Super Materials To Improve Lifetime When Your Pump Is On Acid

Severely corroded impeller next to the original wear part
Severely corroded impeller next to the original wear part

Recently I had a discussion on LinkedIn about the pump killer #2: ‘wrong material’. There I chipped in with this disaster picture1. It was an application where we provided a suboptimal material for the acid environment. The consequences were disastrous, as seen above. Luckily, we were able to identify the problem and propose a different material. Now, I want to share our experience here, also.

What was the case? A client requested a DOP for handling tailings in their facility. Tailings is fine stuff. Leftover from mining or waste water processing. We are always careful on the grain size, as these fines may interfere with the operation of the mechanical seal. With appropriate measures, they can handle them. As the grains tend to be fresh, they can be razor sharp. The erosion on the wear parts is higher than normal fine silt. Oh, and most tailings come with acid in their water.

So, for this request we proposed a material that was usually good in wear resistance and had a moderate resistance against corrosion. Casting materials can be classified for their corrosion resistance with the Pitting Resistance Equivalent Number2. This PREN can be calculated with:

PREN = Cr + 3.3Mo + 16N

Two observations to this formula. One, this is only valid for normal Chromium content materials intended as Stainless Steel. Two, it does not mention the aggression of the corrosion. The acidity is usually provided in the request for quotation. But, a catalyst for the oxidising reaction is the conductivity of the fluid. Chloric acid and sulphuric acid may have the same pH, but due to their different ion and electron content, their conductivity differ. We did not check this in the above example, with the consequences in the picture.

Is increasing the chromium content in the wear alloy a solution to this corrosion problem? Mwah, moderately. Alloys like stainless steel profit from the above approach. But, wear materials use their Chromium for generating carbides. Those are the particles we require for the wear resistance. The Chromium provided is than not available for corrosion resistance. e.g. White cast iron with 3%C and 21%Cr will only have about 6% of Chromium to be used in the PREN. For white cast irons, it is better to use the following graph3 to find their corrosion resistance.

Corrosion Properties of Cast Iron Ball Materials in Wet Grinding. (Credit: Corrosion Feb 1992)
Corrosion Properties of Cast Iron Ball Materials in Wet Grinding. (Credit: Corrosion Feb 1992)

If corrosion is such an issue, why don’t we use Stainless Steel? Well, there you bite yourself in the tail. Stainless Steel in itself is relatively soft. It would have the same wear index of normal construction steel. By definition a Wear Rate Index of 1. For the sharp tailing material, that would be disastrous in itself. But, let’s play along. The stainless steel derives it’s corrosion resistance from the Chromium as explained before. Chromium’s trick is to generate a clear closed patina layer of Chromium Oxide protecting the underlaying material. In dredging conditions, the particles damage the protecting patina forever exposing fresh base material for more erosion and corrosion. In the end, the wear is accelerated and part life decreases dramatically.

Accelerated erosion process under corrosive conditions
Accelerated erosion process under corrosive conditions

Back to the pictured example, we expected some corrosion, but did not expect the higher conductivity. So, after three weeks, the client noticed a sudden los of performance. The leading edge of blades and the hub shroud were completely eaten away. As long as the trailing edge was there, it generated head. A single stone hit severed the front of the impeller from the hub and we received the above disaster picture. After damage evaluation, we sent a CW250 impeller and that one lasted.

A corrosion resistant DOP working in an acid tailings pit
A corrosion resistant DOP working in an acid tailings pit

References

  1. Pump killers: How to fight the 13 most common centrifugal pump failures? Number 2., Jos Overschie
  2. Pitting resistance equivalent number, Wikipedia
  3. Corrosion Properties of Cast Iron Ball Materials in Wet Grinding, Corrosion

See also

CEDA DMC Works On A Guidance Paper For Soil Investigation

CEDA Dredging Management Commission WG on Soil Investigation (Credit: CEDA)
CEDA Dredging Management Commission WG on Soil Investigation (Credit: CEDA)

Did you ever start a project and it turned out that the conditions were different than expected? Welcome to the dredging industry. One of the most underestimated preparations for a dredging project is the soil investigation. As this investigation is of the utmost importance for the dredging community, the DMC is preparing a guidance paper on this topic1, which we discussed last meeting (February 7, 2020, IMDC, Antwerp).

Working for a dredging equipment manufacturer, I am not much involved in the actual soil investigation. However, often our clients base their purchase of a specific type of equipment on the soil investigation and as such we are often presented with the reports on soil investigation. Based on these reports, we calculate the possible production for various types and advise the client for a dredge that will meet their requirements on the maximum production. most of the time we provide a good advise and the client is happy.

Off course there have been occasions where the performance was not as expected. Often because the report on the soil investigation was inadequate. Either the report did not contain all the details, or the investigation itself was lousy. Either way, rubbish in, is rubbish out. Just as an example, let me tell you what can go wrong, when the information is not representing the real circumstances.

One of our products are the so called ‘DOP Dredges’2. They are based around the versatile DOP pump. Basically, it a DOP suspended on an A-frame on a pontoon with a powerpack. The DOP can be lowered into the sediment and create a typical suction dredge pit. The production is more based on the rate that water can enter the bank face and the velocity that the banks recede. Our client provided us a Particle Distribution Diagram of the available sediment3. It was a nice narrow graded sand, but there was a considerable fines tail on the lower end. This was being dealt by the washing and screening installation. According to the client was this the sand characteristic from the whole pit. And what could be better? If you excavate all the material, you really know what is there, right?

Difference between expected soil conditions (left) and real situation (right)
Difference between expected soil conditions (left) and real situation (right)

Well no. As it happened, there were cohesive silt layers between the narrow graded sand layers. When dredging, they sucked at the bottom of the pit. Any silt layers gradually broke of and disintegrated by the eroding density flow. As the pit was created over a long period, the falling chunks of silt just slid down the slope, without causing any harm.

Enter: the new DOP dredge. It started in a new corner of the pit and initially had some trouble penetrating the silt layer. Eventually it managed to get through and started excavating a cavity below the silt layer. These broke of, burying the DOP. Without any possibility to recover the DOP, it turned into a very expensive anchor.

Risk of getting your DOP trapped in a cavity under the cohesive silt layers and the solution
Risk of getting your DOP trapped in a cavity under the cohesive silt layers and the solution

If the presence of these cohesive silt layers would have been known, we would have adapted the suction pipe for a deeper penetration. That prevents the DOP becoming covered and facilitates easier extraction. This story proves two things: 1. A proper soil investigation can prevent costly accidents and budget runovers. 2. A DOP can be modified to most requirements, when the circumstances are known.

Meanwhile, the DMC is preparing its guidance document to assist you in preventing problems like this. Follow CEDA for updates4.

Standard suction tube (left) and long suction tube (right)
Standard suction tube (left) and long suction tube (right)

References

  1. Dredging Management Commission, CEDA
  2. DOP Dredger, Damen
  3. A Sample of Soil Samples, Discover Dredging
  4. News, CEDA

See also

My WODCON 2019 Presentation: Launching Robotic Dredging

Me, presenting my WODCON 2019 contribution.
Me, presenting my WODCON 2019 contribution.

Yesterday, I gave my presentation at the WODCON 2019 in Shanghai1. The WODCON is the triannual world dredging conference, were everybody in the dredging industry meets and exchanges knowledge and ideas. Just as I mentioned in my New Year’s post, I sometimes like to delve into some old archives, get inspiration and hatch new ideas2. So did I for this presentation.

Overview of the ‘Ketelmeer’ (Credit: Google Maps).
Overview of the ‘Ketelmeer’ (Credit: Google Maps).

A seminal dredging project concerning environmental dredging is the ‘Ketelmeer’ clean up dredging project, resulting in the creation of the contaminated sediment storage depot in the artificial island ‘IJsseloog’3. As careful removal of the contaminated sediment required novel dredging techniques, the government challenged the dredging industry to test four innovative concepts. The results were evaluated by the institute now called ‘Deltares’ and published in a report4. The original conclusion of the report was, that the auger dredge was the best in reducing the turbidity. Later, the bigger auger dredge ‘HAM291’ was constructed and used to actually clean up the lake. With the knowledge and the experience of the auger we also developed a range of auger attachments for our DOP pumps5.

Traditional auger attachment for a DOP pump excavator combination.
Traditional auger attachment for a DOP pump excavator combination.

Reading the Ketelmeer report again, it occurred to me, that one parameter had not been properly accounted for: the size of the dredge. The auger dredge was by far the smallest dredge in the game. With a weighed scoring method, the dredges were also compared in size and installed power. The reasoning is that a bigger dredge has more interaction with its environment. Naturally, the environment gets more disturbed and turbidity levels should be higher for a bigger dredge. And the data was there to support this hypothesis. Smaller is better! Still, this does not undermine the initial results of the concept, as that was evaluated for turbidity per cubic meter. The bigger dredges also delivered more production. But when comparing dredges of the same concept might the smaller ones will perform better on turbidity. And this is in accordance with our experience. Every project where we’ve supplied these auger dredge units, the contractor and the client where surprised and happy about the achieved turbidity levels. Now, we know why: smaller is less turbidity.

The next step in performance might be reached by further decreasing the size of the dredge. However, the DOP is already as small as it is for a viable application on an excavator. The conclusion is to have an auger operating directly on the bottom: an unmanned submarine dredging machine!

Possible general arrangement of a robotic dredge submarine.
Possible general arrangement of a robotic dredge submarine.

This machine should navigate by itself and self-supporting. The wear parts of the auger should be exchanged by itself and solar panels can provide extra energy for extended missions. It has only a small hopper and discharge should also be done quick and automatically. An unmanned barge or even a dump truck trailer at the shore of the waterway can be replaced at longer intervals. Obstacles and other tricky spots can be alerted to a human supervisor for later intervention. One machine alone does not have an impressive production. The real power is in applying them in numbers. As we are standing on the brink of a revolution in robotics and artificial intelligence, this scenario may be not as farfetched as from your first impression. Imagine a whole school of these mechanical fishes cleaning up your waterway, while you sleep…

Working method of robotic dredge submarine (Credit: Judith Korver).
Working method of robotic dredge submarine (Credit: Judith Korver).

References

  1. WODCON, Damen
  2. New Year’s post 2019, Discover Dredging
  3. Ketelmeer project, Wikipedia
  4. Rapportage baggerproeven Ketelmeer. RIZA Rapport, 97.023, ISBN 9036950708
  5. DOP Pumps, Damen

See also