9 replies to this topic

[antator]

Hi all, just been reading up on pumps and the following struck me:

The Best Efficiency Point occurs when the pump system line and the pump operation line intersect



I just wanted to confirm my understanding that this is essentially saying that the BEP occurs when the NPSH required is equal to the desired discharged head, i.e. that there is no need for a pump since the required inlet pressure to the pump is equal to the desire outlet pressure?

If my first point is correct, then the pump will always operate on the system curve to the left of the BEP.

[Eprocess]

Actually, I cannot confirm your understandings. The NPSHr curve is the thin one below the "NPSHr" phrase. (BEP)is the point at which the impeller diameter provides the highest efficiency. Each impeller size would have its own BEP. The curve you are referring to as NPSHr actually is system curve - which is a function of fluid flow characteristics.

[Art Montemayor]

Antator:

Eprocess is correct. The Best Efficiency Point (BEP) is a pump characteristic determined by the pump manufacturer - not by you or others. You (or others) have generated the System Curve - which is determined or controlled by YOU - and not by the pump manufacturer. Therefore, there is no (or little) relationship existing between the BEP and the System Curve.

Your understandings of Centrifugal pumps and Performance Curves needs a lot of improvement. But don't give up or take my comments as negative. We have all had to go through all the headaches and head scratchings that go together with understanding fluid mechanics, fluid flow, and fluid transport. Hang in there and keep a handy bottle of aspirins ready.

[breizh]

Hi ,
Your comments are very confusing .
Please refer to the links attached :
2 good tutorials :

http://www.pumped101...tml#pump_ed_101

An other one from Goulds pumps manufacturer.

http://www.gouldspum...at_pf_0001.html

Hope it helps

Breizh

[Zauberberg]

...and keep a handy bottle of aspirins ready.

Or a bottle of Skotch - that one releases the pain as well.

[Padmakar Katre]

Hello,
The BEP (Best Efficiency Point) is the point where the speed and pressure at the impeller and the spiral stator are equal.The efficiency of a centrifugal pump is ratio of the water horsepower to brake horsepower. The highest efficiency of a pump occurs at the flow where the incident angle of the fluid entering the hydraulic passages best matches with the vane angle. The operating condition at which a pump has highest efficiency is called the best efficiency point (BEP).The specific speed and pump capacity are the basis for calculation of the pump efficiency.In most of the cases the higher the capcity, higher the efficiency rule applies.The efficiency is influency mainly by the factors such as,
1. Hydraulic effects
2.Mechanical losses and
3. Internal leakage.

Hope this helps you to save the cost for your aspirin/skotch bottle. Good luck.

[antator]

Hi all, thanks for all the responses, I can recommend the second link that breizh offered up, I found it v. helpful.

I've been reading through the material and believe I was getting confused because I thought the pump curve was referring to the Total Discharge Head and not the Dynamic Discharge Head (btw I'm going by the definitions I found here: http://www.pumps-in-...erminology.html ). Also mistook the system curve for the NPSHr curve!!!

My understanding is (when using the above definition) that when the system curve and the pump curve intersect the pump will be providing just enough head for the system to be satisfied. If you were to move to the right of this point on the pump curve the system curve would be above the pump curve and hence the pump would not be able to produce enough head to satisfy the system. If you were to move to the left of this point on the pump curve the pump curve would be above the system curve, thus the pump would be producing too much head for the system and the system curve would have to be moved upwards by way of a throttle - an obviously inefficient process.

I also understand that a pump must always operate where a pump curve and system curve intersect (otherwise the pump is producing more head than the systems needs - which would be solved by a throttle that would change the system - or the pump would not be producing enough head). This leads me to my last point, can anyone think of a good definition of the BEP since all I seem to be able to come up with is something like "it is the point where the system curve and the pump curve intersect but where the system does not include a throttle to reduce the pump discharge pressure to that required."

Edited by antator, 03 May 2010 - 12:49 PM.

[kkala]

The Best Efficiency Point occurs when the pump system line and the pump operation line intersect
I just wanted to confirm my understanding that this is essentially saying that the BEP occurs when the NPSH required is equal to the desired discharged head, i.e. that there is no need for a pump since the required inlet pressure to the pump is equal to the desire outlet pressure?
If my first point is correct, then the pump will always operate on the system curve to the left of the BEP.

For any flow rate the system curve indicates the system head, being the difference: discharge head -- suction head (head = pressure expressed as liquid height). It is not related to NPSHr, although the two curves happen to be beside each other on the specific diagram.

For any flow rate the pump performance curve indicates the head (discharge head – suction head) delivered by the pump (for a given impeller and RPM).

The point where mentioned curves intersect is the operating point; pump will deliver this flow rate and head.

The operating point should not be far from best efficiency point of the pump, operating flow and head should not deviate more than 20% from flow and head indicated by the best efficiency point (according to a practice, of course the closer the better). Best efficiency point represents pump conditions of maximum efficiency, i.e. maximum of (mass flow * head)/absorbed power.



It is noted that frictional ΔP, static difference (H), operating pressure at end of discharge (Pd) & beginning of suction (Ps) line contribute to system head
It is also pointed out the operating pressure at pump suction would be Ps+Hs-ΔPs and at pump discharge Pd+Hd+ΔPd (s=suction, d=discharge, H=static difference relative to suction centerline), so pump differential pressure (Pd+Hd+ΔPd)-(Ps+Hs-ΔPs)=(Pd-Ps) + (Hd-Hs) + (ΔPd+ΔPs). The latter is often expressed in height of liquid being the pump head.

A worked out case would clarify the matter, if confusing, that is making the system curve by calculating several points of it. Examples can be found at E. Ludwig, Applied Process Design for Chemical and Petrochemical plants, Gulf 1977, Volume 1, Chapter 3, Pumping Systems and Performance.

Edited by kkala, 03 May 2010 - 11:51 PM.

[antator]

That seems to have confirmed everything I was thinking, thanks guys.

[Zauberberg]

This brief guide may assist as well. There is a lot of quality information regarding centrifugal pumps in ChE Resources. Explore the site.

Attached Files

•  PUMPS Efficiency Basics.pdf   179KB   173 downloads