Dear paulocezar,

First have a look at:

https://forum.bulk-online.com/showth...ison+umloaders

In addition:

-The choice for a type of unloader is highly depending on the material to be unloaded.

-The energy consumption per ton is depending on the conveying distance after unloading.

-The chosen technology of the equipment is of great influence on the energy consumption.

-The number and technology of energy conversions in the installation is important.

-The unloading sequence curve (unloading capacity = function (progress of unloading)). (free digging rate – clean-up rate – free digging percentage of cargo – clean-up percentage of cargo).

-In case of a pneumatic installation: Designed for maximum capacity (higher energy consumption) or for lowest energy consumption (higher investment)

-The size of the ship is important as it determines the size of the unloader. (a bigger ship requires a bigger unloader and thereby more energy per ton)

Your question is a very important one, however, a general answer is very difficult to formulate.

In case of a real project, the various optional designs have to be made and compared for investment, capacity, energy, maintenance, environmental impact, etc.

Perhaps you can give us more information of your project, we can focus more in detail.

Success ■

It seems you are influenced by power consumption. This is leading you away from reality. Power consumption becomes irrelevant if the ship is waiting while e.g. the rotary valve on a vacuum unloader is changed out or the screw bearing is replaced. My criterion is that a ship unloader must work reliably and consistently for the cargo in question.

If you have a single cargo then you can play around with some fancy machinery but if you have different cargoes your only real answer is going to be grabs where you can change the grab while the ship approaches the berth. Hopefully, there is a serious study done, though not always, before a purchase order is issued for a ship unloader. As Teus mentioned, the prospect of clean up is very important. If you have to have clean up machinery available why not use it all the time in the AGV category.

Stevedoring through the ages has predominantly concerned with lifting cargo, of any kind, over the ships rail. That is always the problem. FOB and FOR are practically the same for an engineer. Even container vessels usually lift over the rail. There are smaller alternatives such as the old Safmarine SS Vegerlegen. In ancient time the only means of getting heavy cargo into the hold was by crane, that is all they had. We are stuck between the conveyor and the crane, however modified and sophisticated the latter might pretend to be. Without a quantum change from the naval architects we are stuck with grabs and Bobcats.

At the end of the 18th century Ellerman Fyffes developed a real self unloader for bananas. Their fleet was very fast; immortalised in WW1 songs; hunted and sank submarines in its spare time and impressed me as a child watching it approach Liverpool with the crew ready to set the elevators in motion (late 1940's). I innocently assumed that one day all cargo ship would be made like that. I'm still waiting. When somebody offers me a design for bulk unloading I have to sigh and say "Here we go again: back to the future." I suppose what I am saying is don't worry about it because the shippers, ports and naval architects all have their say and their preference goes. ■

Dear John,

Of course, a ship unloader must be reliable as ship’s downtime costs, because of malfunctioning equipment are huge.

This does not mean that equipment can not be energy efficient and reliable at the same time.

I am afraid that in most cases the investment price is more ruling that the lifetime costs of an unloader. (Short term commercial thinking rather than a long term best practical means approach)

Take care ■

Dear Paulo Cezar,

A pneumatic unloader is not a feasible option.

-The fertilizer is not free flowing nor fluidizable. That causes a high pressure drop at the intake nozzle (and a rotating cutting nozzle) to force the fertilizer into the air stream.

That is the main reason that the energy consumption will be relatively high.

-A capacity of 1200 tons/hr. requires a really big pneumatic unloader. Especially for a big ocean going vessel of probably over 100000 dwt. Investment will be very high.

A screw unloader can reach the (free digging) 1200 tons/hr. capacity.

-Clean-up capacity is not optimal. A vertical screw needs to be completely filled for proper operation. In the clean-up phase this becomes difficult.

-The poor clean-up capacity reduces the tts capacity and increases the energy consumption.

-A screw unloader is sensitive for foreign matter in the cargo. Repairing a stalled screw can take much time and effort. (Dismantling the vertical screw with a heavy counterweight at the other end of the arm requires extra measures)

The grab unloader seems to be the better choice. Especially when high-tech technology can be applied such as hybrid drives, where brake energy is stored and re-used later.

Give special attention to dust suppression.

Use the experience, which you have with your existing grab unloader to make the new one better.

Questioning why you should purchase a grab unloader and not opting for a different system is the right approach.

Success

Teus ■

I quote Teus "Use the experience, which you have with your existing grab unloader to make the new one better."

So, there are various ways to improve grab unloaders. There is not much extra boom loading if you use 2 slightly smaller grabs side by side and hang an extendable conveyor under the boom. That way you will deposit material into the conveying system almost as soon as it clears the hold. Dust generation is time dependent and if you limit the racking distance you will proportionately reduce the dust. Also by restricting the

racking there is the possibility to shroud the discharge so that no shore winds can access the suspended fugitives and carry them beyond the hold. These criteria apply to a single grab machine just as well but my design prefers 2 smaller counterbalancing grabs in the interests of more continuous, clod free, conveying not to mention a substantially increased output. If you examine the tip loading there is only the additional empty grab which makes a significant difference. The conveyor download is less than the point load of a laden racking grab plus trolley; one grab lifts a lesser full load while the other empty grab is in descent.

If you used AGV's to their full capability in the hold there should hardly be much clean up delay. You would have to calibrate the beacons in the event of different hold geometry but that should not be a problem and after the database is developed it should be no problem at all.

My earlier reply dealt with the 'back to the future' aspects and I have little doubt that the above remedies could be disparaged by unloader suppliers. Why build a better machine if you can sell 2?

I've just realised that these forums will be intensely scrutinised by the American NSA because of our frequent use of 'Laden' not to mention 'Bin'. However we were there first! ■

Dear Sir

All reponses are correct. Your Engineers must decide which type of machine will work best for your application. What we offer is a way to cut the cost of the equipment. We offer low hour Surplus Ship/Barge Unloaders:

-grab type, have 2 available

-Screw type, 91)

-Continuous Bucket Wheel, low hours(1)

If we can be of service, please feel free to contact us.

Sincerely,

Al Goodmundson

Phone Canada (306)-664-7260

Email norsequip@sasktel.net ■

Hi Paulo,

Let me get the 'Why build a better machine if you can sell 2?' out of the way. Suppose you need 2500m³h^-1 and there is only a 2000m³ machine available from manufacturers without a serious upgrade.No manufacturer can justify the development cost without increasing the price very steeply. It could be many years before he sells the large machine again so who bears the extra cost? The first customer. So the manufacturers, they are all the same genre, will offer two smaller units at prices just below their 2000m³h^-1 unit. It makes good sense all round. If one unit is out af operation you can still work the other one. But that doesn't satify CAPEX, the owner has to buy two machines.

AGV,s are Automatically Guided Vehicles. In ship clean up work Bobcats, or similar, are used to move material from under the coamings and present the material to the grab working envelope. Operations are reasonably random and potentially hazardous to the Bobcat driver in terms of grab collisions working in the bottom of a very dust laden hold in very poor visibilty. Shippers also complain about mobile shovels damaging the hull walls. AGV's would solve all these problems while at the same time. Performance would improve and the AGV's could start clean up much earlier with precise control. I am thinking about limiting standing walls in this earlier start up situation: which makes an overwhelming case for AGV's.

There would definitely be a single boom carrying 2 grabs. I originally suspended both grabs from a single trolley and then alternated the hoisting and lowering actions side by side. That layout used a single racking winch. There is nothing to stop someone using individual trolleys and an extra racking winch. For trolleys I use a single axle right through. The cross travel wheels are on the end of the axle and the sheaves are on the same axle. So the extra trolley would not cost so much and might improve operations in some cases. I presume this covers your query about independent reeving (steel cables)

Naval architects present unloaders with a conflict of interests. When working with Panamax vessels the hatch open width is necessarily about (34-2)/2m but with Handymax ships, which are usually geared, the hatches are racked back using the gearing and so the opening width is doubled. Since Panamax and Handymax have very similar beam and draught it obliges stevedores to provide unloading boom lengths which will serve Handymax working while at the same time providing booms strong enough for the denser cargoes available with Panamax. I mention this because it is possible to design a lighter boom if the Handymax unloading is organised to work grabs at port and starboard separately. From your precise questions I know you understand this last paragraph. Anything else? ■