1. Why is it expensive to increase the digging depth?
If the digging depth increases, the ladder gets longer which results in larger hull tanks for proper floatation. The spuds must be longer as well. Since the spuds are now longer, they have more weight and the lifting system may need to be larger. This is true for the ladder lift system as well. Now that the dredge is larger, the swing winches may need to be increased to pull this larger mass through the cut. A ladder pump may also be required.
Ellicott® dredges are a balanced unit with all components working together for top efficiency. If one item is changed, many others may need to be changed to keep the unit balanced.
2. How much can a 12" or a 14" or any other discharge size dredge produce?
Solids production depends on many parameters such as:
- Material characteristics
- Discharge pipeline length
- Terminal elevation
- Digging depth
- Dredge pump location on the dredge
Above are just some of the parameters. We will be happy to assist you if you send us the necessary information by completing our Project Data Questionnaire.
3. What is material in-situ specific gravity?
Material in-situ specific gravity is the weight of the material per unit of volume as it lies in the ground before dredging, i.e.: undisturbed. This includes voids, etc. in the volume.
4. How does digging depth affect solids production on a dredge with a hull mounted dredge pump?
In general, a dredge with a hull mounted pump will produce less material as the digging depth increases. At deeper depths it takes more energy to overcome two things; one, lifting the material from the deeper depth up to the pump, and two, the increased friction due to a longer suction pipe. As long as this energy - expressed in feet for suction head losses - is less than about 30 ft (9 m) (1 atmosphere), the negative impact on production is very minimal. When the digging depth increases to where the suction head losses exceed 30 ft, then the negative impact on production becomes more pronounced; i.e. less production with every added foot of digging depth.
5. What depth can you dredge to?
We offer standard auger equipment that can dig up to 20 ft. below the water surface. For special applications certain auger models may be modified to dig to depths of 30 ft.
Our standard cutter suction dredges range from our smallest at 20 ft. up to 58 ft. digging depths. For special applications, digging depths can be increased substantially.
6. What is the GPM?
Gallons Per Minute is the volume in gallons of the slurry pumped per minute. GPM can be used independent of pipeline size to calculate production. Examples of flow rates are 4,200 GPM for 10" pump up through 56,500 GPM for a 30" pump.
7. What percentage of solids can you pump?
We recommend that you confer with Ellicott® to determine your dredging parameters and the amount of solids you can expect to achieve.
8. How far can the dredge pump?
Ellicott® can size the dredge pump to meet your needs for discharge distance. If necessary, a booster pump can be used to pump longer distances. Consult with Ellicott® for application details.
9. Are Ellicott®'s dredges electric or diesel powered?
Ellicott® offers standard diesel and electric designs.
10. During the dredging process, can our lined ponds be protected?
With the use of auger wheels, our dredges can achieve maximum depth without damaging your liner. Special attachments like skids and wheels can be attached to the auger.
11. Is the dredge self-propelled or do I need cables?
All of our dredges use some form of cabling to propel the dredge, either using swing winches or traversing winches. Self-propulsion is only used for maneuvering into position and not for dredging.
The exception to the above is our latest line of swinging ladder dredges which use no cables for dredging.
12. What size of dredge will I need to do my job?
This depends on specific project conditions. Please complete our Project Data Questionnaire to enable us to review your project details and provide a full review and recommendation.
13. Can you visit our plant or project site?
A regional sales manager is available to meet with you at your site or our office, whichever is appropriate.
14. Does your company operate the dredges?
No, we are an OEM dredge manufacturer - not a dredge contractor. However, our field service technicians are available to train your crew in dredge operations and maintenance.
In addition, if you require a dredging contractor, we can direct you to a dredging contractor that can assist in your area.
15. How do I order parts for my dredge?
Ellicott® stocks frequently used dredge parts for our standard dredges. To order parts, contact our toll free number 1-888-4Mudcat (1-888-468-3228), or send us a fax with your requirements at 410-752-3294, or contact our Parts Manager, Jack Fiddes at email@example.com
16. How long does it take to build a dredge?
Our auger equipment and small cutterhead dredges are normally available from stock, subject to prior sale. If stock is not available, normal completion would be 3 to 4 months after receipt of order. Delivery on larger equipment varies depending on manufacturing workload at time of order.
17. What are the main differences between a closed vane impeller and a open vane or (vortex impeller) and when are each recommended?
Closed vane impellers are the most common used on dredge pumps and the vanes have sides on both ends to totally enclose the vanes. This creates closed channels from the eye of the pump to the periphery or the runner or impeller. Some pumps are three vane allowing larger particles to pass and some pumps are four vane. The increased vane increases the pump head but further restricts particle size. These impellers are excellent for silt, all grades of sand, and also gravel within limitations.
Open vane or vortex impellers are designed to have the vanes open to the eye of the pump and are generally recessed into the pump casing. The energy is created by the spinning of the slurry in front of the impeller. They are excellent for viscous sludges such as sewage sludges and will dissipate gasses in the slurry without cavitation. Most vortex pumps are generally submerged so that the eye of the pump is very close to the cutter feeding the pump. The submerged pumps on horizontal auger dredges generally have vortex impellers, but not in all cases.
18. Define turbidity as it relates to a dredging operation?
Turbidity is defined as the re-suspension of bottom sediments into the water column which appear sometimes as a "visual" plume in the water column or an increase in the turbidity in the area of the dredge above residual or "background" levels. It is measured in NTU's or nephelometric turbidity units, and is a measure of light (optic property) passing through a given length in a column of water. All Ellicott® dredges are designed with standard features and optional features to keep turbidity to a minimum.
19. What are the main causes of turbidity in a dredging operation?
There are many variables which enter into this equation but the most common is the wrong selection of a dredge for a given operation, and the incorrect operation of the dredge. For more information, see also Surgical Dredging Controls Turbidity
20. Why is it so difficult to dredge and pump viscous sludges and how do their properties differ from conventional materials like sand gravel/silt?
It should be noted that clear water flow is almost turbulent and as such, the Hazen Williams and Darcey Weisback equations are based on this fact. Slurries consisting of silt and sand also fall into this category. However, sludge, even though it is considered a fluid, often exhibits laminar flow, in which the friction losses can be several times higher than that for water. To complicate matters further, sludge of three percent solids by weight behaves more like a "Bingham Plastic", and the flow of such a material is described by two constants - the yield stress and the coefficient of rigidity. Mud Cat™ horizontal auger dredges with submerged vortex type pumps have pumped these types of solids up to 8% by-weight but with corresponding high friction losses in the pipeline. In some cases the friction losses increase exponentially and extreme care must be exercised. Each type of sludge must be evaluated for pumping characteristics.
21. What is the best method to measure and pay a contractor for a dredging project?
A contractor should always be paid on the actual "in-situ" amount of material dredged and not what is placed in the disposal area. In general, the contractor is given a set of cross sections showing water elevation and the sediment interface with the water and then told to dredge to that footprint. This is usually done on stations at 50 or 100 foot spacing. Immediately following dredging a given area, actual water depths are again taken and the difference at each station is the amount to be paid. This is usually done by computer programs using the average end area method to calculate yardage removed. In most cases an actual probing device is used and depth soundings are taken using sonar which gives fair results.
On many projects the maximum dredge to elevations are given and the contractor is told not to over dredge. On some projects an over dredge depth is allowed, say up to 6", and payment will be made to the contractor for the extra material. For contaminated sediment, dredging exact "dredge to" elevations are even more critical and GPS is also used to locate precisely the "dredged to" depths.
22. What is the main criteria for the proper placement of a booster pump?
The Inlet pressure to the booster must always be positive when working with a dredge pump. It is a good rule to have at least one atmosphere (15psi) of pressure on the inlet to the booster pump. Request Ellicott®'s technical paper on Booster Pumps in Discharge Pipelines for a detailed description on placement.
23. What is the hands-on method to measure the flow in a pipeline w/o flow meters etc on the job site?
With the end of the discharge line horizontal, measure in inches horizontally from the top of the pipeline as distance where the vertical elevation to the top of the flow coming out is 12 inches. The equation is GPM = 0.818 x the ID of the pipeline in inches squared x the horizontal distance measures in inches. Example: the pipe ID is 10 inches, the horizontal distance is 24 inches. GPM = 0.818 x 10 squared x 24 = 1,963 GPM.
24. What happens to the gauge readings for various dredge failure conditions? Ex: The suction line is blocked-what happens?
Suction line blocked - pump vacuum reading will be high
- pump discharge reading will go to zero.
Discharge line is blocked - pump pressure reading will spike up quickly and pump impeller may still turn but there is no flow.
- pump vacuum reading will go to zero.
Burst/disconnected discharge hose/pipeline - suction vacuum high, discharge pressure low.
25. What happens when the front impeller clearance gets larger and larger from the front liner?
There is a condition which develops called recirculation where slurry swirls at high velocity in a circular pattern between the impeller and the liner causing very accelerated wear of both components, especially if an abrasive material is being pumped. Also, the pump loses its design head and flow is decreased. The clearance should be as close as practical. This spacing can generally be adjusted by moving the impeller forward; most pumps allow this. When one goes beyond the limit, the liner and/or impeller must be replaced.
26. Why is there no vacuum gauge on a submerged pump ?
Since the pump intake is submerged there is no need for atmospheric pressure to lift the material to the pump and therefore no vacuum gauge is necessary.
27. Define cavitation, and can a submerged pump cavitate?
A pump operates by excavating fluid through its discharge. This fluid is replaced by other fluid, and the pump pumps. If there is some restriction and the fluid cannot replace the excavated fluid fast enough, a vacuum is created and the fluid can "boil". The boiling is water vapor filling the void creased as the bubbles form and collapse. This is cavitation.
Yes, a submerged pump can cavitate if the suction cannot keep up with the pump requirement.
28. Relate slurry specific gravity to dredge performance.
Dredge performance is a function of the slurry specific gravity. The higher the specific gravity, the higher the production.
29. What is the best procedure to take actual samples from a dredge discharge line?
Picture the discharge pipe as the hands of a clock with 12 being at the top, the best locations are at 4 and 8 on the dial. This is not exact but will give fair results. If monitoring is required on a continuous basis, use a tap off the discharge line with a valve. Do not put your hands or hold a bucket in the main discharge outflow. Injury may result.
30. What are flap valves, suction relief valves, and how are they used?
Flap valves serve two basic functions:
(a) closing the discharge pipe to facilitate priming of the pump, and
(b) obstructing the return of slurry to dredge upon shutdown of the dredge pump
Suction relief valves are mounted as close to the suction mouth of the suction line as is practical. It can be opened manually to allow extra water into the slurry, or automatically if the suction or discharge lines begin to plug, as measured by vacuum and pressure sensors. The suction relief can also be connected to a Production Meter System to help keep at a consistent level of production.
31. Why can a dredge pump fine silt a farther distance than say a coarse sand?
The greatest contributor to the friction between the sand particles and the pipe walls is the velocity of the slurry. Since coarse sand requires higher slurry velocity to keep it from settling, the friction between the sand particles and the inner walls of the tube increases, and that means coarse sand will travel less distance than silt or fine sand.
32. I'm getting a lot of vibration on the dredge at the pump-what is wrong?
This is usually caused by an object caught in the pump impeller in one of the vanes and is causing the pump to turn off-balance. If left unattended, serious pump bearing failure can occur and the dredge must be shut down ASAP. This can also occur with an object caught in the discharge elbow of the pump, and one will also feel vibration. Cavitation as discussed earlier can also cause vibrations.
33. What actually happens when you reach "shut-off head" with a dredge pump?
This occurs when the maximum published head capacity for a pump is reached. Say a pump is rated at 185 of total dynamic head and the system is showing the pump 220 feet of head. What happens is that the solids stop flowing in the line and the pump impeller continues to turn at high speed and nothing is being pumped. This condition can also cause severe pump bearing damage, not to mention a possible plugged discharge line.
34. What is the only energy available to move solids from the cutter to the pump eye and up the suction pipe when the pump is "in-hull"?
The only energy/pressure available to move solids from the dredge cutter to the hull pump is the ambient pressure; one atmosphere. As the pump evacuates its housing a void is created. The ambient pressure will push the material to fill the void.
35. Why submersible pumps-explain in detail?
As explained in the question above, the available energy/pressure for a hull pump is only one atmosphere. For a submerged pump the available energy/pressure is the one atmosphere plus the pressure provided by the depth of water above the pump inlet.
36. Can a dredge concentrate in-situ material as it digs?
This is one of the basic laws of dredging and it cannot be violated. A dredge can only pump the material as it exits in the water in its "in-situ" state and can add no concentration of material. Generally, when cutting more water is added to the in-situ material and the resultant slurry is diluted. For example, if a material is in-situ at 50% solids, a given dredge will dilute this material to say ranging from 17% to 25%. Beyond a slurry specific gravity of 1.5, it becomes increasingly difficult to move the slurry in the line, and shut-off can quickly occur.
37. When is the HP draw the greatest on a dredging project?
The power required by the pump will increase as the pipeline length increases until the full load (max. horsepower) - full speed (max. pump RPM) point is reached. Beyond that point, the power requirement decreases as the pipeline lengthens. Note: If the flow rate is higher than required to carry the slurry, it will use more HP than needed, wasting fuel. The most efficient dredging is done at the lowest velocity that will carry the maximum slurry density.
38. What is critical line velocity, and one of the most important natural laws in dredging?
Critical line velocity varies with pipeline ID and nature of the material. Critical velocity of the slurry in the pipeline is that velocity below which material can settle to the bottom of the pipeline and cause blockage. The greater the particle size for say sand, the greater the line velocity becomes, usually measured in feet per second. Critical line velocity is also important for pumping hard clays since they tend to ball up, in some cases 12 inches in diameter, and can plug a line easily if velocity is too slow.
Also as the ID of the pipeline is increased for a given material, say sand, the corresponding line velocity is increased, critical velocity, than for a smaller ID pipeline and the same material. Contact Ellicott® for critical line velocities for sand in varying pipeline ID's.
39. Why are there no "lemon laws" to protect a dredge buyer?
"Let the buyer" beware is not just a saying, but the truth when purchasing or leasing a dredge. There is no protection, and the purchaser must rely on the dredge manufacturer. Ellicott® has built over 1,250 dredges in our over 100 year history, and we will never sacrifice quality for price. Some of our dredges are still in service in the USA after 50 years of pumping on a daily basis.
Ellicott® also publishes dredge performance curves for all models, and this should be the first question to ask when buying a dredge.
We ask that you consult with us prior to purchasing a dredge so we can show you special features which are standard on all our equipment.