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

The Dredge That Refused to Work on Monday Morning

Cutter wheel dredge ‘Sylvia’ at work on the TIWAG Langkampfen power dam reservoir

One of the first commissioning jobs I had to do for my previous employer, was on the dredge ‘Sylvia’, as she refused to work on Monday morning. And no one knew why. She was purchased by the TIWAG, as they recognised that power dams are a blocking the natural sediment transport and could potentially damage the turbines in the power dam1. To prevent anything serious happening, the power dams were fitted with ‘silt traps’. TIWAG ordered a dredge to clean up the silt trap and make some money on the excavated sand, classified by a dewatering bucket wheel on shore2.

Overview of the TIWAG dredge ‘Sylvia’ and dewatering bucket wheel at Kufstein

At a power dam you would expect, that electric power wouldn’t be a problem. Well, those power dams generate power, but they don’t like to let you plug in your dredge. So, ‘Sylvia’ had to have her own power generation with a diesel engine. As power dams are usually high in the mountains, the dredge pump should preferably be placed on the ladder. The natural setup would then be an electric drive. On ‘Sylvia’ the classic and reliable electric shaft principle was chosen. The diesel engine speed is transferred to the dredge pump by the variable frequency. Depending on the generator speed, the field coil is excitated according to a controller, that can act as some sort of electrical clutch.

Explanation of the electric shaft arrangement on dredge ‘Sylvia’

One thing to be aware of with an electric shaft, is the starting current of the electric motor. The electrical engineer that designed the system just worked with the nominal operating point. I wasn’t involved in the choices, but nowadays I will warn people to about this misconception.

If the pump is started on an empty pipe, even with a moderate speed, the dredge pump receives no resistance and the capacity through the pump increases enormously. The power requirements increase to a level above the nominal operating point! After several blown fuses and an incinerated generator the system was slightly modified. Also, to restrict the power surge, a discharge valve was installed to ramp up against. At zero capacity, the pump requires virtually no power.

Start-up procedures plotted in pump curves

The difficulty with ‘Sylvia’, was, that there was no possibility to hook up the discharge valve to the hydraulic system. Instead, the valve was connected to the working air compressor. It primarily served as an air source for the bubble point to measure the operating depth of the dredge wheel.

Retrofitting a pneumatic operated gate valve to the pneumatic system on board

After installation, this worked fine. No blown fuses anymore. Should be fine. Until next Monday morning. Fuses tripping at every start-up. Only after several hours she would run normally. It turned out, the gate did not close completely under pressure. It required the full pneumatic operating pressure to close all the way down.

Aha! As the compressed air vessel was leaking air through the bubble point on the ladder over the weekend, there was not enough pressure to close the gate valve completely. As the mixture only needs a small opening to already draw a lot of capacity, the fuses tripped every Monday morning. Once the compressor was able to top off the vessel later in the morning, the gate valve did close completely and the dredge pump could start. A simple ball valve to close the bubble point remedied the leak and no more problems on that system again.

Characteristics of an opening gate valve

References

  1. Kraftwerk Langkampfen, TIWAG
  2. Several million tons of gravel extraction with a suction dredger, Stichweh

See also

A Reservoir of Dredging Opportunities