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High Altitude Dredging Operation

10/21/2018 3:50 AM

Good day all.

The objective is to process 60m3 of river bed gravel, mud and water at an average altitude of 2500m above sea level. Pump and diesel 70hp engine will be mounted on trailer to be portable.

Pump used will be a 6 inch gravel or slurry pump. Rpm variable based on requirement to achieve desired outcome. 6 inch inlet 6 inch outlet. Suction head minimum of 3m, discharge head 2m. 30m rubber 6 inch hose with 2 couplings but no elbows to inlet. Outlet discharge hose 10m to elevated station 2m. Aggregate size 20mm to 100mm gravel, mud and cold water. Estimate of 60% water 40% material.

Water temperature between 2 and 8 degrees Celsius.

Pumps are not my field of expertise so am looking for advise from the experts on this forum if they would be kind enough to help.

I understand that for every 1000ft above sea level we lose 2ft of the 26ft head achievable at coast. At 8000ft we have 3m or so left. The naturally aspirated engine driving pump will lose about 1% per 100m so about 25% in total.

My first concern is cavitation. My second concern is acquiring the necessary lift. If anybody out there can part with some helpful advise.

Thank you.

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#1

Re: High Altitude Dredging Operation

10/21/2018 5:46 AM

https://www.engineeringtoolbox.com/air-altitude-pressure-d_462.html

<...Suction head minimum of 3m...> is the principle concern, however, <...At 8000ft...3m or so left....> is incorrect according to the above resource. As the <... Rpm variable based on requirement to achieve desired outcome....> all that can be said is "slow things down and it will be fine". As it takes time to tow anything up to <...8000ft...>, processing time is hardly a consideration.

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#31
In reply to #1

Re: High Altitude Dredging Operation

10/22/2020 12:05 PM

The heavy-duty dredge pumps from DAE Pumps work great for this type of application and DAE provides the best slurry hoses.

Check out the excavator dredge pump attachment. Connects right to the boom of the excavator.

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#2

Re: High Altitude Dredging Operation

10/21/2018 7:24 AM

Addressing only NPSH.

At 8000 ft the atmospheric pressure will be around 0.768kg/cm2. That means that you have 7.68m of NPSH available from which you need to subtract your suction head.

Suction head takes into account not only the physical height difference but also the losses in the suction line and the velocity head (which can be ignored).

You are pumping water, so assume an sg of 1 and a suction lift of 3m, your NPSH available will be around 4.4m (I am assuming losses in the suction pipe).

Now you have to go to the pump manufacturer and ask him what the NPSH required of the pump is at the specified differential head (total head). There is no way to calculate this.

If the NPSH available is greater than the NPSH required by some margin (in this case I would suggest by at least 0.5m) then noticeable cavitation will not be a problem

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#6
In reply to #2

Re: High Altitude Dredging Operation

10/21/2018 12:45 PM

Hi Prof

Thanks for the information.

This may come across as a silly question but here goes:

Same altitude same situation except that suction head needs to be 60 ft or 20m instead of 2 or 3m. Is there a pump that could achieve that or is it simply not possible due to altitude?

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#7
In reply to #6

Re: High Altitude Dredging Operation

10/21/2018 1:44 PM

Without the math, the answer is no. You will never suck just water higher than 33 feet at sea level.

60 feet would require 4 pumping stations, at least.

You MIGHT lift it with an auger system.

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#9
In reply to #6

Re: High Altitude Dredging Operation

10/21/2018 6:28 PM

Several posters have assumed your suction head figures are a suction lift, but it's not clear from your post. Is it a lift, or is the suction flooded by that amount? Though it would be odd for (+ve) suction head to be higher than discharge. On the other hand, if it's a lift, you presumably meant 3m maximum, not minimum.

If it's a lift, as others have said, you can't lift 20m at sea level, let alone at 2500m.

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#11
In reply to #6

Re: High Altitude Dredging Operation

10/21/2018 11:01 PM

Roger, not silly - but the answer is that it cannot be done. Atmospheric pressure at that altitude is 0.768kg/cm2 so if you could create a perfect vacuum with your pump (which you can't) you could only ever get a suction lift of 7.68m. I have not seen the curve for the pump, but you probably have an NPSH required of around 3 to 4 m, which would mean your maximum suction lift would be around 3.68 to 4.68 m.

What you need to do is to lower the physical location of the pump so that these suction conditions are met. The discharge head of the pump would then be the differential head (total head) of the pump (taken from the pump curve) minus the suction head. You then need to check if the flow and discharge head developed (as per the pump curve) are adequate for your needs.

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#13
In reply to #2

Re: High Altitude Dredging Operation

10/22/2018 3:56 AM

The SG will be > 1, making the NPSH situation worse. Aggregate has an SG ~ 2.5, so 40/60% mixture mixture has SG ~ 2.5*0.4 + 0.6 = 1.6.

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#16
In reply to #13

Re: High Altitude Dredging Operation

10/22/2018 10:10 AM

NPSH is only concerned with the vapour pressure of the pumped fluid, in this case water. The solids fraction of the aggregate just goes along for the ride.

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#17
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Re: High Altitude Dredging Operation

10/22/2018 10:39 AM

I can't agree with that.

NPSHav (m fluid) = (Patm-Pvap)/ρ/g +/- Hstatic - Δhfric + V2/2/g

Patm and Pvap in Pa

All of the slurry has to be lifted to the pump suction. I don't think it makes a difference whether it's a slurry or a uniform fluid of the same density.

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#19
In reply to #17

Re: High Altitude Dredging Operation

10/22/2018 3:12 PM

There are several factors at play here. Density is not as important as the state (solid, liquid gas). Solids have a much harder time getting "pumped" and have to rely on the kinetic energy of the surrounding liquid to "carry it along", otherwise it will stratify out very quickly (and lubrication factor to allow flow). Larger solid particles have a harder time than smaller particles. Atmospheric pressure at the suction will probably affect cavitation (vaporization pressure) more than it's ability to pump a liquid, because gasses can't be pumped very well. Density itself is a smaller factor than vaporization pressure (of which solids present no real problem).

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#20
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Re: High Altitude Dredging Operation

10/22/2018 3:23 PM

I wouldn't disagree with most of that. There are a lot of other things to take account of, but I was just commenting on NPSH. I still say that a given fall in atmospheric pressure due to altitude causes a bigger change in NPSHav if the fluid SG is higher.

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#24
In reply to #17

Re: High Altitude Dredging Operation

10/24/2018 4:14 AM

The formula is correct but, I take it from the OPs question that we are talking about a heterogeneous mixture not an homogeneous one.

The behaviour and pumping requirement of homogeneous slurries (plastic system) is difficult to predict without testing.

The liquid (water in this case) in Heterogeneous solid-liquid mixtures retains its own individuality.

To quote the late great pump Guru - Stapanoff " in solid-liquid mixtures solids cannot possess or transmit any pressure energy; therefore when solids-liquid mixtures are being pumped, solids can acquire only kinetic energy". The solids go along for the ride.

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#26
In reply to #13

Re: High Altitude Dredging Operation

11/13/2018 12:36 AM

Hi Codemaster

I'm having a look at the SG of materials. As you have said it would be around 2.5. As we have progressed with planning so has our understanding of requirements.

Our suspended solids will be more along the line of 10 to 16% of volume of water. Would this equate to an SG off below 1?

Thanks.

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#29
In reply to #26

Re: High Altitude Dredging Operation

11/13/2018 11:07 AM

Suspended solids as a % of volume of water is an unusual way of expressing it. Most likely solids volume is 10 to 16% of the slurry volume. In that case, with solids SG 2.5, I make it

10% slurry, SG 1.15

16% slurry, SG 1.24

If the liquid is water the slurry SG cannot be below 1. It will be 1 even if there are zero suspended solids.

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#3

Re: High Altitude Dredging Operation

10/21/2018 11:07 AM

20-100mm? You're better to go with a backhoe and dump truck...use the pump to wash the gravel...

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#4

Re: High Altitude Dredging Operation

10/21/2018 11:48 AM

I was thinking along the same lines. A golf ball through a garden hose comes to mind.

20mm maybe a bit in a slurry of sand/gravel, but as the smallest component!? Sounds like trouble..

I know a good bucket dredge is hard to come by.

Larger pump? A test of the current one before the big crew and equipment set-up would be nice.

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#5
In reply to #4

Re: High Altitude Dredging Operation

10/21/2018 12:33 PM

Hi everyone

Thank you for the response.

Quite correct, the aggregate will be more 2mm to 40mm. Not 100mm.

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#8
In reply to #5

Re: High Altitude Dredging Operation

10/21/2018 2:28 PM

Is this for a gold mining operation?

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#10

Re: High Altitude Dredging Operation

10/21/2018 10:38 PM

What am I missing here? While I concur with the other posters answers here, I am wondering why this "portable" unit can't be located at the river source and the water and gravel pumped up to the processing location; if the pump suction is located at the river level, wouldn't the NPSH be a mute point? Also, I haven't read about anything about additional required pumping head to account for the "saltation flow" of the gravel.

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#12

Re: High Altitude Dredging Operation

10/21/2018 11:55 PM

I always thought inductor pumps were used for that, to avoid pump damage.

Seems to me that you'd be better off using an auger to lift the gravel above the water-line that high so you don't have to pump all that water.

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#14

Re: High Altitude Dredging Operation

10/22/2018 4:19 AM

Have you thought of using a submersible pump? Self priming and no suction head to worry about.

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#15
In reply to #14

Re: High Altitude Dredging Operation

10/22/2018 10:02 AM

Good idea. Lowers the pump level even further than I was suggesting. These American pumps are an example of what is available in Submersibles capable of pumping "rocks"

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#27
In reply to #14

Re: High Altitude Dredging Operation

11/13/2018 12:50 AM

Hi GM,

This is precisely what we looking at now.

https://amandlapumps.co.za/products/s-series/

Any comments? I think the 3 inch variation will suit our needs. We're still busy discussing with manufacturers regarding the implications of attaching 20m suction hoses, etc.

Thanks

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#30
In reply to #27

Re: High Altitude Dredging Operation

11/13/2018 3:25 PM

3inch is equivalent of 75mm, so if the largest solids are 40mm as suggested earlier, there is a good chance of blockage. I’d suggest to keep the diameter of the pump and pipe work at least double the size of the largest solids to reduce the risk of blockage.

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#18

Re: High Altitude Dredging Operation

10/22/2018 12:53 PM

Located ''at an average altitude of 2500m'' approximately where?

40mm is about 2-5/8 inches, but screens limit the width of the the gravel size, but not the length, so an 8'' (or greater...) diam. pipe might more readily handle the intended slurry...

Consider (more?...) screens?...

60m3 is not a terribly large amount of gravel to be (processed), if located in all in one spot, but rubber hose might not hold up, for very long, in such a cold application?...

When your job is finished, please get back to us with how it all went, good and bad.

In any case, good luck.

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#21
In reply to #18

Re: High Altitude Dredging Operation

10/23/2018 9:57 AM

Thank you everybody for the input.

Lesotho is the country we will be working in. Basic river dredging as is the case for gold except we're after gems.

In the DRC it's quite simple as there are no major issues with altitude. The weather only allows for an eight month operation so not much time for trial and error.

Thanks for the information, it will certainly come into play during the equipment procuring process. The project kicks off next year but I will update once we are on the ground.

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#22
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Re: High Altitude Dredging Operation

10/23/2018 11:57 AM

Diamonds and sweatshops....

..."Lesotho has taken advantage of the African Growth and Opportunity Act (AGOA) to become the largest exporter of garments to the US from sub-Saharan Africa.[30] US brands and retailers sourcing from Lesotho include: Foot Locker, Gap, Gloria Vanderbilt, JCPenney, Levi Strauss, Saks, Sears, Timberland and Wal-Mart.[31] In mid-2004 its employment reached over 50,000, mainly female workers, marking the first time that manufacturing sector workers outnumbered government employees. In 2008 it exported goods worth 487 million dollars mainly to the US. Since 2004, employment in the sector has dwindled to about 45,000 in mid-2011 due to international competition in the garment sector. It was the largest formal sector employer in Lesotho in 2011.[32] In 2007, the average earnings of an employee in the textile sector were US$103 per month, and the official minimum wage for a general textile worker was US$93 per month. The average gross national income per capita in 2008 was US$83 per month.[32] The sector initiated a major program to fight HIV/AIDS called Apparel Lesotho Alliance to Fight AIDS (ALAFA). It is an industry-wide program providing prevention and treatment for workers.[33] "...

..."Diamonds are produced at the Letšeng, Mothae, Liqhobong, and Kao mines, which combined are estimated to produce 240,000 carats of diamonds in 2014, worth US$300 million. The Letšeng mine is estimated to produce diamonds with an average value of US$2172/carat, making it the worlds richest mine on an average price per carat basis.[36] The sector suffered a setback in 2008 as the result of the world recession, but rebounded in 2010 and 2011. Export of diamonds reached US$230 million in 2010–2011.[37] In 1957, a South African adventurer, colonel Jack Scott, accompanied by a young man named Keith Whitelock, set out prospecting for diamonds. They found their diamond mine at 3,100 m elevation, on top of the Maluti Mountains in northeastern Lesotho, some 70 km from Mokhotlong at Letšeng. In 1967, a 601-carat (120.2 g) diamond (Lesotho Brown) was discovered in the mountains by a Mosotho woman. In August 2006, a 603-carat (120.6 g) white diamond, the Lesotho Promise, was discovered at the Letšeng-la-Terae mine. Another 478-carat (95.6 g) diamond was discovered at the same location in 2008.[38] "...

https://en.wikipedia.org/wiki/Lesotho

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#23
In reply to #22

Re: High Altitude Dredging Operation

10/23/2018 12:47 PM

Well, in this case, make sure you keep the rubber tubing (covered-up/out-of-the-sun) so the sun's ultra-violet rays don't degrade the tubing too quickly...

Same for the pump(s), etc., except provide effective shade to mitigate against over-heating...

But most importantly, take lots of (security)...

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#25
In reply to #21

Re: High Altitude Dredging Operation

10/24/2018 4:34 AM

The Sani pass (from South Africa into Lesotho) was one of my favourite drives. Could be quite daunting in the old days, you are not allowed through the border post unless you are in a suitable 4WD vehicle. Wrecks of cars litter the valleys. The prize was a cold beer or six in the highest bar in Africa (2873m) at the top of the pass.

Good luck with your venture.

One more comment - always consider belt drives for slurry pumps as they give you the freedom to easily change speeds to adjust to the varying conditions of flow and head as well as the makeup of the slurry and give yourself lots of extra power.

Warman (Weir) make a great slurry pump. (Disclaimer - I have no connection to Weir - just admire their pumps).

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#28
In reply to #25

Re: High Altitude Dredging Operation

11/13/2018 12:59 AM

Hi Prof

Yes, the Pass is quite a fun climb. I find the decent a little more exciting than the ascent. I'm South African and spent a few years living in Underberg, so I have hiked the mountains there and driven the pass a few times. Recently the condition of the 'road' was horrendous.

Sadly this time around I see there's a new wreck and a cross for the poor soul who didn't make it. Doesn't happen often but it does happen from time to time.

We're considering submersible pumps now as an option but yes, I like the look of the Warman; if budget permits we may have both options available in our bag of tricks.

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