can still boast a healthy order book. It has recently added two
not sufficiently stringent — the limit of 85dBa is too high. Van
has significantly lower sound emissions. Not only will the
that live close to the terminal.
has bought very little from Europe over the past few years.
have instead been purchased from India and China.
where the cement industry is booming. This is generating
for Van Aalst Bulk Handling.
for cement in Bangladesh. The reason for selecting the Van Aalst
the situation in Bangladesh. This feature makes it possible to
unload the final part of the cargo out of the hold of the barge.
This proved yet again that knowledge and product development
are strong sales arguments.
The standard situation during the unloading of cement and fly
ash from a vessel is that it is necessary to use a front-end loader
during the clean-up stage. The main part of the cement is first
sucked out of the hold by the suction nozzle on the unloading arm
of the ship unloader. Although the suction arm of a pneumatic
unloader is highly flexible and can reach into the corners of the
hold, it is advisable to use the assistance of the front end loader to
push the last part of the cement towards the suction nozzle (see
photo above). This will keep the unloading capacity of the ship
unloader at a high volume.
The use of the front end loader however is only possible when
the floor of the hold is flat and smooth. In Bangladesh, the barges
which are used for the supply of cement or fly ash have no flat
bottom. Here the ribs, like the ribs of the side of the hold,
continue at the bottom of the hold. It is therefore almost
impossible to get the last part of the cargo out from the spaces
between the ribs.
Van Aalst Bulk Handling has therefore made clean-up hoses which
can be connected to the suction arm. These clean-up hoses are light
weight and can be manually operated. With these devices, the cement
and fly ash can be sucked from between the ribs.
This solution, in combination with the low labour costs in Bangladesh,
meant that it was not necessary to buy new barges with flat
The use of the light weight, hand-operated suction hoses is widespread
on board self-discharging cement carriers and the practice has proved to
be reliable. To combine this solution with a shore-based ship-unloader was
made possible by Van Aalst Bulk Handling, using the know-how gained from
the use of the hoses on cement carriers.
Van Aalst Bulk Handling considers not only the operational costs of the
unloader by keeping power consumption and maintenance costs as low as
possible, it also ensures that transportation expenses are kept to a
minimum. The unloader has to be transported from Holland to
Bangladesh, so all of its parts are being designed so that they can be
shipped in containers, helping to minimize costs.
Both of the above units are set to be delivered at the beginning of
2011. These contracts show that, if suppliers listen to their clients, and
find the right solution for their problems, the market is still there despite
It can be difficult to ensure that all the cargo from between the
ribs of a ship’s hold is cleared out.
Keeping a lid on potentially explosive problems when storing cement
Major UK level control specialist Hycontrol has developed an
integrated silo protection system designed to prevent the
problems caused by the overfilling or over-pressurization of
storage silos used to store bulk products — including cement.
The SiloSafe concept provides a comprehensive system where
the operational integrity of the primary sensing elements can be
fully checked at ground level prior to each filling.
Each year, millions of tonnes of powdered and granular
materials as diverse as cement, animal feed, silicates, fertilizer, fly
ash and flour, are routinely off-loaded from tankers into storage
silos across the UK. Normally the loads are discharged by
fluidizing the materials with compressed air and blowing them
into the silo. For commercial reasons, vehicle operators want to
discharge their loads as quickly as possible using the maximum
pneumatic conveying pressure, a situation which can bring
conditions within the silos close to critical levels. Protection
systems designed to prevent the overfilling of silos have been
used for some years and now new recommendations have been
introduced to meet the growing number of safety and
environment-related regulations. However despite this,
incidents, in some cases life-threatening, still occur during the
Silo protection system implementation is designed to address
two main unrelated issues focused on the safety of site
personnel and environmental problems associated with the
emissions of particulate material to the atmosphere. There is
currently no single set of guidelines stipulating exactly how such
systems should operate across all industries. The British
Cement Association, in conjunction with Defra and the HSE, has
drawn up one of the most comprehensive set of guidelines, but
even these leave some aspects open to interpretation. However
all current guidelines make it clear that silos filled via pneumatic
conveying should be fitted with automatic protection systems to
prevent over-filling or over-pressurization. Such systems should
have three independent sensing elements, namely a pressure
relief valve, pressure sensor and high level alarm.
Effective and safe filling relies on the controlled release of air
through the air filter, ensuring a steady pressure is maintained
inside the silo. Problems typically arise through air filters
becoming blocked or from excess discharge pressures generated
by the tanker filling system. Most storage silos are not designed
as pressure vessels and even an overpressure of 1 psi may be
sufficient to cause serious damage. Theoretically any excess
pressure should be detected by the pressure sensor allowing the
filling to be halted. If all else fails the pressure relief valve should
act as the final backstop, allowing rapid silo venting.
A prime example of the dangers involved if things go wrong
is demonstrated by the following example. Due to a
misunderstanding between a cement tanker driver and a plant
operator, cement was discharged into a silo that had been filled
the previous day. On that occasion, the high level alarm on the
silo had been activated at the end of the fill and had been
muted. Under normal circumstances, the alarm would have
reset once the cement level dropped. However, because the
cement still covered the probe, this had not happened.
During the delivery, a loud explosion occurred and the whole
filter unit was ejected from the silo roof onto the ground.
Fortunately no-one was injured. The subsequent investigation
showed that the air filter was not operating correctly allowing
cement powder to leak out and build up in and around the
pressure relief valve. This had hardened causing
the valve to seize up. In this case, no pressure
detector was fitted and the other three lines of
defence failed through poor maintenance and
poor operating procedures. Had an adequate
pre check system been in place the problems
would have been highlighted immediately.
But how can operators be sure their
systems will work when necessary? The latest
guidelines stipulate that the system’s critical
sensors must be regularly checked to ensure
they are in good working order. However as
Hycontrol’s sales and marketing director Nigel
Allen explains: “Under ideal working conditions,
the three-fold safety regime should be more
than adequate to prevent problems, but the
only way to ensure equipment is fully
operational is to carry out physical checks just
prior to each filling. Carrying out adequate
checks on a regular basis can be difficult,
particularly if the main protection components
are not part of an integrated system. In
practice, especially in busy working
environments, the checking procedure may not
be carried out.
“Historically checks have had to rely on
visual inspections placing the onus on the operator to make
informed decisions on the condition of vital components. New
restrictions on working at height now prevent operatives
routinely climbing to the top of silos to carry out inspections.”
The Hycontrol SiloSafe solution provides an easy to use, fool
proof and fail-safe system, which mandates the automatic testing
of the key components directly from the integrated control
panel at ground level. The panel also incorporates twin multisegment
bar graphs for continuous level and pressure
monitoring as well as digital displays for both.
When the delivery driver reports on site he is issued with a
dedicated panel key for the particular silo to be filled. Having
activated the panel with the key, the driver must then press the
Sensor Test button and release it. At this stage the discharge
butterfly valve is locked. Each of the primary sensing elements is
collectively tested by this single button press and any
malfunction will be immediately highlighted. Should any faults be
detected, the valve will remain locked and not allow filling to
take place until the problems have been remedied. If the
sensors pass the test, the white ‘ready to fill’ lamp is illuminated
indicating that the discharge valve has opened and delivery can
The pressure sensor module is the first line of defence,
measuring the pressure in the silo as the filling takes place. If
this exceeds a preset value it first provides an audible alarm,
followed by a flashing red beacon and then an output signal
which immediately closes the discharge valve in the fill pipe.
Hycontrol has developed an innovative method for checking the
integrity of the sensor and cleaning it at the same time. During
the ground level test (GLT) a small air supply connected to the
sensor applies a fixed pressure of approximately 90% of full
range output directly to the sensor face. This change in pressure
is measured by the system and verifies correct operation. As the
air escapes through the porous membrane at the end of the
sensor tube it removes any build up of material. If the
membrane is blocked the system alerts the operator and does
not allow filling to take place until the situation is remedied.
Current requirements call for a high level probe to be fitted
to silos to prevent overfilling. As part of the SiloSafe system,
Hycontrol recommends the use of its wave-guided radar system
to provide continuous level monitoring even during dusty filling
conditions. This will give a clear indication of level in the silo
prior to filling, ensuring the load to be discharged will fit in the
silo. Where only a high level probe is fitted, the SiloSafe GLT will
automatically check its operational integrity. If a high level
condition is reached during filling, warning is given via the siren
and flashing red beacon. A countdown timer now closes the
discharge valve after 30 seconds, giving the driver time to cease
filling and clear down his line. The PRV is the last line of defence
and should never have to be used. The Hycontrol system allows
for optional full testing of this vital component. Using small air
cylinders, the system can lift the valve to check operation, with
proximity switches detecting correct lifting and closing.
As Allen concludes: “We are confident that our system is the
most comprehensive available today and more than meets the
current requirements for safe silo filling. By offering an
integrated solution with the ability to carry out tests on all the
key elements in one action at ground level just prior to filling
ensures the system will work in earnest if required. During
filling, operators and drivers can clearly follow the conditions
inside the silo via the bar graphs and displays, giving them ample
time to react to any adverse trends. If any preset level or
pressure limits are reached, the system will automatically close
the inlet valve to the silo.”
Claudius Peters discusses the transport and storage of pulverized materials
Ansgar Reismann of Claudius Peters in Germany gives Dry Cargo
International details of some of the latest developments in the
handling of cement, its transport via sea and land, and
arrangements made for the product at the terminal.
CEMENT TRADE: HOWTO TAKE ADVANTAGE?
Over recent years tremendous investments have been made in
new cement production facilities. The driving factor for these
investments has been giant infrastructure and building projects.
These building projects are now completed and, with new
capacities commissioned, there is now a supply/demand gap.
According to recent analysis, seaborne cement trade is
expected to be 65mt (million tonnes) to 70mt a year until 2013.
As an example, China’s installed cement capacity is much
higher than the demand. This situation can also be seen in the
UAE, where the building boom is coming to an end at the same
time as new cement production lines are coming into operation.
One solution to this overcapacity problem is to look to
exports. Seaborne transport is undeniably cheaper than train
and truck transport
The idea of self-unloading cement ships began in the 1960s
when Claudius Peters designed cement cargo holds with inclined
aeration panels leading to pumping systems (screw pump).
Worldwide there are approximately 400 self-unloading
cement carriers in operation. This fleet is under modernization
as the average age is 23 years. A number of bulk carriers are
included to handle yearly cement trades of approximately 70mt.
The following article gives an insight into a state-of-the-art
solution for marine terminals and self-unloading ships, and the
benefits of investment.
System: ship to shore
It is important to consider ship and shore as a system where all
process elements are interacting with each other. The
ship/shore arrangement is defined by various variables. To define
the most appropriate ship/shore arrangement the following
factors must be taken into consideration. They are:-
- total annual throughput;
- trade scenario, long-term, midterm, short-term, booming markets;
- available port facilities on the cement production side (export) and on the cement distribution (import) side;
- port regulations (stevedores, demurrage) and environmental regulation (noise, dust); and
- availability and cost of electrical power.
The possible range starts with a floating terminal supplying
cement bags to a region where no infrastructure exists, right up
to sophisticated high-volume import terminals with capacities of
4mt a year and more (e.g. Jurong Port Singapore)
Figure 1 below shows the most often used applications of
export (ship loading) and import (ship unloading) situations.
Energy saving self-unloading ships
Shiploading: Claudius Peters can adapt your ship to match all
pneumatic and mechanical terminal installations to ensure
environmentally friendly and short loading periods.
Ship-unloading: unloading is generally the most time-critical
element of the ship’s journey therefore the following focuses on
HP-Contank (High-Pressure Conveying Tank) is the result of
analyzing and improving upon unloading systems used in past
decades. The HP-Contank feeds a conveying pipe which runs
from the ship to the shore-based storage facility without
transfer points. (See Figure 2 on p65.) This is also a positive
feature of the x-pump-based unloading system.
Whereas the pump-based system is limited to 2.5 bar, the
HP-Contank allows operation pressure up to 6.0 bar which
results in lower energy consumption.
More pressure reserve of the ship’s cargo equipment means
that it is possible to use a wider range of conveying pipes, in
terms of length and diameter. This is essential when a ship has
to operate with terminals of different layouts.
A practical example for unloading a ship would be: ship size
15,000dwt, the silo is a distance of approximately 350m and has
a height of 40m. (See Figure 3 on p65.) When unloading, the
ship with 600tph [tonnes per hour] (two lines in operation each
300tph) the specific electrical power consumption is 3.1 kWh/t
in case of HP-Contank.
Other systems (screw pump, screw conveyor with twin
pressure vessels, vacuum and pressurized operated pressure
vessels) have a specific electrical power consumption of 3.7 to
Shore side improvement by applying Fluidcon
Fluidcon is the result of transferring the advantages of airslides
into a pipe transport. It is benchmarking all other pneumatic
systems in terms of easy operation and low energy
When applying the Fluidcon in the aforementioned
example, the specific electrical power
consumption can be further reduced to
Fluidcon is in use worldwide in more
than 80 applications.
Both terminals and ships will benefit from
advantages such as less power consumption and longer maintenance intervals.
Due to the reduced amount of conveying air needed, the compressors
on the ship and the filters on the silos can be designed smaller
Claudius Peters has designed and executed more than 1,000
cement silos for cement production lines and for import and
export terminals. The term ‘terminal’ for import means cement
storage for minimum one ship load with receiving equipment for
e.g. self-unloading ships and distribution facilities such as bag and
bulk for truck and train
The term ‘terminal’ for export means cement storage for
minimum one ship load with receiving facilities either directly
from the production site (e.g. pneumatic transport) or via truck
or train and discharge system to e.g. a self-unloading ship.
The import-export process is depicted in Figure 5 above.
The blue lines show the typical export process, yellow lines
show the import process.
Export terminal: example Vietnam Hong Chon
This Holcim-owned plant receives the cement from a plant site
located 1,100m away, using a Claudius Peters pressure vessels
The cement is stored in a conventional cone silo from where
two packing plants are fed. The Claudius Peters packer (see
Figure 7 on p69) produces 2x 2,400 bags an hour, and these are
finally loaded on barges.
Globally hundreds of packing plants are equipped with
Claudius Peters equipment including:
- rotary packer;
- automatic bag applicator;
- bag loaders for truck,train,etc; and
Import terminal: example Singapore Jurong Port
Two cement carriers of 40,000dwt each can be discharged in
parallel at this cement import terminal installed in Jurong Port,
Singapore for Jurong Town Corporation.
The cement is distributed to six cement importers, from any
ship to any silo.
The terminal serves as a common facility for cement
importers in Singapore to replace a number of small import
It was officially opened in 1997 and has a design throughput
of 4mt annually, making it one of the world’s largest cement
In 2010 there are plans to increase this annual import rate.
Claudius Peters delivered core equipment for cement storage
and cement distribution:
- packing plant;
- palletizing plant;
- single-cell silo;
- multi-cell silo; and
- bulk loaders for the approximately 140,000t/year
A very space saving arrangement is to integrate a packing
plant in the silo lower room.
Multi-cell silos consists various cells with different material
types which can be fed to a mixing plant allowing for the
production of numerous receipts.
There are approximately 400 self-unloading cement carriers
worldwide which are ageing and must be replaced by either
new-build ships or converted bulk carriers.
Regional over- and under-capacities will result in a seaborne
cement trade which will be stable or even increasing the next
years (approximately 70mt/year).
The reason for this is the cement over-capacity in areas like
China and the Middle East combined with regional import needs
in countries such as Angola, Nigeria, and Pakistan.
State-of-the-art equipment on ships (HP-Contank) and shore
(Fluidcon) allows energy saving cement handling which was not
thinkable years ago.
Terminal designs covering basic needs up to high-end
solutions have been realized by Claudius Peters.
Claudius Peters is an experienced system specialist for both
ship and shore.
Steel web belting and SJ buckets enhance the performance of bucket elevators
4B STEEL WEB BELT
With a growing demand for larger capacities and for more
efficient and cost-effective elevators to carry industrial products
such as cement, over the last nine years, 4B has researched,
tested and supplied the industry with an integrated system of
steel web belting, SJ buckets and elevator designs for compact
industrial elevators, writes Dave Wolstencroft B.Eng, C.Eng, MIET,
product manager at the 4B Group. 4B has designed, developed and
successfully introduced the 4B steel reinforced elevator belt
system and high capacity Starco Jumbo (SJ) elevator buckets to
achieve very high capacities at a comparatively lower cost than
using chain or traditional belting with fabricated buckets.
In the majority of cement and other heavy industrial applications,
elevators have traditionally been installed with chains on a
worldwide basis. Chains, however, can be very expensive and
have immense maintenance implications after installation.
Chain bucket elevators traditionally use large casing
dimensions, large cumbersome elevator buckets, which are
pitched apart at quite large distances. Chain operates at quite a
low speed (1.3m/s), due to frictional and noise problems, which
has a net result of limiting the case size. Also high strength, large
construction chain, large motors etc are required on chain and
bucket elevators. All these factors contribute to an expensive
elevator, in initial cost, further maintenance costs, and expensive
Large cumbersome fabricated elevator buckets can also used
with belting. These, like chain, are also a very expensive option
because they also take up a lot of volume within an elevator
casing, and fabricated buckets are very expensive compared to
their pressed counterpart. As a result the casing will also
become larger and more expensive.
THE SJ AND STEEL WEB BELT ALTERNATIVE
As an alternative to this old-fashioned tried-and-tested
approach, 4B has introduced a steel-reinforced belting system,
incorporating the unique SJ bucket design.
The Starco Jumbo and steel web belting can use multi rows
(one to four rows of three different sizes) of closely spaced,
heavy-duty Jumbo seamless steel buckets and purpose-designed
steel web system, high temperature elevator belt.
Elevator belt will wear significantly less than the chain
alternative. This is mainly due to lack of moving parts as
compared with the chain links and
sprockets. Chain will undergo
constant friction between each part,
thus causing constant wear, then
The elevator buckets are pressed
from 4mm mild steel and will
therefore have extensive lifetime of
use. A wearband, welded onto the
front edge and sides of the buckets
is an option if the product is
particularly abrasive. The buckets
are designed to achieve very high
capacities through their unique
optimum design characteristics,
which maximizes bucket fill and will
ease product release. The elevator
buckets are also designed in such a
way, that they can be spaced at very tight intervals, unlike the
traditional system. The buckets can also be used up to 2.3m/s
on the larger pulley diameters (which is still gravity discharge),
which will enable high capacity. This all has a positive effect on
the reduction of the casing size.
COMPARISONS BETWEEN TRADITIONAL CHAIN SYSTEM AND
4B’S STEEL WEB BELT / SJ SYSTEM
As previously described, using 4B’s SJ elevator bucket system and
steel web belt, it is possible to achieve far higher capacities for
the given space provided. The following comparison
demonstrates the differences between chain and bucket with the
steel web belt and SJ buckets. This shows that you can use much
smaller case sizes, due to the faster speeds and more efficient
bucket size and spacing. In general terms you can save around a
third of the case size compared with the SJ buckets.
Traditional chain elevator to jumbo and steel cord belt
to handle 300m³/hour.
CHAIN AND BUCKET STEELCORD BELT AND SJ BUCKETS
Sprocket 900 PCD Pulley Dia 900 dia
Chain speed 1.3m/s Belt Speed 1.9m/s
Bucket DIN 15234 1250*400 Bucket 2 rows of SJ370-250
Case 1400*560 Case 930*400
= Reduced case and lower machine cost
Super Jumbo bucket, steel cord belt and case size
The table at the bottom of the page illustrates the various
different combinations of capacities that can be achieved using
4B’s system. The differing combinations are achieved using
various size pulleys and multi rows of SJ buckets, as opposed to
using large cumbersome fabricated buckets.
High capacity – low maintenance system for industrial
Compared with traditional chain and bucket elevators 4B steel
web belt fitted with Starco Jumbo buckets offer:
- up to double the elevator capacity;
- throughputs up to 1,300tph (tonnes per hour) and over;
- much lower capital investment;
- longer trouble-free life;
- reduced maintenance and spares cost; and
- the opportunity to upgrade the capacity of existing elevators.
The steel web belt has fewer frictional and wear points
compared with chain. The belt also has very thick covers
(usually 4mm or 5mm) to withstand the rigours of cement and
other abrasive industrial products.
Due to the construction of the 4B steel web belt, the belt
will have near zero stretch. In the past the other fear with using
elevator belts as opposed to chain, is that the belt will stretch
under initial use, this will not happen with steel web belt, due to
the special E-cords in the warp and weft.
4B can offer belt strength up to 2,500KN/M, which will cope
with the toughest of applications. The belt is also guaranteed and
can last for many years depending on application. All belts come
with punched holes on application.
A special Allan key head bolt can be used to enable ease of
use and access into the holes of the belt by using an electric drill
with an Allan key bit to screw the bolt through the belt (see
The elevator belt has a constant operational temperature of
130ºC with peaks of 150°C, which will allow for most cement
and other industrial applications. Thick covers, usually 4mm, are
used on the belt, to cope with high abrasion and temperature
levels that can be experienced with many industrial applications.
- specially constructed to suit the SJ buckets and fasteners;
- weft as well as warp cables to add strength stability and bolt holding;
- strengths from 800 to 2,500kN/m;
- operating temperature up to 130ºC to peaks of 150°C;
- and negligible belt stretch.
Belt fasteners and bucket arrangement
- specially designed clamp for Starcojumbo buckets and 4B steel web belt combination; and
- clamp for 2,500KN/M belt — made in steel and aluminium.
THE STARCO JUMBO ELEVATOR BUCKET
The buckets also have the ability to have a very small space in
between each bucket; therefore very high capacities can be
SJ buckets are pressed from 3mm or 4mm mild steel, and can
be fitted with wear bands if the product is particularly abrasive
(see graphic at the top of this page).
4B has successfully completed numerous elevator designs for
new bucket elevators. The company can offer free engineering
design specifying the correct elevator buckets, belting, bolts, case
sizes, motors etc, in order that the bucket elevator
manufacturers can manufacture
the optimum and most cost effective
elevator. This service can
also be offered to the end users,
such as cement plants, whereby
their existing bucket elevators can
be changed over from chain and
the capacity increased, and even
doubled. Here are two examples
of bucket elevators one showing a
new elevator 4B designed and one
it retrofitted from chain, using its
SJ elevator bucket and steel cord
The first example is for a
bucket elevator where the
elevator did not reach the
anticipated capacity of 180–200tph
and frequently lost buckets due to
bucket fixing bolts being pulled
through the belt. The belts often
mis-aligned, also the casing side
was damaged by off-tracking belts.
4B steel web belt with higher
safety factor and cross rigid construction was fitted for
improved tracking. The belt speed and belt strength was
increased. The large fabricated buckets were replaced by the
SJ370 buckets at a reduced pitch to achieve the desired capacity
of minimum 200tph (actually calculated at 285tph). Retrofit
using — T130’c belt 4+4 covers, SJ370-250 (4 mm thick) pressed
steel buckets and M12x50 Allan key DIN bolts with high
temperature resistant locknuts, and a special aluminium clamp.
The next example is of an existing chain elevator in Australia
that was upgraded from 120tph for cement, up to 250tph by
retrofitting and installing one row of SJ470 buckets and using 4B
steel web belt SW1000 4+4. The original elevator, 25 metres
high, had chain installed, and it was decided to change over to 4B
steel web belt with SJ buckets to improve the performance and
To complement its elevator components supply, 4B has a free
detailed engineering design service. The service will give
engineering drawings, optimum speeds and elevator sizes etc, to
take away the worry of capacity and size calculations from the
machine manufacturer. This facility is also available to the end
user, with respect to the upgrading of existing elevators.
In conclusion the SJ buckets and 4B steel web belt can offer
the following features and advantages:
SJ system for cement
Traditional chain and bucket elevators are limited
- replaces traditional chain and bucket elevators
- replaces very large cumbersome belt and very large slow bucket elevators
Starco Jumbo and 4B steel web belting elevator are
- uses heavy chain and large fabricated buckets;
- limited number of buckets per metre;
- maximum speed 1.3m/s restricts capacity;
- chain and sprockets wear quickly;
- high maintenance and replacement costs; and
- high capacity requires very large elevator construction.
- uses 1 to 4 rows of closely spaced heavy-duty Starco Jumbo seamless steel buckets;
- in conjunction with purpose designed steel cord high temperature elevator belt;
- capable of up to 2.3m/s belt speed;
- achieves far greater capacities per case size then chain – up to double;
- saves 33% of components costs;
- much lower capital investment;
- belt wears less than chain;
- reduced maintenance and spares cost;
- reduces down time and maintenance costs throughputs up to 1,300tph and over; and
- 4B steel web belt does not stretch.
Cement storage: Eurocode 1991-4 and inverted cone silos
‘Inverted cone’ silos were first developed in the mid 1970s and
since then, thousands have been constructed. Most were
designed using loads derived from the German code DIN1055,
Australian Standard AS3774 or the American code ACI313.
Hugh McKay, Aurecon’s principal consultant within its Mining
& Industrial Sector, recently conducted research on the efficacy
of the new Eurocode 1991-4 in preventing silo distress.
McKay writes: The basic ‘inverted cone’ concept involves the
construction of an upward pointing 60º cone structure within
the silo which forces cement to be discharged around the
perimeter. Over the past decade, it has become increasingly
apparent to us that many inverted cone silos are exhibiting
severe structural distress.
Eurocode 1991-4, released in 2006, radically changed the
assessment of loads on silo walls for silos with eccentric
discharge. Concerned about the possible shortcomings of
AS3774, Aurecon undertook an analytical investigation to
compare the traditional design standards, AS3774, DIN1055 and
ACI313 to Eurocode, using an 18-metre-diameter, 10,000-tonnecapacity
inverted cone cement silo as the case study. The
investigation showed that wall reinforcement normally provided
for such silos was seriously deficient to resist both wall
moments and shears as derived from Eurocode.
Concerned by its findings, Aurecon approached Holcim
subsidiary, Cement Australia, and sought approval to measure
wall deflections on a 10,000-tonne, 18-metre-diameter silo at
Devonport. This silo was selected because it was possible to
survey an entire discharge cycle over a relatively short period
(16 hours). The survey demonstrated that significant wall
displacements (–15 mm to +28 mm) were occurring, far in
excess of those predicted by AS3774.
As a result of these findings, Cement Australia requested
Aurecon to review all of its inverted cone silos throughout
Australia, some of which were experiencing operational
problems such as cracking, water ingress and blockages.
- most of the silos were significantly overstressed although many continued to operate without significant problems;
- silos over 20 metres diameter fared better as they had been post-tensioned; and
- many reinforced silos (up to 18 metres diameter) were exhibiting significant cracking
- strict compliance with Eurocode for all future design of silos;
- all silos greater than 14 metres diameter should be posttensioned;
- the minimum wall thickness should be diameter D/60 and not less than 350mm for post-tensioned silos; and
- vertical reinforcement in both faces of the wall needed to be considerably increased, particularly in the 3 metres above the cone/wall interface.
We then analyzed the cost ramifications of strict design
compliance with Eurocode, essentially comparing designs
compliant with Eurocode with those complying with AS3774.
The first example involved two dual-cell ring silos
constructed in Singapore for Pan United Cement. Both silos
were designed by us, the first in 2001 in compliance with
DIN1055 and AS3774 and the second in 2007, compliant with
Eurocode. Both were constructed by Leighton Contractors
(Singapore) Pte Ltd and operate without problems.
Using tendered rates applicable to the 2007 silo, the costs
were directly compared. Strict application of Eurocode resulted
- one additional pile (1,200 Ø bored pier);
- 260 cubic metres of additional concrete in the outer wall;
- 20 tonnes of additional reinforcement; and
- marginally more pre-stress.
The increased structural cost was less than SIN$200,000,
representing approximately 1.5% of the civil/structural cost and
just 1.2% of the total silo cost.
In the case of a dual-cell ‘ring’ silo, the presence of the inner
wall limits outer wall pressure that can develop as well as the
size of the flow channels. The cost impact of Eurocode
compliance is thus, understandably, minimal.
Obviously, on a single-cell silo, the load impact of Eurocode is
much greater. Two single-cell silo scenarios were considered —
a 27-metre-diameter silo with 25,000 tonnes capacity and an 18
metre diameter silo with 10,000 tonnes capacity. Both silos are
located at Cement Australia’s Bulwer Island plant in Brisbane and
they are of similar height.
The 25,000 tonne cement silo is Eurocode compliant. To
study the cost impact of this compliance, a comparative design
was developed based on loads derived from Australian Standard
AS3774. The exercise revealed that Eurocode compliance
- 520 cubic metres of additional concrete in the high rise wall;
- 140 tonnes of additional reinforcement in the wall, including the need to install nearly 14,000 shear ligatures linking the inner and outer hoop reinforcement;
- 30% increase in wall pre-stress; and
- eight additional piles to support the extra weight of the thicker wall.
The additional cost based on the tendered rates was
A$860,000 or approximately 5.6% of the structural cost and
4.4% of the total silo cost including all mechanical and electrical
supply and installation.
The second comparison related to the 10,000 tonne silo,
designed by us in 1996 in accordance with loads derived from
AS3774. Its wall design was repeated using loads derived from
Eurocode 1991-4, with the following findings:
- a Eurocode-compliant silo of 18-metres diameter requires pre-stressing and the wall thickness increases from 250mm to 350mm (366 cubic metres of concrete) a slight reduction was realized in the total reinforcing steel in
- the high rise wall (-7 tonnes); and
- six additional piles were required to sustain the additional silo dead load.
Using the rates tendered in 2008 for the adjacent 25,000-
tonne silo, the cost premium for Eurocode compliance for a
10,000 tonne silo was estimated at A$625,000 or 9.5% of the
structural cost including piling. The major portion of the
additional cost related to the requirement for pre-stressing and
the resulting thicker high rise wall and buttresses.
Operating difficulties resulting from silo blockages can be
experienced by operators of inverted cone. Removal of these
blockages is difficult as increasing market pressures mean that
silos must remain operational often resulting in silo loading not
envisaged by equipment suppliers.
As a result, many silos exhibit significant cracking, internal
spalling and water ingress and, whilst some of these problems
may have been attributable to poor quality construction, this was
generally not the case with the Australian silos that we
It was very evident to us that loading codes, DIN1055,
AS3774 and ACI313 were deficient in the way they dealt with
eccentric discharge and, as a result, the cement industry has a
potentially significant issue to resolve. Uncompromising
compliance with Eurocode loading requirements in future
designs will however eliminate many of these issues. Depending
on the type and size of inverted cone silo (single, dual or
multicell), the structural cost impact for this compliance ranges
from as little as 1.5% for dual-cell ring silos to a maximum of
around 9.5% for the smaller single-cell silos (<20 metres
diameter and not previously post-tensioned).