MODELLING CASCADED SPLIT RANGE (CASC-SRC) CONTROLLERS IN ASPEN HYSYS DYNAMICS
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This document demonstrates modeling a cascaded split range controller (CASC-SRC) in Aspen HYSYS Dynamics to control an LNG pump and vaporizer control valve. The CASC-SRC uses a high pressure pump speed controller and vaporizer flow controller, both operating in reverse action. An SRC is added with the flow controller as the first input and pump controller as the second. Low and high range values for each controller are calculated and assigned in the SRC configuration. With the CASC-SRC in auto mode, HYSYS stabilizes at operating points matching the original set points of 2950 rpm pump speed and 7500 kmol/h flow.
MODELLING CASCADED SPLIT RANGE (CASC-SRC) CONTROLLERS IN ASPEN HYSYS DYNAMICS
- 1. Page 1 of 3 MODELLING CASCADED SPLIT RANGE (CASC-SRC) CONTROLLERS IN ASPEN HYSYS DYNAMICS The following article demonstrates the use of High Pressure (HP) VFD Operated LNG pumps that controls the LNG (Liquefied Natural Gas) pressure and a flow control valve a.k.a vaporizer control valve that controls pump flow. This ensures that the right amount of flow passes through the heater. With some example data, an LNG stream of 7500 kmol/h at 12 bara and -160 0 C is pumped to a vaporizer at 95 bara. The HP pump rated at 3000 rpm, runs at 2950 rpm while the design flow of the vaporizer is 7534 kmol/h. The vaporizer outlet temperature required is 2.5 0 C. A split range controller (SRC) is added that alters the pump speed and vaporizer control valve to maintain the LNG Send Out at 7500 kmol/h. Figure 1. Cascaded Split Range Controller for LNG Pump Setup The HP pump operates with a speed controller and the vaporizer control valve operates with a flow controller - both of which are 'reverse acting'. This means that when the process variable (PV) (speed) goes up, the power fed to the pump (OP) drops. Similarly, when the LNG flow increases through the vaporizer control valve, the valve closes accordingly to maintain the required flow. Adding an SRC in HYSYS, the parameters can be declared as follows,
- 2. Page 2 of 3 Upon entering the SCV Flow Controller as the first input and HP Pumps Controller as the second input, under the configuration window as shown in Fig. 4, enter the PVMin value as 0 kmol/h and 10,000 kmol/h in the PVMax Window. In the Split Range Setup Tab, the low range and high range values as shown in Fig. 5 are calculated as shown in Eq. 1 to Eq. 6. The value computed in Eq. 3 is assigned as the highest fraction to SCV Flow controller as shown in Fig 5 while the HP Pumps controller high parameter is always assigned 100.
- 3. Page 3 of 3 Note that the Low Range Values for both controllers in the Split Range Setup Tab (Fig. 5) is entered as ‘zero’. Followed by this, the lower fraction of 98.33 is assigned to OP cell in the parameters page (Fig. 6). It is recommended to use a PI controller with IMC tuning methods as a start and add derivative action (if necessary) depending on system response to avoid an oscillatory behavior of the controller. Lastly, the individual SCV flow controller (Fig. 7) and HP Pumps controllers (Fig. 8) are to be transferred to ‘Casc’ mode and the simulation has to be run for a while. With the SRC controller being changed to ‘Auto’ mode (Fig. 9), the HYSYS model stabilizes to arrive at a final operating point similar to the individual controllers’ original set points (SP’s) of 2950 rpm and 7500 kmol/h. As a check, note that the Split Range Outputs match with the values of Eq. 1 and Eq. 2.