10% head rise to shutoff

[SDRL]

Hi,

i would be grateful if you can help meon below case:

According to API 610, for parallel operation 10% head rise to shutoff shall be considered. We have ordered a pump last year which showed smoothed curve with exactly 10% head rise from rated point to shutoff. However, one week ago on performance test the head rise to shutoff was 4%. Vendor claim that since according to API tolerance table (Table 16 of ed. 11) shutoff can be decreased to -5% and also rated point can be increased +3% so it is acceptable. There is a note also in API table in which stated that if a rising head flow curve is specified (see 6.1.11), the negative tolerance specified here shall be allowed only if the test curve still shows a rising characteristic. My question is that whether it is necessary to pump follow 10% head rise on test or not.

thanks

 

[Artisi]

I'm going to play the devils advocate here;

Was the pump vendor given the flow and head required at the time of order?
Why was this pump accepted as being suitable - operating at 70% of BEP?
is the pump running at of has it been corrected to its quoted speed?
is the pump on curve at the 41.25 / 1055?
It's a big ask expecting a pump of this size to be reliable in terms of H/Q so far left of BEP, unless of course the vendor has good experience operating in this region of the curve.
I assume this is a multistage pump unit, have the impellers been carefully checked for surface finish, blade shape, spacing, has the pump case stages also been checked likewise?

How sure are you that the operating conditions will be 41.25 / 1055, is this a known or a calculated condition?


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.)

 

[1gibson]

Do you have a curve that has the actual data points taken from the test, or do you have the raw data so you can generate your own curve in excel? That curve looks way too smooth, maybe the data is still being massaged?

The selection isn't that terrible. Low specific speed pumps like this, you can't expect 85+% efficiency at BEP. There are valid reasons for these selections. Keeping MAWP down is one reason to pick something with a rated point left of BEP. Based on a quick hp check, fluid looks like 1.0 sg so I get about 1970 psig MAWP (shutoff head x 1.1, assuming 0 suction pressure.) A pump that meets the conditions at BEP would have higher shutoff head and that might not be acceptable. Maybe the spec also prohibits 3600/3000 rpm pumps, and this is the best you can get at 1800/1500 rpm. RPM not shown on the curve so no idea if that applies here.

 

[BigInch]

Why not just buy a pump that has a BEP head, flow and rating somewhere around what you need?
Operating away from BEP reduces efficiency and increases maintenance issues as Artisi has already noted.
I have speced pumps for these heads and higher, 1750m, and required API 610
They were multi stage horizontals.
They all had far better efficiency then this one.
They are expensive and you don't want maintenance trouble.

Too many maybes for me.
Bye. Happy Trails

 

[SDRL]

Thanks. However as I asked from my colleagues, at that time it was a best pump on technical evaluation to select from. I am still not sure whether it is easy to find a centrifugal pump with much better efficiency since these BB pumps are working at low capacity and high head. BTW, we are ordering these pumps (all three ones) and we have to deal with this fact regardless of what wrong was made beforehand. Your comments help me to have a final negotiation with manufacturer to decide what can we do at this time.
Thanks again for your help

 

[LittleInch]

Fair enough. Please let us know how it works out when you have finished your "negotiation".

Hope it has helped you.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BigInch]

Please tell us who is making them?

 

[SDRL]

I contacted sulzer and found that the best BB5 pump they can offer is with 57% efficiency which is about 16% more efficiency than the one we selected. However they told me that head rise to shutoff is an issue for those pumps as well.

 

[BigInch]

I am sure that you could do far better with other mfgrs.
Maybe next time.

 

[GPRnD]

Just to mirror the other comments:

The selection point is kind of bad. Your life cycle cost will be significantly higher as a result.

The 10% API 610 requirement is a bit of a fudge IMO. What matters at least as much is the matching between pumps in parallel (when they are fixed speed). I didn't see much discussion on this and how well the pumps match on test.

API 610 12th edition is going to have a revamp of the parallel pumping requirements to include provisions for pump matching.

My interpretation of the API 610 requirement is that if the supplier quoted 10% HRSO and tested at 4% then they are out of tolerance. They need to be able to demonstrate a minimum of 10% - 5% tolerance = 5% HRSO.

I tried our selections and I can get a BB5 at 96% of BEP with 14% HRSO @ 60Hz or 86% of BEP with 12% HRSO at 50Hz. Efficiencies would be in the 45% to 50% range. So the point is that depending on your goals there are probably a few different suppliers you could shop.

 

[ccfowler]

Having had some experience with pumps operating in parallel working in similarly poor regions of their curves, I suggest that you not be surprised by maintenance problems being much worse than expected. Your best hope may well be for them to hold up long enough to get from one scheduled shut-down to the next and then needing to do a very thorough repair during each scheduled shut-down. These recurring needs may well include repair of surprisingly substantial casing erosion damage.

If the actual head vs. flow curves prove to be flatter than expected, the actual slip characteristics of the individual driving motors can become a part of the operating problems with sharing the load between the pumps. Normally, the published pump curves are based on operating at synchronous speed throughout the entire range while during actual operating conditions the each pump operates at the speed where the required driving torque matches the available torque its induction motor's slip speed that produces that torque. In most cases, this is a matter of trivial concern, but when the real pump curves are too nearly flat or the pumps are too imperfectly matched, it can significantly influence the sharing of the flow between the pumps due to the affinity characteristic of the head varying with the square of the shaft speed. Also, where pump matching is already problematic, details of any differences of the piping associated with the individual pumps should be given all due consideration.

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.

 

[GHartmann]

My goodness this is not that hard.

The 10% rise in head to shutoff is specified to protect pumps in parallel operation. Otherwise the pumps can become unstable and ultimately cause damage.

Look up the McInnally institute and find the section on parallel pump applications

 

[GPRnD]

That part is understood, however just buying 2 pumps with >10% head rise to shutoff will not assure successful parallel operation.

The matching of the curve shapes of the two pumps is at least as important and is something that sadly API 610 doesn't currently provide guidance on.

 

[1gibson]

If you look at McNally Institute page on parallel pumping, you will note that "10% head rise" isn't mentioned. The word "rise" isn't even on the page. It is all about fully evaluating the specific operating scenarios.

So while I agree it's not that hard, it is harder than blindly following the spec. 10% head rise to shutoff depends too much on the design point, and needs to be looked at more objectively. A "good" steep curve with low rated flow can fail to meet this requirement, while a flat curve with high rated flow can meet it. It doesn't make that flat curve better for parallel operation than the steep curve.

To be in the situation with low rated flow (relative to BEP) means that you've already made compromises, with a less than ideal pump selection. But you work with what you can get, and throwing up more road blocks without a detailed review doesn't help anybody.

Now if the vendor fudged the proposal curve knowing full well it wouldn't perform that way, then I wouldn't be happy, but it still doesn't affect whether or not you can safely operate the pump *due to the head rise to shutoff.* The other potential issues with the pump selection (low % of BEP, shorter maintenance intervals) are not the topic of discussion, and presumably have already been considered before purchasing the pumps.

 

[BigInch]

see the rising head section,

 

[Pumpsonly]

Most of the postings have concentrating on the pump curve shape. For parallel operation need to look at the system curve also. If the system curve is steep ( mostly friction), the % head rise is not so critical.
If the system curve is relatively flat, (short piping , low friction and high static discharge head.) then the % head rise will be critical.

 

[SLR33]

Please recheck the type of pump before taking new action. Apparently for such high pressure - low flow process condition centrifugal pump is not desirable and low efficiency and flattened curve from rated to shutoff is still expected even by other mfgs.