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
If it was specified, than I would presume it is necessary.
RE: 10% head rise to shutoff
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
RE: 10% head rise to shutoff
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
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
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
RE: 10% head rise to shutoff
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
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
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
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
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
RE: 10% head rise to shutoff
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
RE: 10% head rise to shutoff
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
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
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
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
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
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
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
Thanks again for your help
RE: 10% head rise to shutoff
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
RE: 10% head rise to shutoff
RE: 10% head rise to shutoff
Maybe next time.
RE: 10% head rise to shutoff
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
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
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
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
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
RE: 10% head rise to shutoff
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