ISO 21903:2020 pdf free.Refrigerated hydrocarbon fluids一Dynamic measurement一Requirements and guidelines for the calibration and installation of flowmeters used for liquefied natural gas (LNG) and other refrigerated hydrocarbon fluids.
4.2 Coriolis flowmeter
The CMF is a device that measures mass flow rate as well as fluid density. Its fundamental operational principle is based on vibration mechanics and its interaction with the fluid dynamics. Because of its working principle, the flowmeter is capable of determining the density of the fluid when it matches a resonance frequency that corresponds to the fluid mass enclosed in the measuring tube’s finite volume.
The mass flow rate is directly linked to the Coriolis force that is present when the fluid moves at a certain velocity and in combination with the measuring tube’s angular motion. As this occurs, a secondary oscillation mode will take place, thus generating a phase shift in the measuring tube displacement. Such a phase shift is proportional to the mass flow rate, and is therefore used as a primary output signal to determine flow.
NOTE More information on the CMF is given in Annex A.
4.3 Ultrasonic flowmeter
The ultrasonic transit-time flowmeter is a sampling device that measures discrete path velocities using one or more pairs of transducers. Each pair of transducers is located at a known distance apart such that one is upstream of the other. The upstream and downstream transducers send and receive pulses of ultrasound alternately. The times of arrival are used in the calculation of average axial velocity. At any given instant, the difference between the apparent speed of sound in a moving liquid and the speed of sound in that same liquid at rest is directly proportional to the liquid’s instantaneous velocity. As a consequence, a measure of the average axial velocity of the liquid along a path can be obtained by transmitting an ultrasonic signal along the path in both directions and subsequently measuring the transit-time difference.
The volumetric flow rate of a liquid flowing in a completely filled closed conduit is defined as the average velocity of the liquid over a cross section multiplied by the area of the cross section. Thus, by measuring the average velocity of a liquid along one or more ultrasonic paths (i.e. lines, not the area) and combining the measurements with knowledge of the cross-sectional area and the velocity profile over the cross section, it is possible to obtain an estimate of the volumetric flow rate of the liquid in the conduit.
NOTE More information on the ultrasonic flowmeter is given in Annex B.
Both CMF and USM applications require a stable and consistent single-phase flowing medium in order to correctly measure the flow. It is particularly important to consider this requirement when loading at cryogenic temperatures as potentially large temperature variations and heat gain increase the likelihood of a two-phase flow. This will at least be the case if the meter/pipes connecting the meter are at ambient temperature prior to loading.
Several mitigating actions may be employed to increase the likelihood of maintaining a cryogenic single-phase liquid flow. One effective way to accomplish this is by keeping the meter cooled down, not only during loading operations, but at all times, e.g. by using a proper circulating loop. A disadvantage is the increased cost of cooling the cryogenic medium as the circulation will increase the heat gain. In general, a low-flow velocity, large pipe diameter, and poorly insulated meter and flow lines should be avoided as this will add to the probability of boiling and two-phase flow.
Maintaining the temperature of the meter at cryogenic conditions will minimize stresses on the pipe material, which is desirable.
Depending on the loading product, it is common practice to cool down the meter and pipes from ambient to cryogenic conditions prior to transfer. For LNG application, gas and liquid nitrogen are often used for this purpose. Starting from an ambient temperature, cold nitrogen gas can be introduced to gradually lower the temperature and avoid stress from temperature shock. Small amounts of liquid nitrogen are then injected to boil off and further cool down the system.ISO 21903 pdf download.