Research on Reliability Assurance and Calibration of Natural Gas Metering System

As an important energy source, natural gas is increasingly valued, and the accuracy of its measurement has become the main research task of metering workers. In order to make natural gas measurement accurate and reliable, people gradually realize that only researching, improving and standardizing flow meters is limited in improving the accuracy and reliability of flow measurement. Only by ensuring the reliability of the entire metering system can the accuracy of measurement be fundamentally solved.

1. Reliability guarantee of accuracy requirements

1.1 The accuracy of flow meters and secondary instruments should comply with regulations or contract requirements

The accuracy of the gas flowmeters and secondary instruments that make up the metering system shall at least meet the requirements of national regulations or contracts. The accuracy of the metering system and the supporting instruments shall comply with the corresponding provisions in the appendix to the Technical Requirements for Natural Gas Metering Systems. In addition, the metering system shall fully and accurately comply with any additional contractual terms. Each instrument characteristic in the metering system shall match the expected measured value and the required accuracy level. Special attention shall be paid to the range of the instrument used and the dynamic response to fluctuations in the measured value.

Independent measuring instruments in a measuring system can be calibrated and adjusted separately, and these independent measuring instruments can be replaced in case of failure; to determine the maximum allowable error of the measurement result. The maximum allowable error is expressed as a percentage of a measurement result or an absolute value. It is an error limit, or an expanded uncertainty. The systematic error of a measuring instrument in a measuring system should not be eliminated by the opposite systematic error of another measuring instrument, but should be synthesized according to the uncertainty assessment method.

1.2 Determination of maximum allowable error

The measurement system should determine a double standard deviation, which should be within the maximum allowable error range for each instrument of the measurement system or the measurement system as a whole for which the maximum allowable error has been specified. In this case, uncertainty analysis of various data of the measurement system is effective. Uncertainty analysis includes: residual system error based on the measurement results given by instrument calibration; uncertainty caused by installation effects based on technical conditions, calibration certificates and confirmation; evaluation of the drift of the measurement system over time and the uncertainty caused by drift based on the results of repeated calibration or known performance; evaluation of the uncertainty caused by installation effects; evaluation of the uncertainty influence of calibration equipment .

1.3 Calibration

The metering system and its individual instruments are required to have corresponding measurement accuracy, and they should be verified and calibrated using methods that can be traced back to national benchmarks. Instruments used in trade metering systems should be subject to mandatory inspections in accordance with relevant national laws and regulations. Calibration should be carried out under the same conditions as the actual working conditions. If they are not consistent, additional errors will be introduced. For example, if the working conditions during calibration are inconsistent with the use conditions, then the actual velocity distribution upstream of the flowmeter will be inconsistent with the actual velocity distribution during calibration, which will inevitably cause additional errors in the flowmeter. Sensors and electronic instruments affected by ambient temperature or power changes will also cause certain additional errors. However, if the installation effect is known and stable, or can be determined through on-site inspection, and if this factor is taken into account when calculating uncertainty, then we can also calibrate the metering instrument under different conditions.

The standard equipment used for calibration shall be verified at a legal metrology institute. If it is relevant to the function of the calibration range, the calibration certificate shall specify the systematic error of the measurement result. The use conditions are beyond the range of calibration, such as the sharpness of the orifice plate may have changed, which will inevitably introduce systematic errors. The calibration certificate shall also state the uncertainty of the calibration result. From the empirical data known from type tests or the type of instrument used in the metrology system, the drift of the first calibration result and the uncertainty caused by the drift will be evaluated. Based on this, the inspection and calibration intervals can be determined. The effectiveness of the measuring instruments and the metrology system should be evaluated. The metrology station should indicate how the measurement results should be replaced if a measuring instrument or the entire metrology system fails. The impact of the uncertainty of the substitute value on the uncertainty of the measured value should be evaluated. Even when using substitute values, the measurement results should be within the range recognized by the relevant parties throughout the specified time period.

2. Reliability guarantee of the installation process

The installation of the flow meter should comply with the requirements of relevant national standards, industry standards or international standards and meet the requirements of the manufacturer. When the inlet vortex angle is smaller than the meter manufacturer's wide or appropriate product standard indicators, the vortex and velocity distribution distortion generated are acceptable.

For all flowmeters, a fully developed axisymmetric velocity distribution and the elimination of vortices are essential to obtain accurate flow measurement. Rotary positive displacement flowmeters are less sensitive to velocity distribution requirements, while orifice flowmeters are most sensitive to velocity distribution requirements. This is determined by the measurement principle of the open-type flowmeter. The basic flow equation of the orifice flowmeter is derived on the basis of a fully developed axisymmetric velocity distribution. The typical straight pipe section length requirements for various types of flowmeters are detailed in the corresponding technical specifications. It should be noted here that the straight pipe section length requirements are only applicable to installations where the upstream flow conditions are acceptable, that is, they meet the requirements of the inlet conditions we just mentioned. If there is a serious asymmetric flow or vortex at the inlet of the flowmeter, the specified straight pipe section is not enough to obtain a fully developed velocity distribution. Unless the test has shown that the flowmeter in the above situation can accurately measure. For example, the manufacturer may have conducted a type test for a typical upstream pipeline configuration to prove that the flowmeter can accurately measure in this case, otherwise a much longer straight pipe section or a flow conditioner is required. The length of the self-pipe pipeline should still be determined by experiment.

In order to obtain an acceptable velocity distribution, the following measures should generally be considered and combined with the specific instructions for several flowmeters such as the rotary volume flowmeter to be discussed later. The required upstream and downstream straight pipe sections and the inner diameter of the flowmeter should be the same; the nominal diameter of the upstream and downstream cut-off valves of the flowmeter should be consistent with the nominal diameter of the pipeline, and a fully open valve should be used; if a high-pressure valve is installed, preventive measures should be taken upstream of the flowmeter, such as lengthening the straight pipe section, installing a flow conditioner, etc.; pipe fittings or equipment that will produce asymmetric velocity distribution and vortexes (such as: single bends, U-shaped pipes, double bends in different planes, partially closed valves, etc.) should be avoided according to the type of flowmeter, otherwise, sufficient straight pipe sections should be ensured upstream of the flowmeter or a flow conditioner should be installed. If the upstream conditions cannot guarantee the straight pipe section required by the flowmeter used, an appropriate flow conditioner should be used; a straight pipe section that meets the requirements should also be installed downstream of the flow conditioner.

Pressure pulsation and velocity pulsation may cause serious errors in flow measurement. The frequency range and amplitude that affect performance depend on the type of flow meter, the design of the flow meter and the gas density. For example, pulsation can affect the measurement error of the orifice flow meter by 20% or even 100%. These factors should be taken into account when selecting a flow meter. At this time, we can choose a multi-channel ultrasonic flow meter according to the actual situation. Its structural design and channel layout can measure a certain degree of pulsating flow.

Rotary volume flowmeters are not very sensitive to the shape of the pipe at low pressure. At high pressure (actual operating pressure greater than 0.4 MPa), it is necessary to ensure that the airflow at the flowmeter inlet is fully developed. In general, a 4D straight pipe section with the same diameter d as the flowmeter flange upstream and a 2D straight pipe section downstream can meet the requirements. Due to metering factors, small pressure pulsations can be generated, but they have no effect on the operation itself. When another pipeline is connected to the flowmeter pipeline, or the flowmeter is used at the manifold, care should be taken to avoid the possibility of resonance.

The upstream velocity distribution required for the turbine flowmeter to maintain accuracy depends on the design of the flowmeter, and the test results of the turbine flowmeter sensitivity with upstream interference should be considered. EN12261 and 150 both stipulate that the flowmeter should be tested for sensitivity to upstream interference, which requires that the impact of installation conditions on the flowmeter should not exceed one-third of the maximum allowable error, otherwise it is necessary to increase the straight pipe section or install a flow conditioner to test until the requirements are met.

The installation requirements of ultrasonic flowmeters are specified in GB/T18604. For single-channel ultrasonic flowmeters, as with orifice flowmeters, installation conditions that fully develop the velocity profile need to be provided. For multi-channel flowmeters, the installation conditions may not be so strict. The installation requirements of orifice flowmeters are specified in SY/T6143.

3. Test and calibration before commissioning

The metering system consists of complex mechanical and electronic equipment, and appropriate test runs should be carried out to ensure that they meet the design requirements for formal operation. Before going to the site, the system should be fully tested for factory acceptance as much as possible in the manufacturer. This detailed system test includes mechanical components, secondary instrumentation and flow computer inspection of the system, and confirmation of signal processing and data transmission between different electronic components. After installation, a test run inspection should also be carried out before the system is put into production.

All test equipment used for commissioning should have a valid calibration certificate issued by an authoritative laboratory, or use calibrated test equipment approved by all parties involved in the commissioning. This requirement does not include equipment that is only used to generate current or electrical signals. However, the output of such equipment should be stable and repeatable.

The uncertainty of measurement of the test equipment used for commissioning should be at least one third of the uncertainty of the specific item in the instrument being tested (under test condition 2). Wherever used, the uncertainty of measurement of the test equipment should comply with national regulations. All test equipment should be used in the environment in which it is used. If such equipment is to be used in hazardous areas, it should have appropriate safety certificates. It should be emphasized here that in natural gas metering, the equipment used for field testing must be explosion-proof, otherwise it can only be tested in the laboratory.

After installation, ensure that all debris has been removed, the system has been purged, pressure tested, and air flow has been introduced and boosted to the flow meter inlet valve. The system should be visually inspected to ensure that its integrity meets the design requirements. In particular, automatic and manual shut-off valves and vent valves should be carefully checked to ensure safe and reliable operation. The equipment certificates of all electrical systems and their hazardous area cable circuits should be checked to ensure that they meet the relevant standards. When opening the outlet valve, avoid high pressure shock to the flow meter. Pay more attention when boosting the downstream large pipeline through turbine flowmeters and rotary positive displacement flowmeters.

The test and calibration procedures depend on the design of the installation and whether the metering line

Installation bypass. , should ensure that the metering system operates well and the uncertainty meets the metering requirements. The test and calibration procedures should be carried out before the metering station is put into normal use. A clear test and calibration procedure should be formulated to ensure the stability of temperature measurement through peripherals such as differential pressure transmitters and pressure transmitters. The corresponding pre-trial test should be carried out as required to ensure accurate measurement. The requirements for flow meters should be checked according to the manufacturer's requirements of various types of flow meters and relevant national standards. The secondary instrument should be calibrated according to standards traceable to national standards before installation. In order to prevent unexpected transmission effects, an on-site test should also be carried out. Uncertainty verification of the entire system including sensors, signal transmission, analog-to-digital conversion and flow meters.

4. Reliability assurance of operation and maintenance of metering system

(l) The metering system should prepare and provide a reviewable operating procedure for the metering station. These procedures should ensure that the metering station always operates within its design performance range and maintains such performance during its service life. These procedures should be unanimously agreed by all parties concerned. The operating and maintenance personnel should obtain the appropriate qualifications and perform their work in accordance with the prescribed responsibilities.

(2) In order to ensure that the metering system operates within the required accuracy range and maintains high reliability, regular inspection and calibration should be carried out. The inspection and calibration cycle should be based on the requirements for the uncertainty of the metering system, the performance of the metering equipment and the changes in the metering process parameters. For example, the cleaning and inspection of the orifice plate may be a week or shorter cycle if the gas quality conditions are poor. The static and differential pressure zero points should be calibrated weekly. The calibration of flow meters, transmitters, etc. should be carried out according to the corresponding verification procedures to ensure their metering performance.

(3) In addition to regular routine inspections, process equipment that affects the metering system, such as bypass valves, metering pipeline shut-off valves, regulating valves and filters, as well as actual flow and working pressure, should also be regularly inspected to ensure that they are working properly. In order to avoid reading errors caused by system errors of different standards, the system should be calibrated according to the same standard. When estimating the error between the working system and the verification system, the uncertainty of the system and the possible installation influence should be considered.

(4) The appearance of the flow meter should be inspected to see if there is any abnormal operation, such as excessive noise, irregular movement of the pointer, and whether there is corrosion or other damage. The flow meter should be lubricated regularly according to the manufacturer's requirements to ensure that the flow meter works within the calibration period and its uncertainty is maintained within the technical requirements.

(5) Carry out regular inspections on the conversion device and calibration. After maintaining and inspecting the flow meter and secondary instruments, ensure that the metering system operates normally and keep the corresponding maintenance data periodically.

5 Conclusion

The above materials, combined with the requirements of the gb/t18603 natural gas metering system, discuss some views on how to ensure the reliability of the natural gas metering system and the calibration requirements of various instruments from the perspective of the reliability of the metering system and the accuracy of the instruments, so as to provide some useful suggestions on natural gas metering in our factory in the future.