1、STD-BSI BS-EN IS b817-ENGL 1777 1b2LibbS 0578807 ATi = BRITISH STANDARD BS EN IS0 6817 : 1997 Measurement of conductive liquid flow in closed conduits - Method using electromagnetic flowmeters BS 5792 : Part 1 : 1993 ?WVhU*, iwmting Amendment No. 1 The Euro- Standar EN IS0 6817 : 1995 has the status
2、 of a British standard ICs 17.120.10 NO COPYING WEOuT BSI PERMISSION EXCEPT AS PEBMiTED BY COPYBIGHT LAW STD-BSI BS EN IS0 bBL7-ENGL L7 LbZLi,bb 588A 73T BS EN IS0 6817: 1996 Issue 2, January 1997 mis British Standard. having been prepared under the direction of the industrial-process Measurement an
3、d Control Standards Policy Committee, WS publislied under the authority of tlic Standards hard and coines into effect on 15 March 1993 BSI 1997 The following in closed conduits (Identical) BS 5844 : 1980 Methods afmew.rm af.fluid.fnu: IS0 5168 : 1978 - estimatwn ofuncertaint b) eliminate, as far as
4、possible, spurious e.m.fs. These include common mode and quadrature signals; . c) provide means of compensating for supply volt- age and frequency variations where necessary; d) provide means of compensating or minimizing magnetic field strength variatioris in the primary device. This is important s
5、ince it directly affects repeatability of the voltage at the measurement electrodes. Compensation is achieved by the following means: a) a gain-compensated amplifier in which the gain is proportional to the supply frequency and in- versely proportional to the supply voltage; b) a system in which the
6、 output is proportional to the ratio of the flow signal and a reference signal derived from the field current. At a given flow- rate both signals may vary with supply voltage and frequency, but their ratio will remain con- stant: c) a system in which the field current is stabilized. For alternating
7、current (a.c.) energized systems with unregulated coil current, the secondary device measures the ratio of V/B (see clause 5). Voltages other than the flow signal (V) may be picked up by electrode leads. These voltages may be generated by the varying flux intersecting a loop composed of the electrod
8、e leads, the electrodes, and the liquid connecting the electrodes (transformer effect). Such a voltage will be approximately 90” out of phase with the flow signal. That portion which is 90” out of phase is called “quadrature”. The remainder is called the “in-phase” component. The “in-phase” componen
9、t is zeroed at no-flow during initial instal- lation, unless the flowmeters have a device which provides this function automatically. If the coil current is regulated, the magnetic field is considered to be constant and it is only necessary to measure the electrode signal. If the coil current is not
10、 regulated, then, in order to compensate for variations in the magnetic field, the secondary de- vice may use a reference signal obtained from the primary element. This reference signal may be de- rived from the supply voltage, the supply current, the flux density in the metal or the flux density in
11、 the air gap. In a pulsed d.c. system, under ideal or reference conditions, the peak-to-peak value of the electrode signals, (V, + V,), is proportional to the flow velocity in the pipeline and V, is also equal to V, see figure 4a), where Vp = positive voltage and V, = negative voltage. In a practica
12、l situation, if the zero or “no-flow” signal is offset in the positive direction by an amount Ve, then the positive signal is (V, + Ve) and the negative signal is (i,- Ve) Cfigure4b)l. Hence the overall value of the electrode signal is (V, + V,) and the offset zero is eliminated. The same applies if
13、 the offset is in the negative direction. The system thus eliminates zero errors automati- cally at all times and zero adjustment is not usually required, either at start-up/commissioning or at any time during subsequent operation. General guidance on the function and installation of secondary devic
14、es is presented in 7.2. 6.4 System output The system output can be one or more of the fol- lowing: a) analog direct current in accordance with IEC 381-1; b) analog direct voltage in accordance with IEC 381-2; c) a frequency output in the form of scaled or un- scaled pulses; d) digital. 6.5 Effect of
15、 the liquid conductivity If the electrical conductivity of the liquid is uniform in the measuring section of the meter, the electric field distribution is independent of the liquid con- ductivity and therefore the meter output is generally independent of the liquid conductivity. Minimum op- erationa
16、l conductivity requirements should be ob- tained from the manufacturers. The internal impedance of the primary device obvi- ously depends upon the liquid conductivity, and very large changes in this impedance may produce er- rors in the output signal. If the conductivity is not uniform throughout th
17、e meter, errors may also oc- cur. A heterogeneous fluid composed of small parti- cles uniformly distributed in a medium can be considered as a homogeneous liquid. Deposition of electrically conducting layers on the inside surface of the liner may also lead to errors. 6 STD-BSI BS EN IS0 b8L7-ENGL 19
18、97 Lb24bb7 0578820 237 m f . . . IS0 68171992(E) . . i . sp . . . . sc 6.6 Reynolds number effect In industrial electromagnetic flowmeters, the effect of Reynolds number is usually so small that for practical purposes it can be ignored. 1 . - I 6.7 Velocity profile effect Distortions in velocity pro
19、files may be caused by pipe fittings (bends, valves, reducers, etc.) placed upstream or downstream from the flowmeter; the resulting flow patterns may have an influence on the performance of the meter. In general, the user should comply with the manu- facturers recommendations for installation in or
20、der to minimize these effects. I Flow pattern effects are described in 7.1.2.1. 7 Installation design and practice 7.1 Primary devices 7.1.1 Sire Usually the bore of the primary device tube will be the same as that of the adjacent pipework. If, in this case, the mean axial velocity corresponding to
21、the maximum flow-rate is less than that recommended by the manufacturer, a primary device with a smaller bore should be used. A primary device with a bore smaller than that of the adjacent pipework may also be used for other reasons, e.g. to reduce cost or in the interests of rationalization. Inform
22、ation on the allowable tolerances for matching the pipe and meter tube bores is given in IS0 9104. Zero al Ideal condltlons Random noise No-flow Zero signal b) Practlcal condltlons Figure 4 - Principle of pulsed d.c. (bipolar) system 7 STD*BSI BS EN IS0 b817-ENGL 1977 3b24bb7 0578823 373 = IS0 681 7
23、:1992(E) 7.1.2 Layout not be within the manufacturers stated accuracy. If necessary, warning devices should be installed to There is no theoretical restriction on the attitude at preserve measurement integrity. Partially-filled pri- which a primary device may be mounted, provided mary device meters
24、are used, for example in sew- the pipe remains full at all times. Locations close to age applications, but these merit special electrical equipment which may interfere with the consideration outside the scope of this International flow measurement signal, or locations where cur- Standard. rents may
25、be induced in the primary device, should be avoided. 7.1.2.1 Effect of layout on velocity distribution Ideally, the magnetic field should be so arranged that the calibration factor is always the same, irre- spective of the flow pattern. Though this can be done in flowmeters with special electrode ar
26、rangements, it cannot be achieved if small electrodes are used. In practice, when a flow velocity profile which is significantly different from that in the original cali- bration is presented to the electrode plane, an electromagnetic flowmeter may exhibit a shift in calibration. The arrangement of
27、pipe fittings up- stream of the primary device is one of the factors which can contribute to the creation of a particular velocity profile. Precise data on the effects of flow disturbances is not always available, but for most electromagnetic flowmeters it is recommended that any source of flow dist
28、urbance, such as a bend, should be at least ten pipe diameters upstream of the electrode plane if the performance is not to be altered by more than 1 %. When the distance is unavoidably less than this, the manufacturers advice should be sought. Swirling flow can also alter the calibration factor bec
29、ause, although flow components perpendicular to the pipe axis cannot contribute to the flow-rate, they may contribute to the signal. Furthermore, the amount and distribution of swirl arising from various upstream pipe configurations, such as several bends in different planes, is difficult to predict
30、 from the geometry of the pipework. When swirling flow is suspected, it is good practice to insert a swirl reducer upstream of the primary device; some types of swirl reducers are described in IS0 7194. When the primary device is connected to the circuit by means of conical pieces, the effect on the
31、 cali- bration factor due to the irregular flow pattern may be either reduced or amplified according to the type of irregularity (swirl, asymmetry, etc.) and the de- sign of the connecting piece (convergence, diver- gence, value of total angle, etc.). 7.1.2.3 Electrode position Since any gas bubbles
32、 will rise and collect at the top of the pipe, or sediment may collect at the bottom of the pipe, the primary device should be mounted so that neither electrode is in these positions (see also 7.1.3.1). 7.1.2.4 Zero-checking provision In order to check the flowmeter zero, means should be provided to
33、 stop the flow through the primary device, leaving it filled with stationary liquid. However, in the case of a synchronous d.c. pulsed field supply with an automatically adjusting zero, this provision may not be necessary. 7.1.2.5 Multfphase flow through the primary device 7.1.2.5.1 Entrained solids
34、 For the measurement of liquids containing abrasive materials, vertical mounting is recommended to en- sure evenly distributed lining wear. Where there is a possibility that material may settle in the primary device, it should be mounted vertically or provision should be made to flush it through. A
35、ring to protect the leading edge of the magnetic flowmeter is sometimes used. This ring shall be de- signed to ensure streamlined flow. 7.1.2.5.2 Entrained gases An electromagnetic flowmeter measures total vol- ume flow. Entrained gases cause measurement in- accuracies in direct relation to the volu
36、me percentage of gas to liquid. Precautions should be taken to reduce this effect by increasing the liquid pressure, e.g. by locating the primary device on the high-pressure side of a restrictor such as a control valve, or by eliminating the entrained gas. 7.1.2.5.3 Phase slippage 7.1.2.2 Full pipe
37、requirement The primary device shall be mounted in such a position that it will be completely filled with the liq- uid being metered, otherwise the measurement will In the case of entrained solids and/or gases, relative average motion of the phases can affect the per- formance. This condition is par
38、ticularly likely if the tube is mounted vertically. In such situations the user should consult the manufacturer. 8 STD=BSI BS EN IS0 b817-ENGL 1997 1b24bb9 0578822 OUT IS0 681 7:1992(E) 7.1.3 Pipework connections 7.1.3.1 Design When designing the piping system, access for in- stalling and removing t
39、he primary device as well as access to the electrical connections should be pro- vided. Means should be provided for adjusting and aligning the adjacent pipework. Extra care should be taken during pipework construction to prevent excessive strain on the primary device, both during and after installa
40、tion. Every effort should be made to minimize piping loads and resulting strains at the primary device connecting flanges, particularly in plastic meters which are not intended to sustain piping loads. Per- missible values should be checked with the manu- fact u re r. 7.1.3.2 Pipework adjustment The
41、re should be means for adjusting the distance between pipework flanges used for mounting the flowmeter and for aligning the adjacent pipework. It is essential that the primary device is correctly aligned on the pipe axis when it is bolted into the pipework. Wafer types require special care. Flange b
42、olts should be tightened evenly and in moderation in order to avoid damage to the lining. The manufacturer should state the maximum per- missible torque. Care should be taken when handling the primary device; slings around the primary device, or lifting lugs, should be used. Lifting by any means tha
43、t could damage the liner, for example, hooks in the bore, shall not be used. Alr vent 7.1.3.3 Connecting pieces To minimize pressure loss and flow disturbances in cases where an undersized meter is installed, it is advisable to connect the primary device into the pipework by means of shallow tapered
44、 cone pieces (recommended maximum included angle 15“) (see figure5). In this case, the inlet and outlet straight pipe sections shall be the same size as the flowmeter (see 7.1.2). Eccentric taper pipes shall be used when the pipe- line is horizontal, to prevent air pockets from form- ing. 7.1.4 Elec
45、trical installation 7.1.4.1 General requirements The metered liquid and the primary device body should be at the same potential, preferably earth potential. In the case where cathodic protection is used to protect buried pipework, this precaution be- comes essential (see 7.1.4.3). The connection bet
46、ween the liquid and the primary device body may be made by contact with the adja- cent pipework; or, where insulated or non- conductive pipework is used, by conductive (earthing) rings or electrodes. Equipotential conductive links (usually copper braids) should be fitted across both flange joints (s
47、ee figure 6). The manufacturers instructions should be carefully followed for interconnections between the primary device and the secondary device. The power supply should be taken from a point that is as free as possible from transient voltages. Instructions in re- lation to electrical grounding of
48、 the flowmeter sys- tem shall be rigidly observed. Flow _c 5D rnh. I- Drain I L- Recommended maxlmum Included angle : 15 Figure 5 - Shallow taper entry and exit reducers 9 STD-BSI BS EN IS0 bBL7-ENGL 1997 = lbZLibb9 0578823 TLib IS0 681 7:1992(E) insaattng tlange Figure 6 - Cathodically protected pi
49、pelines: conductive links across flange joints 7.1.4.2 Power factor (a.c. systems only) As the primary device has coils to provide the mag- netic field. it is an inductive device and causes the field curretit to lag the supply voltage by an angle approach:-ig go”, thus giving a poor power factor. Typical values range from 0,l to 0,4 depending on the size of the primary device. To improve the power factor, correction capacitors may be connected in parallel with the supply and may be fitted externally or within the primary device enclosure, b
