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pump allowable operating range
3

pump allowable operating range

pump allowable operating range

(OP)
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

RE: pump allowable operating range

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

RE: pump allowable operating range

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

RE: pump allowable operating range

(OP)
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!

RE: pump allowable operating range

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.

RE: pump allowable operating range

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

RE: pump allowable operating range

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

RE: pump allowable operating range

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.

RE: pump allowable operating range

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.

RE: pump allowable operating range

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.

RE: pump allowable operating range

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

RE: pump allowable operating range

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

RE: pump allowable operating range

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.

RE: pump allowable operating range

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

RE: pump allowable operating range

(OP)
@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

RE: pump allowable operating range

(OP)
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

RE: pump allowable operating range

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.

RE: pump allowable operating range

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

RE: pump allowable operating range

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

RE: pump allowable operating range

Without information about the head required, nobody can tell you if the service is impossible, or if the specific vendor just doesn't have a good fit.

So.... Just ask them for a lower specific speed selection and see what turns up?

RE: pump allowable operating range

Gilus02,

BEP is defined as THE PUMP'S FLOW RATE AT THE POINT OF [b]BEST EFFICIENCY[/b].

Are you actually saying that your pump has a rated capacity of 40% of its own best efficiency point?
Why in the world would a pump have a rated capacity of 40% of BEP.

I CAN'T MAKE ANY SENSE OF THAT.

Who is this pump manufacturer and what model is it?

Independent events are seldomly independent.

RE: pump allowable operating range

BigInch, rated flow on the datasheet is 40% of the proposed pump's BEP.

So the pump only has "rated capacity of 40% of it's BEP" if they purchase that pump for that service.

RE: pump allowable operating range

So you're going to buy a pump with a BEP flowrate 2.5 times what you need? Is that it?
I'm outta' here.

Independent events are seldomly independent.

RE: pump allowable operating range

Of course I would never do such a thing, and the original poster did mention that he has rejected the selection.

But yes, the question was "would it ever be acceptable to purchase pumps to be operated at 40% of BEP" because he had management pressure to accept that (terrible) selection.

RE: pump allowable operating range

Mohammed,

You are the professional . Say "no" to the boss. The boss is trying to bully you. All bullies are cowards. If you tell the boss you cannot recommend the pump but he is free to override you what can he do? The boss will either back down or get someone else to sign off. You might like to look for another job as you dont want to work for someone like that anyhow.

Engineers should stick by their professional ethics. Their duty is to the public interest and good first and the company they work for second. If you dont believe this then get out of the profession.

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King
http://waterhammer.hopout.com.au/

RE: pump allowable operating range

(OP)
Thank you all for your help. I think it's better to tell what's really going on. I have been assigned to a project which was formerly handled by 30 years experienced engineer. We have got about 30 pumps in this project which 4 of them have deviated the project specification and API-610. Before the elder colleague being retired I have asked him why you have accept this deviation and he told me “my son I have seen pumps which worked properly in 30% of the BEP’s capacity, and as our vendor couldn’t provide us any better offer for this condition I neglected this deviation”
Being involved in Oil & Gas industry for just 3 years I couldn’t take this risk and approved the standard deviated pumps. My boss (not so bully!) said if you don’t accept these 4 pumps the vendor is likely to be rejected and he will make financial claim. He added because the rest of the pumps haven’t deviated API is there any way for accepting these pumps?
I replied “No Sir!” but inside my mind I was looking for a way or experience which could convince me that these pump will work properly.
I have attached the performance curves for ones who want to evaluate this case in detail.
Thank you all!
Mohammad

RE: pump allowable operating range

Mohammed,

Thanks for the explaination which I'm sure reminds us all what it is like to be starting out in this field. I can't quite work out why rejecting four pumps out of the entire package and asking the supplier to come up with some alternatives would result in rejection of the vendor and a claim, but that's your issue. Hopefully they will come up with something better aligned to your requirements, but if not then you or they may need to source from elsewhere if the rest of the package is technically acceptable and good value. Like anyone or company, one company may not be able to meet ALL yur requirements in-house, but if they can do 90% then the remaining 10% may need to be placed elsewhere. Makes for another interface, but sometimes that's what you need to do.

Looking at the curves, 103 is probably OK - it's a small pump and would physically function at this duty point - which I think is where your colleague was coming from.
However 110 has an appalling efficiency (9% !!) and not much better at BEP ( 20%) so this looks like a very poor pump choice by the vendor
108 is also a decent size and has a low percent efficincy at duty point to BEP (about 60%) so an alternative should be sourced.
116 is not that big so if forced to accept would do the duty Ok

I hope that our advice has been valuable to you and good luck in getting better pumps for your required duty.

Next time try to tell us the whole story with info up front - it makes it a lot easier for us to give you a more tailored response.

My motto: Learn something new every day

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

RE: pump allowable operating range

(OP)
Little friend! I really appreciate your taking the time to share your thoughts on this. In the beginning of the post I thought that people here could make some general guidelines regarding my problem but I'm realy impressed by some comments which one of them is yours.
Thank You!

RE: pump allowable operating range

I know that it is hard for some to believe, but often times there is not a perfect pump selection for your desired operating point.

Pumps with very low efficiencies are likely low flow pumps, which tend to be very inefficient, but let's face it, if you're only talking a 20 hp application, the low efficiency doesn't mean much in the big picture.

In life, sometimes, you have to compromise.

Instead of getting all pizzy over terminology, or insisting that your pump vendor offer you something that isn't made, why not stop and look around, ask a few questions. Maybe the retired guy actually knows what he's talking about.

RE: pump allowable operating range

gilus02:
there has been lots of back and forth, comments, suggestions, ideas advice, however, if you want proper advice that means something and might be helpful it is now your turn to post the pump curves and the duty / system curves superimposed over the pump curves for all conditions for those pumps you are not happy with. Having this information there are plenty of people here who can then point you in the direction with proper engineering advice based on experience instead of trying to second guess what you are talking about.

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.)

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