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

Perspective on Commissioning – Connecting the DOP’s

DOP pump and power pack, ready for commissioning

A DOP pump1 and it’s drive go together like a horse and carriage, you can’t have one without the other. Basically one product, but two units. Whether hydraulic or electric driven. OK, in some cases, you could use the extra power take off from the excavator or crane, but you still have to hook them up, before they can do any useful work.

And connecting the DOP to its drive is an elementary job, that can cause some headaches, when not done carefully. The modern electric drives require some more certified wizardry, that is beyond the scope of this post. Hydraulic power packs have their own peculiar quirks, that are worth mentioning here.

Usually, the hydraulic hoses come supplied with Snap-tite® connections2. These are quick and prevent most oil spills in the environment. They come with a disadvantage in price and resistance. Ultimately, the DOP should be connected to the power pack like this:

Hydraulic diagram DOP pump and power pack

So, even with such a simple diagram, still things can go horribly wrong. Off course, don’t switch the ‘Pump’ and ‘Return’ line, you will blow some seals on the motor. Most common is a failed connection of the Snap-tite®. When it is not completely tight, the internal valve is not completely open and you have a lot of resistance. So a lot of power on the power pack gets converted to heat on the connection and no performance on the DOP pump. Another common problem is the resistance in the leak line. This can cause terrible things on the hydraulic motor.

One client followed all the instructions in the manual and all advise from the service department, but still could not coach his new DOP to work. Lots of flow at the power pack, but almost no pressure. And the impeller could still be turned by hand. (Warning: never put your hand in an impeller when there is power on the drive!) One of our engineers went over to have a look on board. Well, here is an impression of the situation on deck.

Situation on board during commissioning

He quickly began cleaning up the mess on deck and uncovered how the hydraulic hoses were actually connected.

How the DOP pump was connected (not), actually

Well, that makes sense! Now it was easy to see, why the DOP was not working. Solution, connect the lines correctly and dredge away. Oh, and educate the crew about a tidy workplace.

Ship shape and ready to rumble

Well, after this first basic lesson in connecting the DOP, you are ready to connect your own DOP project. It’s easy as child’s play.

Connect the dots to set up a DOP project yourself (pdf version)

References

  1. Hydraulically driven submersible dredge pump DOP, Damen
  2. Quick Disconnect and Valve Division, Parker Snap-tite

See also

Dual Stage Dredge Pump and Double Action Pump Drive for ¡VAMOS!

 

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

These days I have been very busy drafting a manuscript about our ¡VAMOS! project results1 for a dredging conference paper. As every writing process, there is so much to tell and so little space available. At a certain moment there follows a phase called: ‘kill your darlings’. You have to scrap parts that contribute less to the main message of the article. Still some of those orphans are worth sharing. So here is a part from the paper that might be interesting for you.

For those unfamiliar with ¡VAMOS!, it is a Viable Alternative Mining Operating System2, where we are cooperating in a 17 partner consortium to develop equipment and procedures for exploiting mineral reserves in disused or currently unavailable mines in the EU3. Many mines are disused, but still contain some reserves, that are unrecoverable due to an uneconomic stripping ratio4.

Schematic of typical vertical ore body in an opencast or submerged setting (Credit: ¡VAMOS!)

We developed a prototype mining vehicle (MV) and an accompanying launch and recovery vessel (LARV)5. Although the requirement for the slurry circuit on the MV are deceivingly similar to a normal dredge system, there is one fundamental difference in character: vertical transport. At the system architecture phase we assumed a dredging depth of at least 100 m. For clean water, this poses not so much of a problem, once pumping mixture is where the geodetical height difference comes into play. At 100 m a 1500 kg/m³ mixture requires an additional 5 bar of static head.

Influence of vertical riser on the head requirements of a dredge pump

The dredge pump has to cope with the large variation in head requirements. For the prototype machine, the only option is to vary the dredge pump speed. Still at a normal operating condition, we expected a head requirement for 10 to 16 bar. This is why we developed a dual stage dredge pump, it delivers double the head of a normal dredge pump.

The variation of the pump speed has been accomplished by various controllers working together. The power is generated on the LARV by a diesel engine driving a generator. A frequency drive supplies a hydraulic power pack on the MV. The power pack has a variable displacement pump for controlling the flow. At the dredge pump side there is also a variable displacement motor.

¡VAMOS! prototype drive train arrangement

With this setup in place, the dredge pump can vary between a slurry circuit just filled up with clean water and a fully filled riser with heavy mixture at the operating point. On top of that, there will always be the possibility, that the density increases even more. The flow will reduce and so will the power consumption. This enables the drive to increase the speed for extra oompf of the dredge pump to clear the riser. Where normally the dredge pump speed is controlled by the pump swash plate, the motor swash plate is so to speak the turbo boost. This is similar to a constant power drive for normal dredges, but the vertical riser makes the problem more pronounced.

So, not only the starting up of a dredge pump should be considered in the design of the drive train, but also the variations in operating point. Regarding the comments I received on my last post, yes indeed a production model of the ¡VAMOS! system would have an all-electric drive. Just be sure to have enough copper in the motor to cover every possible operating point.

Dual stage dredge pump with constant power drive mounted on a crowded MV equipment bay

References

  1. Successful demonstration of ¡VAMOS! technology in UK
  2. ¡VAMOS!
  3. Strategic Implementation Plan for the European Innovation Partnership on Raw Materials
  4. Developments in Mining Equipment and Pumps for Subsea and Inland Submerged Deposits, Kapusniak et al. WODCON 2013
  5. ¡VAMOS! reaches Design Freeze Milestone

See also