The problem that should be paid attention to in the practical application of compressed air flowmeter

1. Problems The proposed compressed air flowmeter has been widely used in petroleum, chemical, light industry and power systems for nearly 10 years due to its no moving parts and easy installation and disassembly, as well as many other advantages. Vortex street heat."
In the past few years when our hospital designed its own thermal power stations for several refineries in Fushun, we examined the use of compressed air flow meters in Liaoning. The use of manufacturers reflect that when the temperature and pressure of the measured medium are within the technical parameters of the selected instrument, some of the instruments can be used, and some of the instruments generate their own oscillations, while other instruments have large errors even when the operating conditions are stable. Pressure fluctuations cause the instrument to become unusable.
From the perspective of the manufacturer, the production methods are not standardized, such as the use of parts assembly, self-mapping after physical mapping, etc., and the equipment is simple, and sampling method is used to calibrate the flow coefficient K′, which directly affects the accuracy of K′. Sex*.
According to the author's understanding, although some instrument manufacturers in Liaoning Province have established fluidic test devices, due to the large power consumption of power equipment and the high operating cost, it is rarely used unfavorably, and the flow coefficient K′ of the compressed air flowmeter is calibrated.
2. Problems that should be noticed In order to enable the compressed air flowmeter to accurately and reliably detect the fluid flow in the application, the author believes that high quality products should be selected first, and the following matters should be paid attention to.
2. 1 Elimination of interference, correct selection and installation 1 The fluctuations of the temperature and pressure of the fluid should be compensated to overcome the interference caused by the change.
2 The Reynolds number of the medium in the pipeline should be greater than 2×104, see the figure below.

3 The fluid under test should be of the same nature as the fluid on which the caliber was selected. Otherwise, it should be readjusted according to the procedure for selecting the caliber.
4 Install the compressed air flowmeter away from the power equipment and frequently changing valves. If there is vibration in the pipeline, consider installing a fixed bracket on the front and rear pipelines of the compressed air flowmeter to reduce vibration.
5 If installing temperature measurement and other components for pressure measurement on the pipeline, it should be at the downstream (5~8)D of the compressed air flowmeter.
6 pairs of liquid medium and low-boiling liquid such as liquid ammonia, should prevent the precipitation of gas or liquid gasification, to prevent the generation of cavities. The gas eliminator can be installed on the pipeline. For the low-boiling-point fluid, the compressed air flowmeter should be installed on the pipeline at the outlet of the tank as far as possible, and protective measures (such as thermal insulation and heat insulation) should be taken.
The judgment formula for whether the fluid can generate holes is:

Where p - the absolute pressure of the fluid in the pipeline, MPa;
Δp - differential pressure before and after the fluid is generated, M Pa;
P0 - saturation vapor pressure of the fluid at operating temperature, MPa;
d - density of fluid under working conditions, kg/m3;
V—Flow rate of the fluid, m/s.
When calculating Δp, the actual maximum flow rate should be calculated

Where Vmax - the actual maximum flow rate of the fluid, m / s;
Mmax - the actual maximum flow of fluid, kg / h;
D—— Internal diameter of the compressed air flowmeter housing, mm.
7 Compressed air flowmeter signal transmission wires should use metal shielded wires, and have good grounding measures.
8 The load of the output signal of the compressed air flowmeter should not exceed the specified value.
9 Compressed air flow meters should be installed away from heat sources and strong magnetic fields.
10 In order to accurately detect the fluid flow, the flow coefficient K′ derived from the actual calibration must be used, and periodic calibration should be performed to eliminate the error caused by drift.
For other general requirements, there are descriptions in the manufacturer's manual and no further explanation is given.
2. 2 Compressed air flow meter compensation Compressed air flow meter detects the fluid flow rate as the volume flow in working condition.
When using a compressed air flow meter to measure the flow of gas or steam, it is customary to express it as a standard volume flow or mass flow. At this time, changes in fluid properties and fluctuations in operating conditions require compensation. Otherwise, large errors will occur. With the development of instrument technology, smart meters and computers are widely used, and parameters can be calculated according to a given mathematical model to complete its automatic compensation function.
2.2.1 Calculation of fluctuations introduced by fluctuations in fluid operating conditions Corrections due to the fact that gas is expressed in standard state volumetric flow or expressed in mass flow, the unit of measurement for steam is expressed in mass flow, and the compressed air flowmeter directly detects the fluid in operation. The volumetric flow rate under the standard should be converted to the standard state volume flow or mass flow rate according to the density. Because the conversion factor contains the fluid density factor, fluctuations in fluid operating conditions introduce calculation errors. Tables 1 to 3 give the manual calculation correction formula.

Note: The value of a is related to the unit of p. When p takes the unit of Pa, the value of a is removed by 10 - 3.

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