Extensive safety testing confirms pressure enhancement effects and
identifies safe design limits for integrated granulation and drying systems
GEA Pharma Systems (formerly known as Niro Pharma Systems) in
conjunction with FSA, the safety specialist centre in Germany, have carried out
an extensive test program involving over 100 test explosions. This research has
shown conclusively that should an explosion occur during the transfer operation
in an integrated granulation and drying system where a granulator is connected
directly to a fluid bed dryer without an explosion isolation valve then, the
secondary explosion pressures in the granulator can be significantly higher
than in the fluid bed.
These tests have enabled GEA Pharma Systems
to gain full EC type approval for a range of pressure shock resistant
integrated systems and 16 bar pressure shock resistant high shear granulators.
The tests were carried out with hybrid mixtures which behave in the
same way as pharmaceutical products containing organic solvents and showed that
that the final explosion pressure in the granulator is dependent on the volumes
of the two vessels and that the length and diameter of the interconnecting duct
is critical in ensuring that the pressure remains within safe limits.
The tests showed that where the fluid bed is designed in accordance
with VDI 2263 part 5 (i.e. for a 10 bar explosion pressure), which the standard
considers to be adequate for all pharmaceutical powders and organic solvent
combinations, then the granulator must be able to withstand an over pressure of
at least 16 bar in order to provide a range of transfer duct configurations
which are both practical and safe.
Where the fluid bed is required to
handle materials with explosion pressures greater than 10 bar (e.g. metal
powders) and hence is designed outside of the VDI standard or where the
configuration of the interconnecting duct is outside of the safe design
parameters then either the granulator must be designed to a higher pressure
shock resistance or a hygienic fast acting valve or some other form of
protection is essential.
The test program showed that the pressure
enhancement effects are caused by the difference in the propagation speed
between the pressure wave and the flame front. The worst cases are when the
pressure in the granulator is able to rise significantly before the flame can
reaches the granulator and ignite the pre-compressed material. For the range of
transfer duct configurations to be used with the 16 bar granulator design the
presence of bends and obstructions such as mills was not seen to affect the
increase in pressure.
For plant processing powders or mixtures which
are not flammable at the time of transfer between the granulator and the fluid
bed then the risk of explosion is eliminated and hence a wider range of
transfer duct designs can be safely used.
This extensive research
program has significantly advanced the state-of-the-art in safety technology
for Pharmaceutical plants and confirms GEA Pharma Systems commitment to
introducing new products which are based on sound research and development
using solid process know how and understanding.