Automation and control of a bank for teaching techniques regulating pumping stations.

CENTER. Centro Nacional de Tecnología de Regadíos.

The aim is to show different means of regulating pumping stations. It was conceived as a teaching tool for various regulatory systems. It has three pumps in parallel with a joint capacity of 4.5 litres/second for 40 mWC (metres of water column). The flow driven by the pumps passes through two groups of valves in parallel that simulate the consumption points, and the flow is then directed back to the 500-litre pump suction tank.

The regulatory systems that may be controlled include:

a) Fixed speed pumps (FSP):

Control is carried out using several fixed speed pumps coupled in parallel. Three pressure switches start and stop the pumps and these can be installed with their outputs wired to the pump switches. Alternatively, the pressure switches may be ‘virtual’ as the signal from a pressure transducer is used by the programmable controller to activate the pumps and so simulate the pressure switches. A boiler is used to limit the transients caused by the starting and stopping of the fixed speed pumps.

b) Variable speed pumps (VSP):

Control is carried out using a variable speed pump with two fixed speed pumps in parallel. In this case, the pressure transducer signals the controller that handles both the rotational speed of the fixed speed pump, as well as the start-ups and stops. The boiler is used to limit the transients caused by the actions of starting and stopping the fixed speed pumps.

c) Fixed speed pumps (FSP) with pressure reducing valves (PRV) at the outlet of the pump station:

Starting and stopping of the pumps is controlled by three pressure switches, but the pressure is limited by using a two-inch pressure reducing valve at the outlet of the discharge manifold – which causes losses in the load that are sufficient to reduce the pressure to a previously established level. This line will be isolated when the regulation system is not in use, and this approach is undoubtedly more expensive in energy terms than previous approaches. To use this regulatory system, it is necessary to close the valve at the discharge manifold that links it with the flow measurement system. To operate the valve, the pressure supplied by the pumps must be greater than the pressure at the reducing valve. Clearly, if we only measure the pump flow rather than in combination with the valves, then the former is at a greater pressure since with an equal resistance in the discharge circuit, the supply pressure is lower when using the reducing valve.

d) Fixed speed pumps (FSP) with pressure sustaining valve (PSV) in a bypass:

Starting and stopping the pumps is controlled by three pressure switches, but the supply pressure to the system is limited by using a two-inch pressure sustaining valve (PSV) at the outlet of the discharge manifold and this is connected through a bypass to the suction tank, thereby ensuring that part of the flow is directly returned to the tank. Similarly, the flow that reaches the intake valve with the PSV closed is greater than when the PSV is open for the same resistant circuit. Finally, this line must be isolated when the control system is not in use.