Educational Simulators for Industrial Process Control - Simulation and Modeling Methodologies, Technologies and Application - page 148

The system (Fig. 3) is composed by a tank that receives a flow of water, a centri-
fugal pump connected to the tank outlet, a recirculation valve (V1) and an outlet valve
(V2). The level controller (LC1) output is connected, in cascade, with the flow con-
troller (FC2). The FC2 output drives V2. At the pump outlet, there are two pipes; one
is connected to V2 and the other one to V1. The water can be sent back to the tank
and this flow (FI4) is governed by the flow controller (FC1) that drives V1.
Fig. 3. Minimum recycling control of centrifugal pump
The aim of the control structure is ensure that, regardless of the LC1 actions, the
pumped flow (FC1) must be always greater that a minimum value in order to avoid
both thermal, mechanical or electrical problems and the pump cavitation. So, the Set
Point (SP) of FC1 is the minimum pumped flow, which is a manufacturer specifica-
tion that can be changed by the user. All controllers are implemented by PIDs (Pro-
portional, Integral and Derivative). The process disturbance is the external flow to the
tank (FI3) and it is a boundary condition that can be modified by the user.
Besides, there are two alarm indicators at the P&ID scheme, one for high level
(HLA1) and the other for low level (LLA1). The HLA1 indicator will be active and
change its color when HLA1 is inactive and the level of the tank will be upper than
90% (HLA value), and HLA1 will be inactive and change it color again when HLA1
is active and the level will be minor than 85% (HLA-gap value). The values of HLA
and gap can be modified by the user. The LLA1 indicator works in a similar way than
the HLA1 indicator.
The module allows modifying the PIDs parameters and observing the control struc-
ture performance when the feed flow changes, in particular when it is less than the
minimum pumped flow.
As it was previously mentioned, FI3 and FC1 SP can be modified using a
step, ramp, oscillatory or random signal. The user must click on the corresponding