Downstreamer - "operating pressure" vs "pump pressure"

G’day All, I’m setting up another downstreamer and see that the “operating pressure” is, in this case, 5500psi (pressure in system upstream of injector created by the small orifice inside the injector). So, as my pump is only rated at 3600psi, this particular injector is not suitable. An “operating pressure” of 3600psi or less is needed.
I have got this correct, haven’t I?
I’ve been looking at specs and comments too long & my head hurts. Will more beer help?

Cheers, John

I believe that operating pressure is really “max system pressure”. The actual pressure created is a function of the orifice size and the GPM’s passing through it.

The injectors are sized for the gpm you will be running.

I took a look at this nozzle chart that helpfully shows the nozzle sizes in mm as well as inches:

It appears that a 2.1mm orifice (the size used in a 3-5gpm injector) is the approximate equivalent to a size 11 nozzle. So at 5.5 gpm, the orifice in the injector should only be generating 1000psi.

Even if you got the smallest chemical injector available, rated for 1-2gpm, it still has a 1.5mm orifice (somewhere between a 5.0 and 5.5 nozzle), you could run that injector up to roughly 5gpm before you exceed the rating of your pump.

Side note: After looking at that nozzle chart, I’m thinking of bumping down to a 2-3gpm (1.8mm) hi-draw injector. It shouldn’t cause my machine to bypass any, as it will be generating less than 2500psi at 5.6gpm.

I haven’t had trouble drawing with a 3-5gpm injector and 200+ feet of hose, but it’d be nice if I could get a little higher concentration at times. I’m putting off getting an xjet as long as possible, lol.

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If you decide to go that route with the injector, please do share your results! I’d be especially interested since we have the same machine. lol I did go the X-Jet route for stronger concentrations for pre- and post-treating concrete especially. I modded mine so I could continue using my soap tips. Having the ability to get a bit of a stronger concentration with DSing would be great though.

Thanks Alex
Cheers, It crossed my mind to cross check with the nozzle chart (like all good pressure washers I have one beside my desk), but because it was Saturday afternoon the thought of beer pushed that thought away.

The 3 - 5gpm injector has a 0.083"/2.11mm orifice which is a #11 nozzle. This gives a pressure of (approx) 350 - 850psi.

The 5 - 8gpm injector has a 0.091"/2.31mm orifice which is a #13 nozzle. This gives a pressure of (approx) 600 - 1500psi.

Does the higher pressure mean more suction?

I’m looking to set up my 7gpm machine to downstream with 300’ of 3/8" hose (I have a 3gpm machine that will downstream at about 1:5 with 300’ of hose - great for small complicated jobs). I have tested a standard (10%) injector but there is barely any vacuum.

Using the Hi-Draw injector at 7gpm, the #11 produces just over 1500psi, while the #13 produces about 1100psi and the 2 - 3gpm (#7.5) gives 3500psi.

All solved, now to find a 3 - 5gpm Hi-Draw S/steel injector.

I hear you on the x-jet, I’ve tried doing houses with one but find the misting/pressure a problem.

Thanks again

Same problem I had. You can mod the X-Jet to work with your normal softwash soaping tips. I’d just explain it, but this video does a better job than that would. lol How to Modify the X-jet

There are different ways of doing it, but that’s one way. DSing is the best at house washing IMO, but the X-jet has its uses.

I use 2.1mm injectors on 8 gpm machines to get pull thru 250 ft of hose. 200 ft and I could use 2.3mm

Nice. If that’s the case, I could definitely play with my draw rates using a smaller injector. Thanks! Will put that on my “more crap to try” list. It grows by the day.

That is a very good question. I’m not really sure. I have what probably amounts to a seventh grade level of knowledge on Venturi vacuums.

I know a certain level of pressure drop is required, usually 35% or more, to cause the injector to start sucking solution. Whether or not the vacuum sucks harder the larger the pressure differential becomes, I do not know. Having a larger pres. dif. allows you to compensate for more back pressure down the line, as William pointed out about his 2.1 vs. 2.3 injectors.

I’m curious about how the xjet is able to achieve such high draw rates. They work on the same Venturi principle as downstream injectors. So obviously a Venturi system has the capability of drawing larger amounts of solution than standard or even hi-draw injectors. Maybe the soap input needs to be opened up more?

I’m wondering if anyone has ever hacked their xjet to work in the place of a DS injector? Can it stand up to the pressure involved?

All an xjet is is an injector with a nozzle after the gun. It doesn’t have to fight any friction loss in the hose so it can pull stronger. I know several contractors who put their injectors immediately behind their gun. They downstream out of 2 gallon jugs that they carry with their free hand. Instant soap and no waiting for soap to clear the line.

Ok, so do they get draw rates as high as 2:1 using the injectors that way? It seems like there’s gotta be something else a little different than just a lack of back pressure for the xjets to achieve such great draw rates.

They draw more than an injector at the pump but they don’t get 2 to 1

Ahhhh, the venturi principle. I think there is a lot to do with the small details.

The X-jet chart shows higher concentrations at lower volumes; 3:1 @ 8gpm, 2:1 @ 5gpm, 1.2:1 @ 3gpm (but it doesn’t state which orifice is used). I also came across a d/s injector that (supposedly) pulled at up to 33%. The variables I see only are pressure difference and chemical flow path (restrictions due to length, size, bends, height difference). Maybe the internal design/shape also affects pressure difference & flow?

Might be time for a Phd in something, or some buying of (more) stuff & experimenting…

Thanks for the thoughts

a 2 or 3 gpm machine may get close to 50/50 but to what purpose?