900 rpm vs 1200 rpm VT pump
900 rpm vs 1200 rpm VT pump
(OP)
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.
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.





RE: 900 rpm vs 1200 rpm VT pump
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??
RE: 900 rpm vs 1200 rpm VT 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
RE: 900 rpm vs 1200 rpm VT pump
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%).
RE: 900 rpm vs 1200 rpm VT pump
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??
RE: 900 rpm vs 1200 rpm VT pump
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.)
RE: 900 rpm vs 1200 rpm VT pump
My bad. Pumps are 25 MGD each @ 185 ft TDH. Total flow 150 mgd. Suction bell to discharge ht is around 60 ft.
RE: 900 rpm vs 1200 rpm VT pump
You have the 60' of lift; where is the rest of the head going??
RE: 900 rpm vs 1200 rpm VT pump
My main concern is 900 rpm vs 1200 rpm, the no. of manufacturers that can supply the pump, and initial price of the pump.
RE: 900 rpm vs 1200 rpm VT pump
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.
RE: 900 rpm vs 1200 rpm VT pump
RE: 900 rpm vs 1200 rpm VT pump
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.
RE: 900 rpm vs 1200 rpm VT pump
rmw