Whenever the pressure within a pipeline instantaneously falls
below. the equilibrium vapor pressure of the fluid, a portion of
the fluid, vaporizes and forms bubbles i n the pipeline. This is
termed cavitation. Cavitation is eliminated by maintaining
adequate back pressure on the flowmeter. A downstream valve
that provides the necessary back pressure is one means for
preventing cavitation in the metering run. Control valves should
be located downstream, if possible. Some installations may also
make use of a vapor eliminator upstream of the flowmeter.
The minimum required back pressure may be estimated using
the following equation:
Min. Back Pressure = 1.3 X Vapor Pressure + 2X Pressure Drop
In considering the interconnections between the flowmeter and
the flow measurement system some attention must be given to
anticipated noise sources and to the coupling of these noise
sources to the interconnecting wiring.
Noise signals may be coupled inductively or capacitively into
the wiring between the flowmeter and the electronic measuring
systems. In general, utilizing a shielded, twisted pair for the
interconnection greatly reduces this coupling. The shield should
be grounded on one end of the cable only. In general, grounding
only on the electronic measuring system is best.
However, even with proper interconnecting cabling, cross talk
with other signal lines or power lines may still occur and should
be avoided. Physical isolation in the manner in which the wiring
is run reduces the chance of potential problems.
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