Ettlingen, May 2005 - The filling of a large-volume crude oil tank
originates from several supply ships. Since each oil batch has a different
density, the oil tends to stay in layers. Some of the heavy components settle
down in the tank. With a tank diameter of 80 meters and a height of 20 meters
it is difficult to get a proper homogeneous crude oil mass without mixing the
content. Shaft-driven mixers entering the wall of the tank have difficulties in
mixing the tank in a reasonable period of time. Other ways of mixing, e. g.
with air, result in air pollution due to contaminated air bubbles, which should
be avoided.
In close cooperation with the customer, GEA Jet Pumps have designed a jet
mixing system (fig.). Several jet mixers are installed inside the tank. The oil
itself is used as motive medium, i. e. the existing discharge pumps suck off
the oil from the tank and convey it into the jet mixers as motive medium. The
motive jets mix with the oil in the tank and create a circulating mixing
stream.
Based on the adequate number and arrangement of the jet mixers, a
three-dimensional flow is produced in the tank, which mixes the whole of the
contents without producing a rotating motion.For establishing the number of
mixers required, the following criteria are taken into account: geometry and
size of vessel or tank, type of liquid to be mixed, mixing time and maximum and
minimum liquid levels.
Even if the viscosity in the 80,000 m³ tank varies between 200 and 800 cSt
and the specific weight between 850 and 1,040 kg/m³, it takes about 4 hours of
jet mixing to achieve the required homogeneity. This situation will remain
stable for more than a week.
For the customer and for GEA Jet Pumps it was a big challenge: once closed,
the tank will not be opened for 10 to 15 years. Therefore, the system has to
perform a maintenance-free operation for such a long period of time. For this
reason, jet mixers are perfectly suited as they are simple and reliable, having
no moving parts and being hardly subject to any wear and tear.
Liquid jet
mixers are mainly used in vessels, storage tanks and neutralization ponds. A
comprehensive type series comprising 7 different sizes is available in
materials PVC, PP, PTFE, gray cast iron as well as stainless and acidproof
steel.