24 / 60
HAZARDOUS AREAS + SAFETY
to the synthesis of the metal fluoride salt solution, the same vessel is
used to crystallise this solution to obtain the final solid metal fluoride
salt product. The last stage of the process is the product handling. The
solid metal fluoride salt is filtered and then washed and dried before
being stored in the final product silo, from where it is packaged and
warehoused before being distribution.
Process design analyses and safety optimisation
In the production of a metal fluoride salt, anhydrous hydrogen
fluoride (AHF) is used as a raw material. HF is required as a 70% HF
solution to digest the metal oxide powder, and thus dilution of the
AHF is required. In the process described, the AHF is transferred via
the AHF transfer pipeline (MFS-0102-PL-001), from the AHF storage
tank (MFS-0102-TK-001) to the HF dilution tank (MFS-0102-TK-002)
for dilution to a 70% HF solution.
AHF is a volatile substance and is extremely toxic. It has a flash
point of around 16°C at ambient conditions in the Highveld of South
Africa. The aforementioned necessitates that the AHF be stored
either under pressure (old method) or at low temperature (modern
method). In the instance of an AHF release to the environment, at
conditions where the temperature is higher than 16°C, the AHF will
flash off and form a toxic vapour cloud (based on Highveld ambient
atmospheric pressure).
Basic control philosophy
For the dilution of AHF to 70% HF, the most important thing to con-
trol is the temperature of the dilution tank. This is done in this plant
by means of feed ratio control of the two feed streams into the HF
dilution tank, the AHF feed stream and the demin water feed stream.
The temperature of the AHF feed stream from the chiller’s product
side controls the flow rate of the chilled water through the chiller.
In the case of a rise in temperature in the HF dilution tank, there is
a Temperature Alarm HIGH followed by a Temperature HIGH HIGH
trip on the tank, which immediately shuts off the AHF feed stream
into the reactor.
Similarly in the case of over filling in the HF dilution tank, there is
a Level HIGH HIGH Trip on the tank, which first shuts off both the AHF
feed stream and then the demin water feed stream into the reactor.
Hazard Studies
During the decommissioning process, a design review of the Metal
Fluoride Salts pilot production plant was done. This included the
compilation of engineering design diagrams and process descriptions.
As well as the review of the plant safety and operability in the form of
a HAZOP study. During the HAZOP study the following recommenda-
tions were made in order to guarantee a safer plant design for the
production of the metal fluoride salts:
• Include a high level alarm on the AHF feed storage tank
• Include a low level trip on the AHF transfer pump
• Install a plant trip on chilled water failure
• Include a vent line from the HF dilution tank to the vent gas scrub-
ber system
• Cool the demin water feed to 10-15°C
• Include bunds in all areas that handle HF (separate bunds for each
area)
• Include containment of all HF containing equipment/tanks (and
investigate the best method of containment)
• Include a HF vapour knock-down system in all HF containing areas
(either a water or a foam spray system)
• The high temperature trip on the HF dilution tank should trip at
a temperature above 15°C
• Investigate the inclusion of a dilute HF analyser on the 70% HF
feed from the HF dilution tank (with an alarm)
• Install HF detection in all areas with HF (with a high level system
trip) {AHF storage, transfer pipelines, AHF chiller, HF dilution and
vent gas scrubber system}
• Install automated block valves in the AHF feed line and include
a high flow trip on the block valves
• Determine the optimum position for the control valve FV-001 in
the AHF transfer line
• Include an empty emergency tank
• Include a trip on the AHF transfer pump in the event of pipeline
rupture
• Include a high temperature alarm on the temperature indicator in
line MFS-0102-PL-004 and investigate the addition of HF detection
in this line
• Include HF monitoring on the vent gas stack
• Investigate the addition of an interlock system on the isolation
valves
Using the method as described where the HAZOP study and SIL
review is done in conjunction with each other, a SIL review was con-
ducted after the HAZOP was completed and included the members
of the HAZOP team. This SIL review considered HAZOP items where
consequences were identified and trips were either already included
or recommended during the HAZOP study.
Safety Instrumented Systems (SIS) were evaluated based on the
design and recommendation originating from the HAZOP and mostly
produced SIL ratings of 2 and below, with the exception of one system
(on the high concentration of HF in the plant area trip) where a SIL
rating of 4 was identified, which is deemed to be unacceptable and
Electricity+Control
March ‘16
22