7.1.8.1 Principle
Electromagnetic flowmeters function by measuring the voltage induced in a liquid flowing through a magnetic field. a flowmeter is installed in the process pipe and the signals generated are processed by an integral or remote transducer.
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7.1.8.2 Installation arrangement 7.1.8.2.1 Location
the electromagnetic flowmeter averages the velocity profile across the pipe so that it is not particularly sensitive to upstream or downstream piping arrangements. however, for accuracy, the manufacturer’s instructions should be followed.
the flowmeter unit may be installed in pipework in any position (vertical, horizontal or at any angle) but it should remain full of liquid at all times to ensure accurate measurement. When mounted horizontally the electrode axis should not be in a vertical plane. the electromagnetic flowmeter should normally be mounted where the instrument is not subjected to vibration nor exposed to ambient conditions outside its specified limits.
7.1.8.2.2 Pipework support
normally, magnetic flowmeters up to 300 mm bore require no extra support than that provided for a similar length of pipe. For larger sizes the manufacturer’s recommendation for support structures should be followed.
7.1.8.2.3 Electrical requirement
For predictable measurement it is essential that the process liquid be at earth potential. this may be achieved by earthing metallic pipework, or by the use of metallic earthing rings, probes or gaskets where the process pipework or its lining is non‑conductive.
Earthing arrangements should follow site practice as far as possible.
nevertheless, it is essential that the manufacturer’s instructions for earthing are followed.
Electrical connections between the flowmeter and the transducer should not exceed the maximum distance permitted by the manufacturer.
7.1.8.2.4 Cathodic protection
if the detector head is installed in a system that is cathodically protected or where electrolysis is used in the process, special precautions should be taken to ensure that:
current at supply frequency does not flow through the liquid in a)
the detector head;
any current, at supply frequency, flowing through the body of the b)
detector head does not exceed 10 a r.m.s.
these precautions are intended to limit the magnitude of any spurious magnetic fields.
in systems where a metal pipeline without an insulating liner is used, the liquid in the system can be placed at earth potential by bonding the detector head to the adjacent pipeline as shown in Figure 12a).
this arrangement should only be used for systems where it is known that any current flowing through the body of the detector head does not exceed 10 a r.m.s. if the current flowing through the body of the detector head exceeds 10 a r.m.s., the detector head should be bonded to the adjacent pipeline as shown in Figure 12b).
When bonding the detector head flanges, flange bolts should not be relied upon as electrical connectors. troublesome potential differences
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can occur in systems where, for example, a cathodically protected pipeline joins a grid system, not quite at earth potential, or joins an earthed pumping station. in these cases it might be necessary to insert short lengths of unlined pipework bonded to the detector head as shown in Figure 12c). the length of these sections will depend upon the magnitude and ripple content of the cathodic voltage and upon the nominal bore of the pipeline.
Figure 12 Typical cathodic protection
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2
3
2 1
4
a) detector head bonding b) detector head bonding using insulating flange
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4 4
5 2 5
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c) detector head bonding using unlined pipe sections
Key Bond 1
director head 2
Current 3
insulating flange 4
Unlined pipe 5
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7.1.8.3 Installation 7.1.8.3.1 Storage
the flowmeter and transducer should be stored in a clean dry area until required for installation, and protective covers should not be removed until necessary to permit erection of the equipment. the flowmeter should be stored in its packing or in a cradle; it should never be stood on its end flanges. the flowmeter is a precision‑built instrument and should be treated as such.
7.1.8.3.2 Handling
although the flowmeter appears to be a robust pipe spool, care should be taken in handling it to avoid damage, particularly to the liner.
Lifting should always be accomplished with slings. a bar threaded through the flowmeter should never be used for lifting. if the liner is damaged during handling, it should be repaired or replaced using an approved procedure before the flowmeter is installed.
7.1.8.3.3 Cleaning and purging
the flowmeter should be installed only after the process pipework has been cleaned and flushed. the pipework should be pressure tested after the flowmeter has been installed.
7.1.8.3.4 Mounting
Care should be exercised during installation to avoid damage to the flowmeter and its lining. Where the liner is brought out over the flange face, the liner should not be forced between flange faces but a suitable gasket should be installed between the pipe and the flowmeter
flanges. Such a gasket should not protrude into the bore of the pipe.
during installation it is essential to avoid undue stress on the flowmeter. it is desirable to bolt the flowmeter to its upstream and downstream pipework before completing pipework assembly at a flange remote from the flowmeter. Care should be taken to align the flowmeter with its adjacent flanges, and the fixing bolts should be tightened evenly.
7.1.8.3.5 Electrical connections
Special low capacitance cable, in accordance with manufacturer’s instructions, should be used when the flowmeter does not incorporate an integral transducer. Such connecting cable should not be installed close to power cables nor share a common conduit with power supply wiring.
7.1.8.3.6 Earthing
the instrument manufacturer’s recommendations for earthing should be followed during installation.