the static head would be the difference in well depth and river depth plus the elevation of the outlet above the river surface. Keep in mind that there are back-flow preventers in the underwater pump outlet so it is not necessary for the outlet pipe to be above the level of the reservoir (river) to which it is pumping. Am I wrong about this?

Finally, the question has been asked bout how this thread evolved from a simple question about triangles on a curve to a technical discussion. My answer is that you guys took the ball and ran with it. This has been to my benefit since I am not a 'pump guy' and the benefit of your experiences, comments, and insights has helped reinforce my developing theoretical understanding of the situation at hand.

Anyway...I have been away for some time on other projects while one pump impeller has been removed to be cut and balanced. The motor has been inspected during the same time with no problems found. I am thinking that the installation of the motor and pump will be in the next week or two (it's a big job).

The plan is to run each pump individually and compare the motor loading and heating to see if the change was effective. We will also look at the differential pressure to presumably (?) get an idea of the change in flow rate and/or the new flow rate. I'll have to figure that part out when the time comes.

Thanks again for the help, comments, and suggestions. I will be sure to post again once the pump is installed and the results are in.

 

[DubMac]

This has been a great thread; sometimes the most basic issues are the most misunderstood, and it is good to review fundamentals.

Now that you mention there are backflow preventers in the system, make sure that these are figured into your losses. Depending on the style, they can be significant; can find values in most any handbook or from manufacturer.

Hopefully during the test they will have good pressure guages installed at pump flanges??? Paper calculations for system head are no match for actual values. Good pressure measurements at various flows remove theoretical calculations of system curve.

Also assuming that new wear rings were installed on the impeller and/or case?? Any good pump machine shop can make some if you don't have time to get them from manufacturer. Manufacturer can provide recommended clearances.

 

[bimr]

rhatcher

Patterson pumps is still in business. I don't see why you would not pick up the phone and call them. A 700 HP pump is too valuble to waste.

The triangles appear to be the design points that the pump must achieve. The curve is the actual record of a pump test. The pump manufacturer has to a supply a pump that meets the design. The pump probably would have been rejected if the triangles were above the pump curve.