SMAR CD600 Plus - Page 65

Library of Function Blocks
4.45
Using the normal operating conditions, P
P
and T
p
, as used for the flow primary element calculation,
calculate
d
p
.
In order to cancel the density for normal flowing conditions:
d
p
1
= k
FORMULA FOR LIQUIDS
K
)
CT
r
2
+
BT
r
+ (A
. Q =
Q
c
Where,
C
T
T
t. T 0 α +
T
R
- Reduced temperature =
T
C
- Critical temperature of the liquid.
K - Density of the liquid at the design temperature of the primary element.
The fluid density is given by:
d = A + BT
r
+ CT
r
2
Constants
A, B and C may be found in chemical manuals for some products or may be calculated
using three points of operation as described for gas compensation.
In order to cancel the density for normal flowing conditions:
K = d
p
FORMULA FOR SATURATED STEAM
The characteristic curve of saturated steam is almost linear in some operation sections.
EXAMPLE
:
d = 0.49315P + 0.2155 for 10 ≤ P≤ 35
P
expressed in bar absolute, d in kg/m
3
In this case is better to use the formula for liquids. The pressure signal must be connected to input
B
so that
T
R
becomes P. Furthermore, the following shall be done:
T
o
= Value equivalent to P
o
.
α
T
= Value equivalent to α
P
.
T
C
= 1.
And, in the case presented as an example,
A = 0.2155
B = 0.49315
C = 0
If the orifice plate was calculated for
P = 20 bar abs, in order to cancel the density when the
pressure is 20 bar abs:
K = 10.08, this being the density of steam at 20 bar absolute. Coefficients A, B and C may be
investigated for other operating ranges.