Suck it and see

Vigan units in operation handling alumina, Lianyungang, China
Robert Haymoz: above 345 tph and 'it becomes uneconomical to expect pneumatic to keep up'
The Docksider unit, from FLSmidth
Neuero rotary head used in feed mills
Vigan units in operation handling alumina, Rizhao, China

Pneumatic units may not be the best fit for every bulk handling job, as Stevie Knight discovers

While it's often said that pneumatic units come into their own with a variety of ships, smaller holds and a range of bulk; finding a 'fit' is not a straightforward calculation.

Comparing efficiency means looking at so-called 'peak' capacity - the maximum rate given optimal conditions - but also the more realistic (nominal) measure that factors in non-productive arm movements, and a final 'through the ship' assessment that also takes account of the clean-up phase. And these will be different for every type of machine.

Dave Bergenstock of FLSmidth explains: "Pneumatic unloaders are typically designed for a peak capacity range of 200 to 800 tph (tonnes per hour). Higher capacity designs are less practical due to the higher power consumption." However pneumatic unloaders do maintain their higher efficiency rate for longer than the mechanical variety and are generally less reliant on a front-end unloader to help with the bottom few metres of a hold.

But, along with the clean up, moves and stoppages can be the deciding factor - and this depends on what type of vessel comes in. Larger ships can have a number of holds: while a barge-mounted unloader can take a considerable time to move between sections, a dockmobile unit might slice this in half while rail mounted units could shave the move to minutes. All this soon adds up.

Multiple locations add another layer to this. Since vacuum unloaders often come as part of an all-in-one system that suck material from the ship and then blows it to a silo, with some units you can simply install separate pipes, avoiding the expense of multiple conveyors.

Although still high on energy demand, most pneumatic manufacturers have made streamlining modifications to minimise losses in bends and hoses, resulting in reductions in the air required and increasing the amount of material moved - including delicate or abrasive loads.

Vigan, for example, has developed machines for alumina bagging. The Chinese ports of Lianyungang and Rizhao, partly driven by the need to keep the dust down in an urban area, purchased two vacuum unloaders with a nominal capacity of 250 to 300 tph. However, the particularly abrasive nature of alumina meant the introduction of harder elbow sections with a Rockwell index of 52, resistant to several million tonnes of wear.

Further, separating the mixture of the conveying air and the alumina in the receiving hopper is a tricky process. Adequate filtering was an issue resolved by particularly fine sinter plate filter media and automatic cleaning of the filter sleeves with compressed air. However, one further factor is the need to lay the material down gently, so careful control of the negative to normal pressure area is facilitated through a specially designed valve that allows continuous throughput.

Buhler also have a range of vacuum unloaders for lower throughput, around 345 tph, after which, says Robert Haymoz, chief engineer for the company, it becomes uneconomical to expect pneumatic to keep up (the company has a range of mechanical units that take over at this point).

Another difference between the Buhler units and others is that that despite the lower efficiency, Buhler have kept Roots blowers in place, simply because turbo blowers have shorter life: "Turbofans have a very high speed, and despite the filters, the dust gets in and damages the blades which can then become unbalanced - causing vibration, and finally breakdown, " says Mr Haymoz. He goes on to explain that while this doesn't matter so much for a throughput of lower than around 200,000 tonnes per annum, over this, "you are looking at ruining your throughput ratings".

He adds that the Roots units tend to last "a factor of ten longer" and that Buhler has supplied operations where the original Roots fans are still going strong, even after 20 or 30 years.

Non free-flowing materials have usually been a bit of a problem for pneumatic machines: not only can certain materials become compacted but similar products don't always behave the same way due to processing differences - soybean meal, for example or the kind of biomass that is becoming popular with the modern eco-efficient energy plants.

So, Neuero has developed the railmounted Flexiport which has a 'digging' head to the nozzle which can vibrate/rotate its way into a variety of compacted products, still giving up to 600 tph peak capacity depending on the load. Further, the unit has two kick-in kick-out functions, luffing and slewing movements which allow the digging head to reach all points of the ship. And to make the most of its flexibility, the Flexiport (backed up by a mechanical conveyor system) can be reversed to ship loading too.

FLSmidth has the Docksider among its other units. This uses an innovative 'two kettle' system: one side vacuums the material up from the ship via the loading arm while the other side pushes its load out to the silos under pressure. A process control system detects when the vacuum side is full and verifies that the pressure side is empty, then switches over the functions. Further, the vacuum is built to both luff and slew, having sections that not only move up and down but in and out, all based on a 180º rotating pedestal.

This means the vacuum arm can extend into the far corners of a hold as well as under the hatch coaming for optimal efficiency. Mr Bergenstock says that the Docksider's peak capacity ranges from 200 to 800 tph (depending on the equipment design and material), while nominal capacity (the average rate sustained while the nozzle can be immersed in material) is about 80% of this. The through-ship capacity, which factors the time it takes to clean up, is only another 10% less - about 70% of peak capacity.

Predictably, as with vacuum cleaners, it is the filters that have to take a lot of the strain. FLSmidth typically uses PTFE (polytetrafluoroethylene), a coating that makes the filter media more efficient by reducing particle adhesion, and increases durability against the friction of the material in the air stream. Although it costs a little more, the company thinks it worth it to reduce maintenance.

Virtually all the filters need some form of in-use cleaning cycle: while some simply use a low pressure fan to remove the dust, others (more efficiently) send a reverse-pulse burst of air back through the filter (usually at a pressure of 7 to 9 bars) to knock the dust off.

While on the whole, mechanical unloaders come with a lower energy bill, another set of parameters comes into play when you look at the bottom line. It has to be said that, given current funding situations, it may be easier to lose the higher electricity bill in with the other stevedoring costs than to find a home for the large capital spend needed for mechanical units.

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