900 rpm vs 1200 rpm VT pump 2

[RJSH]

Hi all,
I am working on writing a spec. for a fairly large Vertical Turbine Pump . During the pump selection procedure from various manufacturers, we had more manufacturers that they could meet the design conditions at lower speed of 900 rpm and one manufacturer being at 1200 rpm. My question is Would it be a good idea to specify a rpm limit as part of design conditions. My only concern is if rpm is left open ended, there will be a lack of competition during bid since the high speed motor will have lower initial cost.
The intention is to give the client the best value in the long run.
Which one would be preferred one over other - 900 rpm vs 1200 rpm and why? Should I put a limit in pump speed in the spec.?
I appreciate sharing your thoughts/experience.

 

[DubMac]

Wear increases as the cube of any speed increase; is this a service that has wear concerns. i.e sand, solids, crappy Maint. Dept., etc??

NPSH a concern??? 900rpm is going to be more friendly; is the 1200rpm close to the edge?

With the limited details you have given, it would not be fair to rule out a 1200 rpm pump if the concerns above are met by a good 1200rpm selection; it actually could be a benefit if there is moderate head and the # of stages is appreciably lowered.

Just like any good bid tab, you should assess some sort of value add/deduct for each item relative to its overall concern.

If you do have solids present, I would be very concerned about the lineshaft/bearing arrangement that is spec'd.
If this is really a large pump, are you going to allow the motor bearings to hold the thrust, or will you spec a thrust pot arrangement in the discharge head??
What fluid velocity will you allow through the column, thereby essentially spec'ing in column size??

 

[JJPellin]

I would not normally suggest an RPM limit for the specification. The higher speed may have a higher efficiency which lowers operating costs from energy. It may have a lower spare parts cost which lowers the purchase and carrying costs for the spare parts. If erosion is a concern, you could estimate the impact on the long term maintenance costs of the higher speed. If you are going to choose between big and slow versus small and fast, you need to consider all of the long term costs of ownership of the pump.

I prefer big and slow for abrasive services (as DubMac explained well). I prefer big and slow for some services with difficult mechanical seal applications. But, 1200 rpm is not particularly fast. We would normally be choosing between 1800 and 3600 rpm. Choose the pump that provides the best lifecycle value.

Johnny Pellin

 

[RJSH]

Hi DubMac, and JJ, Thanks for your input.

Pumps are rated around 150 MGD at 185 ft TDH. The pump is for raw water application to pump water from the river. NPSH is not a concern in one manufacturer with 1200 rpm however the primary design point is at the right of BEP however has a good efficeny at that point. Another manufactur NPSHr is much higher that available at 1200 rpm . NPSH is not a concern for lower speed 900 rpm motors.
The 1200 rpm pump is lower in initial cost than 900 rpm (almost 25%).

 

[DubMac]

Each pump is 100K GPM?? And 185'?? Man, those are monsters if so; surely I've misinterpreted something. Usually that high a flow would be an axial or mixed-flow pump and heads much lower than 185'. Certainly would like to hear more about the service, i.e. why the 185'?

River water intake WILL have sand and designing of the tail, or bottom bearing in the suction bell is critical, especially with reference to lubrication. Surely this will be enclosed lineshaft; how long will the pump column be (from suction bell to discharge head??

 

[Artisi]

Agreed, need more data on the application before a considered opinion.
However, with the flow /head advised and the installed power required you would be very hard pressed to not go with the most hydraulically efficient unit.

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.)

 

[RJSH]

Dubmac and Artisi,
My bad. Pumps are 25 MGD each @ 185 ft TDH. Total flow 150 mgd. Suction bell to discharge ht is around 60 ft.

 

[DubMac]

These are still really large pumps; I'm guessing 1000HP or so. Maybe plant layouts have just passed me by but I don't recall seeing that high of a head on a raw water intake pump of that size (approx 17K gpm). You have a lot of HP transmitted through 60' of lineshaft; better be well thought out.

You have the 60' of lift; where is the rest of the head going??

 

[RJSH]

We are pumping the water to a reservoir up on the hill.
My main concern is 900 rpm vs 1200 rpm, the no. of manufacturers that can supply the pump, and initial price of the pump.

 

[DubMac]

If it is a municipality, then it will most likely be a "performance bid"??, where efficiency points will dominate the consideration (Artisi above); and rightly so. With this amount of total HP, energy costs borne by the taxpayer are paramount.

As JJPellin alluded to earlier, there is not a huge difference between 1200 and 900; you certainly want to exclude 1800rpm however. If there is enough efficiency savings on the 1200rpm selection, there is no reason to toss it out solely based on speed. Assuming the 1200rpm pump is most efficient, calculate annual savings based upon efficiency and balance that against projected increased wear. Ask the vendors for replacement costing in the bid for: complete bowl assembly, impeller only. Have them hold the price for 2 years.

With your 185' head, the # stages (for the same bowl selection) required to go from 1200 down to 900 rpm practically DOUBLES: ((1200/900)**2)= 1.78 So, if your guy at 1200rpm was restricted to 900rpm max, using the same bowl he would have 1.78 more bowls (essentially double) than at 1200 rpm. With this size pump, there is real merit in evaluating 1200 rpm selections due to the reduction in bowls. Not just from first cost; double the # bowls may offset the wear advantages.

 

[RJSH]

Thanks guys.

 

[ccfowler]

RJSH,

Although the topic is not often discussed very openly, it is not unrealistic to look at the economic and operational implications of selecting a pump with greater maintenance or repair burdens against the initial costs and energy usage of different options. For example, a pump with a greater shaft speed may cost enough less in both initial cost and repair costs per repair episode due to smaller size, fewer stages, ... that the more frequent repairs still result in a lesser life cycle cost.

I've known cases where less-than-ideal operating situations resulted in significant erosion of the pump internals, both rotor and stator, due to long periods at reduced flow rates, but the periodic repair costs were substantially less than any of the potential alternatives. A new pump with optimal hydraulic characteristics would be too costly due to operational and space constraints, and operating at greater flows by use of a throttled recirculation flow would impose too great of an additional energy consumption burden.

No matter how reliable and durable a slower running pump may be, it will surely require repairs at some frequency. Initial cost savings and less costly repair episodes realized by selecting a smaller, higher speed pump may allow for an additional spare pump so that even with their reduced time between repairs, the actual dependability of the pumping system can actually be better and less costly, too. The only certain reality is that there is no "best" pump choice--there is only the optimal balance of pump, system, operational, cost, and energy usage to serve the ultimate purposes of the system under 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.

 

[rmw]

Don't know what your river is like, but where I live, rivers connote to me grit, sand, debris, and wear, wear, wear. Whatever type of inlet protection you have, screens, etc., will fail some day and your pump will be asked to pump who knows what from 2X4's to fish. Make the best decision you can with future wear in mind.

rmw