Yokogawa EJX115A - Manual (Page 27)

<4. Installing Impulse Piping>
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IM 01C25A01-01E
(4) Temperature Difference Between Impulse
Piping (for differential pressure
transmitters)
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 (for differential pressure
transmitters)
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
(for differential pressure transmitters)
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 a 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.
4.2 Impulse Piping Connection
Examples
Figure 4.6, 4.7, and 4.8 shows examples of typical
impulse piping connections. Before connecting the
transmitter to the process, study the transmitter
installation location, the process piping layout, and
the characteristics of the process fluid (corrosiveness,
toxicity, flammability, etc.), in order to make appropriate
changes and additions to the connection configurations.
Note the following points when referring to these piping
examples.
• If the impulse line is long, bracing or supports should
be provided to prevent vibration.
• The impulse piping material used must be compatible
with the process pressure, temperature, and other
conditions.
• A variety of process pressure tap valves (main valves)
are available according to the type of connection
(flanged, screwed, welded), construction (globe, gate,
or ball valve), temperature and pressure. Select the
type of valve most appropriate for the application.
Tee
3-valve
manifold
Drain valve
Orifice
Drain plug
Tap valve
Union
or flange
Liguid Gas
Condensate pot
Steam
F0406.ai
Figure 4.6 Impulse Piping Connection Examples
(for differential pressure transmitters)
F0407.ai
Pipe (opened to atmosphere
at low pressure side)
Open Tank Closed Tank Tap valve
Union or flange
Vent plug
Tee
Drain valve
Drain plug
Figure 4.7 Impulse Piping Connection Examples
(EJ210)
In
sta
llin
g Im
pu
lse
P
ip
in
g
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