Download - Tanker Operator

TECHNOLOGY - TANK GAUGING
Vessels benefit from
intelligent sensors
As elsewhere in life the move to digital technology is evident in many aspects
of ship operation today, but one area that has been slow to take the step forward
is tank gauging and data acquisition systems*.
From a cost perspective the
predominant technology employed
for level measurement of ballast,
service, and bunker tanks is still the
hydrostatic head or pressure principle. The
most basic of this technology is the direct
“bubbler” or air reactive gauge but since on
most modern vessels it is essential that data is
in electronic format the direct electronic
sensor with a 4-20mA analogue signal is
probably the most cost effective and certainly
the most flexible to install.
The past argument that a directly installed
sensor could be less reliable is certainly not
true today. Mechanically, many quality marine
sensors have been specifically developed to
withstand the rigours and corrosive aspects of
these applications. Electrically, too, correct
design considerations ensure their
performance and reliability is not
compromised by thermal and other adverse
affects imparted by the duty.
Meeting all the above criteria and ensuring
compliance with intrinsic safety parameters
has in the past only been practical using
analogue electronic circuits and clearly
millions of such sensors have been installed
on board ships of all type. That all sounds
well and good - however, that is where the
development has remained for many years
now and as with most things in life and at sea,
there is an increasing downside.
In a conventional arrangement each
transmitter requires its own cabling which
runs from the point of measurement to the
point of display. Some systems have adopted
marshalling arrangements where sensor
cabling is routed to one or more instrument
cabinets, which convert the 4-20mA signal to
a digital format for onward transmission, but
these provide only a half-way solution and
conversion errors compromise on the accuracy
of the overall measurement. The downside is
that although the cost of sensors has gradually
reduced as demand increases, there is a
greatly increased cost of cable and skilled
labour to install and commission these
systems. In many larger systems the cost of
installation can exceed the cost of the system
components.
PSM has designed and manufactured
electronic marine and naval sensors for over
30 years and has continuous experience of
supplying sensors to these applications, both
directly and, through a number of specialist
system integrators.
Having now undertaken more than two
years of shipboard trials on board both Royal
Navy platforms and commercial vessels, the
company confidently releasing ther Intelligent
Ceramic Transmitter (iCT) and associated
system products. The iCT is type approved
and ATEX certified and for the first time
enables the full implementation of a digital
tank gauging system. It is arguably the most
advanced sensor system available for the task
and when one reviews advantages and
features, it is clear to see there should be
significant benefits to be derived by all parties
- eg the shipyard, system installer and the
operator. For cabling, PSM calculated that
over 70% of the cost of cable and installation
components (cable trays, trunking, JBs,
penetrators etc) can be saved on an average
system.
In detail, the iCT measures tank level
hydrostatically using an ultra stable ceramic
capacitance cell. It contains an embedded
microprocessor that processes the level
measurement locally to provide a direct digital
output. The output format employs industry
standard MODBUS protocol for universal
compatibility transmitting onto a robust and
proven RS485 standard.
January/February 2010 TANKEROperator 41