Pump design rate vs. Pump Amps 1

[Falcon03]

Hi

What are the relationship between the pump Amps and the rated flow rate?

We are maximizing the feed throughput to our Crude unit; we are running with two booster pumps out of three. Each one has the capacity of 27336 m3/D (design). While based on our current condition (actual) we are running with two pumps as I mentioned above at the rate of 60500 m3/D. Therefore, based on our data sheet we are running at over design condition if we talk about feed capacity, whereas, the actual Amps is lower than the design.

I would like to make sure that we are running under safe condition and to make sure that there is no need to run the third pump. Thanks for your help in this matter.

 

[Kawartha]

Falcon03

You can email me at rburri@pathcom.com
Forward the pump data sheets and also the pump performance curve. I don't know if you are familiar with performance curves, but the curve represents the performance of the pump based on the x-axis - flow rate and y-axis - the differential head. The performance curve will also provide pump BHP from 0 flow to runout. NPSHr curves may also be included on the performance curve. The curves also may include minimum and maximum recommended flow rates (POR).

Regards,

Richard

 

[d23]

Falcon03

A simple answer to your question is yes there is a relationship between pump amps and flow. In fact most deep well multi-stage centrifugal pumps used in the petroleum industry are controlled by “underload” or “overload” devices that since the motor current. To determine the setting point for these devices you need to use the pump curve as Kawartha explained.

In the sub-pump industry the pump curves are very clear. The x axis is flow, the Y axis is head and the second Y axis is HP and Efficiency. We don’t plot multiple stage sizes on a single curve to maintain clarity.

I have looked at other pump curves that may have several impellor sizes listed on a single curve. In some cases this type of curve confuses me. In a lot of cases with this type curve I will make a pump curve in EXCEL just using the specific stage size that I require.

I would like a copy of your data for two reasons. For me I would like an example of what not to do with curves. If the curve is difficult for the end user it is not a good curve. Second I will plot a single curve that will be simpler for you to use.

If you get a minute please send me a copy of your data.

davidlsus@yahoo.com

Thanks
David

 

[electricpete]

d23 - if you are saying there is always continuously increasing current with increasing flow, then I think you will find disagreement from the folks who have posted above. I apologize if I have misunderstood you.

 

[d23]

electricpete

I am not saying that all, but you do make a very good point that I was not clear with my responce. In fact I have argued your same point in other post.

From my perspective:

Most "deep well" pumps in fact do not continue to increase BHP, amps etc... as the flow increases. In fact we have one pump in our product line that has a higher BHP requirement at shut-in than any other point in the curve. I was only saying that in the case of deep well pumps they are typically controlled by current.

I saw another post where someone was talking about recommended pump ranges. Pumps do have a recommended range. There are exceptions, but if they are operated outside that range pump or equipment damage can occur. It does not matter if the current (BHP required) goes up or down the pump needs to shut down to prevent damage. Using both overload and underload devices can shut the pump down or prevent pump damage.

In the case of multi-stage deep well pumps they are operated in a vertical position. If you pump a well off the pump needs to shut down to prevent caviation or running a pump dry. If the well is trying to flow on it's own and you start a pump the impellors will try to climb out of the pump housing. It does depend on the specific pump, but normally I would expect both cases to operate with a low BHP requirment or low current. Both cases will lead to a premature pump failure and need to be avoided.

The question on this post was is there a relationship between pump amps and rated flow. An over simplified answer would be yes. If we had the pump curve and motor spec a more specific answer could be offered.

David

 

[Kawartha]

d23

Just a couple of comments.

First, there is a simple relationship between amps and flow and it is defined by the formula: BHP = Q x H x S.G. / 3960 x Eff (in U.S. Customary Units). Refer to my posts above. This applies to all centrifugal pumps, whether submersible or not.

Secondly, I have had extensive experience in the submersible "deep well" pump industry, and consider many of your comments regarding submersible pumps as incomplete and in some cases not typical. For example you wrote that submersible pumps are operated in the vertical position, when in fact many "deep well" pumps are installed in the horizontal position. These are used extensively to boost water pressure in municipal water distribution systems (installed in booster cans), in ships to pump ballast water, etc.

Perhaps your "deep well" pump comments should be the topic of another thread.

 

[d23]

Kawartha:

Your BHP required formula is true. We need to remember that the pump affinity laws apply to centrifugal loads. If you slow the speed of the pump down the HP required changes faster than the head or flow changes. The same laws are true if you reduce the area of the impellor(s). By lowering the speed or trimming the impellor enough the pump curve on the right hand side (low head-high flow) will eventually require less HP at 0 flow than it does at its BEP point. This can be verified by finding a pump that offers several different size impellors for the same pump. The smaller the impellor gets the more flat the BHP required curve gets.

If Falcon03 is looking for some linear correlation between flow and amps to be used on all centrifugal pumps it doesn't exist. If the question is "do amps increase with flow" they may or may not depending on the centrifugal pump design. If he is asking if amps can be predicted using a pump curve the answer would be yes in most cases. The reason I say most cases if you assume a pump has a 10 HP load, but a 25 HP motor is being used due to availability then the motor characteristics will be such that predicting amps based on pump curve information (BHP Required) won't happen.

David

 

[Kawartha]

d23,

If you apply the affinity laws you'll find that the head-capacity curve shrinks towards the 0 point, as does the bhp curve. The shape of a trimmed impeller curve after trimming the impeller is consistent with the curve prior to trimming. In other words, if the bhp curve is continuously increasing with increasing flow (typical of a low specific speed pump), this shape will also be reflected in the trimmed impeller curve. The reason that the curves (flow/head or bhp) appear to flatten is because the scale is reduced.

Keep in mind that there is a limit to the amount of trimming which is possible, and this limit varies significantly based on pump design. It is not unusual to trim typical impellers by 25% and some even to 50% (of maximum diameter), but more serious trims result in unpredictable performance because of significant alterations in the geometry of the impeller vane discharge angle, the relationship of the impeller periphery to the casing (cutwater or diffuser vanes), etc.

In addition, the affinity laws are useful in calculating expected performance at reduced impeller diameters, but as noted above, the pump geometry changes with impeller trim, and actual performance can vary appreciably from calculated results. This is one reason why pump manufacturers predict performance subject to test.

 

[BobPE]

kawartha:

Please stop marking your own posts as helpful/expert, thats for others to do and it is verry annoying.

BobPE

 

[Kawartha]

BobPE,

I have never intentionally marked my posts as helpful/expert. If I am marking them it is unintentional and I don't know how I'm doing it.

Check my Member Profile and you'll find that I have not used the helpful/expert mark on any string.

Your claim is insulting.

 

[Kawartha]

BobPE

Checked my last post and mark is there again. I don't know what the problem is, but it's not intentional. Doesn't seem to happen on other strings.

 

[Kawartha]

BobPE

After my last 2 posts were also marked as helpful/expert, I thought that there must be a reason for the repetitive mark. In checking other strings, I find that when a post is marked as h/e, then all subsequent posts by that individual are marked as well.

Possibly an apology might be in order BobPE.

 

[BobPE]

kawartha:

Thanks for your input, I will look into fixing the problem...

I do apologize, I perhaps should have worded my initial post to you a little better. I assumed that the computer is always right, a mistake that I am bound to repeat again in the future I am sure!!

Take care...

 

[Kawartha]

BobPE,

I appreciate your comments. No harm done.

If you are looking into this you may want to ensure that individuals can not vote for themselves, although I suspect that this is already the case.

 

[d23]

Kawartha, All

I actually was not trying to confuse the issue by mentioning deep-well pumps. In one of your responses it seemed that I may have irritated you some. That was not my intent either!

With deep-well type pumps we have a small OD pump and try to produce high volumes and high head. In a more industrial world you would consider my pumps to be “Medium flow, high head pumps.” I have several units operating that produce over 100 GPM with over 10,000 feet of head inside a 5 ½” OD casing. To add to the confusion in the past I have operated pumps at over 5000 RPM. This does reduce the MTBF, but the economics of the fluid being produced justified the op-X for my customer.

We do two things that make most of our pumps require less BHP at low head, high flow than they do at their BEP. We have very small diameter impellors and we operate them at high speeds. This is the pump affinity laws in both cases.

I looked up a couple links to the Mc Nally Institute for you that explains centrifugal pumps from my perspective much better than I can.

Quote one from the Mc Nally Institute:

If you are using a low specific speed impeller the pump will require less horsepower if you start with the discharge valve throttled. If you have a higher specific speed impeller the high power requirement comes at higher head so you would want to start with the discharge valve open.

http://www.mcnallyinstitute.com/CDweb/s-html/s083.htm

Quote two from the Mc Nally Institute:

• The steepness of the head-capacity curve increases as specific speed increases.
• At low specific speed power consumption is lowest at shut off and rises as flow increases. This means that the motor could be over loaded at the higher flow rates unless this was considered at the time of purchase.
• At medium specific speed the power curve peaks at approximately the best efficiency point. This is a non-overloading feature meaning that the pump can work safely over most of the fluid range with a motor speed to meet the best efficiency point (BEP) requirement.
• High specific speed pumps have a falling power curve with maximum power occurring at minimum flow. These pumps should never be started with the discharge valve shut. If throttling is required a motor of greater power will be necessary.

http://www.mcnallyinstitute.com/CDweb/s-html/s072.htm

Once again my intent was not to confuse the issue of the original question or to upset anyone. I was just trying to point out that just because you have a centrifugal pump do not arbitrarily expect the motor current to increase with the flow.

If you would like to see some pump curves that you would consider being strange let me know.

David

 

[kbander]

Falcon03,

Excuse me for jumping in so late...hopefully I'm providing something new.

You mentioned that you measured 210 Amps, but you didn't mention what your power factor is.

For a 3-phase motor,

E=Voltage
I=Aperage
eff=motor efficiency
pf=power factor

Amps(I)=746*HP/(1.73*E*eff*pf)

or

Horsepower=1.73*I*E*eff*pf/746

If a very conservative power factor was used for design (i.e., 0.82) and the actual power factor is quite good (i.e., 0.92) there can be a significant difference in the result.

In addition to the above, are you sure that your actual flowing density is identical to what you see on the pump curve (remember to use SG at flowing conditions...not standard conditions)?

Also ensure that you have calculated the flowrate at flowing conditions for reading the pump curve. The flowrates indicated in your control room will likely report at standard conditions...not flowing.


I hope this helps!

Regards,

Bob

 

[Falcon03]

Thank you Kbander for your feed back. The power factor I used is 0.85. Regarding the flow rate, we usaully refer to the actual flow.