Measurement Technology of Glass Fiber Reinforced Plastic

At present, practical glass steel tank measurement technology and methods are the following:

1, artificial scale

The measurement of glass fiber reinforced plastics starts at the manual scale, which is still widely used and is one of the tools for other level gauge performance verification. The method of manual foot test can be found in the International Standard API2545.

  2, float steel level gauge (including light electronic)

The typical accuracy of this mechanical measuring instrument is 10mm. Due to the friction between the pulley, the spring spring mechanism and the mechanical counter, the reliability of this table is poor.

 3, servo level gauge

Servo level gauge on the basis of the float-driven level gauge has been greatly improved. A delicate float replaces the previous large and heavy float, which is replaced by an electric servo motor to lift the float. A set of intelligent force measurement system continuously measures the weight and buoyancy of the float and controls the servo system, and the servo motor drives the integrated transmitter at the same time. This level gauge can not only measure the level, but also can measure the oil and water boundaries. In the glass fiber reinforced plastic tank with an upper limit of more than 40 m, the accuracy of 1 mm is generally obtained. Superior accuracy and reliability make it available for most metrological purposes and are recognized by many national metering and customs authorities.

4, radar level gauge

(1) the principle of measurement

The use of radar waves to measure the glass steel tank level is a new technology, the radar level gauge without moving parts, only the antenna into the tank, so the use of low maintenance costs. Radar table using microwave, the measurement of the liquid level is usually around 10GHz. Radar wave conducted by the frequency of the wave and the reflected wave frequency calculation, the glass fiber reinforced plastic tank is relatively small, and requires a higher resolution, so the measurement of reflection time is almost impossible, the solution is to change the frequency of the wave , Measure the frequency difference between the transmitted wave and the reflected wave, and calculate the distance of the radar wave transmission. This level gauge, despite the one-time investment compared with the float, but the use of the cost is very low.

(2) the relationship between temperature, pressure and material properties on the measurement and the temperature in the tank

Microwave propagation does not require air media, so its propagation rate is almost immune to temperature changes. According to the determination, when T = 500 ℃, the change of reflection time is 002%; T = 2000 ℃, the reflection time is much less than 003%. So the radar level gauge is fully suitable for high temperature medium level measurement.

(3) and the tank operating pressure

Microwave transmission is almost free from the impact of changes in air density, so the radar level gauge can work in a vacuum or pressure state, the vacuum microwave transmission rate relative to air only 0029% change; but when the operating pressure to a certain Range, the pressure on the measurement of the error can not be ignored. Now launched the radar level meter products, the maximum allowable pressure of 64MPa.

(4) the impact of material properties on the measurement

Volatile gases and inert gases have no effect on radar surface measurements. However, the relative dielectric constant of the liquid medium, the turbulent state of the liquid, the size of the bubble, and so on, the attenuation of the microwave signal should be paid enough attention. When the relative dielectric constant of the medium is as small as a certain value, the effective reflection of the radar wave attenuation is too large, resulting in the level gauge can not work properly, so the measured medium relative dielectric constant must be greater than the product required a minimum. With the rich experience of product application and the continuous improvement of software processing technology, some products are almost independent of the relative permittivity, such as Sweden SAAB's RTG products have been very successful in liquefied petroleum gas (relative permittivity = 12 ~ 19), the measurement of the conductive medium (& gt; 10) is not limited thereto. In addition, the turbulence and bubble size of the liquid have a scattering and absorption effect on the microwave, resulting in attenuation of the microwave signal, which will affect the normal operation of the level gauge. In short, the radar level gauge for high viscosity media (such as asphalt), harmful media and liquid surface fluctuations in the medium of the tank, is undoubtedly a wise choice.

5, static pressure level measurement

Hydrostatic measurement (HTG) is a high-precision intelligent digital pressure transmitter with the advent of the advent of built-in microprocessor can be affected by the temperature and system bias to compensate, HTG system is a superiority of glass fiber reinforced plastic Tank for continuous quality measurement, practical HTG combination of the following:

  A simple HTG system consists of only one tank-mounted pressure transmitter P1 with a total mass equal to the measured pressure multiplied by the cross-sectional area of the tank.

  (P1-P2) / h, the liquid level can be calculated from the density and P1: L (1), the density of the pressure transmitter P1, = P1 / (P1 - P2) · h

  ? The tank top can add a pressure transmitter P3 to eliminate the effect of vapor pressure on P1 and P2. For high pressure tanks such as liquefied petroleum gas, the HTG method is not applicable because the static pressure changes caused by the liquid level are very small relative to the tank pressure, which can cause a large measurement error. One drawback of the HTG system is that its density measurement is derived within a limited range near the bottom of the tank. If the liquid level is higher than the P2 transmitter, the calculated value of the density can be obtained; but if the liquid level is lower than P2, the differential pressure will not be obtained (usually at 15m ~ 25m). In addition, some of the glass tank tank bottom density and the upper density is not exactly the same, this density stratification of the glass fiber reinforced plastic tank measurement will have a great impact. At atmospheric pressure tank, HTG system is better than 005% for quality measurement uncertainty.

    6, hybrid metering management system

Hybrid metering management system will modern glass fiber reinforced plastic tank level measurement technology and HTG together. For high-accuracy metering management, liquid level measurement is the most fundamental, the pressure measurement and level measurement can be combined to provide a full range of the real average density measurement, and then the quality of the calculation. The measured temperature is used to calculate the standard volume and density at the reference temperature. The servo or radar level gauge can communicate directly with the intelligent transmitter as a very good metering system that provides parameters such as liquid level, oil-water interface, volume, mass, average density, average temperature, and steam temperature.

Currently available radar or servo level gauge in most cases, can be easily extended to a hybrid metering management system.

In order to compare the different metering systems, the various parameters that affect the final error of each measurement system are analyzed.

The overall error of the oil metering is the result of the individual parameter error, and the main errors in the measurement of the FRP tank are shown below:

· Level measurement: installation of the instability;

· Temperature measurement: temperature stratification;

  Static pressure method (HTG): transmitter position, wind, tank pressure and so on.

The actual shape of the FRP is affected by many factors. If some of these effects are known and are repetitive, these can be compensated in the computer. In order to achieve the best measurement accuracy, a stable measurement platform is a prerequisite. The use of positioning tubes is a practical and accepted technology and has been successfully applied in many fiberglass tanks (with or without floating roofs). The installation of a positioning tube in the old tank renovation project is a measure of high accuracy.

For the radar level gauge, the positioning tube can ensure the mechanical stability of the instrument, in the high-pressure storage tank, it is recommended to use the reference needle with the positioning tube, respectively, in the positioning tube of the upper, middle and lower position to install three reference pins, In the tank can be used at any time to measure three needle height, in order to achieve the goal can not check the radar table.

Temperature is an easily overlooked measurement variable. Accurate average temperature measurement is necessary to achieve high accuracy of oil measurement. A single point of temperature measurement is meaningless when the temperature of the oil is stratified.

In the hydrostatic measurement system, the pressure transmitter P1 is to be mounted as low as possible, but must be above the highest water level and sediment. Studies have shown that wind power can produce a maximum error of 02% for a 10 m high tank. For a high operating pressure ball or canister, a special development range is required for larger transmitters since the static pressure change relative to the operation The pressure is very small.

At present, many varieties of glass steel tank measuring instruments can be applied to different types of glass steel cans, each measurement principle has its own advantages. So there is more complementarity between the various technologies rather than what is better. Modern radar and servo level meters have been greatly improved, they are almost no maintenance, if used properly, they can be run without trouble. Will be servo, radar, static pressure combination of use, can take advantage of a variety of measurement techniques.

Comparison of several measurement methods:

For stock management and trade handover, if the need to measure the quality, the static pressure method is preferred. But the volume measurement in the world will continue to play an important role. The combination of volume measurement and quality measurement will provide great advantages. The standardized fieldbus can play a decisive role in the direct communication of a dedicated FRP measurement system with other systems. However, the accuracy of the measurement can not be sacrificed because of the pursuit of standardization of the bus.

In the current fierce competition in the world market, enterprises to tap the potential synergies as a means of winning. Therefore, the tank area automation system, which includes the accurate metering management of FRP tank, continuous adjustment of oil, leak detection and so on, will increasingly show its importance.