This lecture deals with misunderstandings in the piping systems of common flow sensors or transmitters that are common in practice, such as the generation of liquid cavities and mixed gases, the formation of condensates in gases, and the misunderstanding of normal operation due to other equipment (such as pumps). Make some explanations for the failure of the flow meter, and put forward some preventive measures.
1 Bad Installation Before installing the flow meter, it is necessary to understand the standards, procedures, and installation requirements of the manufacturer in the instruction manual. Then select the ideal installation location and determine the degree of influence of upstream and downstream pipe fittings on the measurement. Are there enough upstream and downstream? Straight section lengths, etc., to meet the requirements, to avoid additional system errors due to poor installation.
There are two types of bad installations: the first one is due to careless installation and should be avoided; the second is due to the objective conditions of the pipeline system (such as the lack of length of the straight pipe), which cannot be achieved due to the lack of specified requirements. If it is not possible to make a corresponding improvement, it is necessary to assess the degree of influence on the measured value.
1.1 Inadequate installation of the first type of unsatisfactory installation operations, common are: 1 counter-installation of the inlet surface of the orifice plate; 2 the inner diameter of the sealing gasket between the instrument and the pipe is smaller than the pipe diameter and the inner diameter of the meter; 3 the gasket is not Centering, partially covering the flow area and disrupting the normal flow velocity distribution; 4 The instrument is in the wrong flow direction; 5 Installed in locations with poor velocity distribution profiles and vortices; 6 Differential pressure instrumentation pressure piping slope is not correct, There are unwanted phases (eg gas when measuring liquids); 7 instruments sensitive to vibration interference mounted on vibrating pipelines; 8 instruments or transmission lines of electric signals placed in strong electromagnetic fields; 9 lack of necessary protective fittings ,and many more. These problems are well-known and should be avoided. However, because the instrument installation of the new construction project is currently undertaken by the installation company, many units lack specialized trained instrumentation personnel and are often served by plumbers. It is not uncommon to ignore the installation requirements of the instrument.
The sealing gasket aperture DN should be slightly larger than the instrument's inner diameter Dm. If DN is less than Dm, beam flow phenomenon will occur, which will affect the measurement values ​​of throttling differential pressure, turbine, vortex, ultrasonic, target and other instruments. For example, the vortex street-type meter stipulates that the pipe inner diameter Dp should not be too small and should meet 0.98Dm≤Dp≤1.05Dm, that is, the diameter of the sealing gasket should not be less than 0.98Dm. The seal gasket is installed eccentrically to cover part of the flow area, and the distribution of the distortion speed leads to serious asymmetry. Since the asymmetrical flow occurs at the inlet of the flow sensor, that is, the length of the upstream straight pipe segment is zero, the measured values ​​of throttling differential pressure, turbine, vortex, ultrasonic, electromagnetic, and target instruments are measured. Great influence. Sealing linings with too small bore diameters or eccentric installations, although they have no or little effect on the flow values ​​of meters such as volumetric, float, and Coriolis mass, can add additional pressure loss.
1.2 The second type of poor installation In order for the flow meter used in the field to obtain the same accuracy as the real flow calibration, it must satisfy the specified flow conditions. For example, most flow meters require that the upstream beam flow be fully developed, ensuring that disturbances in downstream piping components do not affect the measured values, ie, require a certain length of upstream and downstream straight pipe sections. Site installation conditions are often not easy to meet, but try to avoid the following sources of disturbance.
1.2.1 Upstream Perturbation Sources Upstream perturbation sources include elbows, different diameter pipes, branch pipes, and valves. The most encountered are various elbows and elbow assemblies, such as flat double elbows and space double elbows. Due to the effect of centrifugal force, the fluid in the elbow has a diffusion effect on the outer wall, a contraction effect on the inner wall, and a secondary flow in the transverse flow. Distortion and vorticity of the velocity distribution occur downstream of the elbow. The three-dimensional pipe arrangement shown in Figure 1 is like a double-spaced elbow and can also create serious vortices.
Various types of flow meters have different sensitivity to upstream flow disturbances. Figure 2 shows the effect of R/D on the coefficient of the orifice plate under different upstream straight pipe sections with different bending radius R and pipe diameter D ratio (R/D). The smaller the R/D is, the larger the effect is, but the right-angle elbow (R/D=O) has less influence than the bending elbow. Figure 3 shows the effect of swirl flow at different swirl angles on the coefficient of the orifice plate.
In various types of flow meters, a large number of experimental studies have been conducted on the length requirements of throttling differential pressure straight pipe sections. The results of maturation of typical baffle parts have been specified in the International Standard (ISO). Other instruments have not reached such a mature level so far. Whether it is the data provided by the standard specification or the manufacturer's instruction manual, it is not as good as the throttling differential pressure type, and sometimes it can only serve as a reference. Hayward thinks that other types of instruments can refer to the shortest upstream straight pipe length specified by a standard orifice plate with a hole area ratio of 0.5 when there is no reliable data, as a general reference, but this is only a rough guide because some instruments have to Longer straight sections, such as turbine and vortex street meters that are particularly sensitive to vortices, encounter vortex flow [1].
1.2.2 Disturbance on the downstream side It is usually a illusion to imagine that the flow state after the fluid flows out of the instrument does not affect the instrument. Deviations caused by elbows or valves etc. will propagate the distances that affect the length of several times the diameter of the pipe. If the gauge is too close to them, it will be affected. In most cases, the straight pipe downstream of the 5 pipe diameter is sufficient, and some special cases may be slightly longer. However, it can be assumed that the 10 pipe length can reliably cope with any downstream disturbance source.
1.3 General rules for avoiding installation of additional errors Volumetric instruments can accommodate severe upstream disturbances. Floater-type and Coriolis mass flowmeters are insensitive to upstream disturbances, and other flowmeters are affected to varying degrees by upstream disturbances. Hayward believes that the following general rules can be followed in addition to the requirements of the standard specification or manufacturer's requirements.
a. Avoid the most harmful vortex. As far as possible, the typical vortex generation source shown in FIG. 1 does not exist within a distance of 100D upstream of the meter.
b. There must be "adequate" lengths of upstream and downstream straight sections. How much is the "enough" length? One argument is that as long as the above general rule a can be met, the provisions of the throttling differential pressure gauge standard specification are considered "sufficient" and applicable to most other flow meters that are sensitive even to flow disturbances; the other way is the upstream The lengths of the 50D and downstream 10D straight sections are already very safe, and some types of meters are far below this requirement.
c. If the length of the straight section cannot be satisfied but the measurement accuracy is to be ensured, one of the following two work-arounds can be used.
1 Calibrate under field installation conditions, or check accessories such as elbows in the same conditions as the installation conditions on the laboratory flow standard equipment.
2 Install the flow regulator described in the next section above the meter.
2 flow regulator
2.1 Types of Flow Regulators The length of the straight tube to eliminate vortices is very long (for example, the length required to eliminate the initial rotation angle of 10° is more than 150D). The main function of the flow regulator is to eliminate the vortex, but it can also improve the speed distribution distortion and restore to Accepted reference velocity distribution geometry.
Figure 4 shows three major types of flow regulators: class a is dominated by elimination of eddies; class b is the elimination of eddies and improvement of moderately-speed-distribution distortions; class c is the elimination of eddies and the improvement of severe velocity distribution distortions. Installing a flow regulator will increase resistance, and a small length of straight pipe will be installed between the downstream and the meter. The pressure loss of the flow regulator differs from the degree of distortion of the elimination speed distribution, with the smallest category a and the largest category c. The literature [2] lists the pressure loss factors for calculating the pressure loss of various types of flow regulators and the specific dimensions of Zanke and Mitsubishi-type openings.
2.2 Flow Regulator Installation
The hazards caused by careless installation of the flow regulator sometimes exceed the flow improvement, and the following basic guidelines should be followed when installing the flow regulator.
a. Multi-well plate flow conditioners similar to Mitsubishi type work well even if they are very close to sources of flow disturbances. Therefore, it can be directly mounted on the flange of the outlet of the disturbance source such as elbow, valve, etc.
b. All other types of flow regulators must be installed at least 3D downstream from the source of the disturbance, otherwise they are easily attenuated by the newly created disturbances.
c. There is still some distortion in the speed distribution out of the flow regulator, so there should be a straight pipe between the downstream and the meter to eliminate the distortion. The ideal length is over 20D, but it should not be lower than 10D; Together with the real flow check, 5D is enough.
3 Failure of cavitation formation When measuring the flow of liquid, the generation of air pockets inside the instrument will cause erroneous measurements. The reason for the cavitation is that the internal pressure of the instrument is lower than the liquid vapor pressure, and the working pressure should be increased or the instrument should be equipped with a back pressure valve to increase the internal pressure of the instrument, and should not be lower than the specified value. The position of the lowest internal pressure point of the instrument varies depending on the measuring principle and structure of the instrument. For example, the orifice plate is adjacent to the downstream side, the turbine flowmeter is at the turbine, and the vortex flowmeter is at the vortex occurrence body. Some instruments have lower internal minimum pressure than back pressure.
What should be the minimum back pressure required to prevent cavitation in a flow meter? The test pieces for throttling differential pressure meters should not be lower than their initial cavitation number, see equation (1) and Table 1; other meters should calculate the minimum back pressure according to formula (2), such as turbine flowmeter and vortex flow The two constants A and B in the formula are 3 and 1.5, respectively, which are more conservative values.
Cavitation is characterized by the dimensionless cavitation number λ:
λ=(p-pv)/(1-2Ï2) (1)
Where: p is hydrostatic pressure, Pa; pv is fluid vapor pressure, Pa; p is fluid density, kg/m3; average flow rate, m/s.
Number of initial cavities Orifice plate 3.0 Venturi tube 0.33 Venturi nozzle 0.55 Dove tube 1.0 Nozzle 1.8
The minimum back pressure (p3) min to prevent the flowmeter from generating air pockets is
(p3)min=A(p1-p2)+BPv (2)
Where: A, B is a constant, determined by the experiment; p1 is the pressure before entering the flow meter, Pa; p2 is the lowest pressure in the flow meter, Pa; pv is the vapor pressure of the fluid, Pa.
Another source that is often overlooked is the cavitation created by the upstream pipeline components (such as valves), especially petroleum products. The air pockets created by fuels and solvents form cloud-like bubbles that maintain a relatively long distance downstream. Can easily cause instrument measurement errors. Flow control valves are most prone to cavitation when they are close; some three-way valves and four-way valves are also prone to strong cavitation when changing the direction of flow.
4 Inclusion of gas in liquids The above section mentioned that air pockets can cause bubbles in the liquid and cause errors. In addition, there are several other ways to enter air or generate gas.
4.1 Incomplete filling of the pipeline with residual air The pipeline system is drained when it is overhauled and refilled when enabled. Sometimes completely full and very difficult, because all the high points of the pipeline (such as the top of the inverted U-tube elbow) and dead corners tend to stay in the air mass, and in the future there will be sudden fluctuations in pressure or flow. The air mass will rupture and take away some of the gas. This is one of the reasons for the inaccurate measurement of the flow meter at the initial stage of pipeline operation. A small volume of bubbles can introduce considerable errors. Experiments have shown that a liquid with 1% volume of bubbles enters the turbine meter and produces a +5% error. Excessive air bubbles can also block the differential pressure meter's pressure tube, making measurement impossible.
Well-designed pipelines have only a few high points, and exhaust valves are placed at these high points to manually discharge retained gas. If there are many high-level pipelines, the exhaust gas brings about a large amount of work, instead of using an automatic exhaust valve or installing a gas separator upstream of the flow meter to replace the entrained gas. Many domestic manufacturers have gas separator styling products.
4.2 Seal Leakage Air viscosity is much lower than that of liquid. Sealing at a certain point can keep the liquid from leaking, but it does not necessarily guarantee that the gas will not leak. The connection of the negative pressure piping is slightly inadvertently sealed and it is easy to suck air into the pipe. It is well known that the suction of air is sucked by the poor seal of the negative pressure tube (such as the suction side of the pump). However, if the tube pressure is slightly higher than the atmospheric pressure, the pulsating flow will cause the instantaneous tube pressure to be lower than the atmospheric pressure and will also suck in air. Always check the junctions of the negative pressure tubes. If any signs of damage are found, they should be replaced immediately.
4.3 Vortex into air When the height of the storage vessel drops to a few times the inlet diameter (depending on the suction flow rate) from the inlet end of the pipe, a vortex is created, which traps air at the liquid-air interface into the liquid and enters the pipe. This may be the reason that the most common way to enter the air into the pipeline and the maximum amount of air intake.
4.4 Cooling Shrinking Gases This is a relatively concealed way to mix air with liquids. After the liquid-filled pipeline stops running, it gradually cools. Due to the different thermal expansion coefficients, the liquid shrinks much larger than the pipeline. The vacuum shrinkage space is formed in the pipeline. The dissolved air in the liquid is separated out to form bubbles and accumulates at the high point of the pipeline. These gases. This factor should be taken into account if there is a positive error in the flow measurement at the start of operation.
4.5 Application of gas separators Gas separators, also known as gas eliminators, used to be mostly used with volumetric instruments. When the liquid enters the gas from the above several ways and affects the measured value, gas separators are sometimes required, especially for petroleum products with intermittent operation and high value of measurement, etc., as well as places with strict trade accounting requirements. In the continuous operation system of the process industry, liquid inclusions do not frequently occur. In the past, generally, gas separators were rarely installed before the flow meters. However, there is a demand for improvement, and there is a tendency to install gas separators before the turbine meters.
The gas discharged by the separator is air, or saturated liquid vapor and mixed droplets. If it is a flammable liquid, it should be properly disposed of to ensure safety.
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