Monthly Archive: December 2006

Dec
25

System Commissioning – Onsite Work

One of the phenomena’s that I have noted over the years is that plant startups inevitably run over into a holiday. This seems to be especially true of the Christmas holiday. Being onsite can be very demanding work but it is also very rewarding. Many of the problems that we must address in control design …

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Dec
25

First Principal Process Gains, Dead Times, and Time Constants

First principal relationships can define process cause and effects that can lead to improved controller tuning and performance by the selection of better tuning rules and process variables for scheduling of tuning settings. It also affects the choice of control valve trim and the feedforward design. The understanding of these relationships does not require a …

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Dec
18

Linear in a Nonlinear World

Control systems assume linearity. Unfortunately the world is basically nonlinear. For the next few weeks we are going to explore how gains, process time constant, and dead times change with plant design and operating condition. This week we start out looking at valve gains. A plot of the flow versus valve position (installed characteristic) of …

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Dec
18

Simulation Using Foundation Fieldbus Function Blocks

Training an operator on a new control system often includes hands on experience with a training system that supports dynamic process and control system simulation. The hardware for such a training system may be constructed using spare parts from the new control system. A simple simulation of the process can often be implemented using the …

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Dec
11

Getting a Kick Out of Stick-Slip Compensation

Last week, we discussed the deadly and sticky situations of some control valves. I once thought a variable speed drive (VSD) was the solution until Bob Heider pointed out that some VSD packages may not be as sensitive as a well designed throttling control valve. A VSD resolution may be 0.4% whereas a sliding stem …

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Dec
11

Process Control Function Blocks

As the Fieldbus Foundation (FF) function block specification was being developed, I became aware of the work by International Electrotechnical Commission, IEC, on function block modeling. The IEC TC65C/WG6 (now SC65B/WG15) committee had produced some draft documents that would eventually become the IEC61499 Function Block Standard. This standard defines abstract models that may be used …

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Dec
04

Control Using Wireless Transmitters

Over the last few years the process industry has expressed a growing interest in the application of wireless technology for field measurements. The ISA-SP100 Committee was established in early 2005 to set standards and recommended practices for implementing wireless systems in the automation and control environment with a focus on the field level. Also, various …

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Dec
04

Deadly Deadband

A control valve isn’t doing much to help a control loop deal with the minute by minute onslaught of disturbances if it does not respond to the controller’s output. Yet there is normally nothing in a control valve’s specification form to insure the control valve actually moves. A step forward has been the ANSI/ISA standard …

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