Sniffing Out The Details Of Dredge System Fittings

Sniffer valve on the discharge line
Sniffer valve on the discharge line

The mixture carrying system of a cutter suction dredge, is more than just a cutter and a pump in a pontoon. In the dredge system, there are many valves and fittings, that make the system work. One question I was asked, what these extra valves do. Actually, there are several valves, that are worth mentioning. The sniffer valve, the vacuum relief valve and the non-return valve. For working in the designed operating point, you don’t need them. But, to get there and back, they can be quite useful.

Dredge system layout and fitting locations
Dredge system layout and fitting locations

The first valve is affectionally called a sniffer valve. A more descriptive name would be ‘discharge line de aerator valve’. Usually, it consists of a floating ball in a cage with a seat at the top, that can be closed by the ball. Provided the ladder is already under water and starting up the dredge pump from a fresh situation, probably air is in the high onboard discharge pipe sections on deck. Behind the dredge, the line goes down again and the air is basically trapped, preventing the dredge pump from properly priming. The sniffer valve allows the air to escape and the water to enter the floating discharge pipe at the water line. Problem solved.

Arrangement and operation of a sniffer valve
Arrangement and operation of a sniffer valve

On the other hand, when the discharge pressure falls, the ball floats down with the receding water level. This opens the top and allows air to enter the pipe again. Which is no problem as it can be expelled again through the same sniffer valve. When the water can flow away from the high section on board, this will break the water volume in the system. When opening the pump, only the small section between pump outlet and bulk head passage will fall in the pump well. Or, if properly executed: need to be drained and discharged.

A last function of the sniffer valve is in case there is a blockage of the suction pipe. There will be no new mixture flowing in, but the mixture in the discharge line still has a lot of momentum. For a 1 km, 500ø mm pipe, the mixture has the equivalent momentum of a 75 ton truck barrelling down the pipe at 80 km/h. You don’t stop that in an instant either. The mixture keeps flowing and draws a vacuum. The sniffer ball drops and allows air to enter the pipe.

A better way to prevent the vacuum, is to install a vacuum valve in the suction line. That will allow water in and enable you to clean the discharge line without a cavitating pump. Once the suction block is removed, the relief valve opens again and mixture can be inserted in the dredge line system.

Suction pipe vacuum relief valve
Suction pipe vacuum relief valve

In case there is a high discharge height, the mixture mass will not be broken by the sniffer valve. The geodetic pressure will close the sniffer and all of the mixture volume wants to return through the system out of the suction mouth; or open pump, swamping the dredge. To prevent this from happening, a non-return valve can be mounted in the onboard discharge line.

On board discharge non-return valve
On board discharge non-return valve

These fittings will cover most operational situations. There might be even more for exceptional situations, depending on the design choices by the manufacturer1 and to the taste of the owner.2 e.g. We provide a suction deaerating valve. Any ideas about such a provision?

Suction deaerating valve
Suction deaerating valve

References

  1. Cutter Suction Dredger, Damen
  2. Product Finder Dredging, Damen

See also

Sunken Treasures From ¡VAMOS! At Silvermines

Comment

04/03/2020, Mark:

I did receive the right comment about to the purpose of the suction deaerator valve. It is indeed for letting out the air trapped in the suction pipe when the ladder is being lowered. It could be argued that the air will also leave through the sniffer valve at the back of the dredge. If somehow, the air would have trouble escaping all the way to the back, the pump will be very slow in priming itself. Providing a deaerator on the local high at the bulkhead passage, the suction line can purge the air there and the pump starts quicker.

 

Experiencing The Cutting Edge Of Dredging Technology

Exhibit on cutting forces at the Damen Dredging Experience
Exhibit on cutting forces at the Damen Dredging Experience

Ever tried to eat peas with a knife? Didn’t fare well, right? Sorry, this post will not help you. It is just more entertaining doing so. And you might remember this week’s lesson.

After all the other posts, I would like to continue our tour along the exhibits of the Damen Dredging Experience. We’ve seen at “the Bank” that by gravity and hydraulic action the sand can start moving towards the suction mouth. Another well-known mechanism for collecting the sand is by cutting. This mechanism is primarily used in the cutter heads of CSD’s of course, but also in the trailing drag heads of TSHD’s.

Be aware, that the gravel sample in the exhibit is an artificial sediment, specifically designed to be porous and demonstrate the “rolling peas effect”. Naturally occurring gravel sediments have a wide range Particle Size Distribution and will have virtually no gaps between the grains. The smaller grains will lock the bigger grains in place and it will be more difficult to move them.

In this exhibit you will see two different types of sediment. Wet fine grained sand and very coarse gravel in a jar with a handle. When you rotate the blade in the gravel, you will notice a rather high cutting force, that remains relatively constant. The water can easily move through the pores. Rotating the blade in the jar with sand, is very hard at first. But as soon, as there is a chunk of sand dislodged, the cutting force is decreased dramatically. The decrease can be explained by the fact, that once water has entered the shear plane, it cannot dissipate back in the pores and will lubricate the chunk moving over the bed. This is a phenomenon, sometimes encountered with ploughs.

Under pressure in densely packed sediments
Under pressure in densely packed sediments

The theory of cutting sand is largely explored by dr. S.A. Miedema1. He wrote an extensive book2 on this subject and anyone interested in the details is encouraged to read it. Again, the basis for cutting sand is the dilatancy of sand, just like in the previous exhibit. The grains are moved and the water has to be forced in the pores. As the blade moves at a certain speed, the sand exerts a horizontal cutting force on the blade. Force, times speed is power. At this speed and cutting height you have a certain production. Power divided by production yields a Specific Cutting Energy3, which is a parameter for how much effort it costs to cut 1m³. The SCE is largely governed by the undrained shear strength and the angle of internal friction and is different for every type of sediment. They are measured with a Cone Penetration Test4,5. In order to estimate the production of the dredge, we really need to know these parameters. If they are not available, maybe you can receive the results from a Standard Penetration Test6,7.

Basic explanation of the theory of sand cutting
Basic explanation of the theory of sand cutting

From the equations, you can derive that for a hard material, the SCE can be quite high. Consequently, with a known installed cutter power, the production Q will result quite low. From this perspective, there is no upper limit in the hardness of the soil, anything can be cut. It is just, that the resulting production might be too low for a viable business case. In this respect, it is always difficult to say the maximum hardness of the soil the CSD can cut. Usually, the increased vibrations and unsteady process will limit the productivity in such circumstances.

Example of a cutter production for a CSD
Example of a cutter production for a CSD

This provokes a nice practical experiment for you at home or when you have you have to entertain guests at dinner: have a nice recipe with big peas and fine grained rice8. Serve the peas and rice separately and notice the variation of effort to stir the ingredients separately. Then, mix a portion together and notice the increase in cutting force. For enhanced realism, add some sauce. Exclaim your amazement to your perplexed table partners and explain that you are not playing with your food, but are on a study assignment for your work. Bon Appetit!

Ingredients for a pea and rice recipe
Ingredients for a pea and rice recipe

References

  1. dr. S.A. Miedema, TU Delft
  2. The Delft Sand, Clay & Rock Cutting Model, Dredging Engineering
  3. CEDA Webinar Specific Cutting Energy, CEDA
  4. Cone penetration test, Wikipedia
  5. Painted Hills, how to unveil the sediment layers below the surface, Discover Dredging
  6. Standard penetration test, Wikipedia
  7. Lessons in Camping: Basic Soil Investigation, Discover Dredging
  8. Nice rice-a-pea, Albert Heijn

See also

Innovations In The New MAD Series To Increase Uptime And Reduce Fuel Consumption

Innovative Marine Aggregate Dredge for gravel dredging
Innovative Marine Aggregate Dredge for gravel dredging

This week, there will be a lot of interesting presentations at the CEDA Dredging Days 2019 in Rotterdam1. I would like to draw your attention to one particular presentation that I was involved with at certain moments in the project, though not in writing the paper. Kudos to my colleagues Frank & Frank to write the interesting manuscript2.

The topic of the presentation will be the change of perspective for the concept of marine aggregate dredging. Historically, the marine aggregate dredging takes place relatively close to shore. With the depletion of the deposits and the increase in demand, other locations further out at sea are coming into focus. As Damen, with a heritage in the design of offshore operating vessels, it was a natural choice to cross breed the offshore supply vessels with the marine aggregate dredges. The resulting offspring: the MAD series of hopper dredges3. Frank de Hoogh will elaborate on the seakeeping abilities of this innovative design.

Other dredging related innovations are the suction tube and ancillary equipment, the screening towers and the process sensors. Of those, I have some personal anecdotes on the screening towers and the density sensor. For all other interesting stories, you’ll have to attend the presentation.

The screening towers are fundamental to the efficiency of the process. If the screening is improved, shorter dredge cycles are possible and a better product can be landed onshore. Also, if the requirement for the product change, the screens have to be adapted to the new specifications. Ideally, this changing has to be done at one unloading phase, otherwise you miss a complete dredge cycle. A lot of effort has been done to optimise the design. But the real test was to actually build, modify and test the complete screening tower, before it was even installed on the vessel. So, we had this construction right here at our doorstep for a thorough evaluation.

Screening towers for fit and fat testing at our yard
Screening towers for fit and fat testing at our yard

One other component, that I was even more involved with, was the non-radioactive density sensor. There are regulations in place to phase out nuclear density sensors4 and a lot of alternatives are available. Back in the !VAMOS! project5, we had the opportunity to test a unit of an electro tomography system. The results indicated a good reliability and a worthy replacement for the traditional nuclear sensor6. Because of the tomography picture, there was an additional benefit: we received an early warning on the slurry behaviour. We could actually see when we were too close to the deposition limit in the pipe line. This enabled us to work with higher densities at lower velocities, resulting in better efficiency and less wear. As the rough process conditions in the mining pit were similar to the marine aggregate dredging industry, we proposed to use this on this MAD also. How we further developed and tested this system is for you to hear and see at the presentation.

Testing the non-radioactive density sensor
Testing the non-radioactive density sensor

Due to the physical processes involved in slurry transport, the mixture does not behave like a normal Newtonian fluid. It is some non-linear viscous substance. At high speeds and low concentrations, it is similar to the carrier water. Slowing down, there is a certain critical speed, where there is a minimum hydraulic gradient. At that flow condition, the specific power consumption of moving a cubic meter of soil is the lowest. So, although working at critical speed is dangerous, it has its advantages: low fuel consumption and less wear. Actually seeing the mixture approaching this critical speed from the deposition is an interesting feature of this new density measuring sensor.

Explanation on slurry flow conditions, critical speed and specific power consumption
Explanation on slurry flow conditions, critical speed and specific power consumption

References

  1. CEDA Dredging Days 2019, CEDA
  2. Next generation marine aggregate dredger as platform for innovation and basis for fleet renewal, CEDA
  3. Damen unveils Marine Aggregate Dredger, Damen
  4. Regeling bekendmaking rechtvaardiging gebruik van ioniserende straling, Staatscourant
  5. Project ¡VAMOS! Let’s Go Real!
  6. Real time production efficiency based on combination of non-nuclear density and magnetic flow instrumentation, WEDA

See also