scope:

INTRODUCTION

1.1 Application

This manual is intended to be the primary reference source for the calculation and determination of the quantity of ethylene delivered or purchased by. It is assumed that the reader is familiar with measurement terminology and has had a general exposure to gas measurement theory and metering installations.

All procedures in this manual are based upon established standards and other generally accepted industry practices. These standards are listed in the references section of this manual.

The manual covers the application of concentric orifice meter with flow computers or circular charts. Turbine meters and vortex meters are also addressed.

Care should be taken in all phases of handling the measurement equipment and associated records. For details of generally accepted practices, reference is made to the American Gas Association Gas Measurement Manual.

Operating conditions at the measurement site should be selected to insure that the flowing stream is always in the gas phase and free of pressure pulsations such as those associated with pumping. It is recommended that every effort be made to avoid operating at the critical conditions of ethylene. Specifically, this would preclude operating at conditions where the density of ethylene changes very rapidly thus making the measurement of pressure and temperature critical (see Figure 1).

1.2 Concentric Orifice Meter

The quantity of ethylene delivered shall be preferably measured by the concentric orifice meter with flange taps, located at the point of delivery. The meter tube shall be designed and installed in accordance with the recommendations of the American Gas Association Gas Measurement Committee Report No. 3. The calculation of the measured quantity will conform to the method as outlined in the AGA report. This manual includes physical property data which makes the calculations specific to ethylene.

The ratio of the orifice diameter to the inside pipe diameter (β ratio) should be among those outlined in the AGA report and should be selected so that the normal readings of differential pressure should not be lower than 20 inches of water nor greater than 80 inches of water when using a recommended 100 inch full-scale range.

1.2.1 Flow Computer

A flow computer is preferred for use with the orifice meter. Electronic transmitters shall provide inputs to the flow computer which represent differential pressure, upstream static pressure and downstream temperature. The flow computer should continuously solve for density using the API-2565 algorithm unless a fluid density signal is provided as an input. A totalizer should be used to continuously integrate flow rate over the specified contract period.

The electronic transmitters shall be checked and calibrated at least once each month. These procedures should be witnessed by interested parties. Records of the calibration should be filed by the interested parties and retained on file for at least one year.

If upon testing the official flow measurement equipment, an inaccuracy sufficient to change the delivery by more than 1% is found, registrations of such equipment should be corrected for the period extending back to the time when such inaccuracy originated. If the time is not ascertainable, then the correction should be extended back one-half the total elapsed time since the last calibration.

1.2.2 Circular Chart Recorder

Optionally, a circular chart recorder may be used with an orifice meter tube. It should be of standard manufacture of either the mercury type or the dry type and should record in inches of water differential pressure. The orifice meter chart should be the 24-hour type unless a special arrangement for a longer period is made. In those cases in which this longer time period is used, the computations must reflect this time period in order to arrive at the correct volume.

Flowing ethylene pressure should be measured continuously and simultaneously with differential pressure on the high pressure side of the orifice and should be recorded continuously on the same chart as the differential pressure.

Flowing ethylene temperature should be measured continuously and simultaneously with the differential pressure on the low pressure side of the orifice and should be recorded continuously on a separate chart from the differential pressure and static pressure. However, it is permissable to use a three pen recorder where desired to record the differential pressure, static pressure and flowing temperature simultaneously.

The circular chart recorder shall be checked and calibrated at least once each month. These procedures should be witnessed by interested parties. Records of the calibration should be filed by the interested parties and retained on file for at least one year. If any inaccuracy is found the same procedure outlined in Section 1.2.1 should be followed.

The chart is to be interpreted by using a suitable chart analyzer which simultaneously traces differential and static pressures. These integrated values for differential and static pressure are used with the integrated value for temperature to calculate the weight of ethylene delivered per hour as described in Section 3.2.

1.3 Turbine meter

A turbine meter may be used for ethylene gas flow measurement, provided the installation includes facilities for connecting a suitable prover to ensure the required accuracy. The meter is to be selected and proven for operation over the linear range of the meter.

The turbine meter measures volumetric flow rate and provides a pulse output in proportion to volume at stream conditions. The weight of ethylene delivered is determined by correcting the totalized volume to base conditions and multiplying by the base condition density. Procedures for these calculations are given in Section 3.3.

1.4 Vortex Meter

Vortex meters are used infrequently for ethylene gas flow measurement. In these cases the installation includes facilities for connecting a suitable prover to ensure the required accuracy. The meter is to be selected and proven for operation over the linear range of the meter.

Vortex meters measure the volumetric flow rate and provide a pulse output in proportion to volume at stream conditions. The weight of ethylene delivered is determined by correcting the totalized volume to base conditions and multiplying by the base condition density of ethylene. Procedures for performing these calculations are given in Section 3.3.