pump pressure

[Ciderhead]

Do you not get cavitation or aeration from all the pushing into a smaller restricted space?

[donnchadhc]

Oversize ing the inlet pipe, kinda making a new reservoir connected directly to the pump?

[Dr Horrible]

You don't get cavitation from a restriction downstream of the pump, you get it when suction is created on the inlet to the pump (which can happen if you have too many restrictions like valves or a long length of narrow feed line and the pump has to 'suck' the liquid in rather than it flowing in freely) in combination with high liquid temperatures the liquid  will start to boil and form bubbles.  These bubbles then get pressurised by the pump and collapse with nasty consequences - from my experience it sounds like you're pumping gravel and the inside of the pump looks like it has too!  You might get some aeration with a valve if you have too sudden a restriction - but again if you use the right valve this isn't an issue.
You don't need to oversize the pump inlet so much that you have a reservoir, you just need to balance the flows.  If I was starting out to design a pump being fed by gravity, I'd assume a flow velocity in of 0.5m/s and I would design for a velocity out of 2m/s.  With the same size pipe in and out that means that with this setup the flow out is four time the flow in, which obviously will result in the pump running dry.  But suppose you have a 1/2" line coming from the pump and a 1" feed in - a 1" line has four times the area of a 1/2" line so now you have enough flow into the pump for it to pump out without any issues - hope this is all someway clear!
Sorry if I'm getting into too much detail but those pumps with the same size inlet and outlet annoy my head -they work fine in certain situations but  you need to be careful about their setup.

[RichC]

What's a decent pump without breaking the bank for a single vessel recirculating system?(self priming!)

[Ciderhead]

Not sure such a thing exists

[RichC]

Not sure such a thing exists

Can one be bought for <100 ?

[Ciderhead]

Nope
Start with buying a couple of solars

[Dr Horrible]

The solar projects pumps are direct drive, as in the impeller is driven directly by the output shaft of the motor. Two things about this: if it gets clogged the motor burns out, and maybe more importantly the liquid being pumped comes into contact with the motor's bearings which is not considered food safe.

Very true - and as you start to look for better seals and food grade material the prices start rocketing.  If I was building a brewery I'd be looking at preferably peristaltic or double diaphragm pumps (both self prime and the peristaltic doesn't even contact the liquid), but any costs I've seen for peristaltic are too high to consider using them at home and the double diaphragms need compressed air to run.
I was in a farm shop recently and saw some interesting pumps - I reckon the farm/dairy industry is the most likely to have cheaper equipment that would be food grade, must look into that a bit more, see if there's anything that could be used for brewing.

[Will_D]

Only problem i might see with Dairy/Farm pumps is they wont be rated to 110C as most f/d apps would be low temp.

[johnrm]

...pumping gravel...

Is that a typo?

[Dr Horrible]

Nope! That's an extreme situation though, normally cavitation sounds like there's something rattling around in the pipes. I worked on a job once where a pump was cavitating badly and it really was like rocks in the line, the pump had to be taken out every few days. Moved the pump to directly below the feed vessel, increased the inlet line size and problem disappeared. Same principle applies to the little solars, get the set up right and they'll run away no problem, but if it's wrong they'll let you know fairly quickly.

[johnrm]

I have you now.
Thanks!

[RichC]

I've decided to buy the solar projects SP20/20  . Should I buy the fittings, olives and hose with it or is that stuff easily sourced locally

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[Damien M]

The fittings are handy and at the price you can't go wrong, you can always replace later with brass or SS .

I would also highly recommend the transformer that can be purchased (15 poundish). It allows you to set the speed electrically so that its not going full pelt against your discharge valve that is nearly closed. I normally set it low see what the flow is like then tweek with the Discharge valve to suit, really good for Mash Tun recirc clarification.

 

[RichC]

Thx DaMun, does the 20/20 indicate 20mm fittings or are they standard imperial sizes that I'll get locally?

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