Operating near end of curve

[fe2655847]

Hello experts,

The scenario is this:
The BEP flow of the pump is near the end of curve(pump run-out condition).The pump is operating 108% from BEP which makes it very near to the end of curve. It has a considerable amount of NPSH margin of 8 feet. Is this kind of pump acceptable? If not, please state some reasons and advise. Thank you.

 

[BigInch]

If your motor is capable of delivering that power load on a continuous basis and NPSHA > NPSHR ... probably yes.

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

 

[Artisi]

Please explain what 108% from BEP means.

You should realy post a copy of the curve with your duty point(operating) marked on it - no mis-understanding.

It is not unusual to have BEP towards the "end" of the curve, actually it's not really the end of the performance curve, it's just that the manufacturer has decided not to extend the curve any further to the right as there is really no need, good practice says you should not operate out there anyway.

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.)

 

[TD2K]

I wouldn't select it. Reliability is going to be decreased running that far out on its curve. Depending on the process, the fact that it has limited upside in terms of capacity may or may not be an issue but it's one I would want to look at.

I've had a couple of these pumps and I remember the rotating equipment engineers were not happy when I showed them were the pumps were running on their curves.

 

[JJPellin]

You don't provide enough details to make a full evaluation. Some other information might be useful:

* Fluid
* Pump configuration.
* Number of stages
* Speed (rpm)
* Specific Speed
* Suction Specific Speed
* Suction Energy

It would also be helpful to see the curve.

Given the data that you did provide, there is not an obvious problem. I would not agree that an 8 foot NPSH margin is "considerable", especially if the fluid is water. There is nothing magical about the end of the curve. In some cases, this is just the flow rate where testing was discontinued. Saying that the pumps runs at 108% of BEP is much more telling. We have many pumps that run above BEP by 5 to 10 percent and run quite well. But, if this was a Sundyne running at 21,000 rpm, it could be disastrous. This might result in cavitation within the diffuser throat. If this was a vertical turbine pump with 35 stages, this could be disastrous. This might result in up-thrust, shaft buckling and catastrophic failure. But, if it is an ordinary, horizontal, single stage overhung pump, running at 1800 rpm with a relatively low suction specific speed, it might run well and be trouble free.

We set our limits for maximum flow relative to BEP based on specific speed and suction specific speed. For most services, we allow flows up to 120% of BEP.

Johnny Pellin

 

[TD2K]

It would be interesting to see a pump curve. I took "The pump is operating 108% from BEP" to mean the pump is operating at about 2x the BEP flow given they said the operating point is near the end of curve.

If it's 108% of BEP then I wouldn't be concerned if it's a typical horizontal centrifugal pump.

 

[Artisi]

think the BEP and operating point are confused at this stage.

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.)

 

[fe2655847]

The pump is a horizontal end-suction with a speed of 3550rpm, suction specific speed of 4318(USC). Flow at BEP is 45.91gpm and duty point is 50gpm which makes it 108.91% of BEP.

http://files.engineering.com/getfil...e-40fa-bc86-2745fb9625fe&file=Pump_curve.docx

 

[fe2655847]

What makes it uncomfortable is that the BEP is near the end of curve with its duty point operating 8% more than BEP point(as seen on the attached curve). The duty point must be within the "preferred operating region" ie, 80%-110% of BEP, as the general practice says. The pump is within that preferred boundary being 108% but its near the end of curve. Is this acceptable as long as pump operates not beyond the curve? The pump is a horizontal overhung end-suction with NPSHA>NPSHR by 8 feet. Thanks

 

[Artisi]

Why don't you ask Sulzer, at the end of the day they are the experts on their own equipment.

From curve you supplied it looks ok. What is shown is a curve which is drawn only to the duty point, in real-life it would extend further to the right but without having a full performance curve for this impeller diameter it's hard to say exactly what will happen beyond this point for any upset conditions.

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.)

 

[JJPellin]

I would not see this as a problem. Low Nss, single stage, low energy. As noted by several of us above, the end of the curve is somewhat arbitrary. And, further, this is a "book" curve, not an actual test curve for the pump you are purchasing. I would request that the pump be operated out beyond 55 gpm for the final acceptance test. This will give you better data for the performance of the pump even further to the left.

Johnny Pellin

 

[BigInch]

The "end of the curve" is more limited by the faster swing to high NPSHR values than anything else. 8% over BEP isn't enough to worry about, as I indicated before, as long as you have that NPSHA.

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

 

[Artisi]

If you believe the curve, then NPSHr looks as if it is about to increase rapidly - not surprising as the impeller had been reduced dramatically - only a full test curve will give you correct reliable information.

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.)

 

[psv1990]

A selection to the right of the BEP is more common when you are operating the pump with a drive on a pump that is normally operating below the design capactity. If the pump is normally going to operate at the design point, this pump will have less ability to operate if the system curve is changed or miss calculated. Selecting a pump with the duty point to the left of the BEP allows for a greater safety factor.

 

[ApplicationENG]

fe2655:

Recommendation - I would go with the selection based on the information provided, so long as NPSHa>NPSHr. If you have questions, i would contact Goulds Pumps - they are extremely helpful and will sit down and explain the reasons and though process behind it.

Reliability - the statement made previously that the pump wont be reliable is not completely true. Operating well below or past the BEP will produce increased vibration which in turn kills seals and bearing (thus the pump being unreliable). ALTHOUGH, this statement is true based on the Nss of the pump. If you have a low Nss pump (such as you have) the prefered operating region is much wider than if you had a high Nss. This is why it is acceptable to run this pump.

End of Curve - A previous users statement is correct, just because the line stops on a curve doesnt mean that it cant operate past run out flow. The reasons behind this are as follows: NPSHr may go crazy after a certain point so OEM's limit the flow, pump may exceed vibration limits after runout so the flow is limited, efficiencies may plummet after a certain flow, etc.

Selection Criteria - API pump selection criteria is 80%-110% BEP. No questions asked.

Let me know if you have any more questions.

 

[clay87]

Application - For the most part your posts have been informative and a pleasure to read. Be careful not to push too hard. Respectfullly, Clay

 

[fe2655847]

Thanks for the help guys. Appreciate it.

 

[ccfowler]

Commonly, the actual performance characteristics of an individual production pump may vary from the published generic performance characteristics for the pump model by as much as 10%. Most pumps will perform closer to the generic performance characteristics, but it is best to allow for reasonable margins for variations in the design and selection process. The criticality and potential hazards associated with the particular application weigh heavily on the margins that may be tolerable. I agree with others that this may be a tolerable selection, but the obvious increase in the NPSHr curve suggests that an uncomfortable situation may be reached very close to the chosen operating conditions. Unless there are compelling reasons to accept this situation, I would seek a more conservative pump selection.

Where generic pump performance curves are being used, I think it is best to presume that operation at or near the ends of the published curves should be avoided whenever possible. The ends of the curves may simply represent the end points of testing, but they can suggest the development of operating or durability problems beyond those points. Usually, the ends of the NPSHr curve is the best guide to the tolerable operating range for a particular pump.

Commonly, the published performance curves are based on a constant, synchronous shaft speed (1800 rpm, 1500 rpm, 3600 rpm, etc.), and the actual installed performance characteristics may vary according to the actual slip of the driving motor. While these effects are typically minor, this may not be a trivial matter when the chosen operating point is close to the ends of the published curves. This is yet another reason that I would want to avoid the selection that you are considering.

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.