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

Student Interviews On Their Projects With Our Dredge Pump Slurry Test Circuit In Damen Nieuws

Damen dredge pump slurry test circuit on the outfitting quay in Nijkerk
Damen dredge pump slurry test circuit on the outfitting quay in Nijkerk

‘What sets men apart from boys is the size of their toys.’ And that wisdom applies to a lot of students that we’ve had at our company and have grown from boy to man working on our dredge pump slurry test circuit for their internship or graduation. As the test circuit has seen some intense activity these last months and yielded us with some very innovative concepts and possible new products, it was the right time to cover this interesting piece of equipment in the internal Damen Nieuws1 of January 2021 to share with all our colleagues. And that occasion in turn is an excellent opportunity to share with you the article and zoom into some of the details of the circuit.

General arrangement of the dredge pump slurry test circuit
General arrangement of the dredge pump slurry test circuit

Already more than ten years ago, we felt the need to have our own testing facility to experiment with the processes in our dredges or check the performance of new products2. After defining the specifications of the circuit, we had Hylke Visscher assisting us in designing the circuit for his internship. Subsequently he could actually supervise the manufacturing of the circuit for his graduation. Hylke worked in close cooperation with Arjan de Vries who in turn did his graduation on the building, outfitting and commissioning of the circuit. Both students from then are now esteemed and valuable colleagues as we have appreciated their performance on their projects.

After those ten years, we have a new generation of students working on the circuit. Arend van Roon recently graduated on his project with the circuit, as covered in my last post3. Currently Wim Kleermaker is preparing his experiments on the dredge pump. Upcoming is Williem Salim, not yet mentioned in the article, but now already starting his internship on the instrumentation of our laboratory. All project on the test circuit are supervised by Pieter van der Kooi as plant manager, Frank Bosman as student coordinator. Depending on the project, Ewout van Duursen, Suman Sapkota and me are supervising the student projects more on a subject level.

Various executions of a U-bend c-meter in the test circuit, for delivery and installed on a dredge
Various executions of a U-bend c-meter in the test circuit, for delivery and installed on a dredge

The odd thing you might notice in the loop of the test circuit is the U-bend directly after the dredge pump. Contrary to most first impressions, it is not to generate resistance, although it does so slightly. It is to measure how much sand has been transported. As the circuit is by nature closed, there is no way to check how much we’ve transported through the dredge pump. Sure, there is a density sensor4, but this will only indicate the so called volumetric density; how much material is there in the cross section. It will not differentiate between fast moving slurry and a slow sliding bed. In the extreme you could have a static bed, indicating a very high concentration. Multiplying this with a very high fluid speed, that is squeezed through the remaining aperture, you would expect an impressive production. Wrong! Not a single particle gets transported.

Enter: the U-bend. It measures the hydrostatic pressure differences over a certain hight in the upstream and downstream branches. This will cancel out the velocity differences but will yield the actual transported mass flow. So, that is how we can claim that we already dredged millions of cubic meters, all on the floor area of a 40 foot container flatbed.

Explanation of the U-bend measuring principle
Explanation of the U-bend measuring principle

References

  1. Testcircuit, Damen
  2. Innovation, Damen
  3. Graduation of Arend van Roon: Detecting Flow Regime And Optimising Transport Efficiency, Discover Dredging
  4. Production management, Damen

See also

Graduation of Arend van Roon: Detecting Flow Regime And Optimising Transport Efficiency

Arend van Roon defending his graduation thesis
Arend van Roon defending his graduation thesis

Our first happy event this year is the graduation of Arend van Roon. He recently graduated on a project in the slurry test circuit at our Research and Development department at Damen Dredging Equipment1. His research was an interesting investigation in the detection of flow regimes. It gives some insight in the processes involved in the transport of water-solids mixtures. Let me share some details from the background with you, as I think this might be helpful for your own operation also.

Overview of the Damen Dredging Equipment slurry pumping test circuit
Overview of the Damen Dredging Equipment slurry pumping test circuit

At first sight, it is hard to imagine, how something heavier than water, the grains, can be lifted when the fluid is moving. Sape Miedema has written the standard on mixture transport in his book ‘Slurry Transport’, explaining his approach with the ‘Delft Head Loss & Limit Deposit Velocity Framework2’. Without going into the academic details, I will try to help you grasp the gist of the phenomena.

DHLLDV book (Credit: Sape Miedema)
Slurry Transport text book cover (Credit: Sape Miedema)

First the grains have to be picked up. When they are lying on the bottom of the pipe, they are fully immersed, surrounded by the fluid on all sides. The free fluid on top and the pore water between the grains under and on the side of the grains. Now comes Bernoulli’s trick. When the fluid in the pipe starts moving, he says that the local dynamic pressure decreases, while the static fluid in the pores remains at the same pressure. The pressure difference between the pressure in the pores and in the moving fluid, pushes the grains out of the bed into the fluid.

Grain pickup and suspension process explained
Grain pickup and suspension process explained

Once the particles are in the fluid are in the fluid, they should stay suspended, or they fall back into the bed. The driving force here is the turbulence in the fluid. Usually dredging slurry mixtures are turbulent. This turbulence causes the fluid to flow in eddies. These are little vortices that generally move in the direction of the flow, but in a moving frame of reference tumble in all directions. Mmh, as they rotate in all directions, why don’t they cancel each other out? Now, imagine being a particle yourself, surfing on those eddies. When it is in a fluid, it tends to sink with a certain settling velocity. Independent of the local movement of the fluid. This means, that on the downward side of the eddy, the particle has a higher total velocity than on the upward side. As the eddy is sort of symmetric, the particle dwells longer in the upward draft than on the downward fall. In this infinitesimal time difference, the eddy transfers some extra kinetic energy from the fluid to the potential energy of the particle. As this loss of kinetic energy is compensated by an increase in pressure (remember Bernoulli) carrying grains in a fluid increases the pressure loss in the slurry transport.

Flow regimes and excess hydraulic gradient requirements in dredging slurry transport (Credit: Sape Miedema)
Flow regimes and excess hydraulic gradient requirements in dredging slurry transport (Credit: Sape Miedema)

This turbulence is in short the background of suspension in the slurry transport process. Depending on al the various governing parameters: densities, viscosity, diameters, velocities etc, the equilibrium of forces result in several different regimes in the slurry flow. Ranging from homogeneous, through stratified to ultimately a static bed. Each with their own particular pressure losses. And that is what we are interested in. On our dredges, we want to transport as much material to the least amount of energy3. We are constantly looking for improvements in our equipment and sensors to assist the operator in visualising and controlling the actual state of his process4. Thanks to Arend’s project and the promising results, we can set the next step in our product development.

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

References

  1. Innovation, Damen
  2. Slurry Transport: Fundamentals, A Historical Overview & The Delft Head Loss & Limit Deposit Velocity Framework 2nd Edition, TU Delft
  3. Innovations In The New MAD Series To Increase Uptime And Reduce Fuel Consumption, Discover Dredging
  4. Dredging Instrumentation, Damen

See also