Repairing a Hodge Clemco 1440NC Blast Pot

[awemawson]

While stripping paint on some panels on my JCB 3CX ready for painting my Hodge Clemco 1440NC blast pot jammed up - the rubber conical valve that raises to seal the pot when blasting, and lowers when the dead mans handle is released (to admit more sand from the hopper) wasn't raising thus not building up pressure.

At first I thought that the rubber cone itself was jammed but no - having emptied the pot and removed the mud hole to access the valve it was moving freely - perhaps slightly eroded on the rubber but it looked as though it should function were it pushed up into the matching rubber ring in the lid of the pot.

Now pilot air passes down a thin pipe to the operators dead mans handle, and back up to an 'RMS100D' control valve that operates the rubber cone and equalises pressure in the pot. It has jammed before and I had to clean it out but I can find no record of the pictures that I took then - I'm sure that I posted them on the forum but . . no gone 

So today's job was to remove the valve, dismantle it and assess the damage - they are about £240 to replace but a seal and gasket kit is available for about £30. Hopefully it's rebuild able.

There is confirmatory evidence that I'm not dreaming and indeed have been here before by the alignment marks I've put on the body of the valve.

[awemawson]

Now the first thing I noticed was that the 3/4" 'street elbow' that connects the valve to the silencer had worn right through at the bend ! Now this is nothing to do with the fault but shows how abrasive sand is.

Pulling it all apart the upper valve, piston and associated seals look to be fine, but the lower valve was very stiff (probably the fault) and the aluminium piston is scored but the bore of the cylinder seems OK. The seals are O rings and once the scoring is cleaned up I don't think its sealing properties will be compromised too much.

Rooting about in my bag of old iron plumbing fitting I actually found a rather tatty street elbow - a bit of blasting in the cabinet blaster and a light spraying with galvafroid and it looks almost presentable.

So, quite successful so far - I'll order up a rebuild kt that has the necessary O rings, diaphragms and piston seals and I'll probably replace the rubber cone valve while I'm at it. Vendor on holiday until middle of next week so watch this space.

[awemawson]

The new Popup Valve and it's rubber seating ring arrived yesterday so one of today's jobs is to fit it.

The mushroom shaped valve is rubber faced and sits on a vertically mounted 1-1/4" BSP pipe - air entering the pot blows it upwards making a seal with the big rubber O ring in the top of the pot. This allows the pot to pressurise and then pot pressure holds it in the sealed position.

This means that to replace it you have to open the 'mud door' and unscrew the standpipe allowing the replacement of the two sealing components - mushroom and ring. Not particularly complicated just a bit awkward to get at and see what you are doing. To get at the ring I took the upper shield off - just a plate held on by three screws that prevents a weight of sand in the hopper affecting the pop up valve.

Did I NEED to replace the pop up valve and ring - probably not. I can see some erosion in places on the mushroom but not a lot - I'll certainly keep the old one as an emergency spare.

Still waiting for the bits for the RMS100 remote control valve rebuild.

[awemawson]

The rebuild kit for the RMS100D remote valve came in this mornings post so I started rebuilding the valve.

Removing the O rings from the two piston assemblies I lightly cleaned them of scuff marks with fine wet and dry paper, gave them a thorough clean up and refitted the O rings with the slightest whisper of petroleum jelly as recommended by the makers.

Then it came to replacing the actual piston seals. Now these have not been removed by me, either on this repair or on the one previously but I noticed a VERY odd thing. They are perfectly normal 'lip seals' where the lips are usually installed facing the source of pressure. This allows the pressure to push the lips tightly against the cylinder bore making a good seal. Now the upper 'exhaust piston' seal was indeed this way round the but other one was the other way round. Now the operating pressure source for these pistons is introduced between the two pistons so I cannot imagine that it is correct that one is reversed. In the absence of being able to find any detailed drawing I have assembled them 'lips to pressure' - if it proves wrong I'll have to pull it apart again !

So carefully cleaning the cylinder bores I re-inserted the pistons and built the unit up to the stage that I could introduce operating pressure to the port that would normally come from the dead mans handle to prove the pistons moved satisfactorily. Fortunately they did - I had to return them to the un-operated position by hand as this would normally be done by the pressure in the pot (which is not yet attached!)

So far so good - putting the rest back together was pretty simple except that there was some fretting between the exhaust cap and it's mounting spigot that I had to clean up before it would seat nicely.

Putting it back on the blast pot was also fairly simple - the silencer is at a bit of a jaunty angle as I've obviously not got the valves top cap quite where it was originally - I may loosen it all off and get it back vertical later but just now I'm committed to being 'mine host' as the most recent cottage guests have just pulled into the farmyard so I must run 

[awemawson]

So having duly installed the guests in their cottage - (and most importantly shown them the way to the pub !) it was the matter of a few moments to slacken off the fittings and rotate the joints so that the silencer is more vertical.

it cannot be exactly vertical as it clashes with the pressure reducing valve one one side and the body of the pot on the other, but I very much doubt that the exhausted air will mind much !

I can't test the pot in earnest as a further set of cottage guests are expected - it'll probably be a day or two before I get the chance as it means running up the rather noisy road compressor and dragging out all the paraphernalia of hoses, bags of sand etc.

[awemawson]

So having fitted the rebuild kit to the RMS100 remote valve unit did it work - well yes on the bench and NO on the machine  Still the 'pop up valve' wasn't sealing - it would start to lift then not be held up by the pot pressure.

This was getting silly - so as there was some slight damage to the bore of the air slide on the original valve, and despite being convinced that it was so slight as not to affect things I decided to push the boat out and buy a replacement unit. The original had let me down once before mid-job, I'd at least get to a known good starting point, and I'd found a slightly cheaper supplier - so I fired the parts cannon !

New valve arrived - body of the valve itself identical but the connection to the silencer was slightly shorter necessitating mounting it forward of the valve itself - not ideal as it's a bit more exposed to knocks in transport. Anyway I fitted it and with great excitement and a bit of trepidation I set up for blasting:

. . . .STILL the pop up valve wasn't seating properly 


Thinking perhaps I had a regulator problem restricting the flow I removed it from the air circuit and ran the blast pot directly off the compressor which anyway regulates to 100 psi - no change !

Rather dispirited I started packing up the kit to crawl away and lick my wounds. As I disconnected the "CF" blast output connector at the base of the machine where the blast hose connects I noticed for the first time that the bronze of the fitting was suspiciously clean and slightly eroded on it's mating surfaces - do we have a leak ? Nothing was apparent in use, and with the noise of the compressor and the air from the blast nozzle I'd not be able to hear anything. The rubber seal seemed fine visually but when I pulled it out it wasn't - it had disintegrated - is this our culprit?

I already HAD a bag of about a dozen spare seals so I replaced it. OK things are starting to get better. The pop up valve now raises with a good solid thump and sometimes seals properly but not every time. However if I wiggle the pop up valve a bit with my finger it seals every time

Now to replace the pop up valve requires the vertical pipe that it is located on to be removed. This had taken some effort as the threads were locked with sealant, and I'd put it back unsealed. Was it therefore screwed further into the elbow that supports it, holding the mushroom lower and hence further away from the sealing ring. Well as an experiment I wound the pipe thread with several wraps of gas rated PTFE tape - (this is much thicker than the usual type) and refitted the pipe with only the first few threads engaged.

So did this make any difference? YES we are seating every time. I don't want to leave it like this as if that PTFE tape gets blown through the system things will clog up.


Now closely examining the alignment of the elbow fitting and eyeballing it to the big rubber sealing ring I am far from convinced that it is pointing concentrically and vertical, but as it's all inside the blast pot and access is only through the 'mud hole door' measurement is far from easy.

I did find a longer piece of 1-1/4" BSP pipe threaded at one end that I screwed into the elbow letting it protrude out of the top of the pot (no popup valve ) to try and measure concentricity and verticallity but then I realised that the pipe thread was somewhat drunken and the tip of the pipe was describing a circle as I screwed it in - no use for measurements 

I'll see if I can turn up a more precise test piece and in the mean time a friend on another forum who has exactly the same pot is measuring the static height of the popup valve for comparison.

BUT we can blast ! I did a test on my 45 gallon incinerator drum and performance was good.

[awemawson]

So today I made up my test piece for the setting of the elbow inside the pot that the pop up valve stem slides on.

I made a dummy 1-1/4" BSP plug with a dead straight and concentric 7/16" steel rod screwed in that is long enough to extend out of the pot when the popup valve is removed. I'd hoped to use a combination of standard plumbing fittings from my odds & sods box rather than go to the trouble of single point threading this relatively large thread but was surprised how rough and none concentric their threading is - hopeless for measurements.

Not used it yet as I'm awaiting the comparative measurement of valve height from the chap on the other forum and there's little point in pulling all apart more times than is strictly necessary.

[awemawson]

So - what progress . . .well the chap from the other forum hasn't come back to me with a measurement for comparison so this morning I once more removed the mud hole, unscrewed the vertical pipe supporting the popup valve, and made some measurements.


It was far closer to concentric than I had imagined squinting at it through the access hole - about 2 mm both side to side and front to back. Now I don't believe that this error should cause an issue (but what do I know !!!) so having cleaned off the PTFE tape from the pipe I re-assembled it with Loctite 542 pipe thread sealant, adjusting the height on the threads to the 148 mm that seemed to work previously.

Need to let the loctite 'go off' so I probably can't test it until tomorrow if the weather is fair. (Pretty wet early on but OK at the moment)

. . . time will tell . .

[awemawson]

Well it seems to work. Oddly there are places in it's rotation on the vertical pipe that the pop up valve fails to seal but I've marked the pot to avoid them !

So time to put it back to sleep for a few more months. I've always wanted to get a proper cover for it to stop detritus leaves, bird poop etc getting in in storage, but at 15" there are no standard dust bin lids of a suitable size. The makers sell a proper one, but at over £100 that's not going to happen !

Make one - you've got the bits. Plasma cutting a 370 mm circle for the lid is easy peasy. Making the handle is a doddle, BUT my ring roller won't cope with the 1/8" metal strip that I want to use for the rim - what to do 

Slow kinking in a vice would work but look a mess as it will be lots of short straight bits.  The pot has a fairly stout sieve that sits on top - maybe it's strong enough to use as a former?

Clamped the sieve firmly down to the welding bench, and the strip lodged behind a block also firmly clamped down I started manually bending the strip round the sieve - yes it's working 

Obviously it springs back mightily but I reckoned that if I cut off the inevitable straight beginning bit, and started tack welding it to the disk that I had cut I might get away with it.  It turned out the the 'tacks' had to be rather stouter than I'd hoped, and in fact I burnt through in a couple of places trying to get tacks strong enough to stop the strip unwinding but I got there in the end with a combination of clamps and wedges.

Once the strip was all the way round I could cut the surplus off, weld the ends together and it became much more stable. Then I could turn it over and do a continuous weld between strip and lid.

Dressing back it doesn't look too bad, but you know what they say ' my grinder and paint make me the welder I 'aint ' ! I bent up a handle to copy the shape of the one Hodge Clemco weld on their lid and it looks almost presentable.

So a bit of titivating with a sanding disk, a spray inside and out with zinc rich primer and a spray inside with Reseda Green RAL 6011 left over from painting the metal work of my workshop over a decade ago (not too bad a match for the pot in fact) and it can dry over night ready for the top to receive two coats tomorrow.

[awemawson]

So at long last I've managed to get two coats of Reseda Green on both sides of the 'copy' pot lid - oh boy was that stuff slow drying. In the end I brought it in to the (mildly heated) work shop and brush painted it. Spraying in the welding shop was not on due to low low temperature and even slower drying time.

Even now it needs several days to harden off before I can finally put it in place and forget about it for a few more years !

Have pictures of a genuine one (£102 on eBay) and my reproduction.

[Sea.dog]

Proper job