Suction Inlet Size 2

[piffer]

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.

 

[ccfowler]

The velocity does indeed seem to be a bit of a problem, but the main issue is the vortex formation. You can mitigate the vortex formation by installing an intake adapter that greatly reduces the velocity of the water entering the suction flow region. You can fabricate something acceptable with common pipe fittings, commonly available materials, and simple welding operations.

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.

 

[QQstuff]

normally velocity for suction pipe is 4-6 fps.

 

[BigInch]

Its not your pump, its your pit design that's at the root of the problem.

 

[ccor]

QQstuff is correct that normal velocity for suction piping is 4-6 fps; however, suction piping normally flows through a reducer to the suction nozzle of a pump. Suction nozzle velocities usually go much higher, even into the 20 fps range. I would expect the required submergence to be between 8-11' for this pump, so I agree with the earlier posts that the pit design is the problem.

 

[teranko]

What ccfowler is talking about is a "vortex breaker". The description he gives is correct and I suspect your problems will go away once it is installed on the pump suction. These are standard on the pump suction to water disposal facilities installed in the oilpatch in Alberta. Typically the pump suction line is extended into the middle of the storage tank and 90's upward with a votex breaker installed in the vertical.

 

[Artisi]

To say the pit is at fault does not answer the question, especially when the original poster has said the pump is installed in a large open pond.

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

 

[BigInch]

Please substitute "pond" for "pit".

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[Artisi]

The problem could also come from pre-rotation of the inlet flow - if this is the case, a flow splitter at the pump inlet will overcome the problem.

Naresuan University
Phitsanulok
Thailand

 

[rmw]

I already gave big inch a star and when I close this thread it is his, so I can't take it back. I wouldn't take it back in any case.

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

 

[Artisi]

I have to disagree with the point that a large open pond has the same influence as a badly designed pit on vortexing.

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

 

[BigInch]

This installation should only be better if its in a pond. Theoretically there are no edge distances to worry about. Its an infinite reservoir, whether you call it a pond or a pit its hydraulically equivalent. If its in a pond and the submergence is "adequate", then the distance from the bottom of the pond "or pit" must be the problem. Otherwise the pump is not properly designed and it should be up to the mfgr to make it right, as long as its properly installed according to the mfgr's recommendations.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[ccfowler]

By reducing the inlet velocity at the adapter (or vortex breaker) that I described earlier, the required submergence can be reduced dramatically when dealing with a clean liquid. If there are no significant other sources of vortex formation, the required submergence can be reduced to amazingly small values. (A required submergence of less than 1 foot may not be unreasonable under nearly ideal conditions).

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.

 

[Artisi]

ccfowler.
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