Yokogawa EJX115A - Manual (Page 33)

<7. Operation>
7-3
IM 01C25K01-01E
IMPORTANT
• Remove the BT200 from the terminal box,
and confirm that none of the terminal screws
are loose.
• Close the terminal box cover and the
amplifier cover. Screw each cover in tightly
until it will not turn further.
• There are two covers that must be locked
on the ATEX Flameproof type transmitters.
An Allen head bolts (shrouding bolts) are
provided under edge of the each cover for
locking. When a shrouding bolts are driven
counterclockwise by an Allen wrench, it
is coming out and locks up a cover. (See
section 8.4) After locking, the covers should
be confirmed not to be opened.
• Tighten the zero-adjustment cover mounting
screw to fix the cover in position.
7.4 Shutting Down Operation
Shut down the transmitter operation as follows.
1) Turn off the power.
2) Close the stop valves on the up and
downstream sides.
NOTE
Whenever shutting down the transmitter for a
long period, remove any process fluid from the
transmitter pressure-detector section.
7.5 Transmitter Measurement
Range (Determining
Differential Pressure Range)
The following describes the procedure for
calculating the differential pressure range and the
calculation example in low flow measurement.
Conversion factor in pressure unit:
1 Pa = 1.01972×10 -1 mmH
2
O
1 mmH
2
O = 9.80665 Pa
1 atm = 1.01325×10 2 kPa
7.5.1 Determining the Differential
Pressure Range
Use the following procedures to determine a
differential pressure range according to the fluid
conditions being measured.
(a) Calculate a water or air equivalent flow from the
flow of the fluid being measured (100% flow).
 Equivalent Water Flow Calculation
Q
w
= 0.03162 . Qf . ρf (1)
Where, Qw: Water equivalent volumetric flow
(m 3 /h) at 4°C, 1 atm
Qf: Volumetric liquid flow (m 3 /h)
at operating conditions (t°C, p kPa)
ρf: Specific liquid density (kg/m 3 )
at operating conditions (t°C, p kPa)
 Equivalent Air Flow Calculation
Q
o
= 0.5356 . Qn ρn . . (2)
273.15 + t
101.325+ p
Zf
Zn
Where, Qo: Air equivalent volumetric flow at 0°C,
1 atm (Nm 3 /h)
Qn: Volumetric gas flow at 0°C,
1 atm (Nm 3 /h)
ρn: Specific gas density at 0°C,
1 atm (kg/Nm 3 )
Zn: Compression factor of gas at 0°C,
1 atm
Zf: Compression factor of gas at
operations conditions (t°C, p kPa)
(b) Obtain a differential pressure from the
above equivalent water or air flow using the
nomograph shown in Figure 7.5.1 or 7.5.2. In
this procedure, multiply Qw or Qo by 1000/60 to
convert the flow unit into liter/min.
(c) Select an orifice bore, taking into considerations
pressure loss, etc.
(d) As necessary, calculate Reynolds number at
normal flow rate and correct the differential
pressure obtained from the procedure (b).