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Abrasive setting on a Schmidt Thompson abrasive metering valve

BlastOne’s Master Technician, Kerry Cooper, discusses abrasive setting on a Schmidt Thompson abrasive metering valve


So, at the end of this air line, here, is the Thompson valve. The Thompson valve governs the amount of grit that flows from the conical section of the pot. So, the Thompson valve is activated by a pneumatics as well.

So, in the Thompson valve, we’ve got a piston that when the air comes from the auto air valve down to the Thompson valve, the air pushes the piston in an upward position, pulls the piston off the seat, and allows the grit to flow through, down through this body of the back of this Thompson valve.

So, the Thompson valves is quite a unique piece of engineering, primarily because, it’s activated by pneumatics but also too, it gives me a greater governance in relation to the amount of grit dispensation. The good part about these particular valves too is that underneath the nob, there is segmentation or increments of where I am in relation to
how far we have this knob out.

Now, how far I’ll wind it out? Well, basically, the best thing to do is to wind though knob in until it’s home and that means that all the piston is closed on the seat. Operate the deadman. You’ll hear the squeal of just air. So, that means it’ll still allow air to come through there, but it will stop the grit coming through. So, winding that in all the way home to the seat stops the flow of grit coming from the conical section the pot.

So, now, what I need to do is appropriate that grit in relation to how appropriate this knob accumulates to how much grit I have coming through? So, what I need to do is I need to turn this knob out 3 1/2 to 4 turns. That’s funny. That’s what it was with the other valve.

Basically, it’ll give you an indication of somewhere where you need to be. You see, on this particular valve, there’s a mark on there. There’s nothing wrong with putting a texture mark on there to let you know where you are in relation to its operation.

So, 3 1/2 to 4 turns, I wind it out and it’s not 1, 2, it’s full turns that I’m talking about. So, wind at home until it seats and then you’ll go 1 turn, 2 turns, and so on. 3 1/2 half turns.

The only time that this will become problematic is that if you utilize this air system without an after cooler or air dry or air prep as we call it. So, the air prep reduces the amount of moisture that comes through this system. This particular valve is so defined and refined that enables me to govern the grit to such an extent that it’s perfect day-in, day-out, the right amount of grit all the time.

Every time I open the deadman, the same amount of grit day-in, day-out. So, with an air prep on the system, you can appropriate that that’s outcome you’ll have with moisture coming through the system with no air prep you’ll find that ultimately, you’ll be winding this pot. This particular valve, you’ll be winding this backwards more and more to get more grit to come through.

How does that happen? Well, basically, if you’re pumping air in here as well as moisture, the moisture accumulates with the grit. The grit will tend to consolidate. So, you’ll have a spasmatic floor of grit. So it won’t be even. You’ll have dry grit, moisture impregnated grit, dry grit, moisture impregnated grit. So, this poor valve has no idea what’s going on.

So, ultimately, you have to keep opening the valve to get the grit to flow to get it to run through. So, it’s a false economy. No air prep. No aftercooler moisture in here. You’re moving this all the time, so the operator becomes frustrated with the fact that he’s got grit, no grit, grit, no grit. All of a sudden, he’s got too much grit.

So, inappropriate this valve with the air prep in the system. Once you set it, you’ll find that you’ll be able to leave that in the one position all the time.

So, remember what I said about appropriate in the grit as far as dispensation is concerned where it exits the nozzle. If you hold the nozzle up with the deadman depressed, you’ll be able to watch the mantle in the end of the blast nozzle and that’s the mix of air and grit which give a blue color. It’ll be a blue color, definitive blue color, when you hold the nozzle up to the light.

So, once you get to that, you know you’re pretty right. So, if I’ve got too much grit, you’ll hear it. The noise from the nozzle is dull. If I don’t have enough grit, the noise from the nozzle is quite sharp. So, remember in grit with grit dispensation from a valve such as this, less is more.

So, don’t flood what you’re trying to blast with excessive grit. You’ll find you’re more productive with less grit exiting this particular valve. But as I say, if you want that 3 ½ to 4 turns, that’s about where you need to be.

Also, with this valve, it has a coupling on the face side of it and again, these couplings have what we call gaskets in them. Every day, you will need to check this particular gasket. Every day, primarily because, where the hose is connected to this.

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