Sciencemadness Discussion Board - A Pressure Filter from PVC Plumbing Parts

A Pressure Filter from PVC Plumbing Parts

Xenoid - 13-12-2007 at 12:57

Pressure Filter

Introduction

Often, the home amateur chemist has need to undertake filtrations on a scale and rate, greater than can be accommodated with a simple glass funnel and fluted filter paper.
Some examples which I have encountered, include;
1) The processing of largish chlorate cells, where carbon/graphite detritus and Fe flocculent must be removed.
2) Extraction of potassium perchlorate precipitate after double dissolution reaction.
3) Removal of calcium and barium sulphate precipitates after bulk reactions.
4) Removal of very fine pink precipitate from fertilizer grade potassium chloride.
5) Removal of calcium carbonate and other impurities from CAN (Calcium - Ammonium Nitrate fertiliser)

In addition I could envisage the filter being used for bulk extractions of natural materials, using compatible solvents such as water or alcohol.
Although vacuum filtration is a viable procedure, it requires a vacuum pump of some type, most of which have some drawbacks for the amateur.
Too overcome these shortcomings, I built this pressure filter from easily procured PVC (Drain and Waste) plumbing components which can be operated from a bicycle pump or preferably a home air compressor. The filter is basically the combustion chamber end of a “spud gun” with a filter baffle plate inserted halfway along and a funnel arrangement in place of the barrel. The whole unit is mounted on a sturdy stand.

Construction

The components required are shown in the attached image and are mainly 100mm fittings and pipe. As described here the filter has a volume of about 1.5 litres, however the pipe section could be made a little longer to increase the volume to 2 litres. The filter is constructed in two parts, the upper filter chamber and the lower funnel section.

Filter chamber
The filter baffle plate (8) is the only tricky part to construct. It was cut from 6mm (8mm would be better, perhaps) PVC sheet (obtained as an offcut from a plastics fabricator). A circle was scribed the same diameter as the inside of the joining fitting (6), this was cut roughly with a coping saw and the central 6mm hole was drilled. A bolt was inserted in the central hole (with washers and nut) and mounted in a poor mans lathe (electric hand drill, mounted horizontally in a vice). The baffle was turned down using a rasp, file and chisel until it was an exact fit. The remaining 6mm holes were marked out in an hexagonal pattern and drilled. Rough edges were then chamfered. The baffle plate was glued in place against the ridge inside the jointing fitting. The pipe section (3) was then glued in behind the baffle plate, removing any excess glue that squeezes out and could interfere with the positioning of the filter paper. The threaded section (1) was then glued onto the other end of the pipe section. Make sure it is pushed all the way home, the length of the pipe section was such that a space was left to accommodate the large mounting bracket (5). The screw on lid (4) (see warning note in following post) was then drilled and fitted with a tyre valve (13) and pressure gauge (9) (obtained from a junked electric tyre inflator).

Funnel section
The funnel section was made from two reducer fittings (2) (100mm to 50mm) and (10) (50mm to 32mm) an adapter fitting (11) cut from 12mm PVC sheet and a short length of 15mm PVC pipe (12). These are glued together.The completed funnel section is just pushed in place with a little silicone grease and is NOT glued. It can then be removed for cleaning if necessary.

Stand
I found the stand at a recycling centre, it is very solid, Italian made and used for corking wine bottles. The filter is attached to it using pipe stand-off clamps (5 and 7) available from plumbing suppliers. Old electric drill stands, would also be suitable.

PressureFilter.jpg - 39kB

Xenoid - 13-12-2007 at 12:59

Using the Pressure Filter

The filter paper (B) will probably have to be cut to size to fit the inside of the pipe. As additional support for the filter paper, I place 1 or 2 layers of a non-woven synthetic "interfacing" material (A) I stole from my wife's sewing cabinet, on the baffle plate. Any sort of semi-rigid support material could be used here, it's just to stop the filter paper being pushed through the baffle plate holes. For light pressures it may not even be needed, but I have always used it. A split, 12 mm section of PVC pipe (C) is used to retain the filter paper and support layer in position. If you are only interested in retaining the liquid filtrate, diatomaceous earth (D) can be added, 1 - 2 cm thick on top of the filter paper, this is particularly useful for removing very fine flocculate.

Medium2.jpg - 8kB

WARNING

Xenoid - 13-12-2007 at 13:00

Warning

Never, never exceed 3.5 bar (~50 psi) with this design of filter. In general you will never need to exceed 3 bar (~40 psi), 1.5 bar (~20 psi) is adequate for just about any filtration problem you will encounter. Even .1 - .2 bar, produces astounding results with simple filtrations.

After I first built this filter, I used to overload it with repeated additions of diatomaceous earth. On many occasions I took the pressure up as high as 5.5 bar (~80 psi). The lid actually bulged upward! I even considered taking it to 7 bar (~100 psi) or beyond! One day, as the pressure was approaching about 5 bar, there was an almighty explosion, I thought for a second that the chlorate I was processing had somehow detonated. In actual fact it was the screw-on lid which had disintegrated and blown all over my garage. Fortunately I wasn't leaning over the filter, otherwise I would no longer have a face (some would say this was an improvement). After my ears had recovered, I managed to find most of the lid fragments.
Part of the problem was that the lid was weakened by having both a filler valve and a pressure gauge inserted through it. It was also a pretty cheap, thin lid I happened to find at a recycling centre. I have since replaced the lid with a very stout one, about twice as thick, I have also dispensed with the pressure gauge. The tyre filler attachment for my compressor has a built in gauge, so it was not needed.

Remember this type of PVC plastic pipe and fittings are not designed for high pressure applications, make sure you stay within it's limits!

[Edited on 13-12-2007 by Xenoid]

[Edited on 13-12-2007 by Xenoid]

Lid.jpg - 14kB

MagicJigPipe - 13-12-2007 at 13:14

Some types of PVC pipe and fittings are designed for "high" pressure applications and are usually rated as such. Maybe using materials rated for higher pressures would make a more solid (if you need it) device.

I've made plenty of potato cannons that could easily accept ~130psi because I made them out of PVC that was rated for high pressures. Any higher than that would be risky and you would need some sort of steel setup to be completely safe.

Of course you would have to make sure any modifications you make can also withstand the pressures you are attempting. Then again, I don't see much need for going over ~80psi.

I really like your design. It would be cool if you could make a schematic of your design that people who aren't that good with written directions/pictures could use. Maybe I'll try to do it. Seems like fun.

Xenoid - 13-12-2007 at 13:55

I couldn't find higher pressure versions of the PVC around here, I'm sure they would be much more expensive!
As it turns out, they are not needed, there is really no need to exceed a couple of bar. You will be amazed how well this works, solutions just gush through, even before there is any reading on the pressure gauge....

MagicJigPipe - 13-12-2007 at 14:50

This got me thinking about how resistant PVC is. I found this chart as the 2nd hit on google.

http://www.buerkle.de/eng/d2-1.php

Some surprises in there.

Does anyone have hands on experience with the resistivity of rigid PVC to various reagents?

12AX7 - 13-12-2007 at 18:16

Yeah, PVC is nuts around compressed air. Best to not use it at all. It makes shrapnel: not a question of if, but when! Don't even think about hot filtering solutions in that contraption.

For filtering chlorate solutions, at the moment I use a acid jug, cut down and the bottom punched full of holes. A pad of glass wool is the filter element. The large surface area captures most crud, though it takes a few passes before the colloidial stuff gets stuck, at which point it goes kind of slow.

Tim

Xenoid - 13-12-2007 at 19:06

Quote:

Originally posted by 12AX7
Yeah, PVC is nuts around compressed air. Best to not use it at all. It makes shrapnel: not a question of if, but when! Don't even think about hot filtering solutions in that contraption.

Oh.. Tim, you wuss!

I've been using it for nearly a year, and all thats happened is a weak lid has blown off due to exceeding the normal pressure about 4-fold. If anyone is silly enough to put hot solutions in it, it would only blow up like a balloon (well actually it probably wouldn't even do that). I've done plenty of PVC pipe molding, to make battery cases etc. and to get the stuff soft enough it has to be heated in an oven well over 100 oC. The "body" of the filter is effectively a double layer of PVC!

I find it a fantastic aid, a simple 1 litre filtration can be completed in 10 - 20 secs. More gloopy materials may take a few minutes.

Just heed the warnings and don't overpressurise! I guess one could fit a safety valve if your're paranoid!

[Edited on 13-12-2007 by Xenoid]

12AX7 - 13-12-2007 at 21:08

I'm not so optimistic.

I've heard many horror stories of PVC used for compressed air installations. 85 PSI is about 6 atm, pretty typical for room temperature PVC use. It may not go in the first month, it may not go in the first decade, but it will go.

Built of robust materials, the filter would work very well indeed. You can get a whole lot more positive pressure in this sort of device than you can vacuum below a Buchner.

Tim

chemrox - 13-12-2007 at 22:43

I like it! I wonder if the various parts could be coated with fiberglass and the whole setup strengthened that way.

Xenoid - 13-12-2007 at 23:06

Quote:

Originally posted by chemrox
I like it! I wonder if the various parts could be coated with fiberglass and the whole setup strengthened that way.

I don't think it's really nescessary, as I pointed out above, a simple filtration like separating some non-clogging crystals from liquid, can be done at very low pressure, ie. it doesn't even register on the pressure gauge. The thing I like, is the large volume and large surface area of filter medium. Something like a Ca Nitrate double dissolution where you get almost solid Ca Sulphate forming, is a breeze, 90% of the liquid can be separated in a couple of minutes. I only take it to higher pressure to get the last drops. It's sort of like squeezing stuff in cloth, you end up with a material that is barely moist.

Actually, now it has a very heavy, solid lid, the weakest point is the baffle plate inside. That will break first, the results might be spectacular but hardly life threatening...

bio2 - 14-12-2007 at 12:59

..........I built this pressure filter from easily procured PVC (Drain and Waste) plumbing components .........

DWV rated pipe and fittings are maybe schedule 10 and don't really carry a pressure rating.

Although compressed air is definetly not recommended for PVC I don't see any problems if low pressures are used with schedule 40 or schedule 80 PVC fittings.

Schedule 40 rating is standard plumbing grade and schedule 80 thickness is readily
available.

MagicJigPipe - 14-12-2007 at 16:30

I know I have seen pressure ratings on plastic pipe at my local hardware store. As for what kind of pipe or what grade/thickness I can't say for sure. But there are SOME pipes that have max pressure written on the side.

Xenoid - 14-12-2007 at 16:47

Around here PVC pressure is rated PN12 (average) up to PN18 (extreme). The PN number is the pressure in bars for routine use... 12 bar and 16 bar....

These ratings are probably quite conservative!

I am using this filter at no more than about 2 - 3 bar!

If you want to pay the money for material that is more than 4 times the required pressure rating - go ahead....

benzal - 23-5-2009 at 12:46

This post rocks! I took the concept one step further in simplicity and made a pressure filter out of a 2 litre coke bottle with a valve stem stuffed in the centre of the bottom end (drilled the hole for it with a pointy knife) and small holes drilled in the lid to act as the filter plate. Cut out the centre of that blue seal inside the lid and drill the holes in the lid where the hole in the seal is, if you catch my meaning. Works alot better than gravity filtration and you can wash your product by adding solvent after filtering is finished and shaking it like a seperatory funnel. I regularly take it up to 50 psi with no problems. Best part is, if it gets realy dirty no need to clean, just throw it away and make a new one.

grndpndr - 31-10-2010 at 18:40

Thats exactly what ive been looking for to make the calcinit/NH42SO4-AN synthesis work.
Woosy's indeed,a friend of mine used 4ft PVC pipe sched 40IIRC and a thread on end cap to launch 12 oz cans of
condensed milk 400-500 yds with several ozs of BP.He wasntv the brightest soul but he did seek cover at every shot.Point being the PVC pipe contraption never failed and Id venture a guess the pressure was a bit more than a few atmospheres.Venture a guess as a starting point for failure Id say the PVC tubing touch hole.Far as I know he tired of it after 50rds or so and proof loads using concrete filled cans and double the powder attempting to destroy it to no avail

grndpndr - 4-11-2010 at 05:00

Searched a friends garage, retired jack of all trades did alot of plumbing work and discovered EVERY piece needed down to the pipe- all fittings, tire air valves,and pressure gauges,compressors .The threaded lid is sched 80 and far to thick to accomodate an auto valve stem.The tubing (sched 40) is almost to thick but with lube and proper tools it should fit.Did you encounter any proplems installing the valve stem in the sched 40 cap?Im wondering If I will encounter weakness making a hole in the tubing for the valve stem or if Id be better served buying a sched 40 cap,the sched 80 is simply far to thick w/o countersinking the valve stem hole which sounds dangerous .Im gonna limit
pressure to 2-2.5 atm.Witth a test behind a barrier of 5atm+.
Main use will be filtering sulphates from Nitrate solution.Wouldnt be a bit suprised if I have to use the clay filtering media along with paper filters.And change it out every run as my tubing is unfortunately only 4in greatly limiting filter area.Well,Its free remember.

[Edited on 4-11-2010 by grndpndr]

mr.crow - 4-11-2010 at 11:36

Drilling holes in pressure pipes sounds like a bad idea. The potato cannon people drill holes where the fitting goes over the pipe so there is two layers of plastic.

If the valve stem doesn't fit maybe you should buy a new one. You could tap and thread an 1/8" NPT hose barb to connect to the air source.

Xenoid - 4-11-2010 at 13:06

Quote: Originally posted by grndpndr

The threaded lid is sched 80 and far to thick to accomodate an auto valve stem.The tubing (sched 40) is almost to thick but with lube and proper tools it should fit.Did you encounter any proplems installing the valve stem in the sched 40 cap?Im wondering If I will encounter weakness making a hole in the tubing for the valve stem or if Id be better served buying a sched 40 cap,the sched 80 is simply far to thick w/o countersinking the valve stem hole which sounds dangerous

The plastic thickness of my lid is about 5mm (I'm not sure what schedule it is). I had to make a small chamfer ( 45 degree countersink) on the outer side to accommodate the valve stem. With a little silicone grease it fits beautifully.

Remember, high pressures are not required or desirable. This filter works well with just a few strokes with a hand bicycle pump.

The only change I would make if I was building this filter again would be to possibly use a thicker PVC pressure plate (the bit with the holes in it). I used 5-6mm (1/4"), I think perhaps about 8mm - 10mm would be better (say 3/8").

grndpndr - 19-11-2010 at 07:38

Quote: Originally posted by Xenoid

Quote: Originally posted by grndpndr

I could drill the proper sized hole in the thick cap and use a wood spade drill to get the proper thickness.Like you said I doubt Ill need much pressure at all.The filter plate is a problem in that I may have to use 2 thicknesses of the Plate I have cemented together with PVC pipe glue/along with some sort of ring type retainer as shown to assure the filter media stays in place prior to pressurization.But beyond that I really dont forsee any problems with excess pressure.I wouldnt be overly comfortable with 30psi and if it doesnt filter at well below that...

Ive got serious problems anyway that may require a different approach

condennnsa - 23-3-2011 at 22:24

Xenoid, I also built a pressure filter much like yours, except I did not use PVC fittings, but instead a PP plastic jar. I drilled holes in the bottom for the filter paper, and a hole through the lid through which a 16mm bolt goes through. With a nut and a rubber gasket it is sealed. This bolt has a 6.5 mm hole drilled through, where the hose for air pressure from the pump connects.

Anyway, the problem I seem to have, I tested it last night, is that after I place the filter paper , moist it up, but when I add the slurry to be filtered the filter paper kind of moves around, and it is not firmly pressed against the holes in the jar. The holes in the jar over which the filter paper sits are about 1 mm in diameter.

Xenoid - 23-3-2011 at 23:52

@ Condennnsa

Yeah!... That's why I have the split plastic retaining ring (part "C" in the image above, showing the filtration materials) to hold the filter paper in place. The tension needs to be just right, too sloppy and it doesn't do it's job, also material leaks down the side, too tight and it's hard to remove! I ended up drilling a small hole in it so I could hook it out with a piece of wire.
Hope this helps, good luck with your design!

EDIT: Oh, also, due to the inside diameter of the plastic pipe, I have to trim the size of the filter paper. So they are a tight fit and can't move laterally, the "ring" stops them lifting up and seals the edges.

[Edited on 24-3-2011 by Xenoid]

peach - 26-3-2011 at 05:16

I like it.

But there's no way I'd use it to filter volatile solvents or anything too corrosive. At best it'll contaminate it. At worst, it'll melt.

There is also a problem with plastics becoming brittle around some chemicals. Such that it may pop at a later date when it was fine at that pressure when first made.

Saying this, I can see uses for it. I would test the soon to be filtered material on a bit of scrap PVC first.

[Edited on 26-3-2011 by peach]

condennnsa - 26-3-2011 at 08:06

Well if I can get my plastic version to work, I will try to make one from a glass jar. I have some 4 mm diamond coated drills like these
http://www.sculesiechipamente.ro/Carote-caramida-gresie-meta...
They work great for porous ceramics, but in glass they're slow.
Even so, about 20 holes shouldn't be very hard .
For the pressure I have the same setup in mind. A hole through the lid for a drilled bolt sealed with rubber.