Cost savings in ethylene flow measurement

Flow Measurement & Control

October 2003 Flow Measurement & Control Products & Solutions

An ultrasonic flowmeter designed by Krohne specifically for use in supercritical ethylene applications has been chosen by a consortium of chemical companies in preference over a combination of turbine meters and density meters because it offers significant cost savings and outstanding accuracy.

The consortium members, including the chemical giant DOW, are all connected to the European Ethylene Pipeline, a major distribution network supplying ethylene to production sites, sea terminals and consumers across Europe. The group has carried out extensive trials using Krohne's ultrasonic GFM 700 SE to measure the mass and volume flow of supercritical ethylene, and has selected it as the favourite for custody transfer purposes.

Ethylene is a raw material used in the petrochemical industry for production of synthetic materials such as polyethene and polystyrene, as well as solvents and coatings. It is a difficult medium to measure because its density can change dramatically with relatively small fluctuations in temperature or pressure; therefore, constant pressure and temperature need to be maintained at all times.

During transportation ethylene is kept in a supercritical state, which means it is neither a gas nor a liquid. In spite of being in a state of high density, the fluid behaves much like a gas and is inflammable and potentially explosive. The conventional method of measuring the flow of supercritical ethylene consists of a gas turbine meter in combination with an inline density meter installed downstream. However, there are a number of disadvantages with this method; any temperature difference between the turbine and the density meter results in an error of the mass flow measurement, and at low and high flow rates the turbines become inaccurate because they have a small dynamic range. These intrusive technologies also cause a 100 kPa pressure loss.

The use of turbine meters to measure supercritical ethylene also has high cost implications. Expensive filters need to be installed because the meters are sensitive to small particles, and frequent maintenance and calibration is required to maintain high accuracy since they suffer wear from internal moving parts.

In comparison with turbine meters, ultrasonic meters do not obstruct the flow or cause a drop in pressure because they have no moving parts. This means less pumping capacity is required which enables a saving in energy and cost. The ultrasonic meter is also insensitive to particles and does not require filters. Moreover, costly re-calibrations with the GFM 700 SE are also unnecessary due to the long-term reliability of the meters.

In addition to providing volumetric readings the GFM 700 SE can also produce highly accurate mass flow measurements by using a flow computer and an absolute pressure transmitter. Sound velocity data collected by the flowmeter is fed into a flow computer, the OMNI 3000, and by using Krohne software with a unique density model, the mass flow can be determined.

For more information contact Gordon Duff, Krohne SA, 011 315 2685, [email protected]

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