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

 

[1gibson]

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?

 

[BigInch]

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.

 

[1gibson]

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.

 

[BigInch]

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.

 

[1gibson]

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.

 

[stanier]

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/

 

[gilus02]

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

 

[LittleInch]

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

 

[gilus02]

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!

 

[TenPenny]

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.

 

[Artisi]

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.)