- Created: Monday, 28 September 2009 08:16
- Published: Monday, 28 September 2009 08:16
Willard Salemink is a walking encyclopedia of dredging technology. The 76-year-old founder of Twinkle earned his degree in general science from the University of Iowa in 1961 and began his career in steel fabrication. He launched Assemblers Inc, which concentrated on the design and construction of aggregate processing equipment including crushers, screens and conveyors.
In 1978 he got the chance to build his first sand and gravel dredge. It had a diesel engine, an 8-inch pump and a cutterhead that could dig down to 35 feet. It was a simple design that applied the concepts of other dredges that he had visited. More than 30 years later, that dredge is still operating at River Products in eastern Iowa.
ìWe made a little money on it, which was our first goal,î Salemink says. ìAnd it seemed to work pretty well.î
Designing that dredge really tripped his trigger, Salemink says. And he began immersing himself in the science. But there was still a bit of mystery surrounding the technology at the time. There wasnít a lot of literature, and a lot of dredges were producing poorly because of shabby engineering. He spent a lot of time experimenting and demonstrating his ideas to the industry. He was a one-man gang, he says. He sold instruments and controls to producers all over the eastern two thirds of the United States. Eventually he found new opportunities to start building dredges.
He set a goal of building better hydraulic dredges and finding ways to increase their productivity. Dredges became Assemblers Incís sole business, he says. The company began pitching the idea to its clients and hooked three or four buyers. Salemink then spent the next five years designing all of Assemblersí dredges. In 1983, he left Assemblers and founded Twinkle Co. to focus on the designs that worked best for the sand and gravel industry.
He says the suction inlet of hydraulic sand and gravel dredges usually identifies them as one of four viable versions; plain suction, cutterhead, linear-cutter or bucket wheel. Mechanical dredges are still widely used and include draglines, excavators, clamshells, drag (Sauerman Crescent) scrapers, and bucket ladder dredges.
Plain-suction dredges are getting scarcer as owners come to realize that these machines lack the means to attack the deposit. They see that production suffers when solids are not loosened fast enough and when oversize cobbles are allowed to enter and lodge in the pump. The proper mechanical digger solves those problems.
The cutterhead dredge is an old concept. Salemink says the oldest he ever set foot on dated back to the 1920s. Although, he says dredges started digging much earlier. Back then hydraulics were primitive, so dredge machinery was actuated by mechanical means, which made them very cumbersome to operate. Rotary cutters used line shafts extending down the ladder from a gearbox or a set of open gears above the water.
The cutterhead is usually the tool of choice for use in a deposit consisting of free-flowing material containing few cobbles. Salemink says it is a rule of thumb that a dredge pump will pass cobbles that are no larger than half the diameter of the discharge port. Cobbles that are too big will lodge in the pump and cause downtime.
Rotary cutters are common but they have an inherent limitation that often becomes apparent when mining sand, gravel and cobbles. They cannot dig any deeper once a ìblanketî of oversize cobbles covers the floor of the deposit. These cobbles keep the cutter from cutting. The usual complaint when this occurs is that the cutter has hit a layer of ìhardpanî.
The linear cutter becomes the tool of choice when a deposit contains a significant quantity of oversize cobbles and/or clay layers.
A linear-cutter, or chain ladder, essentially consists of a large endless chain moving in a track around a steel frame and operated as a trencher. The chain resembles a giant bicycle chain with link that can weigh more than 35 pounds. The chain travels down the bottom track, up over a 180-degree-arc ìnoseî at the end of the frame and back up the top track. The suction pipe inlet is positioned behind the chain at the nose. Only sand and cobbles small enough to pass through the chain links can flow into the pump. The moving chain (ten to twenty feet per minute) loosens solids and moves oversize out of the way so that it cannot interfere with continuing downward digging action. Linear cutters usually solve ìhardpanî problems.
Bucket wheel dredges are in limited use mostly in ore deposits. They are effective for mining fine grained solids and ineffective when cobbles are present in the materials bank.
Hydraulic sand and gravel dredge mining is best carried out by keeping the suction inlet at the bottom of the material slope so that it undermines the bank to cause cave-ins. The cascading solids create a fairly uniform mixture of the various particle sizes in the deposit. Process plants operate much more efficiently when fed a stream of solids with a constant gradation.
The most common (hullpump) dredge arrangement has the pump located at or above water level. These machines can be very productive for mining down to a depth of about 35 feet. Beyond that, productivity decreases as mining depth increases. Salemink says any pump located at or above the surface of the water has only atmospheric pressure to cause water and solids to flow into its inlet. This pressure, about 15 psi, will raise a dense mixture a short distance. However, as the vertical lift (depth) increases the density of the mixture must decrease or flow will stop altogether.
The ladderpump dredge overcomes the hullpumpís atmospheric pressure limitation by having its pump mounted on the digging ladder where it operates submerged under many feet of water. Immersed, a pump has atmospheric pressure plus the pressure created by the weight of the water above it to cause water and solids to flow into its inlet. A ladderpumpís rate of production is not limited by depth, only by its ability to pump the mixture to the discharge point. Salemink has designed dredges that are mining at depths of over 100 feet. Structurally and functionally ladderpump dredges can go much deeper, however, the likelihood of being buried under a cave-in increases with depth.
In the 1970s the number of operating ladderpump dredges began to increase, as sand and gravel producer became aware of their great productive capability.
Salemink says that dredges are usually purchased with the goal of obtaining a certain rate of production. Productive capability relates to pump size. The choice of whether to use a moderately priced hullpump or a relatively expensive ladderpump rests on factors such as the mining depth, the desired efficiency and the desired rate of production. Cost and efficiency concerns dictate that the pump be no larger than necessary to achieve production goals.
Hydraulic dredge mining is usually less expensive than mechanical methods because only one man and one machine are required to pick up solids from underwater and deliver them to a distant process plant.
That contrasts with mechanical mining machines, which raise aggregate from underwater to a point where auxiliary meansóconveyors, loaders or trucksóare required to move the material to a process.
The dragline, the modern classic mining machine, is gradually fading from use. Why? Difficulty of finding experienced operators, high operating expense and limited digging depth capability (Approximately one third of the boom length).
The backhoe or excavator is a popular replacement for the dragline. It has a lower operating cost, a large pool of experienced operators, however, it cannot mine very deep and must operate while positioned very close to the bank that it is undermining.
The number of floating clamshell-mining machines is slowly increasing for several reasons; It mines to a depth of 200 feet or more, Can be fully automated, brings up just about everything that is down thereósand, cobbles and boulders. While these machines are costly to buy and operate, there are applications where no other machine will do.
The Sauerman Crescent scraper system should be on the endangered species list of underwater mining machines because it is becoming quite rare. Very popular from the early 1900s through the 1950s, this ruggedly simple mining machine featured state-of-the-art components such as a two-drum winch, a special drag bucket and cables and pulleys. The bucket simply dragged aggregate to the surface from underwater.
As far as Salemink knows, only Ohio can boast of having a bucket ladder dredge at work mining sand and gravel.
Many mechanical dredge users view the idea of hydraulic dredging with a skeptical eye. Evidently that is because the operation of a mechanical dredge is observable. Each yard of solids brought to the surface can be seen and counted as production. In contrast, hydraulic dredge production takes place out of view and requires the use of instruments. Salemink has encountered this bias several times when trying to convince dragline users to try a whole new idea.
Man has always used dredges of some sort to create and maintain navigable waterways. Some theorize that thousands of years ago blocks of stone that make up the Pyramids in Egypt were barged from a distant quarry through a dredged canal.
The first dredge was probably a barge supporting slaves who were using long-handled dipper shovels to raise solids out of a waterway to the barge deck for disposal elsewhere.
Productivity gains likely came about when animal power was used to increase the digging power of early dredges.
The late 1800ís saw the development of electric and steam power units. These high-horsepower prime movers enabled dredgers to build huge dredges with bucket ladders and centrifugal pumps. Interestingly, some of the dredges used to construct the Panama Canal were built by Ellicott and that nameplate is still being put on new dredges today.
Small diesel engines rated up to about 200 horsepower came along in the 1930s and made small 6 to 12-inch hydraulic dredges practical. After WWII, small, mostly home-built sand and gravel dredges proliferated as the nation went on a building boom and cheap war surplus engines came on the market. Starting in the 50ís, diesel engines steadily increased in power and decreased in weight, which enabled powerful factory-built portable dredges to enter the market in numbers.
Hydraulic technology made great advancements in the 60s with the result that hydraulic winches and hydraulic rotary cutter drives became a welcome replacement for clunky mechanical drives.
Fixed-site sand and gravel producers often have sufficient, relatively cheap commercial electric power available at their plants to power a mining dredge. In the late 40ís scores of motors and controllers salvaged out of obsolescent electric trains came on the market. Many of these were adapted to power small dredge pumps and a few are still in operation. These drives were very inefficient but they were cheap and provided a way to vary pump speed.
DC electric dredge pump drives became popular in the 70s made possible by the large quantity of reconditioned surplus traction motors originally used to power the axles of diesel-electric locomotives. These drives were very dependable, efficient and provided variable pump speed.
AC variable frequency drives are the latest thing in electric dredge pump drives. They are very efficient, use standard electric motors, have variable speed and are available from a number of sources.
Saleminkís goal has always been to find ways to improve hydraulic dredge efficiency. He constantly stresses that high dredge efficiency and production is possible only through the use of instruments and controls, especially the velocity meter. He continues work to improve his suction bypass valve system, vacuum and pressure sensors, rotary and linear cutters as well as general dredge design. He freely shares his knowledge and experience and answers specific questions through his Web site www.willardsays.com.
Willard Says: Donít get Sunk
Not all dredges have to sink, explains Twinkle founder Willard Salemink.
Most dredges have two types of flotation spaces: sealed (closed to atmosphere) and floodable (open to atmosphere). A dredge cannot sink if it has sufficient sealed flotation space to stay afloat when the floodable spaces are flooded.
ìFloodable space will eventually flood,î Salemink says.
If a dredge has floodable spaces, calculations need to be made to decide if the dredge will stay afloat once all floodable area is filled. If the answer is no, sealed flotation cells need to be added. Of course, many people donít and will pay the price when the dredge goes down.
Salemink is a proponent of round pontoons opposed to square or rectangular. Advantages include fewest pounds of steel per cubic foot of displacement, automated fabrication and no need for internal stiffening.
Willard Says: Keep it Open
ìI became very sensitive to the need to leave machinery accessible,î says Twinkle Founder Willard Salemink. ìSooner or later this stuff is going to leak, break, wear out, corrode or vibrate to failure.î
Everything eventually will need to be inspected, repaired or replaced. So designing a dredge where parts are inaccessible is counterintuitive. Hydraulic lines should not be covered by the deck or located in the far spaces of the hull. They are better placed on the pontoons. Engines should likewise be exposed rather than buried in the pump house. The same is true for drives.