So-called extremely low temperature fluid, in the industry generally refers to under the atmospheric pressure boiling point for fluid under - 50 ℃.
Representative can have very low temperature LNG (
Liquefied natural gas)
, liquefied ethylene, liquid hydrogen, liquid chlorine, and the low temperature gas, L PG (
Liquefied petroleum gas)
Do not include in the extremely low temperature fluid.
In the extremely low temperature fluid flow measurement methods, especially want to know is in the process of industrial use of these fluid, compared with normal temperature or high temperature, it have what different?
What kind of problems likely to occur?
* to enumerate, therefore, extremely low temperature fluid and the characteristics of the process.
Easy to cause the change of state of extremely low temperature fluid matter has shown in table 1 and table 2 physical constants.
The fluid is usually stored in saturated state in adiabatic storage containers.
Pump booster, send out, starting process, and repeated compression and decompression.
During the process, the saturated state of extremely low temperature fluid into supercooled state for a while, a moment into boiling state, state change easily.
Therefore, the piping design and flow meter
design needs to take measures.
Table 1 physical constants of low boiling point substances in table 2 composition of liquefied natural gas (LNG) material constants (
Have a change of state of liquefied natural gas (LNG) by enrichment is composed with the material are shown in table 2 and the mixture.
Because from the outside to the storage container or tube heating, so begin with methane evaporation, occurs is called condensed composition changes as shown in figure 1.
The composition of liquefied natural gas (LNG) substances of methane, ethane, propane, butane liquid density is shown in figure 2.
Because of enrichment, liquefied natural gas (LNG) liquid density is bigger, so the measurement of flow rate, density must be corrected.
Have restrictions on materials of metal materials in a very low temperature yield point, tensile strength, elongation, throttling, and room temperature mechanical properties such as impact test values are quite different, in a very low temperature, the tensile strength of the metal material is rising, but reduced ductility, impact value also declined significantly with the material.
Therefore, the instruments used in a very low temperature of the material, should be have good resistance to destroy the toughness of the material, don't cause the brittle fracture, the material is called a material at low temperature.
But now, use the general materials are: stainless steel, aluminum alloy, 35% 90% nickel steel, nickel steel (
Nickel and iron)
Table 3 shows the mechanical properties of materials at low temperature.
Figure 3 also shows the migration of charpy temperature curve of stainless steel.
Figure 1 liquefied natural gas enrichment figure 2 liquefied natural gas (LNG) composition of liquid density figure 3 stainless steel charpy transfer temperature transfer curve of mechanical properties of materials at low temperature (table 3
To consider the stress due to cold, clearance after the change of the design.
The physical properties of the low temperature materials as shown in table 4.
Use these materials when designing measuring instrument, must fully consider the influence of the cold.
Need precooling if makes extremely low temperature fluid dramatically inflows in warm condition of piping and instrument of local, cannot control flange leak out from the extremely low temperature fluid, in this way, piping and equipment will produce abnormal stress, there is a danger of destruction.
Therefore, must carry out precooling, cooling to the temperature set by the state and then began to use.
If they don't eliminate air, oil, so, in a very low temperature curing water, oil content, carbon dioxide will increase the friction of moving parts, make filter blockage, effective barrier.
The physical properties of materials at low temperature (table 4
Cause convection in gas-liquid interface within the gas-liquid interface of convection, there will be a faint pulsing phenomenon, fault occurs due to smoke.
Currently use many different kinds of flow measurement instrument of the extremely low temperature fluid flow measurement instruments have orifice plate, vortex flowmeter
s, the three types of turbine flow meter.
However, a variety of flow meter, according to its use, the measuring range, also developed a number of different structures.
Hope when choosing instrument, therefore, must be in fully grasp the features of various kinds of instrument on the basis of considering the price.
In the process of maintenance performance, precision requirements, the characteristics of the measurement sites.
A low temperature fluid exist in two kinds of liquid and gas, but almost all of the problems occurred in the process of fluid flow measurement.
Here it illustrates main cryogenic liquid flow measurement.
Orifice plate with the method of the orifice plate to measure fluid is according to the German industrial standards, the American society of mechanical engineers, Japanese industrial standard standard.
Due to its use effect is good, so, extremely low temperature fluid in the standard definition of property value range, if there is no exception when the measured physical value change, can complete the measurement.
In fact in the extremely low temperature fluid flow measurement, orifice plate application, reliability is high.
However, in the instrument design, some important matters need to pay attention to, the following will be described.
1 material and structure.
SUS316L material used.
Welding nozzle using low carbon welding rods to be safe.
Extremely low temperature in the saturated state of fluid flow, for the easy to cause the motion of the differential pressure is measured after smoothing, hope to take pressure type Angle with access.
In order to reduce the leakage caused by cold and orifice turbidity, orifice plate and ring room into an organic whole.
For measuring high pressure piping or pump flow orifice, difficult to clamping method to control the leakage of installation place turbidity, sometimes even need to stop running.
, therefore, to enhance the orifice plate installed in accordance with the piping rank position of the flange and the hierarchy of screws, this approach, with a larger amount of interference than usual, in case of leakage.
At the same time, to insert the good thermal conductivity metal screw space, make the pipe cold does not produce time lag.
According to the situation, also has the choice of screw cold shrink rate than orifice.
In large diameter pipe installed on the orifice plate, with the chip, sometimes can't control the leakage.
To use with flange welding hole orifice or pull.
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