4 pole versus 2 pole 4

[HomeMadeSin]

Back when I was handling contract orders for IDP, a lot of the specifications called out for 1800 rpm (4 pole) maximum speed. Inevitably, sales and purchasing always worked around that spec. for the sake of savings, but I was wondering if that has changed at all. Although the pump size would be larger for slower speeds (typically), the bearings, seals and such would last much longer bringing down the operating and maintenance costs.

Any thoughts?

 

[electricpete]

"This discussion is just academic and does not add any value to the practical aspects of how to select right pumps for a given duty. Even assuming that 5HP, 4 pole motor driven pump bearings are significantly better than the 2 pole versions, it does not translate in to significant savings to justify 4 pole pumps of this type"

I would be interested to hear your estimates of the various costs used to reach your conclusion.

 

[PUMPDESIGNER]

Stanier has said more than most will ever know.
Why do engineer's fold, waffle, fail in their duties?
Read what Stanier said, chew on it, the answer is there.

"Dont compromise your engineering integrity
Dont sign off anything you dont believe is right
Walk away from a company that doesnt have ethics
Dont live beyond your means so that you're financially compromised before you start"

Most engineers will NOT follow Stanier's advice because they CAN'T, they have no power to walk a higher road. As with many other people in our present society, they value money and security NOW rather than later, they become enslaved like the others.

I know some will think Stanier off topic, but I do not.
We spend all our time arguing over the obvious and the deeper causes almost never come to the surface.

PUMPDESIGNER

 

[HomeMadeSin]

Ah, grasshopper, but you may not perceive the undertones of my intentions with this post. I believe the beancounters are the ones one dictate 2 pole versus 4 pole. Who writes the specifications? Engineers. Is the requirement to be max 1800rpm put in just for fun, I think not. Why did the salespeople get exceptions taken on that part of the spec? Because it allowed them to offer a CHEAPER bid!!!!!!

So, grasshopper, I agree with the fact that engineers should hold true to their opinions, but I also have worked in Sales and Marketing and while I think that a titanium pump is sexy as hell (hey, I'm a pump guy), 9 times out of 10 non-metallic works as well. I may not like to compromise, but it is required.

I want to design our next pump for 4 pole speed because I think it is better (was looking for justification) and will have to fight the sales types from wanting to push it into 2 pole land. The reason this gets significant is that at 2 pole speeds the pressure is 4 times that of 4 pole speeds. Wall thickness matters. Also, the shafting, sealing and all change.

We had a Product Development meeting yesterday and the management supports the idea of a 4 pole unit (even 6 pole is ok). But I know the natural tendancies of distributors......

 

[electricpete]

Since we're getting philosophical...

System designers/specifiers will naturally attempt to evaluate alternatives and develop a quantitative evaluation based on life cycle cost. That we all know.

Some items are easier to quantify and some are harder. It’s the nature of the beast.
My thought is that it varies from easiest to hardest as follows:
energy cost, maintenance and failure costs.

You can go to a catalog or oem and determine initial cost.
You can also use oem data along with calculated operating conditions to determine efficiency.
You can give some thought to redundancy and it’s effect on reliability.
But there is no book to go to to determine maintenance costs and reliability of the individual components (i.e. 2-pole vs 4-pole).

The natural reaction of system designers may be to downplay or ignore the importance of those factors that can’t be easily quantified (after all… what good is it if you can’t estimate it). Usually it is the only practical approach because the info simply isn’t readily available beyond very narrow parameters like L10 life of bearings.

Homemadesin, you have gone the extra mile to really try to probe a better understanding of those maintenance costs. I think it is a commendable approach. Although the system designers/specifiers may have a lot to contribute on the subject of initial cost and efficiency, there may be other sources that have valuable insight in the area of maintenance costs. I would once again suggest you post this question on the board I mentioned above for another set of viewpoints.

 

[PUMPDESIGNER]

electricpete is correct in all that he says, as usual.
I am watching that site pete recommended, great potential for learning there.
One little interesting thing.
What is the fallout of allowing changes too often after bidding? A manufacturer came into my office and threatened me. Why? I told a specifier to not use one of their products (cheap junk), and instead told the specifier to use another product by same manufacturer (nice product). So why the threat? Because the manufacturer was worried that if the more expensive product was specified they would not get the job because someone would change the job out to a competitors cheap junk.
Strange world.

PUMPDESIGNER

 

[stanier]

Homemadesin,

To aid your endeavours, have you considered that the 4 pole pump with its larger inpeller etc will have a higher moment of inertia. In any waterhammer analysis you will find that the 4 pole pump will always be less susceptible to causing waterhammer excursions than the 2 pole unit on loss of power. Should there be a transient pressure event the robustness of the 2 pole unit will be challenged more than that of the 4 pole machine.

Another point is that the 2 pole pump will have nozzles far less capable of taking any load what so ever without distorting the frame. In turn will promote more wear of the seals and bearings because of the misalignment that is caused.

In respect of life cycle costs I have been shown figures at seminars and workshops such as "the initial capex of a unit represents only 12% of the cost in its 25 year life time". A huge generalisation I know but food for thought.

 

[RotaryG]

Does anyone in this forum know the cost of mass produced antifriction bearings?

Regards,

Guru

"Evolution rather than Revolution"

 

[electricpete]

The majority of cost involved in a bearing replacement would be manpower.

 

[Artisi]

27 responses so far - that in itself is interesting, so I decided to put in my 2 pennies worth on the subject.
As a supplier I always offered 4 pole (50Hertz) units provided they could generate the head and were a good hydraulic fit on the curve, why did I do this - well for for 2 reasons --
1.-- I think 4 pole is less troublesome than 2 pole duty for duty in a heavy duty application.
2. -- more profit on my bottom line at the end of the day.
My justification was less maintenance problems and maintenance costs, but then I wasn't paying the bill.
I am talking here mainly about the paper Industry - where reliability is or used to be the main criteria - we were dealing with experienced plant engineers - however today this has changed and we now have to deal with the bean counters or younger engineers who have been trained more like bean counters than practical hands-on engineers.

Having said this - I think that duty for duty, in a heavy duty application 4 pole is less trouble than 2 pole, however for lighter duty applications I would not consider 2 pole to be a problem.
At the end of the day - the defining criteria what is the duty, what is the best hydraulic selection and what are you trying to achieve.

 

[tonyh]

I think that we are getting too carried away with only one parameter here. Unless 2 pole or 4 pole is the fundamentally important issue (not in my view, then the whole duty is what matters to determine which pump you choose.

I believe that for the right duty, I am perfectly capable of buying a 2 pole pump which will have a reliability more than adequate for long term continuous use. However, the most important issue about higher speed pumps is that their NPSHR will be a lot greater than that of a slower speed pump. If you buy a pump with no thought to NPSH you are more likely to get away with it at 4 pole. Also, if you buy a pump with a poor design (flexible shaft in particular - running well off its duty point is another classic) it will have problems. As Engineers we need to provide econonmic solutions, so I would always pick a well designed two pole pump if all the other (more important criteria) - NPSH, position on curve, l3/d4, bearings, detail design etc.. were acceptable.

For a given head, tip speed is going to be fairly similar whether you have a two or a 4 pole pump.
The two pole pump will have a smaller and lighter impeller and therefore does not need as big a set of bearings, so this evens out fairly well in the end (although watch out for where high axial loads are required e.g. high suction pressures as the thrust bearing loading may be of interest).
The mechanical seal on a 2pole pump will be smaller in diameter and run faster, and typically this evens out so that the PxV rating is similar on both, and you would not get more wear on the faster pump.

Four pole pumps will have a lower inlet acceleration which allows for better NPSH and maybe less erosion at the inlet if this is an issue.

There are some heads that you cannot do on a 4 pole machine, unless you go for series pumps or two stage. I would sooner have a 2pole machine than a 2 stage 4 pole machine (NPSH permitting) any day.

To be fair, if you are intent on abusing a piece of equipment: misalignment (coupling and piping), failure to lubricate, etc.. then you will probably do better with a bigger butcher 4 pole pump, and you probably have some better career opportunities sorting out the maintenance and operation!
On the other hand if your problem on a well designed pump is that the bearings are failing through end of life, I am very impressed. L10 life even on a 2 pole pump is generally far better than people get for pump life.

Balance standard: G1 is totally over the top for 2 pole pumps. Typically G6.3 would be considered acceptable, although it is always nice to do better.

In summary, buy a well designed pump which is right for the job. Avoiding NPSH problems and running well away from BEP are probably the things that matter in determining the speed, rather than a general must be 4 pole.

Incidentally does anyone wish to join the Society for the Abolition of the "Rated point must be to the left of BEP" clause, so beloved by design contractors? The main outcome of this is that the pump can never run on its design point, and we break far more pumps from running below minimum flow.

 

[HomeMadeSin]

tonyh: good f'n post! Thanks for bringing me back to ground. I will say that the NPSH issue is significant, particularly for those applications that weren't sized by a hydraulic engineer or equivalent (i.e. NPSHa is not known). It just creates a headache for the manufacturer when something doesn't go right and the application is an abortion.

Some of my current bias against 2 pole machines has to do with the applications I typically run up against. Mostly small applications (sub-ANSI) where customers only want flow, flow and more flow. They categorically resist buying multiple low speed pumps (very little head required in these cases) and most of the larger pumps get expensive quick. This gap is what we intend to fill. But back to the point, running a single high speed pump (56J motors) simply has a much lower life expectancy than two low speed pumps (same impeller, housing, etc). And yes, I am talking about running well beyond the BEP of the high speed pump, which technically speaking is not a good thing.

Some other things you mention like l3/d4 could start a whole other thread in itself. A simple static value used to dictate the acceptable shaft deflection is less than ideal anyway. I think it was the PIP spec. that added l3/d4 recommendations to the ANSI spec. So, you get pumps with l3/d4 of like 17 (way oversized bearings for the duty). But enough digression.

I'll join you on the left of BEP thing.

 

[PUMPDESIGNER]

Many good issues came out of this even if incompletely explained or debated.
The skill of the engineering practitioner is the only way this stuff gets sorted out and properly prioritized, and yes that person needs the beancounter.

What Stanier said still goes though. Too many attempts with too much pressure to cut initial costs only imply that they mistrust the engineer, that the engineers does not care about initial costs, which is what Stanier was saying, you don't trust me, design it yourself.

One last thing, not doing the NPSHr calculations is simply not acceptable because putting a 4 pole double suction pump into a flooded suction or pressurized suction without limiting the margin of NPSHa over NPSHr on a water application can easily destroy the impeller quickly unless the margin reaches the point of cavitation inception (NPSHi), which will be at least NPSHr times 5, perhaps more.



PUMPDESIGNER

 

[HomeMadeSin]

PUMPDESIGNER: although more for a separate discussion, NPSH is a tacky issue. I've worked at companies that published a complete set of performance curves for their pumps based on calculations alone - a practice that I do not condone. Poor guys in the test stand polishing the devil out of a pump to get it to pass for the customers that did opt for performance testing, not always succeeding. For that reason, I would only use a NPSHr (I am talking "r&quot calculation for info only and rely on testing to back it up.

Also, applications with NPSHa equal to 5x NPSHr (or more) are rare, from my experience and I must admit I've never seen a NPSHi acronym......

 

[electricpete]

i for incipient

 

[PUMPDESIGNER]

Not saying that Karassik is the "Pump God", but this is how he/they denote the term:

NPSHi = Net Posititive Suction Head Inception
Pump Handbook, 3rd Edition, 2.86


PUMPDESIGNER

 

[electricpete]

whoops... i for inception as pointed out by pd.
inception as in inception of cavitation.

(sorry to beat a dead horse)

 

[PUMPDESIGNER]

electricpete, you are definitely one of the smartest guys on this site, I'll bet that was the first mistake you made this year, though I am saddened by the tininess of it though, hope to catch you big some day, probably will go the other way though.

PUMPDESIGNER

 

[PUMPDESIGNER]

HomeMadeSin,
If you have Pump Handbook 3rd Edition read the section I referenced: 2.86. Good statements regarding excessive margins of NPSHa over NPSHr. By the way, does your handle have any specific meaning?

PUMPDESIGNER

 

[RotaryG]

"Rated point must be to the left of BEP" We chucked that concept longtime ago. Now-a-days, related clauses read something like:

a) Rated point to be within 70-110% of BEP.
b) For rated capacities larger than XXXXX m3/h, rtaed point to be within 10% of BEP.

Regards,

Guru

"Evolution rather than Revolution"

 

[HomeMadeSin]

PUMPDESIGNER: I have the 3rd Edition, although I just got a few months ago. Up to that point, I was still on the 1st Edition, back when HE worked for Worthington. It seems they identified the term between 1st and 3rd ed., not that I am that proficient in that section of the back anyway.

My handle is insignificant. There was a saying my grandmother had about something being ugly as home-made sin. I've made a few things for engineering classes that I would label that ugly, so it stuck.