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10% head rise to shutoff

10% head rise to shutoff

10% head rise to shutoff

(OP)

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

RE: 10% head rise to shutoff

10% head rise is recommended enable ease of flow control and balancing pump load amongst parallel units. That's why it was specified.
If it was specified, than I would presume it is necessary.


RE: 10% head rise to shutoff

Section 6.1.11 is your key issue with the vendor.

So, in your spec / data sheet did you actually specify (not implied or discussed but actually written down)

a) This pump was designed to be used in parallel operation
b) a continuous head rise to shut off is required.

The vendor is, IMHO, just trying it on as despite the fact that the table you refer to does allow this sort of thing, it should not overwrite a requirement of the main specification "...shall be at least 10%." It does though depend on the answers above....

My motto: Learn something new every day

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

RE: 10% head rise to shutoff

(OP)
OK, we have written in our spec that parallel operation is required, however vednor stated that 10% head rise is for offered curve on proposal and on test stand tolerances mentioned on API (-5%) may lead to 5% head rise. I want to know is this claim acceptable acc. to API 610 or not?

RE: 10% head rise to shutoff

Your issue is that there are two conflicting statements in API 610, with no identification of which takes precedence.
Your claim is in section 6.1.11 with actual text below:
"If parallel operation is specified, the head rise from rated point to shut off shall be at least 10%."

The vendor is relying on this para in the testing section 8.3.1.2
"Performance tolerances shall be in accordance with table 16."

So there are two "shall be's" in those two clauses. You will claim the former is the key clause, he will claim the second...

Without any other overriding specification from you or notes to the contrary it now descends into a commercial fight between the pair of you. Is it technically acceptable to 610 - Yes I think so, is it a good idea - don't think so, especially for parallel operation.

My advise would be to "officially" reject the pump, but then ask what would be required to modify the pump to more closely match what you thought you were buying (probably a bit of impellor trimming) and then try and meet somewhere in the middle as to the impact on cost and schedule, recognising that you both have a case.

All depends on your commercial standing with the vendor (size matters). If yours is a one off then probably hard luck, if you're a customer they want to keep then your hand is strengthened.

My motto: Learn something new every day

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

RE: 10% head rise to shutoff

Please clarify one point, is the performance test you are discussing a factory test in a certified test facility for order acceptance?

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.)

RE: 10% head rise to shutoff

The 10% head rise to shutoff is not an absolute pump characteristic, it depends on where your rated flow is on the curve. If you have 10% head rise to shutoff based on rated flow of 500 GPM, then you might only have 5% head rise to shutoff if your rated flow happened to be 300 GPM. It's still the same curve, and will act the same when operated in parallel.

In my opinion, asking to satisfy this requirement when the rated flow is near minimum flow, is just onerous and is not necessary.

RE: 10% head rise to shutoff

I don't think it is onerous and unnecessary if you plan to control your flow and pump loads. A slight or flat slope will give widely varying and almost unpredictable pump surges with only a very small change in system curve resistance.


RE: 10% head rise to shutoff

Oh it absolutely needs to have continuous rise to shutoff, but the 10% is based on the rated conditions. If you're at 9% head rise, and you revise the nameplate for a higher flow / lower head that takes you to 15% head rise, the pump doesn't become any easier to control.

Often, if you don't meet the 10% head rise then it means the best fit was a pump operating near it's minimum flow, so you're already in a less than ideal situation.

You can make the pump curve steeper by not under-filing the impellers, at the cost of some efficiency. This would require a larger impeller compared to one with the standard underfile. So, probably too late there.

Separate issue from the fact that you may not really need the 10% head rise, but if the vendor's argument is that they only had to show you a proposal curve that met the spec, and don't really have to meet it on the test stand, then that is messed up. If the pump vendor has online selection software, select your pump through there and see how that curve compares to your proposal, and your test results. Curves can sometimes get a little tweaking to meet the specs, then you receive a customized curve in your submittal documents, but in reality there were no modifications to the pump, so it will perform as per the standard curve. This tactic is also known as "lying."

RE: 10% head rise to shutoff

Right, however I maintain that 10% is the minimum useful %rise to have any control. Anything less must basically be run at constant speed and whatever the pump wants to give you in terms of both pressure and flow. No controlling it.

In any case 10% is not very much to work with.
1000 ft BEP head pump (discharge pressure around 230 psi + suction) based on gasoline. A 10% rise = 23 psi over the pump's BEP flow capacity. If that is 1000 gpm, then it is 0.023 psi/gpm. If I have a pressure sensor accurate to within +/- 1%, then it's +/- 2.3 psi, so the pump can go drifting off by 100 gpm before I even get the first indication that it might be drifting. If my sensor is +/- 5%, not unusual in many situations, it's 20% change in Qbep. Now the pump is probably outside of best operation range before I even know about it.

10% is not very much to work with. If you need any kind of control, you need at the very least a 10% rise.


RE: 10% head rise to shutoff

10% from what? That's exactly the issue I am trying to bring up, head rise is not from BEP (that would be great if it were, very useful thing to compare curves.) Instead it is from the rated point. Because of this, a blanket statement that "10% minimum is required" is difficult to agree with.

If pump "A" BEP is 500 gpm @ 100 ft head, and shutoff is 150 ft head, that is 50% head rise from BEP. No problem to operate.

But my rated point for pump "A" is 200 gpm @ 140 ft head, so I have ~7.2% rise, less than the 10% rise to shutoff from rated, which is the API 610 definition. All of the sudden I have trouble operating pump "A"? No. I can't really operate much lower because I'm near minimum flow.

Now I have a different pump "B", BEP is 500 gpm @ 100 ft, and shutoff head is 111 ft. My rated point is at BEP, nice selection. 11% head rise to shutoff. Is pump "B" easier to operate in parallel than pump "A" with a rated flow of 200 gpm? Of course not.

I agree you need a certain amount of head rise to operate safely in parallel. I disagree with an arbitrary standard that is taken as an absolute requirement.

RE: 10% head rise to shutoff

Can't argue with you there about no absolute arbitrary standards. I also try to get rating points much nearer to BEP conditions. By reducing the flow "rating point" in your example, you have effectively, and in fact drastically, increased the slope; doubled it, no. Possibly reducing the need for a specified 10% rise to shutoff as well, because in a flow range that short, just about any pump of or over ANSI#300 would have a rise to shutoff well above 10%. Being that this is an API 610 pump, I will be willing to venture that the target flowrate is greater than 200 gpm, and the pump is possibly ANSI#600 pressure range, and he needs the 10%, especially if variable speed (I hope not), but I assume that mostly because 10% was what the spec said anyway. Get out to 2000 gpm and higher (a common range in my experience) and you really need to stick to 10% minimum, as those curves get to be relatively long and very flat from shutoff to BEP.

I am only trying to illustrate and emphasize the probable need to stick to the spec in this case, nothing else. I well realize that there are many reasons why you don't always need to do something for one reason or another, but here it is written into the spec. There is no ground to do otherwise without the customer's full knowledge, understanding and permission.


RE: 10% head rise to shutoff

Some more information is definitely needed. Is it really a flat curve, or is the operating point just far left of BEP? My gut says it's the latter, not a great selection to begin with, so meeting that 10% still won't help the situation much. Neither will rejecting the pump because not much can be done about it that late in the game, so realistically it is what it is.

One thing to note, the test setup will likely use a butterfly valve, there is probably some leakage at shutoff so it isn't really zero flow. A few GPM leakage might not be picked up by the flowmeter, and would skew the shutoff head reading to be low. This can be significant if it is a low flow, high head pump. The shape of the curve should tell you if this is happening or not. Depends on the smoothing, but if shutoff is not the highest head displayed (there is a peak shortly before zero flow) then this is probably happening, and the situation might not be as bad as it seems.

RE: 10% head rise to shutoff

I think we are all pretty far out on the curve right now, so to speak.


RE: 10% head rise to shutoff

Agree about being "out on the curve". I would be a good idea for the OP to post the so-called test curve - then we might just have something to discuss.

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.)

RE: 10% head rise to shutoff

(OP)
Thank for you support, it helped a lot. I have attached pump curve. Take note that rated flow is 41.25 cubic meters per hour and total head is 1055 m. it is a low capacity high head pump as 1gibson said. As you can see from curve pump is operating at 70% BEP. I was thinking about rejected pump performance tested and force vendor to have some repairs on pump ( maybe have a slightly reduced discharge connection with a slightly larger impeller simultaneously to have a steeper curve), but since yoy are saying it may be not the case I am in doubt to do so. What would you recommend?

RE: 10% head rise to shutoff

You want a pump with 52% Efficiency?
Then you're going to operate it at 70% of BEP at 42% efficiency.

First, before you reject the pump, you should reject your pump selection procedure.


RE: 10% head rise to shutoff

it would have been good to also mark on what the test results were, but I think we get the drift.

Seems to me like my learned posters above have alluded to which is that the vendor has been working on the edges of his design envelope and offered you a pump which is in reality a bit too "big". This will make your operation more expensive in operation (lower efficiency), but I don't know what your other options were.

I think the vendor is trying to pull a fast one myself, but the things to consider are this:
What leverage do you have with the vendor?
You say this is for a parallel pumping arrangement. Is the other pump(s) already there? or are you buying two from him?
If the latter then is your other pump the same?

Apart from the current response quoting the spec, have you had any other discussion with the vendor? you may need to go above your current contact to explain your situation and get some options. The contract engineer just wants to get this job complete and move onto the next critically urgent delivery, but the sales manager doesn't want to unnecessarily p*ss off a customer, so don't take a first refusal to compromise as the final statement - go round the obstruction if you can.

In any event, how are you / were you planning to control the flow through the pumps to equal it out. This forum is full of parallel pumping issues and in your case the flat curve will make it difficult to have equal flow within 10% without some form of flow monitoring or flow control to equalise between them.

So called identical pumps are not in reality that identical but you're not in great shape being significantly left of the BEP with a pretty flat curve.

We don't know where you stand on these issues therefore I cannot really provide any recommendations, just give some general advice.

Hope it helps.

My motto: Learn something new every day

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

RE: 10% head rise to shutoff

That's exactly it.
He's worried about 10% rise yet wants parallel operation of pumps working at 70% BEP, 42% efficiency. Crazy.
Start over using a new selection procedure.


RE: 10% head rise to shutoff

I think it's a little late for that if he's at the point of testing them....

OP says it is parallel operation planned, but they only seem to have ordered one pump??

My motto: Learn something new every day

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

RE: 10% head rise to shutoff

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.)

RE: 10% head rise to shutoff

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.

RE: 10% head rise to shutoff

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


RE: 10% head rise to shutoff

(OP)
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

RE: 10% head rise to shutoff

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.

RE: 10% head rise to shutoff

Please tell us who is making them?


RE: 10% head rise to shutoff

(OP)
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.

RE: 10% head rise to shutoff

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


RE: 10% head rise to shutoff

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.

RE: 10% head rise to shutoff

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.

RE: 10% head rise to shutoff

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

RE: 10% head rise to shutoff

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.

RE: 10% head rise to shutoff

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.

RE: 10% head rise to shutoff

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.

RE: 10% head rise to shutoff

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.

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