Advice making a venturi fertilizer injector?

[gellfex]

I've been using drippers and micro sprinklers for many years on my container veggie garden, and wanted to up my game by injecting liquid fertilizer in the system via an injector that dilutes it at a metered rate. So I bought a cheap 1/2" venturi injector on Ebay since it looked no different that the more expensive ones and it's just a piece of molded plastic. After I set it up with the piping and diverter it seemed to work great, sucking up water from my test cup, till the system bled out its air and pressurized, and then it stopped sucking as the flow reduced. My flowmeter said I was flowing 1.3gpm at that point, and that sounds about right. I run 20 min and flow 25 gal. From what I read I thought a 1/2" unit should still be working at that flow. I tried removing the backflow valve to the feed cup, to no avail, it actually was filling the cup at that steady flow.

So my next idea is to machine one with a smaller effective range. Does anyone know what the parameters of this kind of venturi device are? I know you create high pressure in front of the inlet hole, but after that I'm not sure what works. My only other experience with venturis is heating loops. Or should I try to modify what I have? Any advice would be welcome.



These plastic injectors are more or less a commodity item on Ebay. The one I bought has the below specs listed. My flow of 0.3 m³/h is just at the low end of it's envelope, so I guess in reality it's outside it.
Size:
1/2":140*75*30mm;
3/4":143*75*30mm;

Flow range:
1/2":0.29-0.81m³/h 0.7-9.5bar;
3/4":0.65-2.42m³/h 0.7-9.5bar;

Self-priming capability:
1/2":3-37L/h;
3/4":9-92L/h;



Here's a performance table from a higher end mfr, I can't make heads or tails of it in term of actual physical characteristics of the hardware. I have a filter with an integrated 30psi regulator.

https://www.dripworks.com/downloads/dl/file/id/198/product/1996/injector_performance_table.pdf

[Rustybolt]

I need one that can move a powder.

[jjcale]

This might do the trick.
efunda.com/formulae/fluids/venturi_flowmeter.cfm

[chipmaker4130]

The minimum flow rate specified for venturi action probably assumes an open discharge. You restrict your outlet with 'micro-sprinklers' and 'drippers' which changes the entire dynamic. I bet you would need to pressurize the fertilizer source to at or near the pressure at the emitters for it to pull reliably. Your flow rate of 1.3gpm is a result of discharge restrictions, not of minimal inlet pressure/flow. In other words, if you were to open the venturi discharge and restrict inlet flow to the point of passing 1.3gpm, the venturi would likely pull as advertised.

The automatic systems I've seen use a pressure-type injection pump capable of exceeding the pressure in the distribution lines.

[Illinoyance]

Venturis don't work well with back pressure on the outlet. I suggest a metering pump to feed liquid fertilizer into your system.

[lakeside53]

Yep.. I had the same idea once... back pressure kills it. Even a long hose.

[Ohio Mike]

Venturis don't work well with back pressure on the outlet. I suggest a metering pump to feed liquid fertilizer into your system.

+1 metered pump is the way to go. Same way they do chlorine injection for household water systems (when its required).

[gellfex]

Thanks all. Metered pumps are WAY more expensive than this. The next best is a half assed system called EZ flo that uses water pressure diverted into a container, not unlike a quart cup spray gun. Problem is the dilution changes continuously as water enters and solution exits. I'm going to contact Mazzei the maker of the more expensive injectors and see what they say of the back pressure issue.

[J Tiers]

Thanks all. Metered pumps are WAY more expensive than this. The next best is a half assed system called EZ flo that uses water pressure diverted into a container, not unlike a quart cup spray gun. Problem is the dilution changes continuously as water enters and solution exits. I'm going to contact Mazzei the maker of the more expensive injectors and see what they say of the back pressure issue.

One would assume the back pressure would kill it, since after all, the whole deal depends on a lower pressure being created, and obviously back pressure will kill that.

Except it ain't necessarily so.

The lower pressure area in the venturi occurs regardless of ambient.... there HAS TO BE a lower pressure when the flow is faster. So it would still work. The issue might be what the flow rate is, and what size the venturi is. And, of course, the point that the pressure in the venturi is not necessarily "low"... it is just "lowER" than the pressure in the pipe away from the venturi. Relative, not absolute, relative to the pipe pressure. That "what is it relative to" issue is important.

The usual venturi with open exit thus has atmospheric pressure as the "reference", the pressure it is relative to. So then, it will develop a suction from an open container, one exposed to atmospheric pressure. The other issue is that the entrained liquid does not change the pressure in the atmosphere.

In a closed pipe, with "back pressure", the venturi would have suction relative to "pipe pressure", but might easily actually spray out in reverse, instead of having suction, vs atmospheric pressure. And it might be more difficult to have a source of material that is at pipe pressure, va having one that is at atmospheric pressure. Added materil entrained by the flow, will also potentially add flow volume and back pressure in the exit pipe.

[Guido]

Gell---Pull up Mazzei's website. Appears they have schematics showing methods to do exactly what you want, on the cheap. g

[batt-man]

first link i could find that shows a picture of how to setup you injector

https://thewatersprinklershop.co.nz/products/vicom20

I have a setup at home exactly as this picture shows - you need to slightly close the "red tap" so that you effectively have a lower pressure at one end; this causes the suction to draw the liquid fertiliser up into the system

Works great in my garden sprinkler system

Cheers
Batt