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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.

Distribution studyThe 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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