Sensing My Audience, Feedback On How To Drive DOP Pumps

DOP pump and power pack, ready for commissioning.

This website provides me a platform to share my knowledge about dredging as far as my experience reaches. Sometimes, I do training sessions with colleagues or clients. They give me the opportunity, as an academically educated dredger, to experience how our figments of imagination work out in real life. People from different countries and lifestyle share their experience with our products. I really enjoy those sessions, they give me feedback on my normal work. Some of the topics on this website are directly related to the topics I present on those sessions. One of the recurring issues might not be that obvious at first hand. So, this post results from this feedback, aptly concerning the feedback loop in a load sensing circuit in a hydraulic drive system, specifically for a DOP.

The most common arrangement for the traditional DOP drive is to have a separate (preferably co-purchased) hydraulic power pack1. The power pack we supply is specifically designed to drive the dredge pump in the DOP. This requires the extra investment of a hydraulic power pack. Whereas most hydraulic cranes and excavators, that can handle the DOP pump by weight, will also be able to divert some hydraulic power to an auxiliary connection or so called Power Take-Off or Aux PTO. Especially when the excavator is working with a standard suction head, not much motion and forces are required for other functions. So, naturally clients tend to propose to use their DOP driven on the hydraulic PTO. This could be possible, but only under very strict conditions. And here comes the advice after commissioning hundreds of those units: ‘Don’t use the load sensing feature of the hydraulic PTO!’ Why? Let’s study the hydraulic diagrams.

Hydraulic diagram DOP pump and power pack.

The standard arrangement of a DOP is quite straightforward. As long as you do connect them in the right fashion. By controlling the angle of the swash plate, you control the delivered flow from the hydraulic pump and the rotational speed changes accordingly. Any load changes at the dredge pump result in pressure changes in the circuit, without changing the pump speed.

Typically, excavators do have a different power characteristic. When freely raking the stick, they encounter little resistance at preferably a high speed to reduce cycle time. When they hit the ground, the resistance or load increases and they typically lower the flow to maximize power delivery. They do this by connecting the pump line to a control line that influences the angle of the swash plate. Effectively, this results in a feedback control loop.

Infographic feedback control loop in a load sensing circuit.

And here comes the trick. Feedback control loops are designed to be stable. But the varying load at the dredge pump, in combination with the masses in the hydraulic lines, the springs in the rubber hoses and the damping of the flow losses, make the sensitivity of the control loop irrelevant. The signal from the dredge pump load is lost and becomes out of sync with the required action at the LS port of the pump. Any random static will make the swash plate rattle and usually in an instant, the drive stops. Other than maybe a blocked dredge line, no harm is done and service calls to our colleagues2 will resolve the issue: ‘modern excavators will allow you to turn of the Load Sensing feature’. Or, purchase said dedicated hydraulic power pack.

Damen hydraulic power pack mounted on a crane.

I would like to acknowledge Wim Roeterdink for his advice and review of this post. He is an expert on hydraulics engineering and he is always available for assistance in your particular hydraulics problem.

References

  1. Hydraulic power packs, Damen
  2. Service, Damen

See also

Parker hydraulic pumps

Project ¡VAMOS! Let’s Go Real!

¡VAMOS! equipment on trial at Lee Moor, Devon, UK.

One month ago, we concluded the ¡VAMOS! project with a presentation at the European Commission headquarters in Brussels1. A relieving event after an amazing project. It all started long, long ago, when several people from the early partners came together and their creative minds forged a brilliant concept: let’s apply the experience from the off-shore mining machines to submerged inland mines, to retrieve minerals, which currently are not profitable to exploit2.

There were three major questions in the project:

  1. What will the production rates be in relation to the minerals that can be expected?
  2. How do we find out, where the valuable mineral is in a submerged environment?
  3. Is the alternative system indeed as easy to handle and environmentally safe as envisioned?

All these issues have been addressed in various work packages. As this was not a desk study or a lab experiment, we needed real hardware to test in field trials. A mining vehicle, a hybrid ROV and a barge to launch the others. The engineering and building was interesting in itself, but the real test was in the two field trials. All the equipment and the people had to perform there. And after all these years since I helped writing the project proposal, it worked! I love it when a plan comes together!

All three research questions can be answered positive. We know how to handle such a system and the hardware required. It will definitely look different than the test vehicle. Here we optimized the engineering for the test, not for production or operation. But we definitely know how to configure the components in a viable operating system. The cutting system was tested to the limits, and production rates estimated. This machine was too light as a production model, but the cutting technology will be able to handle the hardest mineral, as long as weight and power can be applied. As there is no direct vision under water, we developed a data fusion system, where measurements from video, laser, sonar and GPS where the environment was presented in meticulous detail and the vehicle completely modelled in geometry, position and movement. At the pit floor, we were literally driving in virtual reality. The machine created some turbidity, deteriorating vision, but it happened to be less persistent than initially thought. The influence from precipitation runoff into the pit caused more turbidity. All together, we also prepared several business cases for this system against a conventional solution and there are certainly opportunities for a ¡VAMOS! solution.

Results from ¡VAMOS! (a) Cutting tests (b) Virtual vision (c) Equipment handling (d) Viability example (Credit: ¡VAMOS!).

The EU was also very interested whether clients were lining up for the real product. However, investments in the mining industry are slow, long term projects. Our main objective was to find out the operational parameters and present this as a viable alternative to conventional system. And that is what we’ve achieved. And the technology has matured enough, that when there is an opportunity for such a requirement, we are confident, that the tested components can be scaled to production size and readily applied. As a research project, we are finished. At each partner, we will still be working on the test results and improving our technology further. For the interested customer: we are ready to offer a production model. ¡VAMOS!: let’s go for real!

A happy team after concluding the ¡VAMOS! project.

References

  1. The outcomes and the future of the ¡VAMOS! project, ¡VAMOS!
  2. Developments in Mining Equipment and Pumps for Subsea and Inland Submerged Deposits, WODCON 2013

See also

Discussion at LinkedIn post