I The Physics of unloading sand from a barge

[oz93666]

TL;DR Summary

How to efficiently move sand with water jets.

I think it's easist first to watch a short vidio clip

I find these videos very relaxing to watch .. I got to thinking is this being done in the most efficient way?

The sand has to be suspended in the water to move it to the outlet ... The faster the water , the more turbulance and the sand stays suspended, so it seems to me the rule of thumb is the hose be aimed towards the outlet at all times .. Many times the workers hit the sand directly which will greatly reduce the water velocity , it will dislodge the sand but it will be more likely to settle out elsewhere and need moving again ...

Another way to maintain velocity is to carve narrow channels (with the water jet) to the exit point , wide expances of flow reduce velocity.

Any thoughts ?

 

[Baluncore]

They are moving the sand to the pump inlet using water jets. The volume ratio of water to sand, in order to be pumped, requires that high volume of water. The water volume that is needed to pump out the sand, is more than sufficient to dislodge and transport the sand within the hull.

The same pump, being used here to unload the bulk sand, would have been used to load the sand earlier, at the suction dredge site in the navigable channel.

The river has plenty of water to circulate. The cost of wages for local employment is not too high, and almost all of those wages circulate within the local economy. The cost of the diesel needed to run the slurry transfer pump cannot be reduced. How could the process be made more efficient?

Transport of sand like that is used throughout the lower Mekong River. There is the regular YouTube 'Nguyen Che Linh Channel', video from the Tra Vinh dam, that shows the almost over-loaded transport barges, with their slurry pumps, fighting to get from the tidal to the fresh water navigable river.
An example:

 

[sophiecentaur]

I love to watch dredgers dumping silt out of the hull by opening doors in the bottom. Very elegant to let gravity do the work. The ship slowly rises up out of the water.

 

[sophiecentaur]

The cost of wages for local employment is not too high,

The guys doing the job don't seem to be working efficiently. Was there any staff training involved? They seem to be splashing water everywhere with no particular end in view. It's the same problem that you get trying to get rid of sand from a shower tray; it just won't do what it's told. But do we ever ask about where the sand hides when it's in the pipes?

Maybe the economy can't support a 'proper' bottom-unloading dredger. For them, it's all over in a couple of minutes.

 

[Baluncore]

They seem to be splashing water everywhere with no particular end in view.

So long as there is sufficient sand in the exit pool, to be removed by the pump and available water, there is no need for efficiency. Indeed, too much sand in the exit pool might prevent removal by blocking the exit.

 

[sophiecentaur]

So long as there is sufficient sand in the exit pool, to be removed by the pump and available water, there is no need for efficiency. Indeed, too much sand in the exit pool might prevent removal by blocking the exit.

I reckon they could sack half of their staff if they told them what was actually needed. Half the time they seem to be acting against each other (good fun though, for a short while). I've done similar games with kids at the beach, damming and re-directing a small stream which appeared at the base of a cliff.

 

[256bits]

I reckon they could sack half of their staff if they told them what was actually needed. Half the time they seem to be acting against each other (good fun though, for a short while). I've done similar games with kids at the beach, damming and re-directing a small stream which appeared at the base of a cliff.

The pump is fully submerged at all times, so they need 4 hoses, no more, no less.

I would have liked to see the startup of the unload. With sand all around the pump, how in h..ls name did the sand, with little water I am thinking, become fluid enough to begin the pumping without clogging.

 

[phinds]

The guys doing the job don't seem to be working efficiently

I believe you are correct. I've done this sort of thing before (not to THIS scales, but ...)

This guy is doing it the most efficient way. You start near yourself then move the jet slowly up towards the target with some, but minimal, side-to-side wavering. All the others are spreading the sand all over.

 

[Baluncore]

I would have liked to see the startup of the unload. With sand all around the pump, how in h..ls name did the sand, with little water I am thinking, become fluid enough to begin the pumping without clogging.

The sand in the barge was initially saturated. The pump inlet was placed on top of the load at the start, then the hoses were used to move sand into the pump inlet. The pump inlet fell as the unloading continued. Notice that there are two suction pumps operating to empty the barge.

 

[sophiecentaur]

there is no need for efficiency.

That's only true in economies where labour is really cheap. Ask yourself would three / four AI robots be doing it that way?

The pump is fully submerged at all times, so they need 4 hoses, no more, no less.

I would have liked to see the startup of the unload. With sand all around the pump, how in h..ls name did the sand, with little water I am thinking, become fluid enough to begin the pumping without clogging.

You must have a good reason for saying 'four hoses'. I can't think of one.
Maybe they start with a higher level outlet to get mixing / flowing started - or just start / operate with the pump inlet hose positioned at the appropriate height. That would be possible because it's probably suspended out of the load when dredging, to protect it.
The video may be showing the last phase of the job.

You start near yourself then move the jet slowly up towards the target with some, but minimal, side-to-side wavering

I've been there with wet leaves or mud and a blower or hose. Waving about may feel good but slow or stationary build-up pressure where it's directed.

 

[256bits]

You must have a good reason for saying 'four hoses'. I can't think of one.

If water flow into the barge doesn't match output flow, then either of 2 things happen
1. Water inflow > outflow from pump -> barge fills with water
2. Water inflow < outflow from pump -> pump rums dry, slurry sand/water exceeds specs

In either case, the efficiency of sand unloading decreases.

With say only 2 larger hoses for equal water output as they have for 4 hoses, then that would require 2 men per hose for control.

 

[sophiecentaur]

If water flow into the barge doesn't match output flow, then either of 2 things happen
1. Water inflow > outflow from pump -> barge fills with water
2. Water inflow < outflow from pump -> pump rums dry, slurry sand/water exceeds specs

In either case, the efficiency of sand unloading decreases.

With say only 2 larger hoses for equal water output as they have for 4 hoses, then that would require 2 men per hose for control.

But none of that need apply if the height of the pump input is adjustable. For filling up with spoil, the same considerations apply. You need a clear exit. Obvs that only applies if hydraulic dredging is used. It would be a good system to dredge selectively.
But either way, how could get started with the pump input immersed in metres of solid sand?

 

[Baluncore]

But either way, how could get started with the pump input immersed in metres of solid sand?

You start with the pump input pipe resting at a low point on the surface.

 

[sophiecentaur]

You start with the pump input pipe resting at a low point on the surface.

Which is the model in my head. I don't understand the comments about water input rate and exit pump capacity (Apart from the obvious constraints). More water in can provide more suspended sand out. It's easy to regulate the input water flow so where does this tight rule about the power of four come in?

Would it be too high tech to use a vibrator to make the water/sand mix mote fluid? Concrete pumping uses that, I believe.

 

[Baluncore]

Would it be too high tech to use a vibrator to make the water/sand mix mote fluid? Concrete pumping uses that, I believe.

Vibration is used to remove air bubbles from concrete, and would separate excess water from more dense sand, thickening the fluid.

The sand being moved is river sand, which is sharp and angular, for making concrete, unlike windblown sand that is rounded and so flows more freely.

The sand transport system is designed to pump significantly more water than sand. The fluid then has lower density, and does less damage to the centrifugal pump, which is scoured by the sand. Since the slurry is pumped some distance, there needs to be plenty of water to keep it moving, to prevent sedimentation along the way.

https://en.wikipedia.org/wiki/Bar_Lev_Line
"To deal with the massive earthen ramparts, the Egyptians used water cannons fashioned from hoses attached to dredging pumps in the canal".

 

[256bits]

Would it be too high tech to use a vibrator to make the water/sand mix mote fluid? Concrete pumping uses that, I believe.

Concrete would act as a Non-Newtonian fluid, which when under a level of stress becomes liquid and can flow. It is also non-settling, meaning that the particles and water do not separate.

Sand is settling. The turbulence of the water keeps it in suspension. A minimum flow of water is needed to keep the sand flowing. Below that flow velocity, the sand will settle to the bottom of the pipe and the pipe may clog.

Rather than vibration, one needs agitation to provide the turbulence in the tank to keep the sand particles in suspension. The high velocity hoses are doing double duty. They are providing localized agitation, and providing to the suspended pump a supply of water to carry the sand away

 

[sophiecentaur]

Vibration is used to remove air bubbles from concrete, and would separate excess water from more dense sand, thickening the fluid

I now realise the difference; interesting. I rummaged around in pages about concrete and the bubble-clearing effect of vibration is made clear. There is mention of aiding flow and settling but that's clearly a local feature - filling small spaces - which doesn't apply to the dredging exercise.