pump allowable operating range 3

[gilus02]

Dear Friends

As a mechanical engineer I have to approve vendor documents (VP) for centrifugal pumps, working in a petrochemical plant. Unfortunately some performance curves don't meet allowable operating region (70%-120%), determined by API-910. Some pumps have rated capacity in 40% of BEP capacity but the Vendor urge us to accept VP because according to his experience no problem is likely to happen.
Being under the pressure of my supervisor, shall I accept deviation from API-610 which governing out project?

Mohammad

 

[JJPellin]

I would not accept this deviation. You are comparing a theoretical pump curve against an expected operating capacity. The actual test curve for the pump you purchase may be slightly different. And the actual operating point will almost certainly be different than expected. You need to stay close to BEP for the pump selection.

In our plant, we require the selection to fall between 80 and 105 percent. If this cannot be met, we have to install a minimun flow spillback to allow us to control to that range.

Johnny Pellin

 

[LittleInch]

Mohammed, let's see if i understand your post. You have some pumps where you are being offered pumps where the duty point is 40 % below the BEP. Assuming this is capacity, then you are being offered a pump which would seem to be grossly oversized. To a certain extent the vendor is correct, the pump will work effectively, but long term the client will expand much energy as at this duty point, the efficiency will presumably be much less than the BEP??

It all really depends if your duty is relatively fixed or can vary. However in this case I would tend to agree with JJPellin, that you should not accept this deviation. If this is repeated many times I would also add up the absorbed power required compared to what would apply if you had pumps all at the proper range, you might be surprised by how much extra power you would consume. Even if you generate all your power, it isn't free..

Definitely sounds to me like the vendor is trying to pull a fast one, but stick to the required specs unless there is a very good reason to deviate. This doesn't sound like one to me.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[gilus02]

Thank you both! JJpellin and LittleInch

If I persist on performance curve rejection our vendor is likely to be totally rejected and this may cause considerable cost impact. As being a young engineer I fear to deviate project specs but my manager said don't consider higher energy consumption just tell us whether this pump operates technically good or not?
I'm completely agreed with JJpellin and LittleInch but I have made this post to see if anyone else have face such a condition and know about the operation and probable failure of a pump which operates out of the allowable operating region (70%-120%)?

Thank you in advance!

 

[DubMac]

It is not easy to say how much effect operating off BEP will have without knowing a few details:
What is the service? what fluid are you pumping?
What "type" of pump is it? single stage overhung, between bearings?
Capacity is 40% of capacity @ BEP but what is the efficiency loss in % ?? (from what % down to what %)

There are occasions where selection criteria could be more important than %BEP, heavy solids app?, viscosity?, super low NPSH....all you are getting are guesses without you supplying some of this info to us.

Other than the obvious power waste, the other concerns running far left of BEP would be increased radial loads on the shaft which can wear out bearings, seals, increase vibration.......of course this is not near the concern on a btween bearing pump that it would be on an overhung. So to provide you better info to build your case, please give us some detail.

On a side note, you don't always get what you pay for when you buy expensive, but you always do when you buy cheap.

 

[gilus02]

Thank you DubMac for your comment. I'm going to provide detailed information tomorrow.
I will appreciate if you could check this thread again.

 

[LittleInch]

It looks like you're being put in a difficult situation. Is there any one else in your company that you can go and talk to about this, chief engineer, lead engineer, discipline manager etc?

On this forum we can confirm your concerns but can't always solve your problems.

I agree with all things said by dubmac, but unless there are good technical reasons why they are operating in those ranges, then efficiency is the key issue. At the least you need to send a technical query to the vendor detailing the parts of 610 that he is not in compliance with and or sections of the data sheet and requesting a technical response. If you get overruled later on by your managers then at least you can point to this if the client complains later on.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[mirghaffari]

During the study of vendor proposal you mast cheke this point. You ralready ecived this curve . Itis better you check the approved data sheet and cureve with the final test result. In case of discepancy you can reject it.

 

[gilus02]

Here are the technical data of the mentioned pumps. It is noteworthy that I have rejected these pumps but I want you to say is there any flexibility for accepting these pumps:

http://files.engineering.com/getfil...6d64-4874-846a-f2d4a48fd9d6&file=eng-tips.jpg

Flow (m3/hr)
VIS [mPa·s]
Fluid CAUSTIC SOLUTION 5-10%

 

[BigInch]

By the definition of BEP capacity, no pump can have a 40% of BEP rating. That's different from the system capacity. Do you mean one pump has a BEP of 40% of the required system capacity?

If the pump being offered is 40% of required total capacity, Then you're looking at buying two and operating both of them at 125% of their BEP flowrate to equal system capacity. But I'd be looking at buying two pumps, each with a capacity of 50%-60% of required system capacity (at BEP) and operating them at 80-100% of their BEP flowrates.

Example:

System flowrate is 300 m3/h. Look at buying two pumps.
Power needed at that flowrate is 500 kW from each pump motor.
Each pump with a BEP flowrate of 150 m3/h and with a maximum rated capacity & motor power sized for somewhere around 170-180 m3/h at 500 to 600 kW. That power level is actually around 15% - 20% over the 100% continuous duty rated power (at 150 m3/h) of 500 kW.

Then you can operate comfortably at a BEP of 150 m3/h while at a power setting of around 80% to 100% of rated power. 80% if the motors are 600 kW, 100% if the motors are 500 kW. Pick a standard motor size at or between 500 to 600 kW and let the % of rated power at continous duty fall where it may.




Independent events are seldomly independent.

 

[BigInch]

PS You won't be doing anybody any favors if you buy the wrong pump(s).
In fact you may not be doing anything at all shortly thereafter.

Independent events are seldomly independent.

 

[JJPellin]

BigInch, I don't understand your first point. Based on the data provided, the proposed pumps would be rated to run at a flow rate this is less than 40% of the BEP flow rate. So, the proposed pumps are drastically oversized. If I have interpreted this correctly, running multiple pumps in parallel does not improve the situation.

I would use a different example. The required rated flow for their process is 40 gpm. They went to their pump vendor and asked for a pump to satisfy this flow rating. The vendor came back with a pump that has a Best Efficiency Point flow of 100 gpm. If they purchased this pump, it would be running at 40% of BEP. The vendor indicates that their pump should run well at 40 gpm. I would not recommend purchasing a pump that is so severely oversized for the required service conditions.


Johnny Pellin

 

[LittleInch]

Big bro' - You've got his problem the wrong way round. The list of data, which he posted, is that he's being offered pumps with a BEP and max flow significantly in excess of his duty point, by a factor of up to 3.

Gilus02 - with the relatively skimpy detail in your table it's difficult to be sure, but pump 103 appears to have only a small drop in efficiency for duty / BEP, but some of the others are dreadful efficiences and grossly over rated. As you haven't put head or power or any of the curves it is difficult to say much more....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[TenPenny]

It looks to me like it's an issue due to the relatively low flow rates required, combined with the wide flow range. Good luck finding an API 610 pump to handle these flow ranges, your choices will be limited. You don't give any head requirements, but you might well have to look at other options instead of API610 pumps. Look at your first example, you have a min flow of 3 with a max of 22 gpm. You might find an API pump with a low-flow impeller design that will do this, but realistically, you'll more likely have to be flexible on your BEP range.

 

[LittleInch]

Maybe I've got this wrong, but gilus, can you advise whether tenpenny is correct - I thought the max figure was the max from the pump, not your max flow?? Without the full info we can all get confused.

Is your flow range min to max or min to rated?? It does make a huge difference and could be why you're getting the response from the vendors like this.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[gilus02]

@BigInch
Dear friend; in this case we don't want two pumps working in parallel but you have indicated a point that I haven't already known. Thank you very much!
@JJPellin
Dear Johnny; You have provided a clear and meaningful example. I wouldn't recommend purchasing this pump too, but in a one special case (based on my personal knowledge) I think two soluion could be applicable, (please make me correct if you see anything wrong)
1- Impeller trim
we may asked vendor to study whether another size (larger or smaller) satisfy our condition better? Probably vendor have already done.
2- By Pass
Let the pump operating a flow near the BFP and draw a pipeline from discharge to suction to make the net discharge flow as minimum as we want. This solution requires higher power consumption but on the other hand it may reduce pump vibration, bearing wearing, temperature and probability of failure.

Mohammad

 

[gilus02]

Little Bro!

We have just determined normal flow and rated flow for the vendor and all the other data were proposed by vendor like max flow, min flow or flow at BEP.

Mohammad

 

[BigInch]

JJP, I have difficulty reading it that way. How can there be a pump with a rating of 40% (of its own) BEP. What would that mean.

Independent events are seldomly independent.

 

[gilus02]

BigInch it means that the rated capacity of this pump is 40% of the its capacity at BEP.

 

[LittleInch]

That now makes better sense, but you haven't included are what the head figures are for your normal and duty point and what is being offered. You need to look at head and flow, but it does look quite odd that a vendor with what appears to be a range of pumps is offering you something that far off the duty point. If you can avoid re-circualtion lines your life will be much easier so I would never recmoend going down that road unless there was no other option.

As you have rejected them your job is done until the vendor comes back to you, but the general consensus is that you have been correct to reject them as they seem grossly over sized which would lead to prmeature failure in the long run as well as increased power consumption.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way