Question about r.p.m. (1.500 vs. 2.900??) 2

[jacilore]

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

 

[Artisi]

What are you pumping - it might be a good idea to give this information?

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.)

 

[jacilore]

Hi:

Sorry, it's just normal quality water.

Regards

 

[Artisi]

For a correctly selected and installed unit on clean water at 2 pole speed you shouldn't expect any more problems than a 4 pole unit. However, there are others here with a great deal of operating experience who hopefully can give more advice.

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]

The general idea is that things wear twice as fast. Wear rings, pump/motor bearings. There are specific concerns for specific pump types.

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...

 

[TenPenny]

If it's clean water, there is no particular reason to avoid 2pole speeds. Pretty much all refineries and power plants use a vast array of pumps at 2pole speeds.

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.

 

[bimr]

1gibson (Mechanical) has stated the case for the slower rpm pump.

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.

 

[jacilore]

Thanks very much to all for your soon answers:

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.

 

[electricpete]

There was a brief discussion here and it has actually been hashed over quite a few times in different threads.
thread237-138757

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)' ?

 

[electricpete]

Above I was attempting to address only one aspect: maintenance cost (and that only in a very general fashion).

The other guys around here know much more than me about the other stuff.

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(2B)+(2B)' ?

 

[Artisi]

electricpete - no dispute on the statement that the problem pumps always seem to be the 2 pole units, but bear in mind they are usually working harder than the 4 pole units in terms of performance - that's why they are running at 2 pole, also the OP is talking about 2 pole 50 Hz not 2 pole 60 hz.

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.)

 

[bimr]

This issue involves the Total Cost of Pump Ownership, and has little to do with 2 pole vs 4 pole motors. Water quality is somewhat important when purchasing a pump, but is not as big a factor as the pump speed.

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.

 

[electricpete]

...has little to do with 2 pole vs 4 pole motors

Since there is no mention of vfd, the number of poles directly affects the machine speed. My comments were intended to address effects on machine speed on the entire train.

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)' ?

 

[electricpete]

electricpete - no dispute on the statement that the problem pumps always seem to be the 2 pole units, but bear in mind they are usually working harder than the 4 pole units in terms of performance - that's why they are running at 2 pole,

As a motor guy, I interpret “working harder” as related to power. Fair comparison would be same operating point flow rate and dp and same fluid horsepower (I’m imagining 2 different-shaped pump curves that intersect at a single point which is our point for comparison). I don’t understand what you mean by working harder.

also the OP is talking about 2 pole 50 Hz not 2 pole 60 hz.

Good point. It stands to reason that would not be quite as bad.


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(2B)+(2B)' ?

 

[DubMac]

Jacilore,

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??

 

[GPRnD]

You have to balance a lot of things in considering 2 pole vs 4 pole.

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.

 

[Pumpsonly]

If it's clean water, there is no particular reason to avoid 2pole speeds. Pretty much all refineries and power plants use a vast array of pumps at 2pole speeds

.

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.

 

[jacilore]

Thanks all because I am getting plenty of information, and most of all it seems to be a question of where the balance goes, as I see many good indications in both ways, so I think it's my turn to measure the weights properly.

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.

 

[DubMac]

Jacilore,

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.

 

[GPRnD]

DubMac, I will disagree with your claim regarding wear ring contact.

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.