An Example Tutorial to Model Differential Pressure (DP) Pressure Transmitter for Flow Control in Aspen HYSYS Dynamics

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ASPEN HYSYS DYNAMICS MODELLING OF DIFFERENTIAL
PRESSURE (DP) TRANSMITTER FOR FLOW CONTROL
Differential Pressure (DP) transmitters are commonly used in Process industries (E.g., Oil & Gas, Refineries and
Petrochemicals) to measure parameters such as flow, liquid level, etc. based on the differential pressure across
a primary element (E.g. Orifice plate/Venturi/pitot tube). To control parameters such as fluid flow, a control valve
catered by the primary element is adjusted by a controller that receives differential pressure information from a
DP transmitter which becomes the secondary element. Below is an example schematic of a DP transmitter setup
that adjusts flow based on the recorded differential pressure from a primary element (Orifice Plate).
Figure A. DP based Flow Control [Ref: https://automationforum.co/basics-of-pressure-transmitter/]
The feedback DP controller to maintain constant differential pressure across a process element can be
implemented in Aspen HYSYS Dynamics the following way.
Figure R.1 Aspen HYSYS Dynamics Model Setup
A schematic of the DP controller is shown in Fig. R.1. A holdup volume of 1 m
3
+ 1 m
3
is added to both VLV-100
& VLV-103 to take into account time delay in the output flow variation. It is desired to maintain a DP value
of 0.3 bar across VLV-102 [Orifice Plate] by adjusting VLV-101 opening. The steps therefore are,

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1. Add a PID controller (PIC-100 & PIC-101) for Stream 3 as well as Stream 4 & declare the process variable
source as ‘pressure’. The controller mode is set to ‘manual’ & ‘direct’. This is done since the controller’s duty
is only to read the pressure values form the streams & transmit it to a selector block (OS-1) which described
in the next point. Note that the PVmin & PVmax is set to 2 times the value on either side (i.e., stream 3 has a
PVmin & PVmax as 0.0 barg & 9.4 barg respectively, while stream 4 has a PVmin & PVmax as 0 barg & 8.8 barg
respectively). Details of PIC-100 are shown in Figs. R.2 & R.3 which is the same for PIC-101 except that the
PVmax value is changed to 8.8 barg.
Figure R.2. PIC-100 Connections Tab Figure R.3. PIC-100 Parameters Tab
2. A selector block (OS-1 ) is added & the Output Target Source of the two pressure transmitters (PIC-100 &
PIC-101) is selected as ‘Input’ of OS-1. When this is done the selector block process variable sources show
‘OP’ as the variables for the two inputs PV1 & PV2. These need to be changed to ‘PV’ by clicking on ‘Edit PV’
& selecting ‘PV’ as the variable. These are shown in R.4 & R.5.
Figure R.4. OS-1 Connections Tab Initial Setting Figure R.5. OS-1 Connections Tab Final Setting
3. Under the OS-1 parameters tab (Fig. R.6), select the selection mode as ‘Sum’ & in the Scaling Factors Tab
(Fig. R.7), enter ‘-1’ for PV2 in Input Parameter. The intent of this step is to negate the discharge pressure of
the valve & sum it with the suction pressure to obtain the pressure drop. Therefore, the two values PV1 &
PV2 (negated) are added causing a net subtraction from PV1. This value becomes the output variable & is
fed into ‘Output Value’ indicated in the ‘Monitor Tab of OS-1.

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Figure R.6. OS-1 Parameters Tab Selection Figure R.7. OS-1 Parameters Tab Scaling
4. The ‘OP’ target in the connections tab of the selector block OS-1 needs to be declared to operate VLV-101
which adjusts the valve opening to meet the pressure drop criteria of 0.3 bar. This is done by adding a PID
controller (IC-100) whose process variable source is declared from the controller IC-100 as the ‘Output value’
& Output target object as VLV-101 ‘Actuator desired Position’. Upon doing so, the selector block ‘OP’ Target
variable should show ‘PV’. This is shown in Fig. R.8.
Figure R.8. OS-1 OP Target
5. The PID controller (IC-100) is now declared with the following parameters as shown in Fig. R.9 & R.10. It is to
be noted that the PVmax value is to be set at 2 times the SP to have 50% as the ‘OP’ value & this provides
sufficient margin for the OP variation.
Figure R.9. IC-100 Connections Tab Selection Figure R.10. IC-100 Parameters Tab Scaling