pump power requirement for a dredge pump 2

[riqson]

Hi all , I am brasilian , sorry about ortografic errors...
someone can help-me to determine how much HP and what RPM must be ideal for my 18" drege pump? pls take a look in anex esqueme ,

is a 1,1 mts x 35 cm 4 blades impeller in a pump shell 1,5 mts diametre
discharge and suction pipes 17" internal diamere
pump from sluice box goes 5 mts head by 9 mts lenght
max deep pum from botton in the river 32 mts

thanks in advance

 

[LittleInch]

riqson,

Without a flow rate or pump curve this will be impossible to answer.

also what is approx. density of the presumably very dirty water going through this pump?

How much mud is present in the water as a percent.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[riqson]

LittleInche, thanks for reply...
well we do not have any kind of instruction , we make dredges by pratice try and error...
everybody here have no idea in flow rate , etc

the material is heavy gravel whith big cobbles

normally we use Cummins 400-480 hp in 14" and 16" dredges
this dredge we are preparing to go more deep , let me upload a video in youtube to you have some idea in flowrate...
we had a cummin 400hp but this engine was dead and we dont get running to test.. we are looking a 600 hp engine but before buy a engine I want get some advices from experts to avoid set big engine whith too much hp and fuel consuption...because this I try get some technical advice about the minimun hp need to pump from 30 mtrs deep and send 5 mts ahead trough 9 mts descharge pipe long..
by while its is all I can say , pls wait I upload a video to have idea in flow rate of this dredge.

thanks again
this image below is from a 14" dredge sluice box -

 

[Artisi]

If you are using the same pump as you normally use on your dregdes, the depth below the water level won't make any difference to the amount of power needed - so the same engine will be ok.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[LittleInch]

With all due respect to artisi this isn't quite true (it's a bigger pipe), but more of that later.

first off some assumptions needed to make the calculation work:

Flowrate / velocity. From a quick search it looks like you need about 5m/sec min in order for the gravel / cobbles to be lifted up the pipe and maybe a bit more so lets go for 6m/sec.

17" ID this works out at 3150 m3/hr
The differential head of the pump is likely to be with approx. 3m head (absolute) on the suction side to (10m + 5m lift from pump + 2m friction loss) so say 15m differential head.

That pump looks pretty basic to me so if we take an efficiency of 0.5 we're being generous.

fluid density - assuming we have material with an SG of say 1.9 and we have it in a volume ratio of 20% in water, this makes the average fluid density SG of 1.18

shaft power of the pump is then Q x m x SG / 367 x eff in kW
power is 303kw = 405 hp

However there is a lot of uncertainty here so I would go for at least 500hp shaft output.
Remember the pump will only take the power it needs so using a larger engine doesn't mean a bigger fuel bill for the same duty.

HOWEVER, I'm not sure you fully appreciate the impact of extra depth.
For a pump mounted at river level, the only driving force you have to flow water up your 30m long tube is atmospheric pressure minus the pressure at the pump inlet. Whilst the theoretical limit is 10m head water, the practical limit before the pump starts to cavitate and loose suction is probably about 6-7m "lift" capacity.

for a 30m long tube running at 6m/sec you will probably loose 2m at least in friction losses so that leaves 5m head. That 5m is required to lift the heavier fluid up the pipe. That 5m is the same whether you have a tube 10m long or 30m long.

So what changes? - the percent of gravel / rocks you can lift in the water. So the deeper you go the more water per tonne of gravel you need to move which hence costs more and more per tonne of gravel as you go deeper.

A fairly quick search on the internet shows that for a surface mounted pump you are generally limited to around 20-25m max in suction lift before it either doesn't work (not enough velocity or the pump simply cavitates or just becomes very expensive. For 30m + you either need a submersible pump or accept that your percent of material is going to be as low as 5%. Many dredgers locate their pump inside the hull at the bottom and have deep dredgers of >5m water depth to help the pump.

However if you want to give it a try that's up to you - just don't blame the pump when it doesn't do what you want it to do.

Speed of the pump? No idea but I would start at 1500rpm and wouldn't go much above 2000.
NOTE: without doing some tests on the pump to establish your pump curve, the pump might want to consume more power than your engine can supply.
Flow is proportional to speed and is a minimum of about 3000m3/hr
Head is proportional to the square of the speed
Power is proportional to the cube of the speed

Hence you might find yourself trying to increase speed to get the flow rate / velocity you need but then running out of power. The only answer then is to modify the impellor by making it smaller diameter or use a smaller pipe.

Good luck and let us know how it goes.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Artisi]

LI, my opening comment was, "if using the same pump as you normally use". If we can establish this then what follows is basic info.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[LittleInch]

Artisi,

I'm not going to get into an argument with you - I respect your views too much. My point was simply that he's using a bigger longer pipe, but needs to maintain the same velocity, therefore flow rate will go up and friction losses in the pipe will go up, therefore power required from the same pump will go up.

Even with the same pump and the same pipe, a longer/deeper pipe will need more marginally power to do the same flow than before. I think we're splitting hairs here....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Artisi]

Far too many unknowns to even start guessing other than if the pump is larger - yes bigger engine but as the OP has said they do it all by trail and error so it's anyone's guess.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[Artisi]

One approach would be to run the new pump at a reduced pump speed (not engine speed) using the 400 hp engine to get an idea of power required, from here you can make a few assumptions of power requirements at increased pump speed.
But it's all guess work and trial and error.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[SandCounter]

Very rough, but this will get you in the ball park.

Assuming you are increasing your line to 18" pipe, your worst material is not the cobbles, it is the 0.7mm material. You will need about 16 ft/s velocity, up from about 13.5 ft/s in the 14" pipe. That increases your minimum flow requirement from 6477 gpm in 14" pipe to 8460 gpm in 18" pipe.

Your power increases by the cube of the flow increase

P2/P1 = (Q2/Q1)^3

So if you are currently maxing out your 400 hp engine, you will now need 891 hp to just increase the flow.

Dredge pumps are going to have similar proportions so you can use the pump affinity laws for Family of geometrically similar pumps.

Assuming the same speed,

P3/P2 = (d3/d2)^5

where P is power and d is wheel diameter, assuming direct proportions, you can use diameters of the discharge.

I figure you will need about 1900 hp. . . again, rough.

If you are only increasing the pump size and not the line size, use P3/P1 = (d3/d1)^5 for 1400 hp.

The more info, the closer we can get.


I used to count sand. Now I don't count at all.