Yokogawa EJA110A - Manual (Page 28)

IM 01C21B01-01E 5-3
5. INSTALLING IMPULSE PIPING
(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
Piping
If there is a temperature difference between the high
and low impulse piping, the density difference of the
fluids in the two lines will cause an error in the
measurement pressure. When measuring flow, impulse
piping must be routed together so that there is no
temperature difference between them.
(5) Condensate Pots for Steam Flow Measure-
ment
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 neces-
sary either to enclose the transmitter in a box,
or to connect a impulse piping 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.