DRY CARGO INTERNATIONAL - May 2003, page 67 to 70
Pneumatic versus mechanical
discussing what works best in bulk handling
A t the recent Bulk Terminal Operations Conference & Exhibition
in Antwerp, Belgium,
Alain de Visscher, commercial manager for Belgian equipment manufacturer
VIGAN, gave the following presentation, comparing the merits of
pneumatic unloaders and mechanical units which he has kindly allowed
us to reproduce. We a/so hear from Claudius Peters, another specialist
in the pneumatic handling of bulk.
Pneumatic versus mechanical systems for grain unloading
Modern industry now, more than ever, demands maximum efficiency
in the unloading of grains and other similar free-flowing materials.
Pneumatic conveying through vacuum pipe-lines has been used since
the late 1800s. On the other hand, mechanical systems - mainly screws,
chain conveyors or double belts or even bucket-elevators - have
also been used for many years.
Pneumatic systems are achieving capacities up to several metric
tons/hour per pipe, in unloading operations for a large variety
of materials. However, despite their numerous advantages including:
quicker cleaning of the holds; greater flexibility for various ship
sizes; freedom from pollution; few moving parts; and safety, such
systems are thought by some people to be less adequate than mechanical
systems.
This article will review and bring out the main differences between
the two systems:
A. MECHANICAL: mainly screw, chain or bucket elevator;
and
B. PNEUMATIC: mainly fan blower turbines or 'Roots'-type pumps.
All the following comments are quite meaningful. However, it
will not be easy to establish an exact and fair ranking of each
system, as every unloading operation is conducted under different
parameters. Factors such as labour costs, energy costs, other
costs, pollution regulations and many more are ail variable, and
each port is as different as the country where it is based.
However, in order to make as fair a comparison as possible, it
is essential to take ail the factors mentioned below into consideration
before making a choice between the two alternatives.
1. CONTAMINATION
There are various aspects to this consideration:
- contamination of product handled by foreign materials or
agents getting into the conveying system;
- the mixing of different products after multiple unloading
operations, mainly due to product residues; and
- contamination of the environment by fugitive dust.
Mechanical systems are more liable to be vulnerable to the first
two points, even more so when they are not well designed.
For example, if wet grain gets into a mechanical system, it can
either stick to the inside walls of the conveyor, or it can accumulate
in 'dead' zones inside the mechanical conveyors. The design of these
conveyors is such that they never empty out completely; mechanical
systems are much less effective at self-cleaning than mechanical
systems.
Pneumatic systems have several advantages in that they have no dead
zones, and the large volume of air circulating inside the pipes
ensures efficient self-cleaning of the system, and that ail moisture
dries out completely.
Both mechanical and pneumatic systems are closed circuits, so therefore
there is no dust contamination along the conveying line.
Pneumatic systems that operate with negative pressure have a further
advantage in that, as a large volume of air is 'sucked' up during
unloading, they can also reduce dust left over in the holds: this
is not only beneficial for the safety of the workers, but it also
reduces the amount of dust escaping into the environment, making
operations more efficient ail-round.
Appropriate filters incorporated in the pneumatic systems guarantee
absolutely no dust emission. It is for this reason, among others,
that many ports in the United States have only approved the use
of pneumatic systems for the handling of very dusty products such
as alumina: it is anticipated that this trend will spread to include
other products, such as grains.
2. SAFETY
Mechanical and pneumatic unloaders are both considered to be safe.
However, there is no doubt that mechanical systems have many more
moving parts and motors, ail along the conveying line, requiring
the installation of appropriate fail-safe devices with limit switches
in many places.
With pneumatic systems, many motors are located inside a machine
room; it only takes a few minutes to check appropriate running conditions.
In mechanical systems, there is a risk of dust explosions: any metallic
piece caught between the enclosure and any moving part (screw, chain
or bucket) can produce sparks.
For bucket elevators, some recommend an explosion vent every six
cubic metres of elevator leg with one square metre of light relief
material panelling designed to open automatically under slight pressure.
There is virtually no risk of explosions with pneumatic systems:
large air volumes, no parts in movement into the suction pipes.
3. CLEANING OF THE HOLDS
This is probably the major and main issue concerning both alternatives:
mechanical versus pneumatic.
Cleaning out holds is often something of a nightmare for the manager
in charge of the unloading operation: too slow, too much manpower
and, during this time, the tie-up costs are increasing at a rate
up to U$ 1 2,000 to US$ 1 5,000 per day!!!
Pneumatic systems have clear advantages in terms of higher overall
through-ship performance du ring clean-up and, thanks to the vertical
and horizontal movements of the telescopic pipes, and the rotating
cabin, the suction nozzle can easily reach larger areas at the bottom
and suck up material down to the very last product particles from
the bottom of the hold.
A far more limited area and cargo volume will be reached by the
inlet nozzle of a mechanical system due to the up/down movements
or the vertical orientation of their vertical leg (kick-in and kick-out
movements).
When mechanical unloaders are used, it is important that the mechanical
unit's moving parts do not touch the ship's bottom, as this could
cause serious damage.
One of the important advantages of a pneumatic unloader is its ability
to 'suck' up the grain (or other product) like a vacuum cleaner,
which enables it to reach all the corners of the ho Id easily.
Using pneumatic units could save up to US$ 1 00,000 a year over
a total unloaded volume of 1 million tonne (mt), by improving the
turnaround times and efficiency of the vessels.
4. MAN POWER
During the peak time of hold unloading, both mechanical and pneumatic
systems have similar manpower requirements: one operator is normally
enough but, in practice, one assistant is also generally present.
When discharging into trucks and/or railroad cars, one supervisor
is needed for this operation.
The main difference in manpower requirements is during clean-up,
where more workers and more time are needed for mechanical units.
The additional costs related to this can be significant; in many
ports, labour unions can often be problematic, and make demands
of maritime operators or port authorities.
5. MAINTENANCE AND REPAIRS
Sometimes, equipment maintenance is inadequate. It is therefore
essential that equipment is well designed for efficiency, safety
and low running costs - and also for ease of maintenance or repairs
(also in relation to tie-up costs or 'dead times').
Due to their heavy construction, mechanical unloaders are commonly
considered to be low maintenance.
However, experience shows that this is not generally the case, due
to the multiplicity of moving parts, with resultant higher potential
technical failures, and the risk of abrasion wear points.
These potential technical failures also account for the many safety
devices required by mechanical systems.
This can lead to more downtime, stoppages and more maintenance checks.
Any repair of a mechanical system can be very messy and lengthy,
due to poor access and weight of the parts. The position of the
motor(s) at the end of the boom of a mechanical system can be cause
a lot of difficulties when it has to be changed.
Pneumatic unloaders require far less maintenance: all the main equipment
components are enclosed in the machinery room (housing). There is
better accessibility to the motors and the auxiliary systems in
the machine room will facilitate maintenance checks and will increase
the probability that the person in charge will carry out the maintenance
programme properly.
Easier access to the equipment components shows, for instance, that
it is much simpler to replace a vertical pipe than to repair the
vertical arm of a mechanical system.
When choosing and defining the features of an unloader with its
supplier, a customer has to take human factors into account: machines
that are too sophisticated are often not well accepted, and considered
too risky in many countries and/or for local harbour conditions.
6. FLEXIBILITY: SHIP SIZE AND MODELS / DESIGNS
From day to day, maritime operators or importers of bulk products
should endeavour to negotiate purchases and sea freights at the
lowest rate. The vessel unloader should allow for the discharge
of a wide range of ship sizes in order to enjoy the most of the
business opportunities.
Possible harbour dredging works, in a few years ahead, could modify
the maximum ship size. Therefore ship unloaders should be able to
operate with larger vessels, or be easily modified.
Mechanical unloaders do not incorporate telescopic and horizontal
pipes; this specific concept and design are factors limiting the
flexibility of the equipment. To be efficient during up-down and
kick-in/kick-out operations, most mechanical unloaders are well
suited to a certain range of vessels but not for other ship sizes.
In certain circumstances, a small mechanical unloader will be unable
to enter with its arm into the holds of a large vessel.
Pneumatic equipment, with their conveying systems including telescopic
pipes supported by a boom, are very flexible and easily designed
according to the customer's specific requirements.
Pneumatic systems are usually available in a great variety of models
(size, diesel or electrical version, on rubber tyres or on rails).
They offer the advantage of having many standard components, thus
avoiding long delivery delays: on-site assembly time is also shorter
than for mechanical unloaders and consequently less costly.
7. WEIGHT
ln some cases, the light weight of the pneumatic unloader is important:
there is no need for expensive structural work or reinforcements
on the quayside. Mechanical unloaders for similar capacities are
usually at least 20-50% heavier than pneumatic units offering a
similar capacity.
8. NOISE
Mechanical systems are considered quiet, due to the low running
speed of the moving parts.
However, in the case of pneumatic equipment, proper insulation of
the machinery housing can be very efficient in reducing the noise
for fan blower type equipment. High-wave frequency noise is easily
dampened so that this kind of pneumatic unloader can be placed in
urban area in accordance with the highest noise standards.
The above does not apply for unloaders featuring the 'roots' type
blowers (also known as positive displacement pumps): the low-wave
frequency noise produced by the back flow during the lobes (pistons)
rotation is very difficult to reduce.
9. DELICATE CARGOES
Unloaded products must keep their characteristics during their
transport. Delicate bulk cargoes can include such commodities as
barley for breweries, malt, cocoa beans, cargo rice, alumina, soda
ash and so on.
In mechanical systems, the contact between the product and the moving
parts of the unloader (such as the screw, and along the metal walls)
can cause physical degradation to the cargo.
In pneumatic systems, potential damage to cargo can be significantly
reduced if the manufacturer uses the most appropriate technology,
and if the equipment is designed properly.
For example, when handling delicate cargoes, it is important to
avoid the separation of air product using a cyclone. The best alternative,
designed by VIGAN, is the use of vertical and horizontal telescopic
pipes with small diameters at the suction nozzle; this allows the
product to be conveyed into the pneumatic system with
low energy consumption. The increase of the pipe diameter reduces
product speed, and the inclined horizontal pipe enables the product
to keep moving and to flow gently down to the airlock.
A large receiving hopper, with a well-sized filter, ensures perfect
separation of the product and the air conveying it.
The use of the above technology allows for many kinds of delicate
cargo to be transported with minimal product degradation.
Thanks to the flexibility inherent in the unloading of delicate
cargoes, it is possible to use the same pneumatic unit to unload
a range of products for several customers, without risk.
10. ENERGY CONSUMPTION
This factor is fairly complex, and must be addressed closely and
bearing in mind some of the factors mentioned above. Roughly speaking,
energy consumption for wheat with a density of 0.75 is:
• about 0.3 to 0.5KWlton of unloaded product with mechanical
systems; and
• about 0.6 to 1.0 KW/ton with pneumatic systems.
A fair approach in line with understanding the limits of those figures
is through an example in the attachment. After analysing those figures,
we may conclude that mechanical systems offer no energy-cost advantages
over pneumatic unloaders.
Of course, as with any simulation, it differs from reality but fair,
logical and reasonable modifications of the main parameters will
lead to similar conclusions: no real advantage of the mechanical
systems because the lower tie-up delays of the ships with pneumatic
systems due to shorter cleaning times.
Furthermore, manpower and other costs, like port fees due to the
longer cleaning delays are in favour of pneumatic systems.
CONCLUSION
Other points and/or differences between both alternatives may
be considered, however, taking into accounts those above comments,
the main features of mechanical and pneumatic systems indicate that
the pneumatic unloaders have definitively many decisive advantages.
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