Micro Motion Fisher Rosemount RFT9739, Инструкция по эксплуатации

Страница: 105 / 130

RFT9739 Rack-Mount Transmitter Instruction Manual
95
Theory of Operation
continued
ρ
o
=
ρ
s *
exp [–
α∆
T (1 + 0.8
α∆
T )]
where:
ρ
o
= operating density
ρ
s
= standard density
∆
T = temperature difference from base (standard) temperature
α
= K
0
/(
ρ
s
)
2
+ K
1
/
ρ
s
, where K
0
and K
1
are constants
The equation is iterative, and requires significant calculation time to
generate one reading. The transmitter software contains a simplification
of this correlation to maximize sampling frequency of the measurement.
Accuracy of the Micro Motion correlation is ±0.0005 g/cc (±0.5 kg/m
3
)
relative to the API-2540 equation. After temperature correction to 60
°
F
(15
°
C), the density is converted to
°
API by the following expression:
Degrees API = (141.5/standard specific gravity) – 131.5
The K
0
and K
1
terms in the API-2540 equation are constants
characteristic of different types of Generalized Petroleum Products.
Separate API tables exist for crude oils, distillates, gasolines, lube oils,
and other products. The correlation in the RFT9739 is based on the API
constants for Generalized Petroleum Products from 2 to 95
°
API over an
operating temperature range of 0 to 300
°
F. As fluid density or operating
temperature extends beyond these values, the RFT9739 correlation
error will increase. Density calibration must be performed in units of g/cc
for the API correlation to be correct.
API standard volume
If
°
API is selected as the density unit, the RFT9739 automatically
calculates standard volume at 60
°
F or at 15
°
C based on the following
expression:
Standard volume = mass flow/standard density
Accuracy of standard volume measurement is based on the accuracies
of the following factors:
• Mass rate measurement
• Operating density measurement
• Temperature measurement
• RFT9739 correlation to API tables
The accuracy of each factor varies based on the process operating
conditions and fluid that is being measured. For Generalized Petroleum
Products, standard volume will be accurate within ±0.5% of the flow rate.
Because the temperature correction correlations for density are based
on API equations, the RFT9739 standard volume output can be used
only for Generalized Petroleum Products or materials that exhibit the
same thermal expansion characteristics as Generalized Petroleum
Products.