Discussion on the measurement of ethylene gas mass

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Discussion on ethylene gas mass flow measurement in petrochemical enterprises (II)

1) clarify the place of measurement handover. According to China's current measurement regulations, after confirming that the main flow meter meets the provisions and requirements of relevant measuring instruments, and the measurement data is accurate and reliable, the data measured by the common measurement and detection system should be recognized as the basis for measurement handover. If the demander installs a measurement and detection system and monitors the supplier's instruments, it should install a measurement and detection meter not far behind the supplier's flowmeter

2) the Demander shall install a flow meter to monitor the supplier. In order to facilitate the measurement, it is required to select the same type of flowmeter as the supplier, and it is required to have temperature and pressure compensation links

after the implementation of the above two opinions, there has been no measurement dispute. During the past few years, the turbine flow meters of both parties have been operating normally, which shows that the cause analysis is correct, and the measures taken and the type selection of measuring instruments are reasonable

2 improper instrument selection leads to inaccurate ethylene measurement

within Beijing Yanshan Petrochemical (Group) Co., Ltd., a set of turbine flowmeter used by the supplier for gas ethylene measurement and measurement is lwq-40, and the maximum flow is 14 under standard conditions × 103m3/h, accuracy of 1.5, and temperature and pressure compensation links; There are two users. One is vortex flowmeter with accuracy of 1.0 and maximum range of 10t/h. The unit ethylene consumption is 75kg/h according to process production calculation (the range of the table is increased); Another unit selects American micro motion D mass flowmeter (this model table is not applicable to measuring gas) - it mistakenly selects ethylene gas as liquid. According to process calculation, the maximum dosage is 10m3/h (11.74kg/h) under standard conditions, and the selected range is also large, so this meter cannot be used. After calculation, the sum of the maximum ethylene consumption of the two units is 82kg/h (73m3/h under standard conditions), which is only about 5% of the flow measurement program selected by the supplier, so it is confirmed that the flow measurement program selected by the supplier is too large. Because the measuring range of the flowmeter of the three units of supply and demand is too large, several metering disputes have occurred. Therefore, it is suggested that the supplier (material supply system) replace lwq-40 with a more reasonable flow measurement instrument as soon as possible, but also under the pressure of environmental challenges. Of course, the two users of the demander should also reselect the table

3 several problems that should be paid attention to in the measurement of ethylene gas mass flow

1) verifying the flow range to be measured and accurately mastering the operating parameters (such as pressure, temperature value, etc.) are the primary conditions for correct type selection

for example, the operating conditions of a user's ethylene gas are as follows

maximum dosage: 10m3/h (11.74kg/h or 0.2kg/min) under standard conditions

normal dosage: 5m3/h (5.87kg/h) under standard conditions

operating pressure: 1~1.8mpa

operating temperature: 10~30 ℃

standard density: ρ 20=1.174kg/m3

since China implements the handover of quality and quantity, the display unit is required to be "kg/h" or "t/h"

from the above operating data, what kind of flowmeter is selected to measure such a small gas consumption

the following types are applicable to small flow gas measurement: metal rotameter, Coriolis mass flowmeter, thermal mass flowmeter, etc. Among the three types of flow meters mentioned above, the metal rotameter has the lowest accuracy. When measured by volume, the error is within ± 1.6%. After the volume is measured, it has to be multiplied by the density, and the density measurement error factor has to be increased, and the temperature and pressure compensation link has to be added. Therefore, the measurement error is within ± 1.5%, and there is no need for temperature and pressure compensation link. The system measurement error is much smaller than the former

considering that gaseous ethylene is a valuable chemical raw material, it is better to choose a flowmeter with small measurement error and high accuracy. Coriolis mass flowmeter has greater advantages than thermal mass flowmeter. However, from the analysis of the quotation of two kinds of mass flowmeter (cmf01 reduced volume 0 Coriolis mass flowmeter of micro motion, about 80000 yuan, and 8000 thermal mass flowmeter of EP1, about 40000 yuan), thermal mass flowmeter has greater advantages than Coriolis mass flowmeter. Due to the current financial difficulties of the enterprise, not to mention the small amount of ethylene gas measured - the maximum amount is 11.74kg/h, and the production is intermittent. Only a few tons of ethylene gas are used in a month. The error is big. How much difference can it make? Therefore, thermal mass flowmeter is preferred

2) as a measuring instrument for ethylene gas quality measurement and measurement handover, periodic verification must be carried out according to relevant regulations. The above two kinds of mass flow meters can be verified on the standard device

Coriolis mass flowmeter can be verified on gas and liquid standard devices. The results of verification on the gas standard device can be directly applied in gas measurement. Because it measures mass, it can also be verified on liquid devices, such as standard devices with water as the working medium. Because the density of liquid medium is far greater than that of gas (hundreds of times), the range of mass flowmeter of the same specification used to measure gas is smaller than that used to measure liquid, that is, when Coriolis mass flowmeter measures gas, it actually applies the lower limit of flow scale when measuring liquid, Therefore, the accuracy of liquid verification device decreases when it is used to measure gas. For example, micro motio can compete with many American made equipment, and the verification error of N company's E-type mass flowmeter is less than ± 0 If there is a demand of 16%, and the error for measuring gas increases to less than ± 0.5%, it is qualified

there is another conversion link for the calibration of thermal mass flowmeter. The calibration of thermal mass flowmeter can be based on whether the voltage value of some fixed points in the circuit of the detection instrument is measured normally (generally not used). In practical application, the actual flow calibration shall prevail. If ethylene gas is used as the medium to calibrate the flowmeter for measuring ethylene gas, there is no such standard device in China at present. Generally, air is used as the working medium to calibrate the flowmeter. However, because the heat transfer coefficient (or specific heat capacity at constant pressure) of air and ethylene gas is different, there should be a conversion problem in the calibration process. The American EP1 manufacturer has this conversion coefficient, which is provided to the National Metrology Institute and users. The displayed value of the thermal mass flowmeter is calibrated with the air standard device, and then multiplied by a "0.671" coefficient, that is, the value that ethylene gas should display. Its coefficient is

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