High capacity raw water pumps - Submersible or vertical shaft driven pumps

[paliha]

Hi all,

Decision between submersible and vertical turbine pumps for raw water pumps has been discussed under thread 407-233951.

But my problem is more towards high capacity end in the region of 1250 L/S at 40M. (750kW)

Decision is almost made in favor of vertical shaft driven pumps, mainly considering maintenance and handling issues.

But there are submersible pumps capable of this duty by leading pump manufacturers.(KSB)

Can any of the forum member with experience with submersible pumps of this capacity please comment on this?

Thanks

 

[JJPellin]

This service is approximately the same size as our largest cooling tower pumps. All of them are vertical turbine pumps. We use submersible pumps for deep water wells where a shaft driven pump would require over 125 m long shaft. I am not familiar with the term raw water. If these are not deep wells requiring extremely long shafts, I prefer at vertical turbine pump. We are in the process of reinstalling a similar pump (750 KW, 1625 l/s). It is one of a set of four in our newest cooling tower. The lower bushing below the impeller seized. This is the second failure of this bushing in this group of pumps in the past 10 years. We are attempting to resolve this problem. Other than these bushing problems, the pumps have been very reliable. A submersible pump of this size seems very large. Even if it is commercially available, I would be concerned that it would be near the end of the design limit for that technology. I generally don’t want to have the very smallest, fastest, largest or longest example of any particular technology.

Johnny Pellin

 

[DubMac]

Agree with JJP, would prefer a vertical turbine pump (actually yours would be termed a mixed flow or axial flow pump) to a submersible for the same reasons. Maintenance costs much higher, must have specialized repair shops, usually proprietary parts that can only be obtained through the XYZ rep, etc. A good "prop" pump is very simple in design and used in 90% of raw water intake services for a reason.

What JJPellin is describing as his problem is probably the area you need to consider most in your design. You are most likely drawing from a river or lake/reservoir which will contain sand fines whether you like it or not; even with the best of filtering. That bearing below the impeller is many times called the "sand bearing" for a reason. Many schemes have been tried to perfect the isolation and lubrication of this bearing. The best designs usually involve forced lube with fresh water or grease.

Go look at info from some of the most experienced and respected manufacturers of axial or mixed flow pumps: Johnston (now owned by Sulzer), Cascade, Patterson, Floway (now WierFloway), Flowserve (who actually use Cascade bowls and propellers).

 

[Artisi]

Would have to agree completely with the prior comments, a submersible could give you excellent results with long life but the downside is the need of specialised service requirements / staff, whereas a vertical lineshaft driven axial or mixed flow pump is a pretty simple device, easy to maintain with medium skilled staff. There must be many thousands running throughout the world giving years of trouble free service.

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.)

 

[paliha]

Hi all,

Thank you for all who have given comments.
It is good advise from JJPellin not to go for "smallest, fastest, largest or longest of any particular technology"
It is applicable in this case.
River sand is a problem for vertical turbine pumps, but main option we consider is vertical shaft driven pump, although at additional construction cost. (in line with the type of already installed pumps)
So once again thank you for your comments, which made us more confident on our decision.