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Fault finding Schmidt Thompson metering valve – PT 1

BlastOne’s Master Technician, Kerry Cooper, demonstrates fault finding the Schmidt Thompson metering valve


So, this is a Thompson valve and what I’m going to do is show you how to repair a Thompson valve.

But how do you know it does need repairing? There are two factors here — one is moisture will cause premature wear to these particular units. Also, to a terminology called differentiation of pressure. So, differentiation of pressure is arriving at the pot, the pressurized blast pot having air over the top of it from the standpipe and the pop-up valve inside it. If the pop-up valve doesn’t seal properly, you have air escaping from the top of the pot.

So, therefore the pusher line here that comes down to the Thompson valve, which is on one of these threads, when the air comes down. It’s different to what’s been pushed in the pot. So, the pop-up valves not sealing, the air pressure is different in the pot to what is coming down the pusher line.

The other differentiation you’ll get is that where the air is held with inside the pot and coming down the pusher line, you put a nozzle on there that is greater air requirement than what the compressor has a capacity to pressurize a pot and fill your line.

If the nozzle is too big or worn significantly then you’ll get differentiation of pressure because the draw off of this is far greater than what you have in the pressure vessel. So, the differentiation is what the pressure vessel is holding, what’s coming through the pusher line on what’s been exhausting. So, the exhausting side of it is far greater than what the pressure side of it. So, then you’ll get a differentiation of pressure.

What’s the consequence of differentiation of pressure? Primarily, what happens with the valving itself is this is the conical section of your pipe that’s feeding the grid of its own volition via gravity into this valve. So, when you have the pusher line coming through, the grid falling into the hole and the exchaust side of it is quite extensive, you’ll get what we call a vortex.

So, a vortex is the air as it comes through starts a swirling motion and that swirling motion with inside this valve carries grit with it as well. So, what it does within that rotational motion, it starts to become abrasive within the body of the valve itself.

So, it can have a detrimental effect on the integrity of the housing, the piston, the piston sleeve, and so forth. So, that’s why it’s imperative we check the pop-up valves are sealed properly. There are no leaks, or your plumbing is correct and that’s a safety thing and that’s a safety check you do every morning anyway. So, it’s a mandatory requirement to make sure you don’t have any leaks, and everything is functional as per the manufacturer’s recommendations.

So, the other reason we may have to pull this Thompson valve to pieces is because if we have excessive moisture coming through the pusher line, there’s another consequence for every action, there’s a reaction. Now, on the back of this particular valve, this is where the air comes in from the auto air valve. It’s fed in here so that’s only a tiny fitting. it’s an eighth fitting. So, the air comes into this and pushes the piston open to lift the piston off the seat and allow the garnet to fall through.

So, if you have a lot of moisture in your air, that can have a detrimental effect in relation to the operational aspect of this particular piston. And again, if you’ve got dust within your grit, if it’s gone, it’s been reused.

If you expose this to fine particles of grit coupled with moisture, it can accumulate inside this particular piston and causes it to jam or stick; not only here but also in the main piston functionality as well.

So, what’s the consequence of that? Primarily, the piston in here, James it’s stuck. It’s stuck in this sleeve and I’ll show you now how to address that.

So, remember when we were talking about or what our valve, the pilot valve and if we check after the auto air valve by depressing the deadman to see if there’s any air coming out and there is, and I said that this may be jammed. Now I’m going to show you what you’re looking for in relation to repair and checking the functionality of this particular valve.

So, you can see on the top of the valve here is this big blue knob and this big blue knob not only adjusts the amount of grit we have coming out but there’s something else here.

Underneath this blue knob, I’m now exposing two breathers. So, what these do is when the air, when the piston comes up, it needs to exhaust somewhere because you’ve got excessive air that has to go somewhere that’s coming in. It’s got to go somewhere.

So, as the piston raises, these exhaust ports here and here have to exhaust any air that’s compressed or condensed within that environment. So, what we do is we’ll take these off and check that we’ve got them clear and clean.

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