Problem with my Venturi operated Drum Vac

[kdv1988]

Hey guys, I have built a Compressed air venturi operated Drum Pump for my garage. The system consists of a horizontal venturi tube with a pipe (shut-off tube) that is inserted into the Drum. Turning on the venturi creates vacuum in the drum and thus liquid is sucked from outside and is collected in the drum automatically. This pipe contains a float which moves up-down. Purpose of the float is to block the venturi when the liquid reaches up to the highest level in the drum to prevent overflow of the liquid from the drum. Since it is a purely compressed air operated system, there are no electricals and hence no switches. Attached are photos depicting my system, the float & a standard Exair Drum Vac.

https://ibb.co/fkd46kX

https://ibb.co/tCH67Qt

https://ibb.co/64fzFxV

Now, the problem I am facing here is that when I turn on the venturi, the vacuum sucks up the float and blocks the venturi immediately thereby stopping the vacuum generation process in the drum. Releasing the pressure causes the float to fall down in the pipe to its original position and ONLY then the system works (i.e liquid rises and the float rises along with the liquid and blocks the venturi when the drum is full).

The material used for the float is either Nylon/Teflon (don't know which as we have a bunch of either rods available with us) and the shut-off tube is SS304. What can I do to ensure the float ONLY rises up in the pipe along with the liquid? Im worried if I increase the weight of the float, it might not float on water and will remain seated at the bottom of the pipe.

I cannot invest a lot of money on an Exair system as we're a small shop and have most of these parts handy. I feel like I am missing something rudimentary here and some guidance will be really helpful. Thanks!

 

[LittleInch]

I can only suggest that you either restrict the size of the hole at the bottom which lets the water in or increase the area of the air holes above it.

Maybe also turn the air supply on slowly over say 10 seconds to reduce the initial surge of air flow.

A longer dip tube holding the float might work as well.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Artisi]

The valve looks too sophisticated to me and troublesome as you have discovered, Search the net for Godwin pumps and a local supplier or contact them direct. Their vacuum control system is very very simple and works well with probably thousands now in the field operating.
You can possible buy the venturi assembly from them and adapt it to your requirements.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[kdv1988]

@LittleInch increasing the area of the holes above the float would just allow more air to get sucked out of the chamber right, meaning a faster more powerful vacuum? Wouldn't that also cause the float to get sucked up?

How will reducing the diameter of the hole at the bottom help?

Controlling the valve opening can be done, but ideally I would want the system to work fool-proof as we usually employ some casual laborers to do the dirty work and they may not remember to control the opening

 

[kdv1988]

@Artisi Godwin pumps are too large and probably not available where I am right now (India!). I am looking for something that will sit right on top of my drum.
Plus, we have the venturi design sorted. It is just the float that's giving us some problems right now.

 

[3DDave]

little inch is right - make the inlet holes above the float larger and make the hole in the bottom smaller. I'd also cut flutes or slots through the sidewall alongside the resting position for the float so that any air coming from the bottom can bypass the float. If you use slots there will be no need for the hole in the bottom.

 

[kdv1988]

@3DDave, thanks for your help. Would love to understand why you and @LittleInch have suggested those two tips.
For the flute or slot on the tube, PFA photo. Is this what you mean? In the pic, the slot is above the resting position of the float.

Also, any particular material that you would suggest for the Float? As I mentioned, right now it is made out of either Nylon or PTFE(Teflon) since that is what we had available with us. Need something that will float easily as well as withstand abrasion over time (it is moving up-down inside a SS304 tube). There are some fine tolerances that I have maintained while machining, but still....

 

[LittleInch]

The float has weight. Maybe not a lot but some. To cause it to go upwards you need a force.

That force comes from the differential air pressure from under the float to on top of it.

Your holes are probably causing a short term lowering of the air pressure above the float. So increasing the hole size would reduce this effect. Reducing the size of the hole under the float might do the same thing by restricting the airflow under the float.

Or experiment with the float by drilling a hole in it and adding some heavier material until it just floats.

Or lengthen the tube the float is in so it takes longer to move.

Adding more holes might increase air flow a bit but you are limited by the venturi itself. A bit of a bigger gap between float and tube might be enough to allow the air rho flow past it.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[kdv1988]

Hi @LittleInch, thanks for your response.
Some queries :-
When you mention that the holes are causing a short term lowering of air pressure above the float, isn't that inevitable since we're sucking the air out of the drum using the venturi? Wouldn't that create a low pressure (vacuum) inside the drum?

Reducing the size of the hole under the float would mean less air getting sucked out through that hole, meaning less force applied on the float to rise up on its own correct?

Wouldn't drilling a hole in the float (im assuming you mean a through-hole) make it heavier and thereby not allowing it to float?

Increasing the length of the shut-off tube will require making a new one from scratch I guess. Will try out the other suggestions before doing this.

Request you to respond on the above queries. Thank you so much for the help

 

[3DDave]

Small holes = restriction. So lower pressure inside the tube allows remaining pressure in drum pushing the float up. You want all volumes in the drum to have the same pressure so it will not push the float up so remove restrictions above the float.

 

[kdv1988]

@3DDave, Got it. Will give this a shot and report back here. Any other suggestions, please lay them on me. Thank you guys. Really hoping this will work. Shall revert here with the results of this as soon as possible.

 

[LittleInch]

Well your OP did you had an issue on start up so there may be a point where there is lower pressure above the float before the pressure in the rest of the barrel gets the same. Hence why this issue only happens on start up.

A greater gap between floating plug and the the barrel I think would help hugely as well as the slots suggested.

Let us know what happens!

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[kdv1988]

@LittleInch you mean the holes or the slots?

I will increase the size of the holes above the float and if possible the number of holes as well. Should I also make the slots on the side of the tube? Most similar products I've seen online usually have one of the two, never both.

 

[LittleInch]

Either. More or bigger holes are probably easier.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[hydtools]

Heavier float?

Ted

 

[LittleInch]

Just let us know how it goes.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[kdv1988]

@LittleInch, we increased the number of holes above the float and that has made a visible difference! The float does not get sucked up upon starting the air supply through the venturi. So yes that suggestion has worked out.

We've observed another problem though. Some water leaks past the float and enters the venturi which then throws it out along with the compressed air. So I guess we're gonna have to change the design of the float & the tube. Will a round float(like a ball) work? Something similar to what you find in an NRV?

 

[Artisi]

A ball is far better against a near square corner (slight champer is ideal) but the you will probably always get a slight discharge of product from the venturi discharge.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[Compositepro]

Connect another float to the existing one, below your device. You are now allowing water to rise through the bottom hole, which has a significant delta P (therefore air flow, which entrains water).

 

[kdv1988]

@Compositepro, another float below the device meaning outside of the Dip Tube? I am reducing the diameter of the bottom hole now. I need the hole so as to allow the float to rise freely when the water rises up to that level.

(therefore air flow, which entrains water).
Could you explain this bit please. And possibly your entire post if you can