November 2001

Bonus Report: Instrumentation

Using a single final control element to regulate two process variables

This analysis showed the loops would be stable

Needham, M. J., Technip Upstream

Fig. 1 is a diagram of the control loop that uses a centrifugal pump to both hydraulic decouple inflow surges into the surge tank and also decrease inflow to the free water heated knockout (FWO) as the oil/water interface level increases. When our client proposed this loop, we were concerned. Based on the different mathematical input/output (transforms) relationships (Figs. 2 and 3) between the two-level processes, there could be loop instability in trying to calculate a reasonable setpoint for the bypass flow loop. While the surge tank's large volume capacity added to stability of the loop, the small heated vessel had a small capacity and high throughput. Theoretical Laplace trans

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