| Case Studies
TRACERCO™ Level Gauge
Solution to polyethylene overflow problem
A leading manufacturer of low-density polyethylene (L.D.P.E.)
in South Africa recently upgraded its No. 1 polyethylene
plant. Production on this extremely high-pressure plant
has been increased by approximately 50%. This was achieved
without increasing the size of major vessels.
 The
higher flow velocities through the separators had resulted
in these vessels becoming highly susceptible to wild level
fluctuations. These separator vessels are used to disengage
or separate unconverted ethylene gas from the molten liquid
polyethylene product. They operate at a pressure of around
300 bar. The previous capacitance probe based polyethylene
level control systems worked in conjunction with radioisotope
based high level trips. These simply could not cope with
the new demands being placed on them.
Since the upgrading, the vessels frequently suffered liquid
polyethylene carry-over into the gas exit lines. This results
in liquid solidifying in the gas lines. This over-flow condition
resulted in unacceptably high losses in production.
The plant management team decided to install a reliable
level measurement system: one that could be trusted to indicate
the exact position of the interface between the relatively
high-density gas and the relatively low-density liquid.
The gas is ethylene at a density of about 0.2 grams per
cubic centimetre. The liquid is polyethylene at about 0.5
grams per cubic centimetre.
After a thorough investigation (including overseas visits)
and detailed evaluation, the plant management team selected
the TRACERCO™ Level gauge.
The reasons why the Tracerco gauges were selected are
as follows:
- Disastrous fires had occurred on the plant in the past.
Tracerco supplies the only non-invasive level or interface
gauge that is intrinsically safe for zones 0, 1 and 2
use. It is therefore guaranteed not to initiate any fires
or cause the highly flammable ethylene gas to explode.
- Due to the random nature of radioactive decay, long
time constants are desirable for high accuracy when using
nucleonic systems. However, the system had to be both
accurate and fast. Fast response requires a short time
constant. The Tracerco gauges have dual dynamically selectable
time constants. The Tracerco control units automatically
choose a long time constant under steady state conditions
and a short time constant under fluctuating conditions.
This ensures no compromise between the two key performance
areas of accuracy and response time.
The Tracerco system has pressure compensation built in.
The density of the ethylene gas can fluctuate dramatically
due to system pressure swings. A sudden drop in pressure
could cause dissolved gas in the melt to begin escaping.
This causes foam to begin to rise from the level of the
melt. However the immediate effect is for more radiation
to pass through the lower density gas. This results in the
anomalous situation that competing nucleonic gauges would
initially indicate the level moving in the opposite direction
i.e. falling as a result of increased radiation at the detector.
The TRACERCO™ Level gauge detectors use Geiger
Muller tube technology, which do not operate too close to
background radiation levels as do some competing scintillator
detectors. They are therefore less susceptible to minor
disturbances to the radiation field due to e.g. process
inflows. They will allow for a build-up of a polyethylene
'skin' on the walls to be "calibrated out".
The plant has been run under various extremes of operating
conditions (such as maximum output and sudden shutdown)
with the Tracerco equipment. Not a single moment of production
has been lost during this period due to separator vessel
overflow. The investment in the Tracerco equipment was paid
back within the first month of commissioning.
Level gauging can be reliably and accurately performed
using appropriately designed radioisotope based systems
even when the application is particularly arduous.
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