An analysis of the problems with a dense phase coveying system.
Report 97.3.TT.4839, Transport Engineering and Logistics.
Since December 1995, the New Zealand Aluminium Smelters Ltd. has been using
a Dense Phase Conveying system for conveying fluorinated alumina. The system
has three main problems which come up from time to time. These problems are:
Analysis showed there are several causes for these problems:
- Scaling in conveying lines. This causes an increase of conveying times, and
in some cases even results in wearing out of parts in the conveying line.
- Wear of cone valves. When this happens, the valve fails to close the
pressure vessel airtight when pressurising and conveying. Replacement is
- Wearing out of dosing nozzles. When a nozzle wears out, material and air
will leak out of the conveying line.
There are several possibilities which can prevent or at least minimise the
problems. The most practical way to minimise scaling in conveying lines and
on cone valves is to reduce the amount of moisture in the conveying air. The
1 bar air supply is not dried before it is used in the system and it is very
well possible that this air introduces a lot of moisture into the system.
Improvements of the design of cone valves and dosing nozzles also can decrease
the number of problems. Some solutions are being tested at the moment.
- It appears there are three main causes for scaling. These causes are:
- The amount of moisture in the conveyed air and alumina
- The presence of impurities in the alumina
- Conveying velocities.
- The excessive wear of the cone valves is caused by:
- Scaling on the cone, when this happens, the cone is not able to close
air-tight anymore and an abrasive flow of leaking air and alumina will
wear out the cone valve.
- The design of the valves causes the blend alumina to have its impact on
one and the same spot when flowing into the valve. This causes wear at
that particular spot.
- The cause of the excessive wear of the dosing nozzles is not established.
This needs to be addressed by ALESA, the supplier of the Dense Phase
Problems with scaling in conveying lines and wear of cone valves and
dosing nozzles will probably always come up once in a while. To minimise
damaging consequences, it is important to predict these failures. One way to
do this is by using SCADA (Supervisory Control And Data Acquisition); it
offers adequate methods to predict the problems. The system is already
installed and will therefore not require any investments. However, it will
require work to optimise the system.
Scaling in conveying lines causes increasing conveying times. When these
conveying times are presented in graphs, it is possible to detect scaling by
monitoring these graphs regularly. When the conveying time of a certain
pressure vessel exceeds a set period, an alarm will come up.
When a cone valve is leaking, the most practical ways to detect this is by
measuring the pressure-drop by performing a pressure test and by timing the
period it takes to pressurise a vessel. If these measurements reach a set
limit, an alarm will come up.
It turned out to be not possible to predict wear of dosing nozzles using SCADA.
Another method is necessary to predict this, which is outside the scope of this
Reports on Transport Engineering and Logistics (in Dutch)
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