Concept Development

Block Diagram:

Due to the size and number of components involved with this project things had to be broken down into several subsystems. Each sub-system is also made up of several pieces. It was determined that whatever we built would need to perform three major tasks: remove sediment from the flow stream, weigh the sediment and then separate the sediment from the water stream. The block diagram below shows the breakdown along with some simple failure mode breakdowns.

Sediment Catchment

Sediment in the flume is placed along the bottom. Near the end of the flume a bed load holder would be placed. This would simply be a small wall designed to "dam" up the sediment tumbling down the flume. By doing this the movement of sediment is stopped until it tumbles over the holder. It will do so at a low velocity and thus sink back down to the bottom of the flume.

Shortly after the holder will be an opening in the flume bottom. Sediment will drop down into that hole or funnel. The funnel/flume interface is a critical part of the project because it needs to be large enough to catch all of the sediment, but small enough that it does not disturb the flow of the water in the flume. The geometry of the funnel walls has to be such that it prohibits water movement and lets the sediment drop instead of kicking back up and escaping back into the water stream.

Once it passes through the funnel the sediment will need to be collected. Due to the variability in the amount of sediment used in the experiments a catchment system that can empty itself is needed. The rotating drum idea was presented to us early on in the project. This is simply a shaped series of plates that will catch the sediment until the volume is full or a certain time is reached, and then dumps it. Besides the drum a rotation system will also need to be designed.

Weight the Sediment

While the sediment is collecting in the drum it will need to be weighed continuously. To do this a load cell will need to be selected along with recording equipment to provide a continuous weight record. Because the drum will need to be dumped periodically there will be "hiccups" in the weight record. This cannot be helped but these hiccups will need to be minimized as much as possible. The drum will need to be turned quickly and infrequently.

Slurry Removal

Each time the drum dumps it will drop the sediment into the bottom of a tank enclosure. Since the sediment stream may be fairly continuous over the course of an experiment the removal system will have to be able to keep up with it. The tank itself will have to funnel the sediment at the bottom. By some method the sediment will need to be removed out of the tank. A pump and piping system will most likely be needed for this.

The output from the catchment system will be a stream of "slurry" water. This sediment slurry cannot be disposed of in this manner. Instead the slurry will have to be broken back down into soil particles and water. The water can then be pumped back into the holding tank where it can be reused. The soil particles will not be recirculated and thus will need to be disposed of in a proper receptacle bin.

The researchers would also like to be able to take sediment samples out of the catchment system as some point. They would like to know the sizes and quantities of each particle are important to the types of research they would like to do. A sampling procedure will need to be developed that does not have an affect upon the water flow or weighing system.

Design Ideas:

A number of ideas were developed for each component, the best of which are shown below. Each component had to be developed with several major factors in mind:

• Water flow in the flume will not be disturbed
• Sediment larger than one millimeter will be captured before entering the water holding tank at the end of the flume
• Existing flume not damaged or hindered by any additions

Each met certain needs and specifications with some being better than others. Final decisions and justifications for which are listed on the Final Design page.

Funnel Design:

The sediment is carried over the bed load holder by the water, and falls to the bottom of the flume.  Shortly after the bed load holder, a funnel will direct the sediment to the catchment system. The flume must be wide enough to allow sediment to fall out of the flow stream in the flume, but too small to disturb the flow   Three ideas were considered for the funnel:

• (left) Parallel walls, both at an angle to the flume
• (right) One vertical, one negative slope
  
• (not pictured) Hinged funnel system that keeps the funnel vertical at all times

Tank:

The initial concept for the tank that is used to hold the drum, sediment, and auger system was going to be a simple tank that was attached to the bottom of the flume.  This design included cutting a hole in the flume to allow for the sediment to fall through.  The geometry of the tank was chosen such that a horizontal auger would be able to pull all of the fallen sediment to one side of the tank, and out into the slurry pump’s water flow. The best way to do the tank design was to include a section of the flume attached to the top of the tank, and simply bolt the piece onto the end of the flume.

Sediment Catchment:

The catchment system is designed to capture all sediment tumbling out of the flume and then deposit it into a tank each time the catcher gets full. Two designs for the catchment drum are shown below. The difference between each drum is the number of basins to catch sediment in. More basins requires a smaller turn each time the drum is dumped; whereas fewer basins allow more sediment to be captured and therefore require dumping less frequently. Rotation will need to be achieved quickly and infrequently due to a loss of data during dumping periods.

The drum will need to be rotated whenever it becomes too full. To do this three methods were considered.

• Binary Air Cylinder to rotate the drum back and forth
• Air powered motor that turns on when the shaft needs to rotate
• Pneumatic Rotary Actuator ( rack and pinion system) powered by compressed air

Sediment Weigher:

This goal of this system is to measure the weight in real time, output the weight onto a digital display, and logging the data onto a computer.  The measurement needed to be accurate within 1 newton, and the weight should be continuously measured, except for up to 5 seconds for dumping the drum. The weighing device for this system will work in conjunction with the catchment system. It was determined that a load cell was the best way to record the weight and three load cell configurations were seriously developed. Click on each picture to get a larger image.

• (left) Load cell is located above the flume and connected to the ceiling.  Two cables are used to hold up a shaft that runs through the drum underneath the flume.  As the sediment falls on the drum, the cables increase in tension and the load cell records the weight.
• (center) A drum with a shaft running through it and a load cell on each side of the shaft.
• (right) One load cell mounted underneath the drum.

Slurry Removal/Transport:

Sediment will need to be removed from the tank as it is deposited from the drum. To do this two methods were looked at: using an auger to pull sediment into a pump driven pipe stream and gravity feeding sediment into the pipe stream. The pipe stream needs to be designed to meet many factors. These factors include the size and concentration of the solid particles, abrasivity of the slurry, pumping pressures, pipe diameter, reactivity between solids and liquid and surfaces, viscosity of the liquid and the critical velocity (velocity needed to prevent sediment from settling in the pipes).

Sediment Separation:

Before we can return the water from the slurry piping system to the holding tank the sediment must be removed from it. Two methods for doing this were evaluated.

• (left) Centrifugal separators are devices that create a vortex of slurry within them. The water will eventually rise and go out the top of the separator. Because sediment weighs more it will drop out of the stream and sink to the bottom of the separator where it can be removed.
• (right) Slurry water will be deposited into a dumpster where the sediment can either be removed via vibrating screen or just by settling out. A pump will be used to pull the water back out of the dumpster and back into the holding tank.

Once in the dumpster the sediment can easily be removed from the site and hauled away.

Morphological Analysis:

The morphological table was built to lay out the advantages and problems associated with our design ideas. Each design was broken down and compared with the needs/specifications for each subsystem. Efficiency and ability of each design idea to perform it's specified task has to be compared with how well the design fits within the area allotted, makes economic sense, and how well it interfaces with ideas for other subsystems.

Design Failure Mode & Effects Analysis (DFMEA):

The final task before selecting final designs was to determine the potential failures for the design and the what effect they have on the overall performance of the trap. Luckily the nature of the equipment in this project there is little possibility of human injury. However there are plenty of areas where something could fail, ruin an experiment and potentially cost a lot of money to fix. Each subsystem presents it's own problems and potential breakages that have to be accounted for. These potential problems are outlines here. Some of the failures can be reduced or removed with better designs; some cannot. The final design needs to put forth a sediment trap that will work effectively and minimize costly mistakes.