Suction Inlet Size
Suction Inlet Size
(OP)
I have a portable submersible solids handling pump mounted on a self-supporting stand in a large open pond. The pump curve shows BEP to be 1500 GPM @ 29 FT TDH. The pump curve ends at ~2500/10. The pump is operating just beyond BEP at ~1700 GPM @ 26 FT TDH. This pump has an 8" discharge and an 8" suction inlet.
Is it me, or does anybody else agree that the suction inlet size is too small (inlet velocity at 1700 GPM is >10'/s!)? The pump is submerged 13 ft and we have observed surface vortices being drawn into the inlet. The pump doesn't appear to be happy and operate noisily.
Appreciate any comments from pump designers.
Is it me, or does anybody else agree that the suction inlet size is too small (inlet velocity at 1700 GPM is >10'/s!)? The pump is submerged 13 ft and we have observed surface vortices being drawn into the inlet. The pump doesn't appear to be happy and operate noisily.
Appreciate any comments from pump designers.





RE: Suction Inlet Size
I would start with a much larger blank flange (at least 12") with an opposing blank disk held parallel to the face of the flange by several thin radial spacer plates (1/8" or less). ALL edges should be generously rounded. This will permit the formation of an almost purely radial inflow to the system at very low velocities to inhibit vortex formation. If you want to be more elegant, you may want to consider adding cone (or an approximation of a cone) at the center of the plate opposing the flange to help redirect the converging inflow, but this is probably not necessary.
The flow can then be reasonably gently accelerated through simple reducers (12" to 10" and then 10" to 8"). Since the pump is designed with 8" suction and discharge connections, I am inclined to presume that it can function adequately with these velocities as long as vortex formation and excessive suction turbulence are avoided.
If the suction flow is cleaned up, the flow rate is likely to increase somewhat, so it would be wise to allow for this in determining the spacing for the separation of the plate and flange face. From my experience, it is better to have a larger flange diameter and smaller flange to plate separation since this will produce a more gentle acceleration of the inward flow. You will have to make your own judgements for your system to allow for the nature of the solids that must be handled.
This should be an interesting problem to resolve. Good luck, and please, let us know of your progress.
RE: Suction Inlet Size
RE: Suction Inlet Size
RE: Suction Inlet Size
RE: Suction Inlet Size
RE: Suction Inlet Size
The problem lies with the formation of a free vortex which could be starting anywhere ie, from the bottom of the pond or influenced by the self supporting stand on which it is mounted etc - this type of problem is always difficult to diagnose and to remedy.
However, the standard fix always seems to be some sort of vortex breaker which will interrupt any vortex gaining entry to the pump inlet. I would therefore follow the advice of ccfowler as a practical way to intially approach the problem.
Naresuan University
Phitsanulok
Thailand
RE: Suction Inlet Size
Going the Big Inch!![[worm] worm](https://www.tipmaster.com/images/worm.gif)
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RE: Suction Inlet Size
Naresuan University
Phitsanulok
Thailand
RE: Suction Inlet Size
The pond is the pit. It is just bigger than most pits in the sense that we think of pits. The problem of poor pit design and its effect on vortexing at a pump inlet are the same whether it is a classical pit or a pond or a lake or a gulf or a ocean.
What is the minimum submergence of this pump? What is its height off the bottom or the pit/pond? It may well be that you just don't have it operating below its minimum submergence.
The anti vortexing measures will help, but if they are not at the root of the problem, it may still be with you.
rmw
RE: Suction Inlet Size
The submergence is 13 feet - which seems to be far in excess of the minimum requirement - your point about the distance from the bottom of the pond to the pump inlet could well be very valid - although if the stand is part of the standard pump supply, you would expect it to be correct.
Without a lot more information, my feeling is still pre-rotation of the inlet flow.
Naresuan University
Phitsanulok
Thailand
RE: Suction Inlet Size
Going the Big Inch!![[worm] worm](https://www.tipmaster.com/images/worm.gif)
http://virtualpipeline.spaces.msn.com
RE: Suction Inlet Size
If there are other sources of vortex formation nearby (such as a spillway discharging into the pond or flows into the adaptor inlet passing through or around accumulations of the solids that your system is handling), then the situation is much less certain. Once formed, a vortex can be highly persistent. A vortex may be formed elsewhere, carried by the flow to the vicinity of the pump inlet, and then compromise the pump suction comditions.
Since you are handing solids, the very low velocities needed to inhibit vortex formation are likely to result in settling or sedimentation. This can then provide sources of vortex formation that can compromise your pump's performance. You will surely be in a situation where you will have to balance solids handling requirements against vortex suppression requirements. Since you already have 13 ft of submergence available, it seems likely that you will be able to find a tolerable balance.
RE: Suction Inlet Size
Well put, plus a star.
For the flow rate we are talking about, I agree that a submergence of 1 - 2 feet can be adequate - although probably not desirable by design - because as you well know, if something can go wrong it will. You comment re - solids are also very valid and must be a consideration.
Having thought about the problem, I would still, as a first off diagnosis suspect pre-rotation of the inlet flow and not necessarily inlet velocity, this is based on the submergence reported (13'), the pump is operating to the right of BEP and it's located in a pond where there is obviously no vortex forming obstructions, inflows etc.
A vortex breaker can simply be an on edge 3/8" or 1/2" steel plate positioned below the inlet flange to interupt any pre-rotational flow of the inlet or as built in to many process pumps, a partial blade / fin positioned in the inlet port to prevent rotation of the inflow.
Stopping rotation of the inflow- cancels any vortexing eminating from this point and this would certainly be my first-off change to the installation.
Naresuan University
Phitsanulok
Thailand