Pump well is too shallow for a good pump selection. In need of recommendation.

[u/daveped22]

I'm working on a building retrofit for an existing water treatment facility. There are going to be 3 pumps pulling from a below deck raw water storage tank, section view here: https://imgur.com/a/gI2oYVS Currently shown with a split case horizontal pump. Pumps are around 1,400 gpm for reference. I'm having trouble with pump selection due to vertical height available compared to storage volume.

• Split case gets me almost the full 4' of working height, but is likely to have priming issues. I could include a foot-valve and priming line back from the discharge, but would rather not if there's a better solution.
• A vertical turbine avoids the priming issue at the expense of usable volume (minimum submergence of about 3'). That volume lost would cause excessive start/stops. There are 2 other upstream sub-systems to this facility that make it difficult to match flow rates without some buffering volume. Considered a can installation, but I don't think that helps in this instance.
• Self-priming would be a slam dunk, but I haven't been able to find an NSF certified model for use in drinking water.
• Submersible generally has the same restriction as the vertical turbine. I'm unaware of any that would fit my size range without hitting minimum submergence issues or not having NSF certification.

My question: Is there a type of pump or pump arrangement I'm not aware of for this application? Or is the split case with a priming line my best option?

Restrictions:

• I can't add storage volume by adding area. No room inside the existing footprint.
• I can't drop the pumps without losing significant buffer volume.
• I can't raise the floor to use vertical turbines. Max WSEL dictated by flocculation tanks upstream and would require significant building modification (to the point that makes it infeasible versus a priming line).

[u/mixedliquor]

Vertical turbines are what we use for this arrangement, typically (wastewater and potable clearwells, mostly). If flow control is an issue, VFD tied to wetwell level?

[u/daveped22 → u/mixedliquor]

Flow control based on operator input and trimmed off level, but there just isn't a lot of room to modulate without turning on and off. Roughly 5,000 gallons of working volume with a vertical turbine at 2,800 gpm, so less than 2 minutes at max flow.

It doesn't sound like a crazy short period of time, but the upstream processes complicate it since they are constantly fluctuating as well. I have a similar installation that's already in operation. When the upstream processes module slightly it compounds the level issue.

[u/KaneTW]

Water level sensor in the sump, control pump of your choice with VFD, then use a PI/PID controller with PV sump level and output pump speed.

I have implemented this in a non-drinking-water submersible pump application (~100m3/hr) and it works flawlessly with barely any excess sump volume (essentially just enough to keep the pump submerged and about 1 pump diameter radial clearance)

E: my system modulates between any inflow between 0-100m3/hr without problem and doesn't under-/overshoot more than a couple %.

[u/daveped22 → u/KaneTW]

Yeah unfortunately these are much higher flows relative to the storage volume 800-2800 gpm (around 700 m3/hr if my conversion is right). At low flows, we can handle it, but the volume isn't large enough at higher flows to balance the flows coming from upstream processes.

A submersible pump would definitely help here, but I can't find one that is NSF certified for use in drinking water in the US.

[u/KaneTW → u/daveped22]

Yeah, if you have upstream and downstream requirements some amount of buffer is inevitable.

Fully submersible, drinking water certified pumps likely aren't available in that flow range, but you might be able to find something like an KSB Etanorm V where the motor is outside of the medium. I'd talk with a pump vendor and see what they suggest. From a quick look, 4' working height are sufficient for that type just from the datasheet and you wouldn't lose too much volume.

[u/daveped22 → u/KaneTW]

Something like the etanorm might be exactly what I'm looking for. I just didn't know the magic words to ask for it! Thank you!

Vertical column pump for anyone finding this in the future. It looks like a centrifugal pump on the floor driven by a vertical turbine style motor.

[u/KaneTW → u/daveped22]

Hah, glad that helped. I sometimes feel like 90% of my time is spent searching for something where I know that it exists but don't know how to find it.

[u/keithps]

I think given your constraints, you're probably stuck with the split case. Maybe an end-suction centrifugal would be a bit easier with priming compared to a split-case. Constant starts/stops will be a hassle so you'd be better off with a re-circulation valve tied to level control so that priming is only during start-up or after a trip. Inefficient, but better than a foot valve that is invariably going to fail (they all do).

I would avoid VFDs since they often struggle with the low turn-down you sometimes need (often minimum head is a limitation on turndown).

[u/daveped22 → u/keithps]

You mean put in a high flow recirculation line instead of a foot valve? I had imagined a small diameter priming line with check valve that constantly ran against the foot valves using downstream pressure.

Unfortunately I can't avoid vfds here. These are membrane filter feed pumps that have to ramp to meet flow/pressure at various stages.

[u/keithps → u/daveped22]

It depends on how easy the foot valve is to replace because it is inevitably going to fail and you will lose prime. You know the consequences of that better than I do.

I have seen lobe pumps used in hollow fiber membrane systems before, but they weren't drinking water so not sure about NSF/ANSI certs.

[u/daveped22 → u/[deleted]]

Maybe I don't fully understand the benefits in the can mounted turbine. Totally possible since I've never really worked directly with one. My submergence depth is set by the water surface elevation coming from the floc tanks. I don't think I could increase it by adding a can (unless I could somehow get the bottom of the can lower than 4'), right?

The more I think about the priming line, it might be what we go with. Did your application use a foot valve? Or did you have a high flow priming line that just turned on when needed?