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!

 

[goutam_freelance]

Since you have lot of options you may also include providing a water turbine to generate power from the head difference available.This will be the replacement of throttle valve which you already considered in one of the options.Since you did not include head difference and flow, the viability of this can not be checked.This will have the benefit of minimum work in the existing system but at an initial cost to be recovered.

Engineers, think what we have done to the environment !

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

 

[Artisi]

5-10 years or never probably gives you a reasonable answer.
Size the pumps for current duty and if the change comes about in 5 years select and install new pumps, the costs in future power saving will cover the costs, if 10 years it is likely time for a major pump rebuild or even replacement, review the situation at that time, if never - proceed as normal.

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.)

 

[georgeverghese]

Is there a simpler type of VFD that gives you 2 discrete speed settings ?

 

[LittleInch]

Honda,

Your description and options are ok , but you don't provide many details which might make some of the options invalid and others more likely.

So my assumptions so far are that the flow rate won't change?

But the head/pressure required will if the the new FM ever gets built - correct?

Now by how much is the head going to change from what is needed now with your undersized FM compared to some future event?

10%?
30%?
60%?
90%?

Depending on this answer it makes some of your options not practical as impellors etc and indeed motors will only only turn down or be adjustable to a certain range, probably about 40%, without being horrendously inefficient or just not capable of working.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Artisi]

As LittleInch indicated, flow and head for future duty are necessary, although you have listed too many constraints.
The best option you listed is new pumps. The station and some axillaries could possibly be designed initially to suit upgrade.

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.)

 

[LittleInch]

Looking through those options again, the vast majority relate to changing speed or changing the diameter of the impellor.

The problem here is that they all result in lower flow through each pump as well as lower head. Therefore you might need more pumps for your bigger main.

I'm with artisi here - You don't know for sure if the new FM will ever be built so just build for today and your known values and let tomorrow take care of itself.

Also if you're operating at ~10ft/sec, that's already pretty high. To get double the flow ( we don't know what your flow rate needs to be) you will need 4 times the power.

And any flow more than 10ft/sec and you could run into very serious pressure surge / water hammer issues.

Only option I'm not sure is there is a multi stage vertical pump and then remove some of the stages later and end up with a much bigger motor than you need, but just sits there consuming less power.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[EdStainless]

LI, I was thinking the same thing. Using a 4 or 5 stage pump and then in future just remove the uppermost impeller and replace with a spacer. But the size and amount of change would have to fit.

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P.E. Metallurgy, consulting work welcomed

 

[1503-44]

1) Try to educate your client on the appropriate reasons for using VFD. This probably one of the best examples of when they can be of maximum value.

2) It would help to have answers for LittleInch's questions to know if the following might be viable solutions.
Size for current BEP and in future, operate on/off to make your flow delivery requirements.
Size for future BEP, but increase the number of pumps now and turn off or remove one in future.
You will probably be forced to operate with lower than best efficiency in one case or another, but if your client wants variable flows and does not want VFD, then he must be very much accustomed to operating inefficiently already. That's the price you've got to pay.

3) There is a serious lack of an adequate future requirement knowledge.
YOU should NOT design for any condition that includes "possibly never".
You should not design for any condition that has 50% or less chance of occurring at any time, especially 5-10 yrs out, especially if it involves reduced capacity in head or flow.
Keep in mind that once capacity is installed, demand (almost) always rises to full installed capacity, then goes even higher! It almost never, never, never reduces. Extra head installed now probably will help that situation later.

 

[EdStainless]

Perhaps the idea of designing for the future addition of of a VFD is the most practical route.
This hinges on the actual performance required and pump curves.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[1503-44]

Like immediately after Mr. NoVFD moves on.

 

[hondashadow1100vt]

Will plan to provide more details and a few additional thoughts tomorrow. Thanks all!

 

[fel3]

My experience with "5-10 years later (or possibly never)" is that predicting the future is a fool's errand. We can certainly make some educated guesses but truly valid predictions, no. If I could do that, I would have already retired.

Here are a few examples from my career. Others here may have similar experiences.

About 35 years ago I wrote a report that evaluated the performance of a small, 20-year-old "temporary" hydropneumatic pumping station for potable water in Southern California. My report included recommendations for its replacement with a full-capacity pumping station and a water storage tank on top of the nearest hill. The recommended facilities were pretty much the same facilities the neighborhood's developer was supposed to construct upon completion of the houses -- until he conveniently declared bankruptcy. That bankruptcy taught the water district a valuable lesson about not permitting developers to construct temporary facilities and to require the developer to post a bond to cover the cost of required facilities.

About 30 years ago I wrote a storm drainage master plan for a small unincorporated town in Central California. The capital improvements program was a modest $3.5M and county staff told me the RFP for design would be issued within six months and construction would begin the next year. There was no RFP and the facilities are still a dream. About 5 years ago, the county talked about resurrecting the project, but still nothing has actually happened.

About 20 years ago, I designed a "temporary" sewage lift station in a small city in Central California. An ex-coworker was designing a subdivision, but he didn't do pumps so he asked me. The lift station was supposed to go away after about two years when the city moved their wastewater treatment plant to a larger and better site that was downslope a ways from the subdivision. The city is still talking about moving the wastewater treatment plant.

On the other hand, just today I received an email from one of our project managers telling me that a project I had performed intermediate-level QA/QC reviews for more than two years ago and which we thought was permanently dead, was being resurrected. We are having a conference call with the client later this week.

The point of all this is you can't predict the future. So, I suggest (as others here have) that you design for current conditions, but also plan for expected future conditions. For example, when you select pumps for current condition see if an impeller swap for that model will work for expected future conditions. If that doesn't work, see if you can select new and future pumps that have the same bolt pattern for securing to the equipment pad, same inlet and outlet locations, etc. If that doesn't work, them make sure your design can accommodate the future pumps as easily as possible.



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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[1503-44]

Fel3, Great examples there. I've had 5yr geologist's predictions of gathering system production forecasts that were 100% too high, but that's the only example of a reduced future capacity that I can give you. That as opposed to 10yr natural gas and crude oil transportation pipeline capacity forecasts that were up to 100% too low, which now, after 20yrs, most have been looped with separate parallel pipelines. As they say, "Making predictions is extremely difficult, especially about the future." I have to say that EdStainless' future VFD plan might be worth some thought, especially if the future flow rate works out. It might have to deal with flow control by means other than VFD now, which you would probably need to have w/o a VFD anyway, as reducing speed to make future head will also tend to reduce future flow from the pumps.

 

[hondashadow1100vt]

The cautionary examples regarding the risks associated with predicting the future resonate. Our local client has a legacy of constructed of facilities that were designed and intended to be temporary...but that have been in service for the last 20 to 30 years as their capital and operating budgets are reprioritized in response to an ever changing landscape of infrastructure, fiscal and political influences. There is an ever present risk that anything anticipated for the future simply does not materialize. We'd be wise to make sure that our design works for the present conditions in perpetuity. We'd be wise to furthermore aim to build in the capability to allow it to be adapted to future conditions if/when those conditions come to fruition; advising the client on their options and what steps (if any) would be required to adapt to those alternate conditions.

 

[hondashadow1100vt]

Mr. No VFD has existed at this municipality for far longer than my career. The entity has an issue with training electrical and instrumentation technicians who then command a salary bump elsewhere than the local municipality can keep up with. As a result it is frequently impressed upon the consultants to design facilities that can essentially be "fixed with a hammer", "run in manual", "run to failure", low tech solutions. While this might seem a little odd, it does produce low tech, robust facilities that are a unique challenge to design, build and operate. The entity is simply unable to attract, develop and retain the I&C talent to deal with VFDs (not to mention PLCs). 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. This situation may be one of the instances where VFDs are the best feasible alternative, despite the general aversion to them.

 

[fel3]

hondashadow...

Great point. The temporary sewage lift station I mentioned above, I designed as if it was going to be permanent.

Fred

============
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[bimr]

It would appear that you need to reevaluate the project scope.

Operating a forcemain at 10-ft/sec is extremely uneconomical. You may have to overpressurize the forcemain in order to get the necessary flow. You don't mention the length of the forcemain, but you can probably pay for a portion of the forcemain in energy savings and equipment savings.

 

[hondashadow1100vt]

In response to 1503-44's notes:

1) I think that we can demonstrate the turndown capability of the pumps and perhaps illustrate the life cycle cost savings in order to ease the client's potential acceptance of the viability and advantage of VFDs. Proximity to the BEP (and therefore superior pump performance and superior service life are another benefit that is more challenging, but possible to illustrate. We'd also need to demonstrate to them that the pumps still perform adequately when the VFDs fail...which is always a little more challenging, put may be possible with an otherwise unused discharge throttling valve that could be deployed if/when necessary at some point in the future as a backup (i.e., backup to the VFD).

2) The idea of adjusting the number of duty pumps does seem to make some good sense. Given the uncertainty about the future, it seems like designing for the current condition makes the most sense with the future being a "best fit" achievable with the same pumps using fewer duty pumps. Similarly, we’d need to demonstrate that this would need to work optimally with VFD and feasibly in bypass. Maybe the dollars associated with the inefficiency will be more convincing given the current/forecasted economic budget constraints.

3) Regarding “Possibly never”: This is a what if scenario. There is no way of predicting the likelihood that the future force main does not materialize. Purely going off of past experience from other facilities that have indicated that this risk does exist. Your last sentence on capacity increases being inevitable is comforting.

Thank you!

 

[hondashadow1100vt]

Good point bimr! I really do not want to operate at 10 fps. Lots of operational cost. Also erosion and corrosion of the FM.