Question about r.p.m. (1.500 vs. 2.900??)
Question about r.p.m. (1.500 vs. 2.900??)
(OP)
Hello all:
Regarding an installation I posted about in another thread of mine, (that's just a reference), I have collected 3 different offers for it.
The installation consists in an elevation from a deposit 300 mts. (985 ft.) high and 300 m3/h. (1320 us gallons/m)
Manometric height estimated in 340 mts. (1.115 ft.)
1) KSB MULTITEC A-125/3 10.1 (350 KW 300 m3/h 2.980 rpm)
2) RITZ 49200-11 (400 KW 330 m3/h 1.500 rpm)
3) PLEUGER 200 NM B (400 KW-330 m3/h 1.500 rpm)
From what I have read, the KSB meets all characteristics and requirements the installation needs, but the owners of the installation are used to 1.500 rpm pumps and are very skeptical about the reliability of the 2.980 ones.
Are they right to be so skeptical?. What can be in general the main withdraws for higher speed pumps?
NOTE: I wrote the main performances data in case I could be missing something, but I'm not requesting specific advice for the election itself. You are welcome for any suggestion, but my main concern is about the requirements or possible problems of the "fast" pump regarding the others.
Thanks and regards
Regarding an installation I posted about in another thread of mine, (that's just a reference), I have collected 3 different offers for it.
The installation consists in an elevation from a deposit 300 mts. (985 ft.) high and 300 m3/h. (1320 us gallons/m)
Manometric height estimated in 340 mts. (1.115 ft.)
1) KSB MULTITEC A-125/3 10.1 (350 KW 300 m3/h 2.980 rpm)
2) RITZ 49200-11 (400 KW 330 m3/h 1.500 rpm)
3) PLEUGER 200 NM B (400 KW-330 m3/h 1.500 rpm)
From what I have read, the KSB meets all characteristics and requirements the installation needs, but the owners of the installation are used to 1.500 rpm pumps and are very skeptical about the reliability of the 2.980 ones.
Are they right to be so skeptical?. What can be in general the main withdraws for higher speed pumps?
NOTE: I wrote the main performances data in case I could be missing something, but I'm not requesting specific advice for the election itself. You are welcome for any suggestion, but my main concern is about the requirements or possible problems of the "fast" pump regarding the others.
Thanks and regards





RE: Question about r.p.m. (1.500 vs. 2.900??)
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: Question about r.p.m. (1.500 vs. 2.900??)
Sorry, it's just normal quality water.
Regards
RE: Question about r.p.m. (1.500 vs. 2.900??)
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: Question about r.p.m. (1.500 vs. 2.900??)
I'd be more skeptical of that fact that the 2980 rpm pump isn't quoted to your design flow (300 m^3/hr while other vendors quoted 330?). Maybe that is a typo.
If you do convince them the ignore their experience (or superstitions) about 2 pole pumps, and the pumps have any issues or require more frequent maintenance than 4 pole units they are used to, they will never forgive you...
RE: Question about r.p.m. (1.500 vs. 2.900??)
Most pulp mills avoid anything over 4 pole speeds if at all possible.
If the liquid is clean with no abrasives, and the pump is properly sized, there is no reason to avoid 2pole, however, if there are particles, wear would be much faster.
That said, the usual issue is 2pole = smaller and lower capital cost, which is often why pump vendors will offer a 2pole selection where suitable.
RE: Question about r.p.m. (1.500 vs. 2.900??)
The slower rpm pump will be a larger and sturdier pump and will last much longer. The larger size is the reason that the pump will cost more. Experienced users will prefer the slower rpm pump because they know that the slower rpm pump will have the lower overall cost over the lifetime of the pumping system than the high rpm pump.
In addition, pumps in this capacity range are typically specified as 1500 rpm.
RE: Question about r.p.m. (1.500 vs. 2.900??)
Well, there seems to be a little controversy, but I guess the way to analyse the possibility of the 2 pole pump implies to be aware about the water quality, so I'll have to get deeper on that.
The fact is that 2 pole ones are widely offered and sold and this wouldn't resist if they didn't have a solid market.
1gibson: The different flow is because they give the desired height at that rate; I couldn't get a lower flow from those makers that fits my requirements.
Regards.
RE: Question about r.p.m. (1.500 vs. 2.900??)
thread237-138757: 2-Pole vs 4-Pole Motors
I have a perception that people who design machines and walk away tend to have no objection whatsoever to 2-pole machines, and people who maintain machines over long periods of time don't like 2-pole machines.
I fall in the latter category. We certainly have plenty of 2-pole machines running fine. But if you look at the list of our troublemakers that give us the most headaches again and again, there is definitely disproportionately high fraction of 2-pole machines in that list.
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(2B)+(2B)' ?
RE: Question about r.p.m. (1.500 vs. 2.900??)
The other guys around here know much more than me about the other stuff.
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(2B)+(2B)' ?
RE: Question about r.p.m. (1.500 vs. 2.900??)
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: Question about r.p.m. (1.500 vs. 2.900??)
The most common (nominal) speed for electric motors is 2900 RPM. However, 2900 RPM is not the most common speed for a medium sized water pump.
At 2900 RPM, the pump are cheaper since they require fewer stages to achieve the required pressure. However it is important to note that the higher speed dramatically increases (more than doubles) the rate of wear for the pumps (in particular) and the motors.
Consequently the 1800 RPM pumps are preferable when considering the Life of the System (Total Cost of Ownership).
See the linked page from Pumping station design By Garr M. Jones.
The recommendation is to buy a lower speed pump (900 RPM, 1200 RPM, or 1800 RPM).
Listen to the Owners of the installation.
RE: Question about r.p.m. (1.500 vs. 2.900??)
jacilore - Can you find out the bearing sizes (for example 6313 or AFBMA 65BC03) on the pumps and motors as well as the lubrication of each (oil or grease)? From these we can calculate D*N and see how they stack up against industry thumbrules. That is one of the more easily quantifiable effects of increased speed.
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(2B)+(2B)' ?
RE: Question about r.p.m. (1.500 vs. 2.900??)
Good point. It stands to reason that would not be quite as bad.
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(2B)+(2B)' ?
RE: Question about r.p.m. (1.500 vs. 2.900??)
I trust this is the pump application you spoke of some time back?? Water pump I think.
Wear in pumps increases as the CUBE of the speed increase. That is, raising the speed by a factor of two increases the wear by a factor of 8!
Owner has every right to be concerned. Typically, the only reason you would use a 2-pole motor here would be if the head was so high that a 4-pole could not do it.
Yes it will be a larger, more expensive pump now, but the operating costs over lifetime will be greatly reduced. Especially if there are any solids in the water at all.
Are these submersible motor type well pumps??
RE: Question about r.p.m. (1.500 vs. 2.900??)
Wear: Yes wear is a consideration. But most wear in a multistage pump occurs on startup, shutdown and severe transients. In operation the pump should be operating without contact at the wear rings.
So in this condition the "wear" rate at the wear rings is a function of the solids in the water and the fluid velocity across the wear ring.
Keep in mind that the 4 pole machine will be bigger so the fluid velocity across its wear rings may not be much less than the 2 pole machine. If errosive wear is a concern here, a better choice would be to upgrade the wear rings materials.
Life cycle cost: Clearly the 2 pole pump will have a lower first cost. But then you need to factor in other issues. The 2 pole machine will require more NPSH which might require higher tank elevations or even a booster pump (extra $). The 4 pole pump will likely be a lower specific speed and hence less efficient (extra operating $).
Only you can decide what parameters are important.
However I will say that I generally only see demand for 4 pole pumps in mine dewatering type applications (due to solids wear). For most other clean fluid applications 2 pole seems on balance to be a better choice considering the total lifecycle cost.
I'd caution you also to require HI Class A test tolerances. In my experience many manufacturers are "optimistic" with their efficiency claims. HI Class A prevents that by not allowing a negative efficiency tolerance. That way you know what you are getting.
RE: Question about r.p.m. (1.500 vs. 2.900??)
Bear in mind that pumps in refineries are built to API standard which are much more robust and sturdy.
The type of pumps in discussion are multistage ring section pump built to water industry standard.
There is always a trade off for a higher speed pump ( less stages) over low speed pump ( more stages.
RE: Question about r.p.m. (1.500 vs. 2.900??)
I will reply and ask with more detail a little later when I have more time; only for the references (Dubmac); yes, this is the case of my other thread, the water is good quality, but I'm not sure of the exact TSS rate.
And the pumps are horizontal multistage centrifugal (non-submersible)
Regards and thanks again.
RE: Question about r.p.m. (1.500 vs. 2.900??)
Looking at your Conditions Of Service: 1320 gpm @ 1115 ft head, I doubt you will be able to get to the head at 1500rpm without a VERY large pump (many stages). Therefore it may be absolutely required to go with 2 pole; your customer may be painted into a corner.
However, there is no doubt you will have vastly accelerated wear in 2 pole pump; especially with any type of solids. The one advantage of the 2 pole over the 4 pole selection is that your rotating assembly is much shorter (less stages), and smaller.
Bradshsi is right, most all wrecks and troubles in pumps happen at startup; but must contend with the next point that wear rings shouldn't contact during operation. True in theory but misleading in practice.
In "perfect" operation, the rotating assembly benefits from the "Lomakin" effect in which circumferential fluid forces tend to "center" the assembly. However, any upset in operation: solids, downstream pressure fluctuations, imbalance, other vibrational issues etc. will cause the assembly to "bump" the rings; and a bump at 3000rpm is much more destructive than one at 1500rpm.
Once the wear begins in a 2 pole pump, it will get worse much quicker than in a 4 pole. When the shaft sees wear, there will be a slight imbalance and vibration will begin to increase at faster rates. effects are compounded much quicker.
Unless you are forced to go with the 2 pole due to physical size limitations (too large, too many stages), go with the slower pump. You will sleep better at night and so will your customer.
RE: Question about r.p.m. (1.500 vs. 2.900??)
Solids, unless they are solids of a size that causes transient rotational imbalance, you are not going to see deflection of the rotor.
Pressure fluctuations, similarly do not significantly affect radial load in diffuser style pumps and hence do not cause rotor deflection.
Imbalance, this does affect rotor deflection since the rotor will take on shape dependent on the imbalance. That said if you have rotor imbalance sufficient to deflect the rotor the amount of the wear ring clearance, you have bigger problems since your rings will wear out very quickly (by quickly I mean a few hundred hours at best).
If you work out the Lomakin centering forces (as we do when designing this type of pump), you can see that they are very substantial and can easily overcome both rotor weight and normal levels of imbalance (say G6.3)
Looking again at a 4 pole pump. Keep in mind the pump will be bigger with a lot more stages. That contributes to rotor sag and more work for the Lomakin effect to overcome. In addition because you are running slower the Lomakin effect is weaker.
This is why we typically worry more about slow running multistage pumps. As you go slower at some point the Lomakin effect become insufficient to prevent rubs or to prevent a lateral critical speed.
I had an instance recently where a customer was slow rolling a multistage pump at about 1000 RPM for an extended period. It made quite a mess of the wear parts.
Another illustration that slow is not always better is the move to "Advanced" class boiler feed pumps in the 1970-1980s. Previously boiler feed pumps had lots of stages running at 2 pole and suffered for it during the kind of severe transients you get on this service. The "Advanced" class pumps were 3 or 4 stage pumps running at up to 6000 RPM with stiff shaft designs. We designed those to pretty much run forever since it was next to impossible to get wear ring contact even under a transient of loss of suction event.
RE: Question about r.p.m. (1.500 vs. 2.900??)
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: Question about r.p.m. (1.500 vs. 2.900??)
Jacilore, it looks like you will have to tell your customer that they are stuck with 2 pole due to head. See if KSB can even get a selection at 1500rpm, maybe not, it would be very many stages.
Must admit, I was thinking more along the lines of an axial split multistage Volute pump for this service. Just too much head here however to do that at 1500rpm. I stand corrected for even considering the 10 stage monstrosity that would require.
Continuing the discussion on 2 versus 4 pole in general however, and just for discussion's sake, I would have to stick to my guns on always recommending a slower pump if possible; especially in this service.
Bradshi, you obviously know your pump designs and your contentions on rotor stability are certainly correct...as long as we're talking about diffuser pumps in high energy services,i.e. barrel type diffuser pumps. Im having a hard time agreeing that faster is better in any way in terms of reliability.
The stiff-shafted rotors we've been building since the 80s are definitely better machines than than the spaghetti shafted 12 stagers used prior; and they CAN operate 24hrs a day for a year or two without need for substantial element rebuild. But we are now talking about an elevated class of turbo-machinery found in refinery charge pumps, decoking jet pumps, boiler feeds, etc.
Sold those pumps for years and they were always my favorites; truly the pinnacle of pump engineering. Speed does not kill in that world.
What is being offered to Jacilore here has nothing in common with that class of machinery however. This is for use in a municipal water delivery service, and I would say we are looking at an entirely different mentality. There is no Reliability Dept., and Maintenance Dept. is a guy with a truck and set of wrenches. Engineering is subbed out to consultants. It is required to buy low price, unless you have a strong consultant to hold spec. There will be very little monitoring eqpt., and what there is probably will never be looked at. That has been my experience with all except for the large-city municipalities.
Two pole speed pumps are almost non-existent in the Muni world; 4 pole is considered very fast. One of the reasons is that water, aside from being the most corrosive and destructive fluid we know of, typically contains sand fines that cannot be filtered out without great expense. Pumps in the muni world are designed to a large part with this in mind;.....THERE WILL BE SAND.
Now, I'm just remembering stories about the "pancake" pumps that we're looking at here (segmented ring section, if you will). I never sold them, but I sure replaced a lot of them with other styles. They can really be a Maintenance nightmare and they are parts monsters as well. If they are taken apart by anyone that is not a true Master Pump Mechanic; they almost never run right again; too many internal alignment issues.
Now I know KSB and Pleuger are both fine manufacturers (don't know the other one), but 3000rpm with sand fines in a ring section pump being maintained by government employees may be stretching the envelope too far.
Unfortunately, these Conditions of Service really limit the options. Hope there is no performance bond or extended warranty required for Jacilore's sake.
Please don't take my disagreement as argument for the sake of argument (I am, howver, a crabby old man); it is intended to: 1. help Jacilore with his decision, 2. further the discussion on these matters; the science is NOT settled, and 3. give me something to do this morning.
RE: Question about r.p.m. (1.500 vs. 2.900??)
Shaft speed is an important decision. Speed affects pump component wear as well as the pump size. High speed pumps cost less initially, but the maintenance costs are substantial.
If you double the speed of a pump, expect almost four times the shaft whip, wobble or run out and eight times the wear.
The wear rate of the components varies by the cube also
Example: At 1750 rpm. the impeller material is wearing at the rate of 0.020 inches per month. At 3500 rpm the rate would increase to: 0.020 " x 8 (23) or 0.160 inches per month. Likewise a decrease in speed would decrease the wear rate eight times as much.
The amount of shaft run out (deflection) varies by the square of the number
As an example : If you put a dial indicator on the shaft and noticed that the total run out at 1750 rpm. was 0.005 inches then at 3500 rpm the run out would be 0.005" x 4 (22), or 0.020 inches.
http://www.mcnallyinstitute.com/02-html/2-01.html
RE: Question about r.p.m. (1.500 vs. 2.900??)
As you point out part of the issue is the difficulty in finding a 1500 RPM. I ran the KSB online selection tool just for fun
(Note: this is in no way an endorsement of KSB over the other guys, I simply am more familiar with them as a competitor).
The program could not make a 1500 RPM selection as the head required was just too great. Perhaps the other manufacturers have more capability.
So it looks like you kind of need 2 pole. But if you expect sand as DubMac warns, then materials considerations are critical to long term reliability.
RE: Question about r.p.m. (1.500 vs. 2.900??)
RE: Question about r.p.m. (1.500 vs. 2.900??)
You cannot extrapolate runout in the way you describe for flexible shaft machines. Such an extrapolation would only be valid for a stiff shaft machine such as an OH1, OH2 or BB1 style pump.
RE: Question about r.p.m. (1.500 vs. 2.900??)
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The 150 NM pump actually looks like a better recommendation than the 200 NM pump.
RE: Question about r.p.m. (1.500 vs. 2.900??)
RE: Question about r.p.m. (1.500 vs. 2.900??)
RE: Question about r.p.m. (1.500 vs. 2.900??)
Sorry but I didn't even realize 2 of your selections are 1500rpm; you gotta go with the 1500 selection.
Bimr, what style pump is Flowserve's NM pump?? Pleuger ring section?? As an old Worthington guy, I would be interested to know its heritage.
Jacilore, would also be interested to know what the range of price difference is between the 2 and 4 pole selections. Obviously not asking for any confidential info, but just a approx. % range to go from 4 pole to 2 pole for purposes of this discussion.
If somehow you must use the 2 pole, do yourself a favor and make sure you make a statement on water purity required for service at that speed.
RE: Question about r.p.m. (1.500 vs. 2.900??)
bimr, The NM selection you posted was for 1800 RPM. The OP needs 1500 RPM.
RE: Question about r.p.m. (1.500 vs. 2.900??)
I'll need the weekend to read all the posts, unless it gets much crazier, (in the positive way).
However, I'll answer now the last easy question, for Dubmac: The KSB MULTITEC A-125/3 stage is about 27.000 €, the KSB MULTITEC A-125/4 stage is about 33.000 €, (different supplier), the RITZ 49200/11 stage is 80.000 €, and the Pleuger 200 NM (don't have the 150 NM offered) is about 90.000 €).
Regards and thanks to all again.
RE: Question about r.p.m. (1.500 vs. 2.900??)
On the basis that you get what you pay for, Pleuger are very well known in the mining industry and perhaps they are providing something that the alternatives are missing.
And if by chance the end duty location is an underground mine, are you ABSOLUTELY sure its clean water you're dealing with?? AFter 35 years underground , I dont think Ive ever once encountered truly clean water conditions
RE: Question about r.p.m. (1.500 vs. 2.900??)
The NM selection posted was for a 1800 RPM motor. A 1500 RPM motor will not have put out enough head for the application and will not work.
jacilore,
where did you get the 1500 RPM motor and pump selection? The online pump selector on Flowserve.com recommends a 1800 RPM motor.
DubMac,
The Flowserve website is calling the NM pump a Worthington brand. See the link.
RE: Question about r.p.m. (1.500 vs. 2.900??)
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: Question about r.p.m. (1.500 vs. 2.900??)
many water authorities decided that 1500 rpm was better becasue there was a eprception that things lasted longer. they have not done the numbers for decades. A true life cycle costing would see the reduced Capex for a 2 pole machine over a 4 pole machine. Also they tend to be more efficient but not always. They may have a steeper curve that gives better control and more suitable for parallel operation.
From the Opex size parts are smaller and thus less costly. you can replace a whole pump quickly.
Dubmac,
Concentrating of the initial capital cost of a pump is either ill informed or criminal. You need to do a life cycle analysis of the pump installation.
If you are worried about speed think about the Syndyne pumps that run at 26,000 rpm. Speed is all about selection of materials and quality pump products.
In the discussion on multistage pumps be careful to asses any change in operating conditions and how this may affect the balancuing disc. You want to avoid metal to metal contact.
Have you considered a Geho triplex diaphragm pump instead of a multi stage pump? Multistge pumps have fine clearances that can clog if there are suspended solids.
"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.
http://waterhammer.hopout.com.au/
RE: Question about r.p.m. (1.500 vs. 2.900??)
If you could not select this, you need to look at your selection settings. You may have something like API 610 construction set as a requirement.
As I noted previously it is important to specify HI class A for the test tolerances in order to get to true efficiency. I am not sure if the FLS tool is doing this correctly since it reports the same efficiency regardless of the test tolerance selected.
RE: Question about r.p.m. (1.500 vs. 2.900??)
No criminal activity going on here; possibly ill informed though. There was a time when I knew it all; however in my old age I've guess I've forgotten a whole lot of it.
Again, and mostly for Jacilore's benefit, we must keep in mind what level of machinery we are discussing. We are talking about municipal water service here; a part of our industry since Archimedes.
One of the most common themes in a muni consultant's spec is a speed limitation on the pumps. You'll rarely see the same in a refinery or chem plant service spec; 3600 is the most common speed you'll find there, and as you say, much, much faster.
I would say the speed concern has come from years of "life cycle cost analyses" rather than just perception. The Black&Veatch, CDM's, and Ch2MHills of the world are not incompetents; they have very good experience with pumps in water services.
Anytime there are solids present,(and when you're talking source water there WILL be abrasive solids); SPEED KILLS.
Not to scare Jacilore, but the ring section pump's reputation has never been too sterling as far as reliability. Just go look at the number of parts and pieces in those things; more crap stuffed in there than a Christmas turkey as they say.
Not saying there aren't tons of them around working just fine, but it doesn't take much to piss one of them off.
In thinking of Jacilore's decision, it might be criminal if I don't go down as saying; SLOW THE DAMN THING DOWN.
By the way, I know the Sundynes run just fine at 26,000rpm, and they are great pumps, but they are wearing out 8 times faster than if they were running 13000rpm, "and thats a fact Jack".
RE: Question about r.p.m. (1.500 vs. 2.900??)
Great response. My point was only that things have changed and the consultants have not looked the new scenario. I know this is a generalisation but my opinion is they do not question but copy. Why their returns are based on using as few a number of hours for the fee. They are also under the pump to get the design out there. I have worked for the lot of them as a sub consultant.
You make the good point that pumps in refineries run at 2 pole speed. have you considered the costs involved in refineries wher eoil is $100 a barrel and petrol (their product) is $1.50 a litre.
Water on the other hand is $1 a kilolitre but has a lot of votes attached to its reliability of supply. Sewage is even worse as it has no value and even greater voter concerns about reliability.
n respect of wear BHRA published a study on slurry pipelining where the conclusions were that wear was proporional to velocity to the power 2.5 to 4.5. The variation comes from particle size and its distribution, particle shape and hardness, pH, temperature, angle of impact, pipe shapes, pump impeller and vortex design, material etc etc. Slowing the thing down wil reduce wear if solids are present. But the overall life cost may be greater as the bits cost more, the pump station is bigger, the VFD is larger requiring more cooling etc etc. the engineer has to do the numbers.
Water authorities are their own worst enemies. Thier tender documents rave on about life cycle costing then they accept the low cost tender with its poorly selected pumps. The hydraulic institute has some good resources on the subject.
"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.
http://waterhammer.hopout.com.au/