Monthly Archive: February 2008

Feb
29

Common Control Myths – Part 2

Myths are a fertile topic maybe because of all the fertilizer in process control. You can make almost any point you want, by changing what are often obscure details on process and automation system dynamics. For example, you can show a variable speed drive can do better or worse than a control valve. The results …

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Feb
22

Common Control Myths – Part 1

Process control is rich with mythology probably because what happens in the field is pretty remote from what was described in control text books. It is best summed up by a button given to me by my daughter 25 years ago that says “Reality Reeks.” Here are some myths that come to mind this Friday …

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Feb
15

Actual Control Valve Scenarios – Part 3

(5) A column distillate receiver has a relatively constant amplitude level oscillation (a) The amplitude of the oscillation is relatively constant for a given tuning (b) The amplitude and period of the oscillation increases as the controller gain is decreased* (c) Actual readback of level valve position indicates the minimum change in valve position routinely …

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Feb
08

Actual Control Valve Scenarios – Part 2

(1) A compressor shuts down (a) The first out sequence indicates the compressor tripped on high speed (b) A precipitous drop in suction flow followed by a rapid 1-2 second oscillation in suction flow preceded the speed deceleration from compressor shutdown (c) The readback of actual surge butterfly position indicates the valve closed before the …

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Feb
01

Actual Control Valve Scenarios – Part 1

Have you seen a loop more oscillatory in automatic than in manual? Is the problem loop interaction, aggressive controller tuning, or a sticky-sloppy valve? If you have a digital positioner with position readback on a throttle valve supplied by a control valve manufacturer, you have a chance of figuring out if the valve is the …

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