Terminals and factories, receiving their (raw) materials by ship, operate unloaders.
One category of unloaders is the pneumatic unloader.
Although the unloading does not belong to the core business of the company, it can be considered as an umbilical cord to the company’s process or trade.
Without incoming materials there will be no end product nor sales.
A stevedoring company will even stop to perform immediately.
Owners of such installations should be aware of the possible impact on their day to day operations and possible risk in case of failures and therefore should evaluate the offers for their installations with great care.
Purchasing under quality- or under designed and built units will create unpleasant problems (and costs) later on.
In those cases where a pneumatic unloader does not fulfill the specified expectations, the following causes are possible:
- Installation does not reach the design specifications
- Frequent breakdowns
Ad 1) Installation does not reach the design specifications
In case the capacity is not reached, this could be influenced by:
- Pneumatic design
- Product properties
- Pipe size and – configuration
- Air volume
- Pressure / vacuum
- Back pressure silo or flat storage
- Kettle outlet configuration
- Nozzle type
In case the energy consumption is not reached, this could be influenced by:
- Pneumatic design
- Type of vacuum pump or/and type of compressor
- Drive system (electric, diesel or diesel electric)
- Average rate of product feeding
- Skill of operators
- Down time
Operational influences affecting the performance could be caused by:
- Type of ship
- Reach of arm
- Maneuverability of arm
- Use of auxiliary equipment for nozzle feeding
- Ship unloading procedure (shifting holds)
- In case of floating equipment: stability of pontoon
- Maintainability in case of breakdowns
- Availability of spare parts
- Noise level (work place, environment)
Ad 2) Frequent breakdowns
- Equipment failures
- Arm failures
Design specifications :
The design specifications are the values against which the performance of a pneumatic shipunloader has to be compared.
The design specifications are the result of a set of considerations in terms of:
- Types of commodities
- Expected annual throughput
- Future lifetime
- ship size of import/export
- Unloader operational properties
- Dust generation
- Noise limits (day or night)
- Labor conditions
- Dock loads
Capacity / energy consumption
Based on the required capacity and pipe routing, a pneumatic design is made to determine:
- Air volumes
- Pipe sizes
- Filter sizes
- Capacity at different pressures
- Energy consumption at different pressures
- Bend forces
These calculated data are then used for additional calculations:
- System capacity at different pressures
- System energy consumption at different pressures
- Expected maximum- and average capacity at maximum- and average pressure
- Expected maximum- and average energy consumption at maximum- and average pressure
- Strength calculations of pipe supports
- PLC program
Before a definitive pneumatic design is accepted, several alternative pneumatic designs can be made.
The parameters (capacity, air volume, pipe size, pressure) can be combined in many ways, with many different overall results.
An installation can be designed for maximum capacity or minimum energy consumption.
As these two parameters will not be possible at the same time, a choice has to be made between those parameters or for a combination of the two.
The consequence of a choice is in most cases:
Maximum capacity design –> lower investment cost –> higher energy consumption per ton
Low energy consumption design –> higher investment cost –> low energy consumption per ton
Investment costs are fixed costs and will be shared by every handled ton during the lifetime of the installation.
Energy costs are variable costs, which will be imposed on
every handled ton during the lifetime of the installation.
As the pneumatic design calculations are the basis for the further design of the installation and for the economics of the operation, it is imperative that these have to be thoroughly and extensively evaluated and executed.
The physical design of the installation is important in order to achieve the designed performance of the unloader.
The most important component, in this respect, is the pressure kettle outlet configuration where the product is mixed with the conveying air in the right loading ratio.
If the mixing capability is to low (not enough product is mixed into the air) the maximum loading ratio is not reached and therefore the maximum designed capacity is not reached
This will result in a lower pressure (or vacuum) than designed.
The application of extra active fluidization of the kettle cone can improve this situation.
Also the loading ratio control of the kettle outlet must be fast and accurate in order to maintain maximum loading ratio and prevent blockages of the pipeline at the same time.
A given pneumatic design determines the energy consumption per ton conveyed.
As there are various types of air compressors, using different types of compression principle.
Screw compressors with internal compression require less energy than f.i. a water-ring compressor.
The way of power generation also influences the energy consumption per conveyed ton.
F.i. a diesel direct drive is more energy efficient than
a diesel – electric drive.
Operational circumstances s.a. nozzle feeding, operator’s skill and down time influences the energy consumption, mostly by increasing the time that no-load power is demanded while
no product is conveyed.
The type of ship to be unloaded is of influence on the time being used to unload..
Box type ships with straight and vertical walls are easier to unload than f.i. a bulk carrier with narrow hatches, many holds and open frames.
The unloading arm is designed for a ship size in dwt of average dimensions.
In reality the ship to be unloaded will be of different main dimensions, s.a. width, depth, hatch size, ballast draft, etc.
The reach of the arm will therefore not always be optimal, causing delay in the unloading by increasing the amount of cleaning up in the hold.
This also occurs when a ship is chartered, bigger than the original design was meant for.
A considerable part of the unloading time is spent on the cleaning up of the holds.
The clean up equipment used determines the rate of cleaning up and thereby the time used.
Bigger equipment will speed up the cleaning operation and will save time and energy.
In case a break down of equipment occurs, the time used for correction is depending on the possibility of easy repair.
The accessibility of the equipment is then very important.
Also the skill of the maintenance engineer and the availability of spare parts is crucial to minimize the down time for repairs.
For the diagnosis of the malfunction, it is necessary to have extensive and clear diagrams, drawings and manuals available.
Also the PLC program should have an extensive alarm diagnosis function.
Other operational parameters s.a. noise levels and vibrations are to be coped with by the proper application of normal standard technology.
Equipment failures should not occur frequently.
If equipment failures do occur, than it than it can be caused by :
- Not designed for existing ambient operating circumstances
- Inferior technical quality of used components
- Accuracy of assembly not sufficient (f.i. alignment, bad welds)
- Lack of maintenance. (oil change not in prescribed intervals)
- Improper use of components (f.i. operating at higher pressure than designed for)
Damaging the arm by external causes does not often happen and is mostly caused by improper operation or use or accidents.
Unloader arm failures often occur during operation due to improper design in respect to fatigue.
The load on the arm during operation and thereby the material stresses are to a great extend alternating and subject the material to fatiguing.
If the design does not incorporate fatigue calculations and the construction of the arm is such that there are points where stress rising can occur, the arm will fatigue and eventually crack and break.
This phenomenon is, when unnoticed, very dangerous and constant visual checks should be executed at a regular interval. (f.i. between each unloading)
As soon as fatigue cracks are discovered, corrective action must be undertaken.
Not only should the crack be repaired and strengthened, but also the affected area should be redesigned (if possible) and changed to prevent future fatiguing.
Further causes of arm failures are:
- Hydraulic hoses, cables, hydraulic valves, etc. in vulnerable places
- Insufficient control properties
- Inferior technical quality of used components
- Lack of maintenance. (lack of greasing)
- Improper use of components (f.i. operating at higher hydraulic pressure than designed for)
To avoid or even prevent the above scenarios, the technical departments of the operating company should be given the assignment to write an extensive specification with clear descriptions of the required performance and quality of the unloader.
Also the appropriate responsibilities in case of not fulfilling the requested specification must be clearly defined.
Also very important is that the operators of existing unloaders are consulted for their experience.