Siemens 7ME3510, Руководство по эксплуатации, Страница: 19 / 214

Description
3.3 Theory of Operation
FUP1010 IP67 Portable
Operating Instructions, 03/2012, A5E02951522-02
17
The Reynolds number is then computed as follows:
5Q 3LSH,'9
)
9LVFRVLW\
where viscosity = cS = cP/density
cS = kinematic viscosity
cP = absolute viscosity
The flow meter then uses this computation of Reynolds number to compensate the raw flow
velocity for conditions of laminar or turbulent flow profile as defined by an internal Reynolds
compensation table. The flow meter then converts the compensated flow velocity to
volumetric flow rate.
Rate = V
F
* Comp(Rn) * Pipe area
Volume Correction Algorithm
The flow meter provides Standard Volume output by inferentially determining the liquid API
@ 60
o
F (or the specified base temperature) and then applying the API standard (2540)
algorithm as outlined below. Before describing API algorithm the steps required to obtain
bulk or actual flow should be reviewed.
1.
Based on the measured transit time difference (Δt), average transit time (T
N
) and pipe
dimensions, the RAW flow rate (FLOW
RAW
) and liquid sonic velocity (Vos) are computed.
2. Using the liquid temperature input and the optional user provided pressure input, the
measured Vos is then compensated to the value expected for base temperature and
pressure conditions (i.e. 15.5°C and 1 bara or 60°F 14.7 psia). This compensated Vos is
called LiquIdent.
LiquIdent = Vos + TempSlope * (t - base temp) + PresSlope * (p = base pressure)
where: t = measured temperature (
o
F)
TempSlope = change in liquid sonic velocity per
o
F (Linear for petroleum products.)
PresSlope = change in liquid sonic velocity per PSI (Linear for petroleum products.)
LiquIdent provides an output that identifies the liquid independent of the operating
temperature and pressure.
1.
Through optimization the LiquIdent values (for a wide range of different liquids) can then
be associated with an API@60
o
F as well as Viscosity @ 60
o
F. This information can be
keyed into a lookup table within the flow meter. This table then serves as a reference for
dynamically inferring the liquid properties as different liquid batches enter the flow
transducer. (The lookup function for this table will also interpolate and extrapolate for
values between and outside the table values.)
2. Based on FLOW
RAW
and the inferred Viscosity, the current Reynolds # is computed. The
Reynolds # value is used to correct the raw flow for non-plug flow profile. The
compensated actual flow rate is then computed (FLOW
BULK
).
3. The volume correction can now be computed using API standard 2540. Note: 60
o
F is
used for the base temperature value in the example below, but this can be altered within
the flow meter menu.