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McDonald Imperial Super-Diesel

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3 years 4 months ago #216668 by cobbadog
Picked up a couple of plumbing bits today to work towards making up pumping system to load up the McDonald. It is an old belt driven centrifical pump with a 1" inlet and 3/4" outlet but I am dropping that down to 3/8" to create some back pressure and see if the pressure rises enough to put a plasma cut into the bottom of the bucket. It is just a quick set up to load the McDonald and get some hours on the rings. Hope to have it running outside next to the next project to get fixed up and sell off.

Cheers Cobba & Cobbarette
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3 years 4 months ago #216673 by Mrsmackpaul
If you want to load a motor with a pump it has to run open with no restrictions
When you choke it back to increase pressure it uses less power not more

So put a tap on the out let and run it full open and slowly turn it off until its not over loading

I dont think it will even feel that small pump when its full open

Paul

Your better to die trying than live on your knees begging
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3 years 4 months ago #216681 by asw120
I must back up Paul, here. With no flow it will just cavitate. A bit counter intuitive, yes.

Jarrod.


“I offer my opponents a bargain: if they will stop telling lies about us, I will stop telling the truth about them”

― Adlai E. Stevenson II
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3 years 4 months ago #216684 by cobbadog
Advise has been taken on board but too late now as I have already made it up with what I had and that was a small 3/8" line on the outlet. Will pick a couple of elbows and pipe next week to make it 3/4", simple thing to do.
Typical of me I have connected it all up in a different way to normal so will take a pic tomorrow and show you how not to do it even more. But it is fun being different.

Cheers Cobba & Cobbarette
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3 years 4 months ago #216686 by Blackduck59
Yes and no on the pump loading, it will only cavitate when the flow drops below the design pressure and flow of the pump.
A lot of pumps were made to 1 design and you changed the characteristics by turning down the impellor OD.
A bigger than 1"lined to the suction will not hurt and a throttling valve will let you get the maximum out of the pump. Valve may have a short life.
Anything on the pump to say what Rpm it was designed to run at? May need to run faster the the engine to get maximum benefit
Cheers Steve
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3 years 4 months ago #216692 by cobbadog
Hi Steve. It is a Harland centrifical pump of around 6-8" diameter on the housing. I has about a 2" flat belt pulleyand there is a 5" one on the engine. If that is not big enough then I will belt it to the OD of the flywheel, that will give it what for. No info as to RPM on the pump housing anywhere.
My scrap metal man came and did a pick up this morning and he said he will drop in a twin cylinder pump next time he is passing home. No idea of size or make but my reply cannot be printed here but in short I said YES.

Cheers Cobba & Cobbarette
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3 years 3 months ago #216698 by mammoth
Like all the discussions here about horse power etc the principle comes back to "work done'. In this case of a centrifugal pump = volume of water moved to a certain height (pressure) plus internal friction. However by increasing the pressure the flow reduces to nothing and hence no work done, just internal friction.
With a piston pump you can wind the pressure up with constant flow.
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3 years 3 months ago #216703 by Mrsmackpaul
Just a couple of things to clarify

When a pump cavitates it is when it over pumps and the water seperates, this creates air pockets that cause hammer on the impeller

So over revving the impeller will cause cavitation

How many horse power is the motor meant to be ?

The cheapest low reving high volume pump is a paddle or coville pump
They go from a few inches up to at a guess 14 inches
Made in Barham NSW and are fabricated and last for ever and have been sold in there 1000's
They are self priming and pretty much last until they rust thru which is at a guess at least 20 years

Paul

Your better to die trying than live on your knees begging
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3 years 3 months ago #216711 by cobbadog
I think it is rated at 2.5 hp. Hopefully the scrappy will come good with the twin cylinder pump soon and that will put more of a load than the centrifical one. Yes I understand about cavitation the outboard engine I had on the tinnie would do it if I turned too quick and booteed it too early, lots of bubbles and no forward propulsion.

Cheers Cobba & Cobbarette
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3 years 3 months ago #216713 by Mrsmackpaul

cobbadog wrote: I had on the tinnie would do it if I turned too quick and booteed it too early, lots of bubbles and no forward propulsion.


Thats exactly it

I assumed wrongly that it was a lot more HP than that, turns out photos can be deceptive now that no one smokes as people used to put a smoke pack or box of matches in the photo to give it some perspective

Your little pump might be fine running fully open

Paul

Your better to die trying than live on your knees begging
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