Connected 2 Venturis in Series (need a solution)

[kunalv]

TL;DR

I am using 2 venturis in series to suck up spilled oil from a tray. Outlet of 1st venturi is inlet of 2nd. Small orifice of 2nd venturi causing backpressure at 1st venturi.

Hello guys, running a small project at a client's site wherein he has a problem of leakage from pumps and the machine's door causing cutting oil to start collecting in a tray kept beneath the machine. They are now using a Vacuum Cleaner to suck up this oil once every 30 minutes or so and this oil is then returned to the tank.
We have given them our Venturi based Liquid Reclaimer which takes coolant from the coolant pump's delivery end and the resulting vacuum induced by the venturi is used to suck up the oil from the tray and this oil is sent back to the tank along with the oil passing through the venturi. Please see the below image from a similar product by SMC.

Now the problem is that the venturi is not able to create enough suction to transfer the oil fast enough, so we decided to connect another venturi in series with the first one. Connecting them in parallel was not an option as the amount of oil passing through both venturis caused a pressure drop alarm inside the machine.

So now when both venturis are connected in series, the oil exiting the first venturi and entering the 2nd one passes through a reduced diameter (at inlet of 2nd venturi) thus throttling the flow and causing a backpressure in the first venturi. This backpressure led to a reverse flow of oil and we had oil coming out of the suction port of the venturi itself! Video -

How can I solve this?

 

[Dullard]

You probably can't. Eductors aren't typically used in series for reasons that you are discovering. Additional capacity could be added in parallel, but it seems that you don't have enough pump capacity for that. If I couldn't fix the leaks, I'd consider adding a small diaphragm pump.

 

[jack action]

So one venturi is not enough and two in parallel is too much? Then add another tube in parallel with the two venturis and put an adjustable valve on that line. Adjust the restriction until you find the sweet spot.

Even better: if too much oil goes through the two venturis in parallel, restrict the flow on one of them at the inlet from the tray.

 

[Nik_2213]

"Connecting them in parallel was not an option as the amount of oil passing through both venturis caused a pressure drop alarm inside the machine."

Add a relief valve ??

 

[kunalv]

So one venturi is not enough and two in parallel is too much? Then add another tube in parallel with the two venturis and put an adjustable valve on that line. Adjust the restriction until you find the sweet spot.

Even better: if too much oil goes through the two venturis in parallel, restrict the flow on one of them at the inlet from the tray.

Could you explain your first suggestion a bit more please? You mean to say that I have to add another tube in parallel to my current configuration (both venturis in series)? How is that going to solve the problem of the backpressure at the inlet of the 2nd venturi?

 

[jack action]

Could you explain your first suggestion a bit more please? You mean to say that I have to add another tube in parallel to my current configuration (both venturis in series)? How is that going to solve the problem of the backpressure at the inlet of the 2nd venturi?

I'm referring to a setup in parallel. IMO, that is the way to go. That is the one that is more easily fixable.

But, if you want to go in series, I guess adding another tube in parallel to the 2nd venturi would do something similar. Again, put an adjustable valve on that tube and set the restriction to relieve the back pressure. But I'm not sure how more efficient at sucking oil it would be compared to a single venturi.

 

[Tom.G]

TL;DR Summary: I am using 2 venturis in series to suck up spilled oil from a tray. Outlet of 1st venturi is inlet of 2nd. Small orifice of 2nd venturi causing backpressure at 1st venturi.

Connecting them in parallel was not an option as the amount of oil passing through both venturis caused a pressure drop alarm inside the machine.

How about using two venturis in parallel and adding a vacuum relief valve to the machine; thus limiting the vacuum? This could be as simple as a flap of rubber over a hole in the housing.

Keep in mind though that the oil-return plumbing may need to be adjusted to either allow more - or limit - the return-oil volume.

Cheers,
Tom

 

[Lnewqban]

Now the problem is that the venturi is not able to create enough suction to transfer the oil fast enough,

Do you know the reason for that to be happening?
It seems to me that the cleanest solution is to improve the suction effect.

Could the suction -discharge pressure differential be reduced in any way?
If so (reducing the column head, for example), could two steps (being the second one a pump or another Venturi sucking at atmospheric pressure) be feasible?

 

[kunalv]

Here is the Eductor we have used. What do I need to change to increase the suction at the Suction Port?

Meanwhile, I am trying the approach of adding another tube in parallel to the 2nd venturi. Shall update here.