SMAR CD600 Plus - Page 64

CD600 Plus - User's Manual
4.44
PPP
ZCBTAP ++
=
Where,
T
P
and P
P
are respectively the absolute temperature and absolute pressure, in engineering units,
used in the calculation of the flow primary element.
As the block inputs are in percent and the signals from the pressure and temperature transmitters
are seldom in absolute units, the block transforms all measurements in absolute units, as follows:
Where,
100/.
100/.
0
0
tTT
pPP
T
P
α
α
+=
+=
P
0
- Value corresponding to 0% of the absolute pressure signal. If the pressure transmitter is of the
gage pressure type, the atmospheric pressure shall be added to the value corresponding to 0%.
For example:
Absolute transmitter calibrated from 2 to 10 bar: P
0
=2
Gage transmitter calibrated from 2 to 10 bar: P
0
=2+1.013=3.013
α
p
- Span of the pressure transmitter (in engineering units). From the above example α
p
=10-2=8
p - Pressure transmitter signal (in %).
T
o
- Value corresponding to 0% of the temperature transmitter + 273.15 Kelvin or + 459.67 Rankine.
α
T
- Span of the temperature transmitter (in engineering units).
t - Temperature transmitter signal (in %).
The compressibility factor must be calculated for the particular gas over the particular operating
range. Three representative points of operation must be selected from the product thermodynamic
table:
P
1
, T
1
- corresponding to density d
1
.
P
2
, T
2
- corresponding to density d
2
.
P
3
, T
3
- corresponding to density d
3
.
These values must be substituted in the following formula:
C + BT + AP
1
T
P
= W
Originating three equations that enable the calculation of
A, B and C.
Sometimes,
CBTT
p
or
CAPT
P
++
1
.
1
.
are more appropriate to describe the product behavior and are easier to calculate. For many
applications P/T is good enough.