Example Tutorial to Model Vessel Liquid Level On/Off Control in Aspen HYSYS Dynamics

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Vessel Liquid Level On/Off Control in Aspen HYSYS Dynamics
Process Vessels in Oil & Gas facilities are sometimes operated using an On/Off methodology where a
vessel is drained upon its liquid level reaching Max level & the drain valve at the vessel bottom is closed
allowing liquid to fill again. As an example, consider a horizontal vessel with a 2:1 ellipsoidal head of
diameter 3.8m & total length of 10m. The Low liquid level (LLL) indicator is at 450mm, High level (HLL)
indicator is at 1800mm & the High–High Level (HHLL) indicator is at 1950mm. When the liquid level
reaches 1800mm, liquid flowing out of the vessel (171-V-0101) is at 780 m3
/h through drain valve LV-
0021 (100% opening) & closes when LLL is at 450mm, allowing the vessel to fill up again. In the following
example, dummy water and gas streams are declared to fill up the vessel operating at 15.4 barg & 38
0
C.
Figure S.1. On/Off Controller for Vessel Liquid Level
The On/Off control can be implemented using Digital Points (Shown as On/Off objects in Fig. S1) & a
Boolean Latch Operator (Shown as Latch-1) to activate & deactivate the opening of the valve, LV-0021.
The setup of the On/Off Low Level & High Level objects is shown in Fig S.2, S.3, S.4 & S5.
Figure S.2. Low Level Digital Point Connections Figure S.3. High Level Digital Point Connections

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Figure S.4. Low Level Digital Point Parameters Figure S.5. High Level Digital Point Parameters
In order to choose between the High & Low digital points, a Boolean latch gate is used to select between
these & gives its output to another digital point (LIC-0021) to prompt valve, LV-0021 to operate.
Figure S.6. Latch-1 Connections Figure S.7. Latch-1 Parameters
Ensure that under Actuator tab of Dynamics Tab for Valve LV-0021 uses a linear mode as a parameter
instead of instantaneous. The LIC-0021 digital point is configured as shown below.
Figure S.8. LIC-0021 Connections Figure S.9. LIC-0021 Parameters
The HYSYS Dynamics Model upon executing causes the Drain Valve LV-0021 to
‘Open’ when the liquid level reaches 1800mm & ‘Closes’ at 450mm liquid level