By Dave McCracken

One of the main impediments to production in gold dredging is the occurrence of plug-ups in the jet and suction hose. A plug-up is caused when a single rock, or a combination of rocks, lodge in the suction hose or jet which prevents further material from being sucked up.

Beginners especially, are plagued with many, many plug-ups because they have not yet learned which types of rocks, or which combinations of rocks, to avoid sucking up the nozzle.

Generally, an experienced gold dredger will watch to see what kind of rocks caused a plug-up. Beginners should do this as well. This way, after a while, you gain an understanding of which type of rocks to avoid putting through the nozzle.

For the most part, the rocks to avoid sucking up are those that are just large enough to fit in the nozzle that are sharp and angular, or  that are shaped in such a way that if turned sideways, they could possibly lodge in the suction hose or jet.

Sucking up a larger round rock, just after a long-thin rock, or just after a medium-sized flat rock, is just asking for a plug-up. The reason for this is because the round rock, having more surface area, will move up the hose faster than the flat rock, will catch up to it, and possibly cause the flat rock to turn and lodge.

Generally, we avoid sucking up large flat rocks altogether. Just like there is a system of knowing how to avoid plug-ups, there is also a system for removing plug-ups quickly.

Most  plug-ups occur in the jet. These are generally caused for two reasons. The first is that most jets are smaller in diameter than the suction hose. Where the larger sized suction hose meets the smaller sized jet, there is a certain amount of confinement which can cause rocks to lodge. The other reason is further up the jet just beyond the inductor(s). High-pressure water comes from the side into the main jet tube from one or more inductors which can spin a rock just right to make it lodge.

Once you gain some experience in dredging, you can usually tell from the feel of the plug-up when you get it, whether the plug-up is in the hose or the jet. Jet plug-ups are usually very sudden; you can feel them “slam”, with a sudden complete loss of suction. Hose plug-ups generally leave you with some smaller amount of suction.

The first thing to remember with a plug-up is to stop sucking material into the suction nozzle as soon as you realize you have one! All of us, sooner or later, experience the joy of loading a suction hose full of rocks and gravel. But you haven’t experienced life to the fullest until you have had the opportunity to do this with a 12-inch dredge!  A plug-up is much easier to remove if you have not sucked up a bunch of additional rocks to complicate the problem.

On a hose plug-up, sometimes you can free it up simply by yanking forward on the hose, or by popping your hand over the intake portion of the suction nozzle. When I get a plug-up, I will do this a few times, and then set down the nozzle (where it won’t suck up further material) and move rocks for a little while to see if the plug-up will free itself.

I always like to keep my suction hose nice and clean. This means using a good wash brush to clean the algae off it once every two weeks or so. The good thing about a clean hose is that you can look into it for plug-ups as you move towards your dredge to knock the plug-up out of the jet. Sometimes, when you think it is a jet plug-up, you find out that the plug-up is in the hose. With a clean, clear hose, often you will spot the plug-up quickly. This all saves time, energy and aggravation.

When leaving your dredge hole to find a plug-up, always leave the suction nozzle positioned so that it will  not suck up additional material, or will not get sucked against a larger cobble or boulder once the plug-up is removed. There are two reasons for this: One is that once the plug-up is freed, you want water movement through the hose to help carry the rocks which  caused the obstruction out of the system. Sometimes, they free up and cause another obstruction further up the hose. On tough obstructions, I will generally follow the rocks up the hose until I am certain they are through the system.

The other reason for leaving your suction nozzle so it won’t get blocked by a cobble or boulder, is that when you are probing the jet for the plug-up from the surface, you are paying attention to how much water is flowing through the sluice box. A plug-up slows the water down. When it is freed up, more water consequently flows through the box—unless the suction nozzle gets sucked up against something down in your dredge hole.

It is really important to get this right. You don’t know what aggravation is until you have gone back and forth from your dredge hole to your dredge three or four—or eight—times trying to knock out a single plug-up!

You need to get a feel for probing the jet from the surface for plug-ups. This is done with a “jam rod”. All smart dredgers weld a “T” onto the upper-end of their jam rods. This is for the simple reason of avoiding the additional aggravation of having to remove the suction hose to recover your jam rod, after it slips from your hand and slides down the jet. For those of us with larger sized dredges, rather than a “T”, it can be better to have an “L” handle on the end of our jam rods. Sometimes there is a particularly difficult jet plug-up which can be hooked out of the jet with the “L” handle.

What I mean by getting a feel for probing, is that you have to learn to feel around and find where the obstruction is in the jet. Some beginners start off thinking the key is to simply slam the jam rod down into the jet over and over again—the deeper the better. This does absolutely no good if the plug-up is further up into the jet. Sometimes the jam rod goes down into the jet, through the rocks causing the obstruction. The person comes to the surface, slams the jam rod deep into the jet a few times, feels no plug-up, decides the obstruction is in the hose, goes back down and follows the hose back to the dredge hole, follows the hose back up to the dredge, jams the rod deep into the jet, etc., etc., and finally decides there is something wrong with the pump!

And, this is why it is important to learn to get a feel for probing. I do this by probing down the jet about a foot at a time, probing at different angles, feeling for the obstruction. Once I feel the obstruction, I direct the jamming action to free it up.

It is also a good idea to have a bolt or some other solid rod material welded onto the probing end of your jam rod. Otherwise, the pounding action can cause the probing end to flair out. This causes problems when you jam the rod down through an obstruction, and the flared portion gets stuck when you are trying to pull it out. The probing end of your jam rod should be a smooth continuation of the rod itself.

If a plug-up is found in the suction hose, generally it can be freed up by tapping against it with a smooth cobble from your cobble pile. If you look over the obstruction, you can     usually see the best angles to tap against the obstruction. If one angle doesn’t work, perhaps another angle will free it up. If the obstruction doesn’t free up easily, the next step is not to beat your hose full of holes! The next step is to turn your dredge engine down to an idle. This releases the heavy suction pressure holding the plug-up in place. Once the engine is idled down, you can usually tap the obstruction free with little difficulty. Then, by turning up the engine, often the rocks which caused the obstruction will get sucked through the system. Sometimes, they will also plug-up the hose or jet again—in which case, you go through the process all over again. This same procedure is used also in jet plug-ups.

If this procedure does not work on a hose plug-up, the next step is to remove the water from the hose. This can be done by lifting the suction nozzle out of the water while the engine is running at operation speed. With no water in the hose, an obstruction is generally very easy to free up. In this case, however, it is wise to shake the rocks completely down the hose and out of the nozzle—to be sure you are finished with them. Don’t forget to then toss the rocks out of your hole!

When the difficult plug-up is in the suction hose near the jet, sometimes it is necessary to remove the suction hose and pull it up onto the bank to remove the obstruction. However, this is only on very difficult obstructions. If you are paying attention to what you are sucking through the nozzle, you should not be burdened with this chore very often!

Several years ago, in an effort to enhance production, we developed oversize jet tubes—with exterior suction hose clamps. In this way, the suction hose fits into a jet tube which is slightly larger in size than the hose. This eliminates about 85 percent of the number of plug-ups which a dredger will get on a normal day. Some of the dredge manufacturers are now creating dredges with oversized jet tubes and exterior suction hose clamps—which is one of the best things that has happened for suction dredges in quite some time.

Don’t be mislead, however, to believe that you are looking at an oversized jet tube simply because it has an exterior suction hose clamp mounted on it. An oversized jet tube for a 5-inch dredge should have an inside diameter greater than 5-inches.

Team work on removing plug-ups is very efficient when two or more dredgers are working together. When I am nozzling and get a plug-up, I usually hand the nozzle to one of my rock men, or send the rock man up to find the plug-up. Once the plug-up is removed, material is immediately sucked into the nozzle. This acts as a signal to the person trying to locate the obstruction that it has been cleared. If no material is moving through the hose and sluice box, it is a definite signal that the obstruction still exists somewhere in the system—or  that  the partner has fallen asleep and lost track of what is going on (good thing you can’t hear miners when they get mad at each other while underwater).

While sampling, or during production, the end result is directly proportional to how much material you are able to feed into the suction nozzle. Plug-ups play a big part in this; because while you are spending time freeing up obstructions, you are not sucking up paydirt. If you are having problems with plug-ups, sometimes you can improve production by just slowing down a little.

The real key is in oversized jets. The amount of work to build and install one on your dredge is nothing compared to the amount of energy and time you will spend knocking plug-ups out of your jet during the course of a mining season!

Everyone gets some plug-ups. The thing to do is improve your control of  the nozzle to the  point  where you only get  a  few  each day. 

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