Dust Cyclones - Prevent barrell collapse with a relief valve

[matkinitice]

I'm sharing this as a few weeks back the thread about cheap dust cyclones was a real game changer for me. I planned on getting one eventually but after reading that I purchased one straight away. Everything has been great and I wish I had this sooner but one thing has annoyed me - the barrel collapsing.

The common fix to stop this is to cut a circle that you can friction fit about halfway down the barrel. There's a few things that bothered me about this. Firstly I broke my first insert - while cutting it out on the router was fine, bending it slightly to get into the barrel proved too much. My second attempt didn't fare any better. Despite measuring about halfway inside the fit of the second insert wasn't great and sat far too low - it didn't really do anything. My final issue comes from the fact it makes emptying a bit more of a faff.

I first came across relief valves from Peter Parfit's channel - where he makes one out of odds and ends. Shortly after by pure coincidence over on that other site someone posted an image from Dust Commander that showed what appeared to be a valve fitted. It turns out they sell them - but as they're new (new ish?) the can only be purchased direct from them. I have no link to them - I'm just a happy customer. Delivery took longer than Amazon but it came all the way from Germany. I'm sure if you have the means you could reproduce one of these yourself - but I don't and for the cheap price I can't complain.

Having not long fitted it I figured it was worth sharing this as I don't believe it's common knowledge and it solved my barrel issue. In fact it's so good that even if I fully cover the end of the hose the barrel doesn't collapse.

Cost was about £12, only specialist item needed was a drill bit to cut a 28mm hole. The two small screws require a 2mm pilot hole.

 

[matkinitice]

The nut at the top can be adjusted depending on the strength of your shop vac. In my case I tried a few levels and may still adjust it as I use it more - but for now a loose setting seems to work.

Mine came with two springs - I put one to the side as with two fitted it was too stiff. This may change, but for now one is fine.

 

[Robbo3]

Or make your own from bits you are likely to have in the workshop, as I posted here
- Tips & Tricks

 

[TominDales]

Just point out one subtlety that is quite often confused on this forum. Its the difference between having an unbalanced air extract system and a temporary blockage needing pressure relief.

A pressure relief value PRV is great for the one-offs when a pipe gets blocked and will save a dust collector bid from imploding when that happens, it also relieves strain on the vacuum's motor. It could be blocked by shavings or inadvertently closing a blast gate etc the PRV opens and relives the pressure when an ABNORMAL EVENT occurs.

However the problem you are describing of the barrel collapsing under NORMAL operation is a design problem, the system is constantly pulling a vacuum and pulling-in the barrel. A PRV wont be helpful in this case, it will end up being permanently open, so reducing the effectiveness of your dust extractor.
There are two better solutions, simplest one is to strengthen the barrel with an insert or replace it with a smaller stronger metal or wooden one that can take the pressure. Another thing is to increase the pipe diameter going into the cyclone and drum to allow more air flow and so reducing the pressure drop.

Many people report on this forum that they don't see the barrel collapsing, that is because the pipe sizes are balenced to their vacuum's suck (more by accident than design) for most LVHP dust extractors (typical house vacuum cleaner or small shop vac) pulling a vacuum is part of the design. However if they have a bigger vacuum - a powerful shop vac, and the pipework is narrow diameter ie 50mm or less upstream of the cyclone then that will suck in the barrel. The simplest solution is re-enforce the barrel. A more complicated solution - but more efficient is to upgrade the pipework diameter so that it is all the same along the line to and from the cyclone.

By all means add a PRV as that will protect the motor and pipework from an abnormal event, when the pipe gets blocked, but its serving an additional purpose to the designed solution.
I hope that is clear - a bit confusing. Let me know if you need a further explanation. Best wishes Tom

 

[DBT85]

I'll add, the motor isn't under strain due to a blockage (it'll actually draw less current as there is less air to move) , but with no air flowing over it it will heat up and burn out.