Yokogawa EJX930A - Manual (Page 36)

<6. Installing Impulse Piping>
6-3
IM 01C25R01-01E
(2) Position of Process Pressure Taps and
Transmitter
If condensate (or gas) accumulates in the impulse
piping, it should be removed periodically by
opening the drain (or vent) plugs. However, this will
generate a transient disturbance in the pressure
measurement, and therefore it is necessary to
position the taps and route the impulse piping so
that any extraneous liquid or gas generated in the
leadlines returns naturally to the process piping.
• If the process fluid is a gas, then as a rule the
transmitter must be located higher than the
process pressure taps.
• If the process fluid is a liquid or steam, then as a
rule the transmitter must be located lower than
the process pressure taps.
(3) Impulse Piping Slope
The impulse piping must be routed with only an
upward or downward slope. Even for horizontal
routing, the impulse piping should have a slope of
at least 1/10 to prevent condensate (or gases) from
accumulating in the pipes.
(4) Temperature Difference Between Impulse
Lines
If there is a temperature difference between the
high and low impulse lines, the density difference
of the fluids in the two lines will cause an error in
the measurement pressure. When measuring flow,
impulse lines must be routed together so that there
is no temperature difference between them.
(5) Condensate Pots for Steam Flow
Measurement
If the liquid in the impulse piping repeatedly
condenses or vaporizes as a result of changes
in the ambient or process temperature, this will
cause a difference in the fluid head between the
high pressure and low pressure sides. To prevent
measurement errors due to these head differences,
condensate pots are used when measuring steam
flow.
(6) Preventing Wind Speed Effects in Very Low
Differential Pressure Measurement
IMPORTANT
When using a differential pressure transmitter
to measure very low pressures (draft pressure),
the low pressure connection port is left open
to atmospheric pressure (the reference
pressure). Any wind around the differential
pressure transmitter will therefore cause errors
in the measurement. To prevent this, it will be
necessary either to enclose the transmitter in
a box, or to connect an impulse line to the low
pressure side and insert its end into a wind-
excluding pot (cylindrical with a base plate).
(7) Preventing Freezing
If there is any risk that the process fluid in the
impulse piping or transmitter could freeze, use a
steam jacket or heater to maintain the temperature
of the fluid.
NOTE
After completing the connections, close the
valves on the process pressure taps (main
valves), the valves at the transmitter (stop
valves), and the impulse piping drain valves,
so that condensate, sediment, dust and other
extraneous material cannot enter the impulse
piping.