Select Pumps For Reduced Future HP Needs (Delayed Replacement FM Construction) 3

[hondashadow1100vt]

Hi all,

Brainstorming some ideas for a pump station to permit pump downsizing pumps (ideally with minimal future work) in the future when a new larger force main comes on line.

Scenario: Pump station capacity expansion that operates on an existing undersized force main (FM) line (currently about ~10-ft/sec peak. Too high.). FM needs to be upsized. Pump type proposed (and favored) as dry pit submersible end suction non-clog type. Need to build the pump station first and operate it on the existing undersized FM, then build & connect to the FM 5-10 years later (or possibly never). Using the existing force main, the friction head and HP required to pump through the existing is significantly greater than the future upsized force main condition. With the the aim of trying to ideally avoid a second construction contract to renovate the pump station guts when the new FM becomes available, the following list of ideas comes to mind initially (quality and practicability varies):
- VFD (client has a serious aversion to VFDs. Very old school.)
- Replace the motor later with slower speed (requires construction work, realignment/rebalancing)
- Provide two speed motor (likely highly custom for submersible motor)
- replace impellers with smaller diameter impeller later (requires some serious work, realignment/rebalancing)
- Change pump type to extended shaft pumps and go with resistor banks with speed settings (I.e., simply switch the speed setting in the future)
- Change pump type to extended shaft pumps and go with eddy current variable speed drives (have not done this one)
- Install discharge header control valve (maybe a V-port ball valve) on the discharge. For existing small FM, operate with valve open, for future “right sized” FM throttle valve (or valves) to add friction head to system. (waste of operating dollar$)
- Install orifice plate in the discharge when the future “right sized” FM line comes online (waste of operating dollar$)
- Replace the pumps when the new force main becomes available (requires construction work)
- Operate on fewer pumps when the new force main becomes available
- Swap out gearbox (requires construction work)

Are there any other ideas out there that I have overlooked?

I welcome any ideas and any critiques of the above “good ideas” (...and the not so good ones).

Thanks all!

 

[hondashadow1100vt]

1503-44: Regarding the comparison of adding additional pipelines. It would be good to keep both the existing and the future force main around in perpetuity (though one will be old). Provided that the current condition does not necessitate erosion of the interior of the piping, perhaps the old one can be kept for automated wet weather flows in the future (at a reasonable velocity with some flow splitting logic).

 

[hondashadow1100vt]

EdStainless:

I like your idea about removing stages. I've seen reports of this for vertical turbine (VT) type pumps. The typical pump used in sewage conveyance is a single stage, non-clog, end suction centrifugal (vertical in this case). Given all the rags inherent in the flow, I'd be reluctant to entertain VT for this application.

Thank you for the good idea nevertheless!

 

[hondashadow1100vt]

LittleInch: Similarly, good idea regarding the multi-stage adjustment. I like the idea and where your head is at though I don't think that it can be applied to this particular application. You are dead on about the additional reasons to avoid such high velocities:
- HP increasing with the square of the velocity
- more extremes surge (over pressure and under pressure) which would likely necessitate a surge tank (again client cannot be running all over the place to maintain air valves, etc. They also prefer to keep it as simple as possible and not rely on mechanical devices wherever possible. Always interesting challenges!)
- Then my favorite addition of erosion due to conveyance of suspended grit at high velocities thereby sand blasting the interior of the piping, wearing it away, making it more likely that a future break could occur due to thinning of the pipe wall and therefore derating of the pipe pressure rating.

Thank you.

 

[hondashadow1100vt]

Artisi -
Agreed that new/replacement pumps in the future is a definite contender as ideas go. We simply need to state the facts of the matter that it triggers an additional contract to update the pumps at that time. That is the primary drawback. Maybe they can live with it. Will find out.
Thanks.

 

[hondashadow1100vt]

georgeverghese: I have used two speed motors on rinky dink little 3-HP drives before. Nothing like this. 125-300 HP range. Will look into it though.
Thanks!

 

[hondashadow1100vt]

goutam_freelance: The turbine idea is cool and very outside the box thinking. I appreciate the suggestion. I have not seen that done before. At least not in sewage (i.e., this application is sewage if I have not yet mentioned that). If clear liquid, I'd consider it the idea further. Thank you!

 

[georgeverghese]

Have a read of pages 29-7 to 29-10 in Perry Chem Engg Handbook 7th edn which describes many other variants on motor winding configuration and auxiliaries to obtain speed regulation in steps other than VFD, with no loss in reliability. Your electrical engineer and motor supplier has the solution to this.

 

[Artisi]

you mentioned non-clog impellers, is it assumed you are handling raw untreated sewage and all the added junk that tends to end up at a pump station, if yes multi stage vertical pumps are of no use.

Your post had far too many "can't / don't want items" and insufficient data to even think about what is required - collect and post real information and we might be able to help


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.)

 

[goutam_freelance]

The most economical approach will be as follows:
1. Select the pump and motor for present duty conditions without VFD but motors should be sized for VFD use in future. VFD motors need to be oversized by about 10%. But if the future motor load is less by 10% or more you need not oversize the motor at present.
2. In future just install VFD system, panels etc. and operate the pumps at reduced speed.
3. Even though this will have lowest first cost you need to check pump efficiency at reduced speed and work out the economics if reduction in efficiency is significant.

Engineers, think what we have done to the environment !

https://www.linkedin.com/in/goutam-das-59743b30/

 

[goutam_freelance]

A few VFDs have crept in but they always make their discontent about it known and insist on no VFDs unless there is no feasible alternative

If there is apprehension about VFD it is possible to incorporate a gearbox(to be hardcore mechanical) and gears can be changed in future to get reduced speed.

The idea of adjusting the number of duty pumps does seem to make some good sense

When head is lowered each working pumps will pump more flow than design. The BHP may go up. So you need to check the possibility of motor overload and select motor accordingly.

Engineers, think what we have done to the environment !

https://www.linkedin.com/in/goutam-das-59743b30/