Pump capacity is outside prefered opertaing range 1

[Eabrar]

Hi to all;
There is a problem facing to me during selecting a high pressure ammonia pump. the characteristics are as follows;

Service - Ammonia
Type - Centrifugal (Horizontal)
Pressure discharge - 156 kg/cm2
Speed - 6500 rpm
capacity - 60.9 m3/hr
Prefered Range - 78.2 to 134 m3/hr
Allowable range - 40 to 134 m3/hr

Now as can be seen that the capacity lies outside the prefered range but still is allowable range. As per API-610 the allowable range is where the limits of vibration are under control but in upper limits as given in standard.
As i am going to purchase a new pump the capacity should lie with in prefered range.
This pump is in very critical service and runs 24 hrs a day and ofcourse high speed.
Now i want to know that should i allow the vendor to give pump on the given conditions or should i say to design the pump under the said range.
Your help will be appriciated.
Thanks.

 

[Artisi]

If you are to purchase a new pump - it is up to you to specify exactly what you wish for flow, head and operating conditions. It is then up to the vendor to make a pump selection and offer what they consider is an appropriate pump/ offer. It is then your decision to accept or reject what is offered - remember selecting what you want for the project (in conjunction with the pump vendor) is your choice and decision - not the vendors.

 

[Eabrar]

Thanks Artisi for the post.
In my case there is a little problem. We have hired a contractor to do correspondense with different vendors for miscelleneous procurements. What happened was, the contractor selected a vendor to do the job and ordered the equipment. When i noticed the discrepency i urgently asked the contractor to inform the vendor about it and correct it. The vendor replied that the operating capacity is in allowable range and there will be no problem and that the pump will meet the desired process requirements.
What i referred from this that the vendor has designed it and now can't go back. Ofcourse the vendor is right about meeting requirements but what bothers me are the conditions in next few years of operation when i will not be able to do anything except to blame myself.
Now what i need is
"Do i press the situation and force the vednor to do neccessary alterations and bring the operating capacity in preferred range or there is no problem with the Allowable range". Ofcourse the things is i want a machine which will offer its best performance.
Thanks.

 

[25362]

I wonder whether reliable (witnessed) performance tests, as
established in the standards issued by official or semi-official institutions, with special provisions made by the client, and using accurate instrumentation, could reveal any potential vibration and noise problems.

 

[BigInch]

VERY CRITICAL SERVICE, CONTINUOUS OPS, AND ...6500 RPM

I'd have a look at the specification you are using. Your spec data sheet should state as a minimum the ONE required BEP flowrate and BEP head. I would also question your Prefered and Allowable ranges, as I think they are too wide to properly specify ONE API 610 pump.

According to the recommended operating ranges for API 610 pumps and operating for optimum maintenance ranges, to cover your desired flow range would require at least 2 pumps, and you would still only be in the worst recommended operational range (0.7 to 1.15 x BEP). In other words, if you had a BEP of 90, the worst maintenance catagory flow range would be minimum 63 to 103 max m3/h AND that 90 m3/hr of my example is 50% more capacity than what the vendor is offering! IMO, that's the wrong pump for you!

With two smaller pumps, you could cover 30% to 60% and 70% to 115 % of BEP, but notice the hole in the range from 60% to 70%. And we're still in the worst range catagory.

I really notice how bad the spec flows are when I see that it would take 4 pumps to get you into the Second Tier Ops range and you'd still have some holes to get through.

I'd say someone needs to rethink the flowrates and operating strategy for this system and include considerations for downtime and maintenance of the pump(s) and the rest of the equipment, before deciding on exactly how many of each should be purchased.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[25362]

One, not precisely economical, way to cope with this problem, is to operate at the recommended flow rates "spilling back" the excess to the suction reservoir.

 

[BigInch]

Assuming you have something to spill back, which would only allow doing that between 60 and 40. The only thing you can say for that pump is its in the range. However not even close to the lower prefered range. Actually this is really a process specification problem, not a pump problem at all. So what does this "preferred range" really mean? Was 60 to 134 never really required at all? Is that part of the range Entirely Optional??? Then why even mention that range? Does the spec mean that you'll take it if you can get it (basically meaning, if you can get it for free???). Right.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[Eabrar]

For 25362....
The vendor is offering performance and mechanical tests but the same will be done at the end when we have a machine built up. At that time if the vibrations are high then i think there will be very limited options. This is the right time to stop him and make as per requirements.

Well BigInch....
The pump is a 10 stage centrifugal pump of BB5 type acording to API-610 i-e Radially split multistage - double casing type.
You are right about the vibration limits of API-610 and thats what are bothering me. Will the situation cope up with this 10 stage pump. The specific speed of the pump is 960 rpm m3/min/m. This high specific speed could also be the reason of wide preferred range.
This pump creates a differential pressure of 154.4 kg/cm2.
Should we discourage vendor to give the pump with this performance.

 

[BigInch]

OK, let's consider something here ... I don't think so, but I may be confused and have misunderstood the true operating conditons vs the vendor's recommendation. I thought you are the one specifying the ranges, and the vendor has offered a pump with the stated capacity of 60. That would have been within your specified allowed range, but not within your specified preferred range. I was down that track, since it is normally the Process Engineer's responsibility to set the BEP and the head also, and any ranges of each which may be applicable, of which neither of those BEP variables have yet been mentioned, and it is the vendors responsibility to propose a pump that fits within the ranges and meets the given BEP conditions as closly as possible.

Service - Ammonia
Type - Centrifugal (Horizontal)
Pressure discharge - 156 kg/cm2
Speed - 6500 rpm
capacity - 60.9 m3/hr
Prefered Range - 78.2 to 134 m3/hr
Allowable range - 40 to 134 m3/hr

So, The first possibility (which I have already discussed) is that there is some mistake in the specification and the vendor has proposed the pump that he believes will best fit into the minimum allowed operating range that the Process engineer had given and disregarded what I thought was some apparently and mostly arbitrary "preferred range" that also had appeared on the data sheets.

Another possibility is that all the above information is entirely the vendor's proposal and the ranges are the ranges within which his pump will perform. Now if that was the case, we here on the Tips page would not be able to make any comment at all, since there is no way we would know the Process Engineer's defined acceptability criteria. I also don't see that as that being the case, since any pump should have a capacity within both a, preferred range and the allowable range, if the design was indeed optimized for some BEP in there somewhere. If it wasn't, I would not know what kind of design he was doing back in his shop. I also note given that "the capacity of 60" was the BEP capacity, then the stated ranges would not fall within API best operating ranges, so I would have to call the vendor on that one to clarify (and get it in writing) that the vendor was indeed certifying his pump to operate without problems over what would be a range well in excess of the API required range.

So to avoid further confusion, please state the Process engineer's specified BEP Q and H and the process engineer's acceptable ranges of both Q and H, and any preferred ranges he may have. Then separately state the vendor's corresponding proposed values for each of those variables.

The only way to decide if this is a good pump for the application is to know how you are going to operate this pump vs. how well your operation limits jive with the vendor's BEP and to determine how the ranges fit within the API recommended %BEP ranges. So let's get these straight first. If you wouldn't mind clarify those items... please.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[Artisi]

It would certainly be interesting to see a performance curve for the proposed unit with these "öperating ranges" shown. As the purchaser you should have access to this as well as a full engineering report on this pump unit.

There is no way I would accept anything else other than iron-clad certified engineering data from the vendor or are you prepared to accept what sounds like a very vague comment:
"The vendor replied that the operating capacity is in allowable range and there will be no problem and that the pump will meet the desired process requirements."

 

[Artisi]

BigInch - a great response which will hopefully help clear the air - if that's posible. (*)

 

[BigInch]

Thx. Clear air??? Humm.. where can I get some of that?

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[Eabrar]

Big Inch.....
The ranges given are proposed by the Vendor. We only provided normal and rated capacity.
NORMAL CAPACITY - 60.9 m3/hr
RATED CAPACITY - 70 m3/hr

Please also note that this pump has a minimum flow line. And minimum stable continuous flow is 40 m3/hr.

And as far as acceptance is concerned, i think that normal operating point must lie in the preferred operating range. Compromises are usually made in old pumps (revamp case).

Plant has a turndown capacity of 60% and overdesign of 110% of the normal capacity. So one can roughly say that pump will operate in between 60% and 110% of 60.9 m3/hr.

 

[BigInch]

What is the BEP Flowrate of the vendors pump?

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[BigInch]

And might as well find out the pump's efficiency at BEP and at 60 m3/h?

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[Artisi]

So what is the difference between normal and rated performance -- 0.1 m3hr - a bit pedantic don't you think.


60% of the rated flow is puting you very close to the minimum - where as 110% should get you to near BEP which is ok.

Can you post the performance curve.

 

[25362]

This is probably out of the scope of this discussion, but are there any considerations on the metallurgy needed to handle anhydrous ammonia ?